FOUS11 KWBC 142045
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
345 PM EST Thu Jan 14 2021

Valid 00Z Fri Jan 15 2021 - 00Z Mon Jan 18 2021

...Minnesota and Iowa/nearby Midwest...
Day 1...

A closed low-mid level circulation drifts south from MN into IA
tonight into early Friday before moving slowly east and
dissipating Friday night.
Low-mid level convergence and warm advection wraps around the low
across southern MN and Iowa, leading to periods of snow. The slow
movement of the circulation should allow bands of snow to persist
for several hours overnight tonight, leading to accumulations of
several inches of snow.

The latest WPC PWPF shows high probabilities for snow
accumulations of 4 inches or more covering a large portion of
southern Minnesota into central Iowa.
The peak probability for 8 inches is 40-50 percent near the
central MN/IA border. The snow, combined with blowing and
drifting due to windy conditions, has lead to the issuance of
blizzard warnings. The system is expected to gradually weaken and
move east on Friday, leading to snow fall coverage and intensity
waning as the day progresses.

...Central Appalachians and Northeast...
Days 1-3...

The closed low from the Midwest will lift east/northeast Friday
into Saturday, with widespread precipitation expected east of the
low within the broad low-mid level warm/moisture advection coming
into the Central Appalachians Fri on the nose of the 700 mb jet,
then extending up into New York and New England Fri night to
Saturday. Ascent should be strong, given the nicely coupled jet
structure at 300 mb leads to a well defined divergence maxima in
eastern NY/western New England.
Initial temperatures are below freezing through a deep layer in
upstate NY and interior New England, allowing the initial burst of precipitation to start as snow.
Favorable upslope conditions should lead to locally heavy snow on
the eastern slopes of the Adirondacks, and the Green Mountains of
southern VT.
The low-mid level warming results in the possibility of a change
over to mixed precip types in the Catskills to the Berkshires,
southern Green and White Mountains, so precip type uncertainty is
in place.

A triple point sfc low develops and moves north across New York.
West of the low track has a better chance of remaining all snow.
By Day 3, the low departs north into adjacent Canada, with the
slow moving circulation resulting in moisture wrapping back around
the low into the mountains of western Maine, the White, Green, and
Adirondack Mountains, with several inches of snow possible in
favored terrain. Strong cold advection across lake Erie and
Ontario should results in favorable lapse rates and development of
snow showers and bands of snow in lee shore convergence areas.
Several inches of snow is possible along the eastern shore of Lake
Erie and the eastern shore of Lake Ontario, including in upslope
areas of the Tug Hill, where up a foot of snow is possible.

Pacific Northwest to Northern Rockies Day 3...

The mid level ridge retreats inland Sat night and the next low-mid
level front approaches the Pacific Northwest by Sun morning. The
low-mid level moisture advection and convergence leads to valley
rain and mountains snow for the WA/OR Cascades, spreading into the
northern Rockies by the end of the Day Sunday. Higher amounts are
expected in the WA Cascades due to closer proximity to the
moisture source and the longer duration snows. This is supported
by multiple models and SREF and GEFS Mean snowfall forecasts.
Several inches of snow is expected in the WA Cascades.

For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Petersen


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FOUS11 KWBC 152042
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
341 PM EST Fri Jan 15 2021

Valid 00Z Sat Jan 16 2021 - 00Z Tue Jan 19 2021

Days 1 to 3...

...Midwest/OH Valley to the Great Lakes/Central Appalachians and
Northeast...

A slow-moving and vertically stacked low pressure system currently
over the Midwest will shift gradually east across the OH Valley
tonight and cross the central Appalachians, northern Mid-Atlantic
and lower Great Lakes region on Saturday. As the strong height
falls approach the East Coast later tonight, a robust shortwave
will lift out of the base of the upper trough and up along the
Mid-Atlantic coast which will foster secondary cyclogenesis along
the attendant cold front edging offshore. This low center will
then lift inland across southern/central New England by Saturday
evening and then through central/eastern Maine by early Sunday. As
all of this occurs, deep layer south to southeast flow will ride
northeast up across New England bringing strong warm-air advection
and a robust fetch of Atlantic moisture for widespread moderate to
heavy precipitation. While there will be little in the way of cold
air initially in place to support widespread snow, the thermal
profiles do appear to be conducive for locally heavy, wet, and elevation-dependent snowfall for portions of NY and New England.
The main focus should on elevations especially above 1500 feet in
the Adirondacks, the Berkshires, and the Green and White
mountains. Adjacent areas of western Maine will also be a focus
for heavy snowfall. The strong and mild Atlantic inflow should
keep snow levels somewhat elevated at least through midday
Saturday across the Northeast, but as the stronger height falls
arrive and low-level cold air begins arriving around the back side
of departing low pressure, the snow levels will begin to fall and
reach some of the interior valley locations across NY and New
England later in the day. The current forecast is for as much as 6
to 12+ inches of snow for the aforementioned higher elevations of
the Northeast going through early Sunday, with the heaviest
accumulations generally over the Green and especially White
mountains where storm totals of 12 to 18+ inches will be possible.

Meanwhile, the deep layer cyclonic flow coupled with multiple
embedded shortwave impulses will continue to favor broken areas of
light to occasionally moderate snow across portions of the Midwest
this evening and then spreading into the OH Valley by Saturday
morning. This energy coupled with cold and relatively moist low to
mid-level upslope flow into the central Appalachians should favor
an axis of heavier accumulating snowfall from central WV into far
western MD and southwest PA. Locally some areas of central WV may
see in excess of 6 inches of snowfall going through Sunday as the
focus will transition increasingly from synoptically forced
snowfall to more of a lake-enhanced snowfall threat as the
boundary layer winds gradually involve more of the flow over the
Great Lakes going through Saturday night and early Sunday.
Portions of western NY involving the Tug Hill plateau will also
see an increasing focus of heavy lake-enhanced snowfall Saturday
night and Sunday around the departing low center over the
Northeast with a highly favorable cold, moist and convergent
low-level fetch coming off the relatively warm waters of Lake
Ontario. As much as 6 to 12 inches of snow will be possible here.
Elsewhere, the broad deep-layer cyclonic envelope of this system
will also maintain the threat of accumulating lake-effect snowfall
for even areas of the U.P. of MI where north to northeast
low-level flow over Lake Superior will support locally 6+ inches
of additional snow for areas east of the Keweenaw Peninsula.

As the main closed low/trough pushing into the Northeast begins to
pull away by late Sunday, there will be a new vigorous upper
trough/closed low center dropping southeast across the Midwest and
the OH Valley going through early Monday. This may bring locally a
few additional inches of snow to the central Appalachians and
portions of western NY.

...Northern/Central Rockies...

Multiple shortwave troughs will be advancing southeast down across
the northern/central Rockies going through this weekend and early
next week, with one impulse sliding progressively southeast
tonight and early Saturday, and then a stronger shortwave digging
southeast and into Intermountain West by Monday. Stronger
jet-aided forcing associated with the shortwave impulses coupled
with relatively moist mid-level flow should favor heavy snowfall
for portions of the Bitterroots and Sawtooth where as much as 6 to
12 inches will be possible going through Monday. However, somewhat
stronger upslope flow should tend to favor even heavier snowfall
totals that may exceed 1 foot across the Belts, Beartooths and
Bighorns. Some of this heavier snowfall should make it farther
south down into south-central WY and far northern CO with the
Medicine Bow mountains likely to see as much as 1 foot of snow as
well.


For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Orrison



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FOUS11 KWBC 160822
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
322 AM EST Sat Jan 16 2021

Valid 12Z Sat Jan 16 2021 - 12Z Tue Jan 19 2021

...Central Appalachians, Great Lakes, and Northeast...
Days 1-3...

Complex mid-level and surface pattern will produce waves of
precipitation and snowfall including synoptic snow, upslope snow,
and lake effect snow, through early next week.

Today, an area of low pressure developing south of New England
will deepen and lift north into Maine tonight. Ahead of this low,
impressive warm and moist advection, aided by height falls and PVA
associated with a spoke of vorticity/shortwave lifting northward,
will cause precipitation to expand and shift northward, spreading
rain and snow into the Northeast. The antecedent column is cold
only across Maine, with marginal low-level thermals in place
elsewhere. This will lead to a challenging p-type forecast as the
heavy snow will be dependent both upon elevation and dynamic
cooling. For the former, snow levels are forecast to rise as high
as 1500ft, so elevations above that in the Adirondacks, Greens,
Whites, and much of northern/central ME will see mostly snow and
heavy accumulations. WPC probabilities are high for 6" in these
areas, and may eclipse 1 ft in the highest terrain due to snowfall
rates in excess of 1"/hr at times. At the lower elevations,
precipitation will likely change to rain for much of the event,
however, periods of heavy snow are possible where ascent is most
robust causing the column to dynamically cool. Strong WAA and weak
elevated instability will combine to produce a few inches of snow
in the Berkshires and Catskills, with light accums even possible
in the Worcester Hills. However, due to varying precipitation rate
and p-type, confidence is a bit lower below 1500ft, and note the
WSE ensemble mean came down quite a bit overnight in these lower
elevation regions. Some lingering heavy snow is expected into
northern Maine to start D2, but as the system pulls away the heavy
accumulation across New England will wane.

Behind the low, a deep closed mid-level low and strong CAA behind
a cold front will lead to upslope snow across WV which may
accumulate more than 6". A secondary impulse moving over the
region Sunday into Monday will renew the CAA and upslope
potential, with additional light accumulations possible both days. Additionally, the CAA moving across the nearly ice-free Great
Lakes will produce periods of LES, some of which is likely to be
heavy, especially downwind of Lake Erie and Ontario. Nearly
unidirectional flow and enhanced instability both due to the CAA
atop the lakes but also due to the upper low overhead should
produce heavy snow rates into the Tug Hill Plateau D1 and D2, with
a renewed surge enhanced LES off Lake Erie D3. WPC probabilities
D1 and D2 are high for 6 inches east of Ontario, and high for 6
inches east of Erie on D3. Total LES could exceed 12" east of Erie
and 18" in the Tug Hill.

Additionally, although accumulations are expected to be light, the
upper low behind the cold front could produce an environment
favorable for heavy snow showers or even snow squalls across a
large portion of the Appalachians and into the Mid-Atlantic this
afternoon and evening. A brief uptick of instability rising to
near 200 J/kg combined with modest 925mb fgen and enhanced
mid-level RH supports some convective potential. While the snow
squall parameter does increase, 0-2km theta-e lapse rates are
minimal, so its more likely that snow showers will occur this
aftn, but cannot rule out a squall. Accumulations will be minimal,
but brief visibility restrictions and hazardous travel are
possible should any snow squalls develop.


...Pacific Northwest to Northern Rockies...
Days 2-3...

Broad mid-level ridge will give way to an approaching shortwave
trough from the Pacific, accompanied by the RRQ of an equator-ward
diving jet streak on Sunday. This will be followed quickly by a
second shortwave and another jet streak to produce periods of
precipitation from WA state through WY into early next week. While
the duration and intensity of ascent appear to be modest at any
given time, persistent moist advection from the Pacific will be
tapped by this forcing to produce heavy snow in the terrain,
generally above 5000-6000 ft. Where upslope enhancement can occur
from the W/NW mid-level flow, snowfall during the 2 days may
exceed 1 ft, and this is most likely in the Absarokas, Big Horns,
and Black Hills. Otherwise, WPC probabilities are modest for 6"
across much of the remaining terrain.


...Southern Rockies...
Day 3...

A trough and accompanying Pacific jet streak will dive down the
west coast Monday. Two shortwaves embedded within this amplifying
trough will merge into a closed low near the coast of Southern
California, driving mid-level WAA and moist advection towards the
Four Corners in conjunction with enhanced mid-level divergence. At
the same time, a cold front will dive across the Plains and bank
against the east side of the Rockies including the Sangre De
Cristos. While this front likely won't make much further progress
SW, it will lead to enhanced upslope flow, and heavy snow is
likely in the Sangre De Cristos, San Juans, and even into the
Front Range Monday and Monday night. While heavy snow is likely in
these ranges, especially east/northeast slopes, on the downwind
side there may be some shadowing, as there appears to be a gap
between the best low-level ascent and upper forcing. This leads to
WPC probabilities for heavy snow to be confined to the terrain and
not bleed into the lower elevations of NM. WPC probabilities for
D3 are high for 6 inches in the Sangre De Cristos and San Juans,
with locally more than 10" possible.


For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Weiss



$$
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FOUS11 KWBC 162113
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
412 PM EST Sat Jan 16 2021

Valid 00Z Sun Jan 17 2021 - 00Z Wed Jan 20 2021

...Central Appalachians, Great Lakes, and Northeast...
Days 1-3...

An area of low pressure is forecast to move north across Maine
tonight and into adjacent New Brunswick Sunday. Ahead of this
low, warm and moist advection is forecast to lead to mid level
temperatures briefly rising above freezing. This will lead to a
challenging p-type forecast as the snow in northeast Maine mixes
with and changes to sleet and freezing rain tonight. The mixed
precipitation aspect of the event ends Sunday as the low moves
away.

Behind the low, a deep closed mid-level low and strong cold
advection behind a cold front moving across the nearly ice-free
Great Lakes will produce periods of lake effect snow downwind of
Lakes Erie and Ontario. Nearly unidirectional flow and enhanced
instability due to the steep lase rates should produce heavy snow
rates into the Tug Hill Plateau into Sunday night.
Snow coverage and intensity taper on Monday as drier air aloft
moves across Lake Ontario and then into the Tug Hill and western
Adirondacks.
As an 850-700 mb approaches from Michigan on Tuesday, a renewed
surge of enhanced Lake Effect develops off Lake Erie and then
moves into eastern Lake Ontario and the Tug Hill towards the end
of the day. WPC probabilities D1 are high for 8 inches east of
Lake Ontario, and high for 6 inches east of Erie on D3. Days 1-3
snow amounts could exceed 12" east of Lake Erie and 18" in the Tug
Hill.

Additionally, although accumulations are expected to be light, the
upper low behind the cold front could produce an environment
favorable for heavy snow showers or even snow squalls across a
large portion of the central Appalachians this evening. A brief
uptick of instability rising to near 200 J/kg and enhanced
mid-level RH supports convective potential until instability wanes
tonight. Post-frontal west-northwest winds will lead to upslope
snow across WV which is forecast to accumulate to 4-8" days 1-2.
A secondary impulse moving over the region Sunday into Monday will
renew the CAA and upslope potential.

...Pacific Northwest to Northern and Central Rockies...
Days 1-2...

Confluent upper level flow drives a 700 mb shortwave onshore and
advects moisture inland. This leads to a period of snow showers
starting the in the WA Cascades and then moving downstream into
the northern Rockies Sunday. Several inches are possible in the
mountains. Locally heavy snow is expected over a few of the ranges
of central MT, the Bighorns, and Laramie/Snowy Mountains of
WY/adjacent northern CO Sunday night into Monday as moist
confluent flow with 700 mb vertical velocity maxima stream across
the region. Persistent moist advection from the Pacific will be
tapped by this forcing to produce heavy snow in the terrain,
generally above 5000-6000 ft. Where upslope enhancement can occur
in windward areas, snowfall during the 2 days may exceed 1 ft, and
this is most likely in the Little Belt Mountains, Big Horns, and
Black Hills. The event winds down late Monday as the 700 mb ridge
bridges into the northwest and northern Rockies, drying out the
column.

...Southern Rockies/Southwest...

Day 3...

A low-mid level circulation forms over southern CA on Monday night
to Tuesday morning, drifting south into northwest Mexico. The
circulation taps Pacific Ocean moisture and wraps it around the
low, advecting it inland. Upper level divergence in the vicinity
of a jet streak extends from southern CA across AZ and into
southern CO and northern NM.
Lit near the jet along with moisture advecting inland leads to
snow developing as heights/temps aloft fall. A max is expected
int the terrain of southeast UT across the San Juan Mountains of
southern CO/northern NM.

A cold front will dive across the Plains and bank against the east
side of the Rockies including the Sangre De Cristos. Post-frontal
enhanced upslope flow should lead to heavy snow
in the Sangre De Cristos Monday night into early Tue. WPC
probabilities for D3 are moderate for 8 inches in the Sangre De
Cristos and San Juans, with a foot of snow possible in windward
terrain.

As height/temperatures aloft fall, snow may develops in the ranges
of northern AZ and southern CA, with several inches in the higher
elevations.

For Days 1-3, the probability of significant icing (0.25 inch or
greater) is less than 10 percent.

Petersen

$$
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FOUS11 KWBC 170850
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
350 AM EST Sun Jan 17 2021

Valid 12Z Sun Jan 17 2021 - 12Z Wed Jan 20 2021

...Great Lakes and Central Appalachians...
Days 1-3...

Broad cyclonic flow entrenched across the eastern CONUS will
persist through the forecast period, while waves of mid-level
energy and vorticity maxima rotate through the flow. The first of
these will eject through Maine early on D1, with a second trough
moving over the Great Lakes tonight into Monday, followed by yet a
third shortwave Tuesday night. Each of these impulses will be
accompanied by ascent through height falls and PVA, and followed
by CAA. The Lakes are mostly ice-free at this time, so strong CAA
across the warmer lake waters will setup an environment that will
become increasingly favorable for LES, and may be extremely
favorable at times. On D1, LES will be robust east of Ontario
early on D1, but the shortwave moving atop the area this aftn will
cause winds to back to the SW shutting off the heaviest LES east
of Ontario, but will begin to enhance the LES signature east of
Erie. Synoptic ascent will contribute to snowfall on D1, but the
heaviest snow will be due to LES, and WPC probabilities are high
for 4 inches in the Tug Hill and east of Erie just south of
Buffalo, NY, with local maxima of 8" possible.

The signature for LES intensifies D2, primarily east of Lake Erie.
850mb temps crash towards -12C to -15C, causing steep LRs over the
Lake, with intense forcing within the deepening DGZ and ELs
climbing above 800mb. The combination of this forcing and
instability will drive multiple rounds of intense LES on Monday,
with a single band possible producing snowfall rates reaching up
to 2"/hr. WPC probabilities on D2 are high for 4 inches south of
Buffalo, NY once again, with local maxima possibly reaching 12".
At the same time, a weak shortwave moving over Lake Superior may
spawn a wave of low pressure and enhanced instability with CAA on
N/NW winds Monday evening. This will support heavy LES across the
western U.P. of MI where WPC probabilities are moderate to high
for 4 inches.

The LES event continues and may enhance again on D3 with more
widespread LES becoming heavy downwind of most of the Lakes. This
will be due to yet another shortwave moving over the Lakes to
cause another surge of CAA and favorable low-level shear with
deepening DGZs and steepening LRs. 850mb temps crash to as low as
-20C, which raises ELs and drives the most intense forcing over
the lakes to intersect the lowering DGZ. LES on D3 is likely to be
widespread in the favored regions on W/NW flow, and WPC
probabilities for 4 inches are moderate to high in the NW L.P. of
MI, as well as east of Erie and Ontario, where some effective
fetch with a moisture tap from Lake Huron is also possible. 3-day
totals downwind of Lake Erie could exceed 2 ft, with 12-18"
possible in the Tug Hill Plateau.

South of the Lakes, a period of significant upslope snow is
possible by D3 across parts of WV. Confidence is lower here due to
soundings indicating the potential for some dry air in the upper
half of the DGZ, but moist ascent aided by upslope flow on NW
winds behind the departing shortwave has led to an uptick in WPC
probabilities which are now 10-20% for 4 inches on D3. Snow totals
over the 3 days in this region could reach 8", but should be
generally light each day until D3.

...Northern and Central Rockies...
Days 1-2...

Confluent mid and upper level flow aided by a modest eastward
moving Pacific jet streak will drive moisture inland and southeast
from the Northern Rockies towards Colorado through Monday. The
temporal duration of forcing is generally modest, but some
enhancement along the upslope/windward sides of the higher terrain
are likely to see periods of heavy snow which could add up to
significant accumulations during the next two days. Locally heavy
snow is most likely on D1 across portions of the Northern Rockies,
Absarokas, Big Horns, Bear Tooth, and Laramies, where WPC
probabilities are high for 6 inches, generally above 5000 ft, and
could reach 12" in local maxima. As the forcing and moisture
shunts southeastward on D2, modest snowfall will persist,
especially in the Absarokas where an additional 6"+ is likely, but
otherwise WPC probabilities for 6 inches shift into the CO Rockies.

...Southern Rockies/Southwest...
Day 3...

The combination of a closed mid-level low forming near Southern CA
Monday night and a cold front dropping southward across the High
Plains of NM will drive large scale ascent and increasing snowfall
across the region. The cold front will produce upslope flow to
enhance precipitation in the Sangre De Cristo range, while
divergent flow downwind of the closed mid-level feature in tandem
with increasing jet level diffluence will produce large scale
ascent lifting into the Four Corners. SW flow ahead of this trough
will drive moist advection as well, and conditions will become
increasingly favorable for heavy snowfall Monday night into
Tuesday. WPC probabilities indicate a high chance for 6 inches of
snow in the Sangre De Cristos were upslope snowfall will maximize,
but also in the San Juans and across the Kaibab Plateau where
synoptic lift will be most intense and prolonged. Snow levels will
gradually fall through D3 becoming as low as 2000-3000 ft across
the region, and this may allow for some moderate snow by late D3
to develop even along the Mogollon Rim and the higher peaks of the
San Gabriels/San Bernadinos of Southern California.

For Days 1-3, the probability of significant icing (0.25 inch or
greater) is less than 10 percent.

Weiss

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FOUS11 KWBC 180938
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
438 AM EST Mon Jan 18 2021

Valid 12Z Mon Jan 18 2021 - 12Z Thu Jan 21 2021

...Great Lakes...
Days 1-3...
Cold, cyclonic, westerly flow will continue to support lake effect
snow showers off of Lake Erie, with several more inches possible
for portions of New York's Western Southern Tier on Monday into
Tuesday. By late Tuesday, strengthening westerly winds will
support the redevelopment of lake effect snows east of Lake
Ontario, with locally heavy accumulations possible in the Tug Hill
region. Veering winds with the passage of a shortwave trough will
push snows farther south on Wednesday, before diminishing late in
the day with the arrival of a shortwave ridge.

...Northern Rockies...
Day 1...
Unstable, northwesterly flow behind a cold front will continue to
support periods of snow, with additional locally heavy
accumulations possible across the central to southwestern Montana
mountains on Monday. WPC PWPF shows high probabilities for
additional accumulations of 6-inches or more across portions of
the Little Belt, Big Snow and northern Bighorn moutains.

...Southern Rockies/Southwest...
Days 1-2...
A developing mid-to-upper level low dropping south through the
Great Basin into Southern California, along with a cold front
pushing south across the Rockies and High Plains will support
large scale ascent and increasing snowfall across portions of the
region by the late Monday. As moisture deepens ahead of the low,
favorable upper forcing along with low level easterly flow behind
the front will support developing snows across the region by late
Monday and continuing into Tuesday. Heaviest accumulations are
expected to center across the San Juan and Sangre de Cristo
mountains, where WPC PWPF shows widespread probabilities for storm
totals of 8-inches or more.

For Days 1-3, the probability of significant icing (0.25 inch or
greater) is less than 10 percent.

Pereira



$$
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FOUS11 KWBC 182058
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
358 PM EST Mon Jan 18 2021

Valid 00Z Tue Jan 19 2021 - 00Z Fri Jan 22 2021

...Great Lakes...
Days 1-3...

Cyclonic flow around a low-mid level circulation over southeast
Canada will continue to support lake effect snow showers off of
Lake Superior and Erie. Sustained west-southwest flow down the
major axis of Lake Erie supports continuing near shore
convergence, resulting in the likelihood of 6-10 inches for
southwest New York through Tuesday. By late Tuesday, a warm front
crosses Lake Ontario into near shore areas and the Tug Hill.
Frontal convergence triggers shows showers development, followed
by lake enhancement Tue night. Snow will become locally heavy Tue
night as convergent westerly flow picks up latest heat and
moisture fluxes from the lake, spawning snow showers that come
into the Tug Hill.

Wed is a transitional day as a low level ridge builds upstream and
flow across the lower lakes veers, causing drying aloft and
leading to a likely end of the lake effect snow event.
The uncertainty related to event duration before conditions become
unfavorable for lake effect snow.

On Thu, the low level ridge retreats to the east and the next warm
front crosses from the upper to the lower Lakes. Frontal
convergence again triggers snow shower development. Probabilities
are low for heavy snow as the front progresses across the region,
limiting event duration.

Downstream from Lake Superior,a couple of different periods of
enhanced activity and break occur with a succession of secondary
low level boundaries. One passes through tonight with the eastern
UP of MI getting a period of enhanced moisture and boundary layer
convergence and lift lasting overnight into Tue before waning Tue
night.
Lake effect snow showers redevelop on Thu in the lee of Lake
Superior as a secondary cold front crosses the region. Veering
low level flow results in northwest winds across the lakes late
Thu., with shoreline convergence and upslope flow resulting in
snow showers increasing in coverage/intensity. Limited duration
of the snow showers keeps snow heavy probabilities low.

...Southern Rockies/Southwest...
Day 1...

A developing mid-to-upper level low dropping south through the
Great Basin into Southern California drives a 300 mb jet streak
across southern UT to central and southern CO tonight through
early Tue. Upper divergence maxima across northern NM and southern
CO combine with strong 700 mb convergence and moisture advection
to large scale ascent and increasing snowfall across
the San Juan Mountains and Sangre De Cristo Mountains of northern
NM and southern CO. Heaviest accumulations are expected to center
across the San Juan and Sangre de Cristo mountains, where WPC PWPF
shows widespread probabilities for storm totals of 8 inches or
more and areas of 12-16 inches.

On Tuesday night, the low drifts south away from the southwest and
the strength of the low level flow weakens crossing CO/NM. This
weakens low level convergence and lift, so snow coverage/intensity
should wane, lessening snow rates and making day 2 snow light.

For Days 1-3, the probability of significant icing (0.25 inch or
greater) is less than 10 percent.

Petersen



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FOUS11 KWBC 202015
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
314 PM EST Wed Jan 20 2021

Valid 00Z Thu Jan 21 2021 - 00Z Sun Jan 24 2021

...Great Lakes...
Days 1-3...
Shortwave ridging early D1 will quickly be replaced by renewed
troughing as a shortwave and associated vorticity impulse moves
across the Lakes by Thursday aftn. Once this shortwave moves east,
continuous and broad cyclonic flow will persist across the Lakes,
with reinforcing shortwaves and cold fronts moving across the
region into the weekend. The heaviest snow through the period is
likely east of Lake Ontario into the Tug Hill Plateau late D1
through D2. This is when the best overlap of forcing is expected,
with westerly low-level winds becoming aligned to the longer fetch
of the Lake. 850mb temps only fall to around -10C so lapse rates
are steep but not excessive, and inversion heights are modest.
However, enough instability across the Lake combined with
prolonged convergence, enhanced synoptic ascent, and the potential
for an effective fetch with moisture from Lake Huron, should lead
to snowfall rates which may at times eclipse 1"/hr. WPC
probabilities are high both on D1 and D2 for 4 inches, with local
totals exceeding 12" possible combined over the two days. Renewed
shortwave ridging D3 will cause LES to wind down on Friday. In
addition to the heavy snow downwind of Ontario, moderate LES is
likely east of Erie D1.5-2, and across the U.P. south of Superior
D1-2 where WPC probabilities for 4" are generally 10-20%.

...Central Rockies...
Days 2-3...

A potent upper low will drop from the OR coast Thursday night to
onshore northern CA Friday, and then open into a large positively
tilted trough across the Great Basin by Saturday. Height falls and
modest jet diffluence will provide ascent into the Rockies, while
moisture streams northeastward from the Pacific Ocean downwind of
the opening trough. This is likely to produce widespread snow from
the Sierra eastward to the San Juans and north as far as the Wind
River range of WY. Snow levels are generally expected to be
3000-5000 ft, but may lower to 2000 ft as the cold core of the
upper low sinks southward. Periodic vorticity lobes swinging
through the flow may enhance ascent and snowfall at times, and
there is likely to be a 36 hour window of off-and-on snow across
the region. WPC probabilities for 6" are high D3 across many of
the ranges of Utah including the Wasatch and Uintas, as well as
into the CO Rockies and San Juans. Local snowfall may exceed 12
inches in the highest peaks. Lower but still significant
accumulations are possible along the Mogollon Rim and the Sierra
of CA.


...Northern Plains to Upper Midwest...
Day 3...

A weakening shortwave ejecting from the Great Basin will shear and
move northeast towards the Plains late Friday. Modest height falls
and weak upper diffluence will precede this feature, with
confluent mid-level flow spreading Pacific moisture to the east.
At the same time, a low-level trough beneath this shortwave will
move eastward, driving increasing southerly WAA ahead of it. While
the forcing for ascent is likely to be transient from west to
east, it could be intense, and forecast profiles indicate a
deepening saturated DGZ with a near isothermal layer on the WAA to
promote snow growth. A period of moderate to at times heavy snow
is becoming more likely Friday, although there is still a large
spread in potential amounts. WPC probabilities for 4" are as high
as 60% across eastern SD into southern MN D3.

For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Weiss


$$
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FOUS11 KWBC 212045
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
344 PM EST Thu Jan 21 2021

Valid 00Z Fri Jan 22 2021 - 00Z Mon Jan 25 2021

...Great Lakes and Northeast...
Days 1-2...

A continuation of moderate to heavy lake effect snow is likely
downwind of Lake Ontario and Lake Erie into the weekend. Broad
cyclonic flow across the northeast persists, with at least 2
embedded shortwaves moving through the flow - one tonight and
another Friday evening. While each shortwave will have the
potential to produce some light WAA snow ahead of a cold front due
to some modest ascent, the primary heavy snow producer will be
again due to LES as flow becomes aligned to Ontario and Erie
behind each front on CAA. For Ontario, heavy LES should begin
early Friday morning and then persist into the evening, with a
slow SE push of the trajectories likely into early D2. This should
produce bursts of heavy snow rates of >1"/hr at times, and WPC
probabilities for 6" are high on D1 in the Tug Hill Plateau, with
high probabilities for 4" on D2 shifting to a more SE trajectory
off the Lake. For Lake Erie, the heaviest snow is expected D1 on
W/SW flow beneath the inversion, producing long fetch across this
lake as well. Total snowfall may be less here than east of Ontario
due to slightly less favorable fetch and upslope, but WPC
probabilities also indicate a high chance for 4" on D1 here with
some Huron moisture likely aiding in this snow band.

While not LES, there is potential for scattered snow squalls or
heavy snow showers across Upstate New York and Northern/Central
New England on D1, potentially lingering into early D2. Increasing
instability behind the cold front with continued mid-level ascent
could produce bursts of snowfall with rates >1"/hr for a short
duration. Accumulations within any of these showers/squalls should
be light, but could produce brief periods of limited visibility
and difficult travel.

...Central Rockies to Great Basin and Southwest...
Days 2-3...

A closed low deepening over Southern California will drop
southeast Saturday and then gradually begin to open as it moves
atop the Four Corners region on Sunday. This feature will produce
height falls and mid-level divergence, with modest upper
diffluence also present for ascent. Downwind of this trough, a
Pacific jet streak will intensify, while 700-500mb flow becomes
oriented to the SW. These features together will cause an increase
in PW evident by saturation in the 700-500mb RH fields, creating
an environment favorable for heavy precipitation. While the
mid-level low opens and begins to eject quickly, pronounced
synoptic ascent in the presence of this deeper moisture will allow
for heavy snowfall to spread from the Sierra eastward through the
Utah ranges, and into the CO Rockies and San Juans D2, shifting
further south towards the Mogollon Rim and White Mtns of Arizona
on D3. WPC probabilities are high for 8 inches on D2 in the Uintas
and San Juans, as well as some of the higher isolated peaks in the
Great Basin. By D3, the remaining high probabilities for 8 inches
are confined to the Mogollon Rim, White Mtns, and continue into
the San Juans. While less impressive, snow levels which may drop
as low as 3000 ft into Southern CA will produce moderate snow as
far south as the San Gabriels and San Bernadinos, as well as light
accums possible even into the somewhat lower elevations east of
Los Angeles and east of Tucson.

...Northern Plains to Upper Midwest...
Days 2-3...

A piece of energy shedding from a closed low over the Great Basin
will eject northeast and embed within confluent mid-level flow as
it traverses into the Plains and Midwest this weekend. As this
wave moves northeast, it will be accompanied by moist advection
within the aforementioned confluent flow from the Pacific, while
low-level southerly flow intensifies out of the Gulf of Mexico
driving warm and moist advection northward. At the same time,
modest diffluence from the LFQ of an upper jet streak will aid in
ascent, and a fast moving but potent swath of precipitation is
likely to spread eastward from the High Plains of SD eastward
through the Western Great Lakes. While this system is fast moving,
it does have plentiful moisture with which to work, and forcing is
briefly robust. Additionally, regional soundings indicate an
environment favorable for very high SLRs, possibly approaching
20:1, due to a deepening DGZ, a nearly isothermal layer within the
WAA, and a cold overall column with temps in the teens near the
surface. These high SLRs should squeeze out heavy snow despite
modest QPF, and WPC probabilities for 4" are 40-60% from central
SD eastward across southern MN and into Wisconsin. Local maxima in
excess of 6" are likely in southern MN which has the best overlap
of high SLR, moisture, and forcing.

...Pacific Northwest...
Day 3...

A shortwave dropping out of the Gulf of Alaska will be accompanied
by the LFQ of a modest Pacific jet streak to drive moisture and
ascent into the Pacific Northwest the latter half of D3.
Increasing moisture and ascent will spread precipitation down the
coast from WA through northern CA. An elongated trough across the
West ahead of this feature will lead to lowering snow levels, with
further reduction likely as the shortwave drops along the coast.
Snow levels may fall below 1000 ft as far south as Portland, OR,
and between 1000-2000 ft into northern CA. The heaviest
precipitation is likely into the OR Cascades with this impulse,
but WPC probabilities for 4 inches are high from the Siskiyous
northward along the Cascades of OR and WA and into the Olympics.
With the very low snow levels and the potential for easterly winds
due to the pressure gradient across OR/WA, light snow accums are
possible into the Columbia Gorge and some of the other surrounding
lowlands on Sunday, but looks to remain just outside the major
metro areas of Seattle and Portland at this time.


For Days 1-3, the probability of significant icing (0.25 inch or
greater) is less than 10 percent.

Weiss


$$
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FOUS11 KWBC 222028
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
327 PM EST Fri Jan 22 2021

Valid 00Z Sat Jan 23 2021 - 00Z Tue Jan 26 2021

...Great Lakes...
Day 1...

Continued cyclonic flow across the Lakes will lead to more LES in
the favored eastward bands. Tonight into Saturday, this is most
likely downwind of Erie and Ontario, as well as across the western
U.P. of MI as CAA enhances behind yet another impulse rotating
within the flow. The heaviest snowfall is likely downwind of Lake
Ontario where despite less than ideal fetch on a more NW flow, an
effective moisture tap from the Georgian Bay and Lake Huron will
help enhance snowfall. Heavy snow is also likely at times downwind
of Lake Erie due in part as well to a Lake Huron feed. Westerly
flow developing across Lake Superior will channel some heavy LES
towards the Keweenaw Peninsula as well. WPC probabilities for 4
inches are high in these locations, with local maxima exceeding 6"
possible south of Syracuse, NY SE of Lake Ontario.

...The West...
Days 1-3...

An amplified longwave trough across the far West will be
reinforced by several impulses rotating down from the Gulf of
Alaska, and then pivoting towards the Great Basin as closed
features.

On D1, a mid-level low will pivot down the coast of CA and become
a closed feature Saturday evening before opening and ejecting
northeast through the Four Corners on Sunday. A persistent and
slowly intensifying jet streak downstream of this trough will keep
a the modest diffluent LFQ stretching from the Southern CA coast
into the Central Rockies, providing moisture and aiding ascent
across the region. As this trough expands eastward, snow levels
will crash, starting as high as 7000 ft D1 across the Four
Corners, and falling to 3000 ft in Southern CA and the central
Rockies. Where mid-level divergence and upper diffluence overlap
atop the moisture stream, heavy snow is likely above these levels.
WPC probabilities for 6" are high on D1 across the San Juans, CO
Rockies, Uintas, Wasatch, southern Sierra, and other ranges of the
Great Basin. Isolated maxima above 12" are possible in the highest
terrain of the San Juans. By D2, the best moisture shifts east
while ascent maximizes to the south. This will lower snowfall
across the region, but additional heavy snow is likely. WPC
probabilities are highest for 6 inches in the San Juans once
again, as well as spreading across the Mogollon Rim and White
Mountains of AZ.

As the first system exits to the east, a more powerful shortwave
will dive along the coast from WA to CA, with multiple closed lows
possibly developing across the coast. This will have a two pronged
effect of significantly lowering snow levels, while also
transporting Pacific Moisture along the coast and then inland
towards the Four Corners once again. Height falls will be the
primary driver of ascent this period as the jet energy stays
mostly west of the trough and over the Pacific Ocean. However,
pronounced increase in mid-level RH suggests moderate to heavy
snow will spread down the coast, and on Monday snow levels may
fall as low as 500 ft in OR/WA, and to near 2000 ft in southern
CA. While the heaviest snow will once again occur in the terrain,
there is potential for lowland snow accumulating above 1" in
WA/OR, including the Columbia Gorge. In the Cascades and coastal
ranges, WPC probabilities are moderate for 4 inches. Further
south, heavier snow is likely to being to impact the Sierra,
Mogollon Rim, and other ranges of Southern CA and the Great Basin
as moisture gets channeled eastward concurrent with intensifying
ascent. It is becoming increasingly likely that heavy snow will
accumulate in these ranges, and WPC probabilities are high for 8
inches in the Sierra, San Bernadinos, and Mogollon Rim D3.
Isolated totals above 12" are likely, and more heavy snow is
expected to impact these same ranges into D4, just beyond this
forecast period.

...Northern/Central Plains to Upper Midwest...
Days 1-3...

Two distinct storm systems are set to impact this region over the
next 3 days.

The first is a modest mid-level wave which will shed from a closed
low over the Great Basin and eject northeast across the Plains
Saturday through Sunday. While the amplitude of this feature is
modest, it will be embedded in moist mid-level confluent flow,
with weak height falls and accompanying LFQ jet diffluence
providing ascent, aided by intensifying isentropic lift providing
WAA. Although forcing and moisture is transient for this event,
snowfall could over-perform. A cold column noted on regional
soundings will allow for high SLRs, aided by a deepening DGZ as an
isothermal layer develops on the WAA. During the period of
heaviest snowfall, the DGZ appears to be quite deep, and as the
best lift intersects this saturated DGZ, heavy snow is likely. WPC probabilities for 4 inches are above 60% from far eastern SD
through extreme western WI, with local maxima above 6" possible.
The greatest snow accumulation is likely 00Z-12Z Sunday, with
rapid waning occurring thereafter as the system pulls away to the
northeast.

As the first system pulls away, a more significant and larger
scale event will begin as the Great Basin low ejects into the
Plains as a more robust shortwave lifting from OK to IL Sunday
into Monday. This shortwave will be accompanied by an intense
jet-level divergence maxima, and surface low pressure is likely to
develop and head northeast into Illinois while deepening by the
end of the forecast period. An increasing Gulf moisture tap with
WAA/isentropic ascent will spread moisture northward from KS to IN
D2 into D3. This increasing WAA is likely to drive a warm nose
above 0C, creating a mixed p-type scenario with sleet and freezing
rain likely across a large area. The guidance still features
considerable spread in the intensity and placement of this low,
but at this time feel the northern solutions (non-NAM) was the
most likely. With this track, a wave of sleet/freezing rain is
likely to impact a large corridor from eastern KS through northern
IN, and WPC probabilities on D3 are as high as 30% for 0.1" of
freezing rain accretion. WSE plumes indicate the potential for
much higher amounts, especially across Missouri, but confidence is
not high at this time. Some light snow is likely north of the
freezing rain with the WAA, but heavier snowfall should begin to
develop on the N/W side of the low as it deepens and lifts late
D3. An intense mid-level fgen band is progged to length NW of the
surface low as rapid CAA commences, and this will quickly cause
precipitation to change to snow within the developing comma. WPC
probabilities currently indicate a 30-40% chance for 4" of snow
from central KS into southeast IA.

...Mid-Atlantic/Central Appalachians...
Day 3...

A low pressure system organizing near the Arklatex late Sunday
will drape a warm front northeastward towards the Mid-Atlantic,
and then race northeast along it. Moisture sourced from the Gulf
of Mexico will spread northward on warm/moist advection associated
with this front, expanding precipitation northeastward into the
Central Appalachians and Mid-Atlantic Sunday night. As this
precipitation expands, a wedge of high pressure will be in place
from the northeast, and is likely to become reinforced and
entrenched by precipitation falling into it. At the same time the
WAA will drive a warm nose quickly northward, and guidance
suggests that despite a brief burst of snow at precip onset, most
of the precipitation by the end of D3 will be in the form of sleet
or freezing rain from western VA northward into the Laurel
Highlands of PA and much of MD. Guidance still features
considerable spread in the low-level thermal structure so
confidence in the exact evolution is moderate. However, there is
an increasing signal for a period of freezing rain which could
become impactful. WPC probabilities for 0.1 inches are as high as
60% in the Panhandle of WV and western MD, into the Laurel
Highlands, with low probabilities for 0.25". Light freezing rain
accumulating to less than 0.1" is likely across much of the rest
of this area. With differences in low-level thermal structure
persisting, it is possible that this may end up more snow/sleet if
secondary low-development occurs more rapidly, or more rain if the
WAA overwhelms the low-level cold air. Future updates will
hopefully narrow the spread and increase confidence in the exact
evolution, but it does appear a wintry mix of moderate to heavy
precipitation is likely beginning Sunday night.

Weiss


$$
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FOUS11 KWBC 230945
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
445 AM EST Sat Jan 23 2021

Valid 12Z Sat Jan 23 2021 - 12Z Tue Jan 26 2021

...Great Basin to the Central and Southern Rockies/California...

Days 1-3...

On D1, a 700 mb wave ejects from northern UT across southern WY,
with convergence along the wave's path supporting heavy snow in
windward terrain of northern UT and southern WY.
South of the CO, upper divergence crossing the ranges of western
CO supports several inches of snow there. A mid-level low will
pivot down the coast of CA and become closed Saturday evening
before opening up and ejecting northeast through the Four Corners
on Sunday. A persistent and slowly intensifying jet streak
downstream of this trough will keep a the upper jet streak
crossing AZ into northwest NM and southern CO, providing moisture
and aiding ascent across the region. As this trough expands
eastward, snow levels will crash. Where upper-level divergence
occurs atop the moisture stream, heavy snow is likely. WPC
probabilities for 6" are high on D1 across the San Juans. Isolated
maxima above 12" are possible in the highest terrain of the San
Juans. D2/Sunday, WPC probabilities are highest for 6 inches in
the Mogollon Rim of AZ, with a secondary max in the San Juans once
again, as well as spreading across the White Mountains of AZ.

As the first system exits to the east, a more powerful shortwave
will dive along the coast from WA to CA. This will have the effect
of significantly lowering snow levels, while also transporting
Pacific moisture onshore and then inland towards the Four Corners
once again. Upper divergence and mid level
deformation/frontogenesis will be the primary driver of ascent
this period as the jet energy stays mostly west of the trough and
over the Pacific Ocean. However, pronounced increase in mid-level
RH and 700 mb ascent suggests moderate to heavy snow will spread
down into lower elevations areas in WA/OR. While the heaviest
snow will once again occur in the terrain, there is potential for
lowland snow accumulating above 1" in WA/OR, including the
Columbia Gorge. In the OR Cascades, WPC probabilities are high for
4 inches. Snow spreads into the northern CA Sierra Nevada
Mountains Sun night, with several inches expected.

On Monday, the amplifying trough lowers snow level so snow grows
in coverage in the ranges of Southern CA inland across the
southern Great Basin and southern Rockies concurrent with
intensifying ascent as upper divergence maxima cross this region
in the left exit region of a jet max crossing southern AZ and NM.
It is becoming increasingly likely that heavy snow will accumulate
in these ranges, and WPC probabilities are high for 8 inches in
the Mogollon Rim and southern UT Wasatch Monday. Totals of 12" to
18" are likely in the ranges of AZ.

...Northern/Central Plains to Upper MS Valley/Great
Lakes/Midwest...
Days 1-3...

The forecast for Saturday features a wave ejecting from UT and WY
across the Plains and merging with a northern stream wave. While
the amplitude of this feature is modest, it will be embedded in
moist flow, diffluence aloft providing ascent, aided by
intensifying isentropic lift from low level warm advection.
A cold column noted on regional soundings will allow for high snow
to liquid ratios, aided by a deepening dendritic growth zone. The
lift intersects this saturated dendritic growth zone, so that
favors higher snow to liquid ratios. WPC probabilities for 4
inches are 60% across southern MN, with local maxima above 6"
possible. The greatest snow accumulation is likely 00Z-12Z Sunday,
with rapid waning occurring thereafter as the system deamplifies,
with 700 mb ascent decreasing and snow amounts/coverage declining
as a result. On Day 2, light snow is indicated over northeast WI
across Lake Michigan to the UP of MI. Modest lake enhancement
could bring 3 or 4 inches to portions of the UP near northern Lake
Michigan.

On Monday/Day 3, a closed 700 mb low is forecast by the ECMWF/GFS/UKMET/Canadian global to develop and move across KS to
northern IL by Tuesday morning. This shortwave will be accompanied
by an intense jet-level divergence maxima, and moisture wrapping
around the cyclonic circulation to produce snow in a mid level
frontogenesis band from northern KS across southeast NE, southern
IA, and northern IL. The last 2 runs of the ECMWF have shown
potential for a foot of snow in southeast NE and southern IA. WPC probabilities currently indicate a 60-80% chance for 4" of snow
from northern KS into northwest MO and southeast IA.

South of the 700 mb low, warm air advection is likely to drive a
warm nose above 0C, creating a mixed p-type scenario with sleet
and freezing rain likely across a large area from northeast KS
across northern MO, central IL, northern IN and OH. The guidance
still features considerable spread in the intensity and placement
of this low. WPC probabilities on D3 are as 45% for 0.1" of
freezing rain accretion in central IN to west central OH.

...Mid-Atlantic/Central Appalachians...
Day 3...

A mid level wave/850 mb circulation moves into the Ohio Valley on
Day 3. Moisture from the Gulf of Mexico will spread northward on
warm/moist advection, expanding precipitation northeastward into
the Central Appalachians and Mid-Atlantic Monday. As this
precipitation expands, a wedge of high pressure will be in place
over northeast. Despite a brief burst of snow at precip onset,
most of the precipitation will be in the form of sleet or freezing
rain from western VA northward into the Laurel Highlands of PA and
much of western MD. There is an increasing signal for a period of
freezing rain which could become impactful. WPC probabilities for
0.1 inches are as high as 80% in the Panhandle of WV and western
MD, into the southern Laurel Highlands, with 50-70 probabilities
for 0.25". With differences in low-level thermal structure
persisting, it is possible that the freezing rain is mixed with
sleet. Future updates will hopefully narrow the spread and
increase confidence in the exact evolution. In central PA,
uncertainty relates also to the QPF gradient, as well as precip
type. Several inches of snow are possible where a long duration
snow event occurs, but the models are not in agreement where this
could occur yet.

Petersen


$$
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FOUS11 KWBC 232101
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
400 PM EST Sat Jan 23 2021

Valid 00Z Sun Jan 24 2021 - 00Z Wed Jan 27 2021

...Central Plains to Midwest...
Days 1-3...

A significant winter storm is becoming more likely for parts of
the Central Plains and Midwest Monday into Tuesday.

A potent shortwave shedding from energy in the Southwest will
advect northeastward Monday, while taking on a negative tilt
across the Plains. This will combine with a strengthening jet
streak to the northeast and downstream of the longwave trough to
produce ascent, and surface cyclogenesis is likely early Monday
over OK before lifting northeast while rapidly deepening. This
system will likely produce two phases of wintry precipitation,
freezing rain in the WAA ahead of the low, and heavy snow in the CAA/deformation as it departs.

For the WAA phase, precipitation will expand as isentropic ascent
intensifies, tapping Gulf of Mexico moisture over the Southern
tier. This will lift northward as an expanding area of
precipitation from eastern Kansas through Ohio the first half of
Monday. This WAA is likely to be strong, so a warm nose of >0C
will spread northward, even as low-level temperatures remain below
freezing, at least initially. This suggests a swath of freezing
rain will develop along the leading edge of precipitation Monday.
There still exists uncertainty into how far northward the mixed
precipitation shield will extend, but a non-NAM consensus suggests
the heaviest freezing rain will occur along a line from the MO/IA
border eastward through central IL, IN, and into far western OH.
WPC probabilities in this area are 30-50% for 0.1", with locally
up to 0.25" possible.

As the low deepens and moves eastward, guidance has become more
aggressive that a band of heavy snow will develop on the NW side
and slide eastward along the northern periphery of the
precipitation into Tuesday. As the low strengthens, isentropic
upglide on the 290K and 300K surface will intensify from the
south, which correlates well with the height of the DGZ. This will
occur concurrently with a developing TROWAL, and the combination
of isentropic ascent into the DGZ combined with theta-e advection
aloft will produce a saturated column with intensifying mesoscale
ascent. While the column will initially be marginal for snow,
these intense dynamics are likely to cool the column sufficiently
such that precipitation will rapidly change to heavy snowfall
within this deformation band. The conceptual model for this event
suggests this band will initially pivot NW of the surface low near
the 700mb low, and then gradually translate eastward, driving
heavy snowfall from eastern KS into northern IL. Within this band,
snowfall rates may peak at 2"/hr, and cross sections indicate some
modest -EPV favorable for possible thunder-snow. While there is
still some uncertainty in the exact placement of this band, of
which can cause significant accumulation differences due to the
intense rates, the consensus is that the band will pivot near the
KS/NE/MO/IA conjunction, and it is here where WPC probabilities
are highest for 8", with isolated maxima over 12" possible. As
this band shifts eastward, it should begin to weaken and move more
rapidly, but WPC probabilities for 6" are 40-60% from eastern KS
through northern IL, including Chicago.

...Central Appalachians and Northeast...
Days 2-3...

An area of low pressure moving through the MS VLY Monday will
drape a warm front well east towards the Atlantic Coast. As this
warm front lifts northward, secondary low pressure may develop off
the VA Coast Tuesday, and the combination of warm/moist advection
with the warm front and secondary low pressure development
offshore will create a challenging mixed-precipitation event late
Monday through Tuesday.

As the primary surface low moves northeast towards the Great
Lakes, it will be accompanied by a slowly filling shortwave, with
downstream ridging expanding across the east. This strengthening
SW mid-level flow will help drive warm Pacific moisture across the
CONUS, aided by a robust jet streak rising to 170kts over the
Northeast. This jet streak will aid in transporting moisture into
the region, with low-level WAA associated with the warm front
transporting Gulf of Mexico moisture northward as well. Forcing
for ascent within this moistening column will become robust
through a strong upper divergence maxima, and isentropic ascent
within the WAA regime atop a wedge of high pressure anchored at
the surface. As the moisture and ascent approach late Monday,
precipitation will spread quickly northeastward into the Central
Appalachians, and then into the Mid-Atlantic and Northeast through
Tuesday. Strong ascent initially will likely lead to a burst of
snowfall in most areas, however, rapid warming aloft will quickly
transition snow to sleet, freezing rain, and eventually rain south
of the Mason Dixon line and outside of any terrain. With the wedge
likely being reinforced by precipitation, a long duration freezing
rain event is becoming more likely, especially in the terrain and
elevated valleys of the Central Appalachians through the Laurel
Highlands. WPC probabilities in this area are greater than 50% for
0.25", with significantly more possible as shown by WSE 90th
percentile plots. However, uncertainty with respect to
rates/runoff and whether the wedge will erode or be reinforced
preclude higher probabilities at this time. Further north,
precipitation will last much longer as snow before the warm nose
advects through PA, and WPC probabilities for 4 inches are
moderate across central PA into southern NY. For the I-95 corridor
from DC to Philadelphia, a burst of snow late Monday is likely
before a rapid transition occurs to sleet, freezing rain, and then
rain. There remains uncertainty here, as the secondary low
development could isallobarically cool the column enough that snow
and/or freezing rain will be able to last longer than current
progs suggest. However, at this time, the total wintry
accumulations for the I-95 corridor due to snow and freezing rain
are forecast to be light.

...The West...
Days 1-3...

A series of impulses in this wave-train will advect into the West
through early next week, bringing repeated rounds of precipitation
and gradually lowering snow levels. Heavy snow is likely in
virtually all of the Pacific Northwest, CA, Great Basin, and Four
Corners ranges through the period.

The first impulse is a closed low over southern CA that will open
and eject eastward towards the Four Corners on Sunday evening.
This 500-700mb wave will gradually lose amplitude as it shifts
eastward, but pronounced mid-level divergence and heights fall
will drive ascent across the area. The downstream jet streak ahead
of this trough will maintain intensity, but the overall forcing
will begin to wane as the entire system shunts east as it gets
replaced by the next impulse dropping down from the Gulf of
Alaska. Backed flow ahead of the trough axis combined with Pacific
moisture advected through the upper jet will provide plenty of
moisture to spread heavy snowfall from the southern CA ranges
through the Mogollon Rim and into the San Juans. The heaviest snow
on D1 is likely in the Mogollon Rim and San Juans due to the more
orthogonal moist flow upsloping into this higher terrain. WPC
probabilities are high for 6 inches in these areas, with locally
more than 12" possible. More than 4" of snow is likely D1 in the
San Bernadino range with snow levels generally in the 4000-5000 ft
range.

As this first system kicks out to the east, it will be replaced
almost immediately by another vorticity streamer coming down the
West Coast, and tightening into a closed low over Southern CA,
while yet a third system drops into the Pacific Northwest. While
these two impulses deepen, heavy snow will again spread across the
West. For the lagging shortwave into the Pacific Northwest,
moisture is a little more limited due to the angle of approach of
this shortwave, with a more northerly mid-level flow developing
and weak LFQ jet diffluence. Despite the relative lack of moisture
(PWs below climo), strong height falls and modest upslope will
squeeze out all available column moisture as snowfall. Snow levels
crash even further with this system, becoming as low as 500 ft in
WA/OR. The heaviest snow is expected in the Cascades of WA and OR
on D1.5-2, and WPC probabilities are high for 6 inches, but
snowfall is not expected to spread inland beyond the Cascades.
While not heavy, light accumulations up to 1" are possible as low
as the Columbia Gorge on D2.

The other impulse, the closed low redeveloping across Southern CA,
will be much more significant. In addition to rapid snow level
drops, forcing through height falls and pronounced jet level
diffluence will spread eastward, with moisture wrapping from the
Pacific southward into CA and the Great Basin/Four Corners. A
prolonged period of moist advection is likely on long
trajectories, which when combined with the snow levels falling to
as low as 1000-2000 ft, supports exceptional snow potential in the
Mogollon Rim and Southern/Central Sierra which will be orthogonal
to the mid-level flow for ideal upslope enhancement. WPC
probabilities are high for 12" D2 in the Sierra and Mogollon Rim,
continuing into D3 in the Mogollon Rim. Additionally, heavy snow
is likely in the San Gabriels and San Bernadinos, with lighters
snows likely even into the lower terrain outside of Los Angeles,
San Francisco, San Diego, and Tucson. Total snowfall across the
Mogollon Rim may exceed 3 ft by Tuesday.


Weiss


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FOUS11 KWBC 240925
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
425 AM EST Sun Jan 24 2021

Valid 12Z Sun Jan 24 2021 - 12Z Wed Jan 27 2021

...Central Plains to Midwest...
Days 1-3...

A major winter storm is likely for parts of the Central Plains and
Midwest Monday into Tuesday.

A potent shortwave will move northeastward Monday, while taking on
a negative tilt across the Plains. This will combine with a
strengthening jet streak and mid level frontogenesis maxima to
produce ascent. This system will likely produce two phases of
wintry precipitation, freezing rain and sleet in the low-mid level
warm advection ahead of the low, and heavy snow in the mid level deformation/frontogenesis maxima in the cold sector.

For the warm advection phase, precipitation will expand as
isentropic ascent intensifies. This will lift northward as an
expanding area of precipitation from the central Plains into the
mid MS Valley and Midwest. Temperatures 850-700 mb rise above
freezing, even as low-level temperatures remain below freezing,
until warming later.
On Day 1, the stripe of freezing rain/drizzle covers central KS to
northern MO, central/southeast IL, and southern IN to the OH
Valley in KY/OH.
Another swath of freezing rain will develop on Day 2/Monday from
northeast KS across northern MO to the IA border, central IL,
central IN, and OH. WPC probabilities in this area are 30-50% for
0.1", with locally up to 0.25" possible.

As the low deepens and moves eastward, guidance has good agreement
that a band of heavy snow will develop in the mid level
deformation zone in the cold sector. The combination of isentropic
ascent into the dendritic growth zone combined with theta-e
advection aloft will produce a saturated column with intensifying
mesoscale ascent. While the column will initially be marginal for
snow, these intense dynamics are likely to cool the column
sufficiently such that precipitation will rapidly change to heavy
snowfall within this deformation band.
Heavy snowfall is expected from southeast Nebraska across
northeastern KS, far northwest MO, southern Iowa into northern IL.
Within this band, snowfall rates may peak at 2"/hr, and cross
sections indicate some modest -EPV favorable for possible thunder
snow. WPC probabilities for 8" peak near 80 percent in these
areas, with maxima of 12" expected in parts of southeast Nebraska
to southern Iowa.

...Central Appalachians and Mid Atlantic/Southern NY/Southern New
England...
Days 2-3...

An area of low pressure moving through the Ohio Valley Monday
night will cause warm/moist advection to stream out of the OH
Valley into the central Appalachians. Upper level jet streaks
are forecast to cross from the OH Valley across PA and WV, which
will aid in transporting moisture into the region, and also
produce forcing for ascent within this moistening column.

As the moisture and ascent approach late Monday, precipitation
will spread quickly northeastward into the Central Appalachians,
and then into the Mid-Atlantic. Strong ascent initially will
likely lead to a burst of snowfall in most areas, however, rapid
warming aloft will quickly transition snow to sleet, freezing
rain, and eventually rain south of the Mason Dixon line and
outside of any terrain. With the initial cold air damming, surface
cold air in the mountains of western MD and adjacent portions of
WV and northwest PA/southwest PA will be reinforced by
precipitation. A mixed sleet and freezing rain event is likely,
especially in the terrain and elevated valleys of the Central
Appalachians through the Laurel Highlands of southwest PA. WPC
probabilities in this area are greater than 50% for 0.25" of
freezing rain.

Further north, precipitation will last longer as snow before the
warm nose advects through PA, but ridging to the north in NY acts
to slow north progression of precip until Tue.
Modest low-mid level convergence and deformation leads to light
snow across much of southern and western NY to adjacent PA.
WPC probabilities for 4 inches are moderate across central PA into
southern NY.

For the I-95 corridor from Washington, DC to Philadelphia, snow
late Monday is likely before a rapid transition occurs to sleet,
freezing rain, and then rain. There remains uncertainty here, as
the secondary low development off the mid Atlantic coast could
allow the cold air damming to remain in place.

...Pacific Northwest to California...Southern Great Basin....Southwest...Southern Rockies...

Days 1-2...

The first disturbance to cause locally heavy snow is an impulse
over southern CA that will open and eject eastward towards the
Four Corners this Sunday evening, moving out on to the high Plains
tonight. Enhanced moisture and ascent along its path are still
expected to combine with orographically forced ascent in windward
terrain
to spread snowfall from the southern CA ranges through the
Mogollon Rim and into the San Juans. The heaviest snow on D1 is
likely in the Mogollon Rim and San Juans due to the more
orthogonal moist flow moving upslope into this higher terrain. WPC probabilities are moderate for 8 inches in these areas, with
locally more than 12" possible. 4" to 8" of snow is likely D1 in
the San Bernardino range.

A second area of snow on Day 1 is across the OR Cascades down into
the CA Cascades, Shasta/Siskiyous, and northern CA Sierra Nevada.
The 700 mb wave driving ascent and moisture convergence streams
ashore this morning and crosses the WA/OR Cascades today,
continuing down into the CA Sierra Nevada until the 700 mb trough
moves inland into NV.
As the wave crosses southern NV into adjacent UT Monday, the ECMWF
develops a closed low, with focused moisture convergence near the
circulation center driving heavy snow across the ranges of
southern NV/southern UT.

A prolonged period of moist advection is likely in the mountains
of AZ due to the slow movement of the mid level circulation
center. Around a foot of snow is expected in the Mogollon Rim,
supported by high probabilities for 12" in the Mogollon Rim.
Total snowfall across the Mogollon Rim may exceed 3 ft by Tuesday.
Moderate snow is likely in the San Gabriels and San Bernardino
Mountains of southern CA.

Day 3...

On Day 3/Tuesday, a closed low over the northeast Pacific moves
southeast towards the northwest US. Strong onshore flow develops
in OR and crosses CA, with the strong onshore moisture fluxes and
large 300 mb divergence maxima leading to high confidence in heavy precipitation in northern CA. The strong jet max crossing the
Siskiyous and Shastas/CA Cascades and then the northern CA Sierra
Nevada Mountains leads to high confidence in heavy snow, as
multiple models show 1-2 inches of liquid equivalent of snow, with
higher local maxima.
The probability of a foot of snow peaks in the Siskiyou Mountains,
where is is 80-99 percent for the 24 hours ending 12z Wed.

The probability of a quarter inch of freezing rain is less than 10
percent Day 3.

Petersen


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FOUS11 KWBC 242054
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
353 PM EST Sun Jan 24 2021

Valid 00Z Mon Jan 25 2021 - 00Z Thu Jan 28 2021

...Central Plains to Midwest...
Days 1-3...

A major winter storm is likely for parts of the Central Plains and
Midwest Monday into Tuesday.

An area of low pressure will begin to consolidate across Oklahoma
Monday morning and then lift slowly northeast into Missouri while
deepening through Tuesday morning. This low will be driven by a
potent negatively tilted shortwave moving out of the Southwest and
closing off over Kansas Monday aftn. This mid-level wave will then
slowly fill and open into Tuesday morning as it ejects into the
westerlies and moves over the Great Lakes late Tuesday. At the
same time, a strengthening jet streak downstream of an amplifying
trough across the West and of Pacific origin will reach 170kts
across the Mid-Atlantic, leaving its diffluent RRQ atop the
Plains, while a secondary jet streak blossoms over the Southwest,
creating a coupled jet structure favorable for rapid cyclone
deepening. These features together will drive deep layer ascent
for the significant winter storm.

As all the features together lift northeast through Monday, there
is likely to be an expanding area of precipitation as isentropic
lift/WAA spread northward ahead of the surface low. This will
cause an expanding shield of precipitation to lift northward. The
antecedent column is marginally cold, so precipitation may briefly
start as snow from northeast Kansas through Indiana, but the
strengthening warm nose will quickly cause a transition to
freezing rain as precipitation lifts northward. Eventually, the
column will be cold enough for all snow, but this should be
confined to Iowa, northern IL, northern IN, and into MI/WI. A
prolonged period of freezing rain is likely in this E-W band, and
WPC probabilities for 0.25" are as high as 40% from northeast
Kansas through northern MO, and striping into far western OH.
While much of the freezing rain is due to the warm nose aloft, it
is also noted that a dry slot will try to work into this area,
especially MO and northeast KS, drying the DGZ and leading to more
freezing rain locally.

The heavy snow is likely to begin early Monday across Kansas on
the NW side of the low, and then gradually blossom northward
through NE/IA and into MO/IL/MI. Across this region, the column
will be marginally colder and supportive of snow. However, the
heaviest snow is likely on the NW side of the low as CAA develops
causing intensifying fgen and a potent deformation band is likely
to setup. At the same time, persistent moist isentropic ascent at
290-300K with mixing ratios of 3-4 g/kg will efficiently intersect
the DGZ, while a TROWAL strengthens aloft. These together suggest
a potent deformation band is likely to develop to dynamically cool
the column and cause heavy snow rates which may reach 2"/hr and
could be accompanied by lightning and thunder. The conceptual
model for this event suggests the band will pivot initially NW of
the low, before laterally translating eastward the latter half of
the event. Where this band pivots is likely to experience the
heaviest snowfall, and at this time that is forecast to be across
extreme SE Nebraska and into southern IA. Here, WPC probabilities
are high for 8", with a narrow corridor of more than 12" likely
Monday into Monday night. Surrounding this max swath, both to the
SW as the band begins to develop, and to the northeast which is
colder but will see the band begin to weaken, WPC probabilities
are high for 4" from central KS all the way into southern MI, with
a secondary maximum possible near Chicago, IL due to some lake
enhanced snowfall.

As this first system pulls away to the northeast D2 into D3, yet
another shortwave ejecting from the Southwest will approach the
Central Plains, aided by a more impressive subtropical jet streak
rotating through the trough base. This second system is likely to
be much weaker than the first, and forcing/moisture is more
transient as well. However, an additional round of moderate to
heavy snowfall is possible from the High Plains of NE/KS eastward
into Missouri during D3. WPC probabilities are low for 4".

...Central Appalachians and Mid Atlantic/Southern NY/Southern New
England...
Days 1-3..

A warm front extending from the surface low across the Central
Plains will arc northeastward towards the Mid-Atlantic beginning
Monday night. S/SW flow out of the Gulf of Mexico will produce
warm/moist advection into the region associated with this front,
and precipitation is likely to spread across the area through
Tuesday, lifting into New England on Wednesday. A shortwave
driving the surface low will weaken as it heads northeast into New
England, and this splitting of energy will somewhat weaken the
potential for heavy precipitation across the area. However, potent
WAA, subtle height falls, and the RRQ of a departing jet streak
will combine to drive deep layer ascent for wintry precipitation.

As moisture spreads northward Monday night into Tuesday, it will
encounter cool high pressure from the northeast wedged into the
Central Appalachians. This will create an overrunning scenario,
and while the initial burst of precipitation is likely to be snow
in most areas, it will quickly change to freezing rain as temps
aloft warm above 0C. There is still uncertainty into how long
freezing rain will persist, mostly due to the potential for
secondary low development offshore to help reinforce the cold air
at the surface. However, models have backed off on this secondary
development so more freezing rain than snow is expected,
especially south of the Mason Dixon line, and much of the
Mid-Atlantic may eventually change to rain. The heaviest freezing
rain is likely in the terrain of the WV Panhandle, northern VA,
western MD, and into the Laurel Highlands where WPC probabilities
are more than 70% for 0.25", and isolated amounts to 0.5" are
possible. Elsewhere, Up to 0.1" of freezing rain is likely,
including the I-95 corridor from DC to Philadelphia.

Further north, the column will be colder so more snow is likely.
Eventually, even much of central and southern PA may change to
sleet/freezing rain, but WPC probabilities indicate a high chance
for at least a few inches of snow. The best chance for significant accumulations of 4" or more are confined to the Poconos and into
central upstate NY where the column will remain all below
freezing, and the weakening shortwave will provide the best
enhancement of synoptic ascent.


...Pacific Northwest to California...Southern Great Basin....Southwest...Southern Rockies...
Days 1-2...

The first disturbance to cause locally heavy snow is an impulse
over southern CA that will open and eject eastward towards the
Four Corners tonight, and then progress into the Plains Monday.
This impulse will be followed almost immediately by a second
shortwave that digs down the West Coast before briefly closing off
over Southern CA Monday evening, and then also ejects northeast
towards the Four Corners on Tuesday. These back-to-back features
will spawn modest surface waves that will move across the region,
but both will be accompanied by enhanced moisture from the
Pacific, as well as modest jet level diffluence for ascent.
Combined heavy snow from these impulses will be greatest in the
ranges orthogonal to the 700mb flow, which will provide upslope
enhancement to the snowfall. This is likely in the Mogollon Rim,
the Sierra, and the S CA ranges on D1, spreading into the Mogollon
Rim, White Mountains San Juans, and other ranges of NM/AZ on D2.
Snow levels will be quite low during this time as well, as low as
1000-2000 ft, so light snow is expected in the less common terrain
also. WPC probabilities for 8 inches are high on D1 in the
Mogollon Rim and Sierra, as well as the southern Wasatch and San
Bernadinos, with high probabilities for 8 inches confined to the
Mogollon Rim and White Mtns on D2. Total snowfall may exceed 2 ft
in the higher terrain by Tuesday evening.


Day 3...

On Day 3/Tuesday, a closed low over the northeast Pacific moves
southeast towards the northwest US. Strong onshore flow develops
in OR and crosses CA, with the strong onshore moisture fluxes and
large 300 mb divergence maxima leading to high confidence in heavy precipitation in northern and central CA. The position of the
mid-level trough and jet streak support a long duration of moist
advection and intense ascent focused into northern and central CA,
and snowfall could be extreme on D3 even as snow levels begin to
rise from the south. WPC probabilities for 12" are greater than
90 percent along the Sierra and into the Siskiyou/Trinity/Shasta
regions, and with intense ascent directed into these ranges
coincident with anomalous moisture, more than 3 ft of snow is
likely in some places on D3.

Elsewhere, very cold snow levels across WA and OR may permit some
light accumulations even into the lowlands and metro areas around
Portland, OR on D3.

Weiss


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FOUS11 KWBC 160910
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
409 AM EST Tue Feb 16 2021

Valid 12Z Tue Feb 16 2021 - 12Z Fri Feb 19 2021


...Central and Southern Rockies to the Northeast...
Days 1-3...
Shortwave energy dropping southeast through the Great Basin into
the Southwest will continue to amplify the western extent of the
broad, large-scale trough that remains positioned over the U.S.
Broad, large-scale ascent supported by left-exit region upper jet
forcing is expected to encourage widespread precipitation across
the Great Basin, Southwest into the central and southern Rockies,
with orographic effects supporting locally heavy amounts on
Tuesday. Areas impacted are expected to include the Utah, western
Colorado, eastern Arizona, and New Mexico ranges.

By late Tuesday as the upper shortwave begins to move east of the
Rockies, precipitation is forecast to develop out across northern
Texas and Oklahoma before expanding farther north and east during
the overnight hours. Guidance is showing a period of mid-to-upper
level forcing along with low level theta-e advection sufficient
for light to moderate snows spreading from the eastern Texas
Panhandle to Arkansas and southern Missouri Tuesday night. Latest
PWPF shows high probabilities for snow accumulations of 4-inches
or more extending across much of central and eastern Oklahoma into
western Arkansas by Wednesday afternoon.

Impacts downstream have become less certain, with the general
consensus of the overnight guidance showing less amplification
with the upper trough as it moves across northern Texas and
Oklahoma Wednesday night. A notable exception is the NAM, which
remains more amplified, drawing more moisture into the colder air
and developing heavy snows out across Arkansas into western
Tennessee and lower Ohio valley. Following the trends of the GFS,
UKMET and ECMWF, WPC leaned away from the NAM solution. While the
potential for heavy snow appears to be decreasing, there remains a
strong signal for significant icing from eastern Texas to northern
Mississippi, with accumulations of 0.25-inch or more likely across
portions of the region.

As shortwave energy continues to translate through the base of the
longwave trough and lift to the northeast, precipitation will
continue to spread farther north and east from the Tennessee and
Ohio valley into the Mid Atlantic Thursday morning and into the
Northeast by late in the day. A cold air wedge in place will
support a prolonged period of freezing rain, with significant ice
accumulations (0.25-inch or more) expected across portions of the
North Carolina and Virginia Piedmont. Farther to the north within
the deeper cold air, widespread snow accumulations of 4-inches or
more are expected across the northern Mid-Atlantic, New York and
southern New England.

...Pacific Northwest to the Northern Rockies...
Deep northwesterly flow, with embedded energy aloft will continue
to support periods of mountain snow, with additional heavy
accumulations expected from the Cascades to the northern Rockies
on Tuesday into early Wednesday. An upper ridge moving east
across the region, will provide a break in precipitation late
Wednesday into early Thursday. However, precipitation is forecast
to return as the ridge gives way to a shortwave trough and its
associated frontal band during the day on Thursday. This system
is expected to bring another round of heavy snows to the Olympics
and Cascades, with lighter accumulations spreading east across the Intermountain West into the northern Rockies late Thursday into
early Friday.

Pereira

$$
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FOUS11 KWBC 170940
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
439 AM EST Wed Feb 17 2021

Valid 12Z Wed Feb 17 2021 - 12Z Sat Feb 20 2021


...Southern Plains to the Northeast...
Days 1-3...
The next round of winter weather will continue to develop across
eastern Texas and the lower Mississippi Valley as shortwave energy
moves through the base of the large-scale trough centered over the
southern High Plains on Wednesday. Strong ascent along the
right-entrance region of the upper jet and a low-to-mid level
baroclinic zone is expected to support moderate to heavy
precipitation developing across eastern Texas into the lower
Mississippi valley later this morning. This will likely include
heavy snow across portions of southern Arkansas, with freezing
rain and heavy ice accumulations unfolding farther to the south
from eastern Texas through central and northern Louisiana into
southeastern Arkansas and northern Mississippi. Latest PWPF shows
50 percent or greater probabilities for snow accumulations of
4-inches or more centered across southern Arkansas. PWPF also
shows 50 percent or greater probabilities for ice accumulations of
0.25-inch or more extending from the central Piney Woods area of
eastern Texas to northern Mississippi.

As this energy begins to lift out to the northeast, heavier
precipitation will begin to spread out through the Tennessee and
Ohio valleys later in the day before reaching the central
Appalachians during the overnight hours. Latest PWPF suggests a
sold stripe of 2-4 inch snowfall accumulations, with locally
heavier totals, extending from northern Mississippi and western
Tennessee through eastern Kentucky. Probabilities for
accumulations of 4-inches or more are higher across portions of
the Allegheny Mountains from southern West Virginia into western
Virginia, where local orographic effects in addition to
large-scale forcing are more likely to support heavier snow
amounts. Meanwhile, moisture and warm aloft moving across the top
of arctic high pressure will set the stage for a prolonged
freezing rain event, with heavy ice accumulations expected.
Freezing rain developing Wednesday night may continue through
Thursday into Friday. Latest PWPF shows 50 percent probabilities
or greater for ice accumulations of 0.50-inch or more extending
from the northern North Carolina Piedmont and southwestern
Virginia into central Virginia. Farther to the north, a period of
snow likely changing to a wintry mix is expected along the I-95
corridor from DC through Philadelphia, while snow is expected to
remain the predominant precipitation type for areas farther north
and west on Thursday.

A coastal low is forecast to develop and track northeast off of
the Mid Atlantic coast on Thursday, staying well southeast of Long
Island and southern New England on Friday. Overall trend of the
guidance has shifted farther southeast with the track of the low.
This has also brought the axis of heavier snow accumulations
farther to the southeast across the northern Mid Atlantic and
Northeast. While the threat for heavy snow appears to be
decreasing across western to central Pennsylvania, Upstate New
York and northern to central New England, the shift in track is
supporting an increase in probabilities for accumulations of
4-inches of snow or more along the I-95 corridor from the New York
Metro through southern New England. Even as the low continues to
track northeast well off the coast, light snows are expected to
continue across portions the Northeast as a northern stream wave
moves from the Great Lakes into the Northeast on Saturday.

...Pacific Northwest to the Northern Rockies...
Days 2-3...
A strong ridge is expected to bring a period of dry weather as it
moves east across the region Wednesday into early Thursday.
However, unsettled weather is forecast to return during the day on
Thursday as mid-energy/upper jet flattens the ridge. A weakening
frontal boundary, followed by deep westerly flow will support
another round of locally heavy snows over the Olympics and
Cascades, with light accumulations spreading east across the
Intermountain West into the Rockies. Periods of mountain snow
will likely continue through Friday. Then on Saturday, a
well-defined shortwave moving inland is expected to extend the
threat for snow farther to the south into California and the Great
Basin.

Pereira


$$
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FOUS11 KWBC 180906
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
405 AM EST Thu Feb 18 2021

Valid 12Z Thu Feb 18 2021 - 12Z Sun Feb 21 2021

Days 1 to 2...

...Central Appalachians to the Mid Atlantic and Northeast...

A strongly confluent flow today leads to a southwest to northeast
upper jet crossing from the Ohio Valley to NY/New England.
Low level deformation/frontogenesis extends across the central to
south Appalachians to the mid Atlantic and then up to long Island
NY.

An axis of moderate to perhaps locally heavy snow will develop
through the northern Mid-Atlantic region. Generally, the greatest
probabilities of heavy snow will be across the Piedmont areas of
the Mid-Atlantic from northeastern WV and northwest VA northeast
into southern PA. Across this region there is a 70 to 90% chance
of exceeding 4 inches of snow. Snow changes to sleet across the DC
to Baltimore/Philadelphia/NJ corridor, cutting down on
accumulations.

Upper jet-induced divergence maxima cross southeast NY to southern
New England, where a broad area of 3-6 inches of snow is expected.
As the wave lifts off the coast on Fri, return easterly flow
causes convergence along the eastern shore of MA, including the
Boston area. Enhanced ocean effect snow should bring a few more
inches of snow tomorrow before the winds veer and the low level
coastal convergence weakens late Fri.

Areas of sleet and freezing rain will occur across portions of the
Piedmont of VA/NC. This will all be associated with a strong surge
of warm-air advection and Gulf of Mexico moisture transport that
will encounter a modified Arctic airmass entrenched over the
region.
Higher than normal deep moisture exists with precipitable water
values in the GFS indicating 1-2 std deviations above normal
across southern VA and adjacent northern NC, where the longer
duration freezing rain will occur. The icing threat decreases
heading south across central North Carolina as temps remain above
freezing.

There is a 70 to 90% chance (per latest PWPF) of exceeding a 0.25"
of ice accretion across southwest to south central VA to the NC
border. This will include a threat to south central VA, which was
hit particularly hard by last weekend's ice storm. The icing
threat wanes on Fri as the wave of low pressure moves offshore,
with ascent decreasing in its wake.


...Pacific Northwest to the Northern Rockies and Great Basin...

An influx of Pacific moisture and energy with low-mid level warm
advection and ascent will support another round of locally heavy
snows over the Olympics and WA/OR Cascades with lighter
accumulations spreading east across the Intermountain West into
the Rockies. As much as 2 feet of new snow is expected through Fri
for the Olympics and the Cascades, with as much as a foot for the
northern Rockies including the Bitterroots, Sawtooth and Tetons.
On Saturday, an upper trough moves east across the Great Basin and
upper jet crosses southern UT and CO.
Upper divergence in the left jet exit region will bring several
inches of in part of the UT Wasatch and also the ranges of western
CO.
The Olympics and WA Cascades are expected to receive another round
of higher elevation snow as upper divergence maxima cross
northwest to southeast across northern WA. Resurgent low-mid level moisture/warm advection and ascent aid in producing lift, which is
augmented by the topographically induced lift in windward
terrain. A low probability of a foot is forecast for Saturday in
the northern WA Cascades.

...Southwest to South-Central Texas...

A period of some snow, sleet and freezing rain is expected today
where a low-mid level deformation and frontogenesis maxima is
located in portions of southwest to south central TX; including
the Big Bend area. An area of 2-5 inches of snow is expected
centered near Del Rio TX.
A broad are of light freezing drizzle extends along the mid level
front across south central TX to northern LA.
The front is progressive so the lift will be short-lived, and so
will the icing threat.


Petersen

$$
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FOUS11 KWBC 192029
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
329 PM EST Fri Feb 19 2021

Valid 00Z Sat Feb 20 2021 - 00Z Tue Feb 23 2021

...Great Lakes, Mid-Atlantic, and Northeast...
Days 1-3...

On Day 1, low pressure moving away from New England leaving broad
NW flow in its wake, while a secondary mid-level shortwave swings
across the Great Lakes during the day The combination of these two
features will lead to increasingly unidirectional W/NW flow and
strong CAA across the region, setting up a favorable LES
environment. Lake temperatures have cooled recently, and ice cover
has dramatically increased, both which will limit LES intensity.
However, favorable flow and the likelihood of some lake-enhanced
effective fetch across multiple lakes should still support periods
of moderate to heavy snow, especially downwind of Lakes Erie and
Ontario, as well as across the NW L.P. of MI. WPC probabilities
are high for 4 inches in these regions on D1, with locally more
than 6 possible.

After a respite on D2, a more significant low pressure system will
emerge out of the Midwest and extend a warm front eastward into
the Mid-Atlantic. Cold high pressure ahead of this system should
quickly erode and retreat eastward in response to increasing
mid-level divergence, so a strong overrunning scenario is not
expected. This will limit freezing rain the Mid-Atlantic, although
some light accretions are possible Sunday night into Monday.
Elsewhere, a swath of moderate snow is likely as WAA spreads
northward into the Great Lakes and Northeast. WPC probabilities
for 4 inches are as high as 30% D3 in northern PA extending into
parts of the higher terrain of NY and VT, with some enhancement
possible the latter half of D3 downwind of Lake Ontario behind
this system.

...Midwest...
Days 2-3...

A longwave trough moving through the West will shed a piece of
shortwave energy northeast through the Central Plains and into the
Midwest late Saturday into Sunday. This feature will be
accompanied by a robust poleward arcing jet streak placing
significant LFQ diffluence atop the region. The combination of
height falls, PVA, jet level diffluence, and at least modest WAA
ahead of the shortwave will drive surface low development and
expand precipitation northward. Guidance continues to feature a
rather large disparity in the timing and intensity of this
shortwave, but the trend today has been for a subtly deeper trough
and subsequent stronger low with heavier precipitation. This
heavier precipitation may also be accentuated by frontogenetical
banding which looks likely N/NW of the sfc low, but exact
placement at this time is still in question. The transient nature
of this system should limit snowfall amounts, but WPC
probabilities do indicate up to a 30% chance for 4 inches across
parts of IA on D2, shifting into Michigan on D3. Light freezing
rain may also occur ahead of this system at the onset of
precipitation, but accretion should be light.

...Pacific Northwest through the Central Rockies...
Days 1-3...

A potent shortwave moving onshore the WA/OR coast early Saturday
will spill southeast through the Great Basin and into the Four
Corners by Sunday, spreading enhanced Pacific moisture eastward to
produce heavy snow across most of the Western Ranges from the
Olympics through the CO Rockies on D1. This shortwave will be
accompanied by a Pacific jet streak which will gradually sink
southward, providing moisture and enhancing ascent. Heavy snow is
likely in many of the ranges, with as much as 2 ft possible in the
NW CA ranges, the Cascades, and more than 1 foot likely in the
Tetons. Otherwise, WPC probabilities are high for 6 inches across
most ranges. As this upper trough continues to swing eastward on
D2, snowfall will generally wind down from west to east, with
lingering heavy snow likely in the CO Rockies and San Juans.

As the leading impulse moves southeastward, renewed moisture in
the form of a modest AR will approach the WA coast as confluent
mid-level flow angles orthogonally into the WA coast, while a 160+
kt jet streak dives southeast from British Columbia eventually
leaving the RRQ atop the Pacific Northwest. Snow will again
overspread the ranges from the Olympics to the Northern Rockies
late D2 and D3 with slowly lowering snow levels. WPC probabilities
are high both days for 8 inches, with up to 4 ft possible in the
higher terrain of the WA Cascades.

Weiss

$$
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FOUS11 KWBC 200828
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
328 AM EST Sat Feb 20 2021

Valid 12Z Sat Feb 20 2021 - 12Z Tue Feb 23 2021


...Pacific Northwest through the Northern and Central Rockies...
Days 1-3...

Multiple waves of deep layer warm/moisture advection are expected
to produce prolonged snow with a couple of days of heavy snow
leading to impressive snow amounts of several feet in the WA
Cascades.

As the leading impulse moves southeastward, renewed moisture
streams onshore in confluent mid-level flow, while difluent flow
aloft Sat night leads to divergence maxim a crossing western WA
State. Snow increases in coverage and intensity in the Olympics
and northern WA Cascades,
with locally up to a foot of snow expected.

On Day 2, the moist confluent flow continues, with a 160+ kt jet
streak dives southeast from British Columbia eventually leaving
the right entrance region atop the Pacific Northwest. Snow will
again overspread the ranges from the Olympics to the Northern
Rockies with slowly lowering snow levels.
2 to 3 feet are forecast in favored mountains of the northern WA
cascades, with up to afoot inland over the ranges of northern ID
and northwest MT expected.

On Day 3, the upper jet continues inland and with an upper trough
developing in WA/OR, the jet max persists in northern ID and
western MT a little longer than WA. With continued deep layer
ascent, heavy snows are highly likely across the mountains of ID
and northwest MT, with an additional 1 to 2 feet expected both in
these ranges and also the WA Cascades. Confidence is above normal
given the persistent deep moisture and strong, slow moving upper
jet. Also, the NAM soundings in the WA Cascades indicate steeper
lapse rates near 8 deg c/km Mon night, so response to lift will be
enhanced in WA State owing to favorable instability.

A potent shortwave moving across OR/CA early today will progress
southeast through the Great Basin and into the Four Corners by
Sunday morning, spreading enhanced Pacific moisture eastward to
produce snow across the ranges of NV/UT/CO Rockies on D1. This
shortwave will be accompanied by a Pacific jet streak which will
gradually sink southward, providing moisture and enhancing ascent.
Several inches are likely in favored terrain in these areas, with
locally as much as a foot of snow.

...Great Lakes Day 1...

On Day 1, snow showers will be widespread in western New York as 2
different streams of flow converge, leading to pooling and
moisture and lift. One stream moves from west to east across Lake
Erie, and has already lead to widespread snow showers south of
Buffalo in near shore areas of southwest NY. Streamers from this
area will continue to moves east into western NY today. The
second stream of lake effect snow is from northwest flow across
Lake Ontario that moves onshore and inland into western NY and the
eastern Finger Lakes region.

Favorable flow and the likelihood of some lake-enhanced effective
fetch across multiple lakes should still support periods of
moderate to heavy snow inches in these regions today, with band of
4-6 inches expected. As a low level ridge builds in western NY,
drier air advects across the region and the low level convergence
wanes as well, so snow shower coverage/intensity should decrease
overnight.


...Midwest/Great Lakes, and Northeast Days 2-3...

A 700 mb trough moving through the northern Plains will reach near
the NE/IA border by 12z Sun, with the UKMET/ECMWF indicating a
closed low developing. Near the circulation snow develops across
eastern SD and NE, continuing in tandem with the low across Iowa
and southern WI.
The continued steady progression across WI and MI and then into
the lower Lakes by 12z Mon leads to modest snowfalls across MI. A
brief enhancement off Lake MI in the Ludington area may lead to
4-5 inch totals with most of the rest of lower MI received 2-4
inches of snow.

On Monday, the wave continues to move east across PA, New York,
and then New England. Low level southerly flow brings warm
advection across
PA and NY, with the low level southerly flow enhancing ascent in
windward terrain from the Tug Hill to the northern Mohawk
Valley/southern Adirondacks of NY, where several inches of snow
are expected. The steady forward movement of this system precludes
a more widespread heavy snow event. Probabilities for 4 inches are
as high as 30% D3 in northern PA and terrain of VT/NH, peaking
near 70 percent in the southern Adirondacks of NY.

The probability of a quarter inch of freezing rain is less than 10
percent Days 1-3.

Petersen

$$
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FOUS11 KWBC 202008
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
307 PM EST Sat Feb 20 2021

Valid 00Z Sun Feb 21 2021 - 00Z Wed Feb 24 2021

...Midwest through the Mid-Atlantic and Northeast...
Days 1-3...

A positively tilted longwave trough will advect eastward from the
Four Corners while shedding shortwave energy to the northeast
across the Central Plains and into the Midwest Sunday. As this
shortwave lifts northeast, it will deepen slowly, and guidance has
begun to converge on an amplitude somewhere in the middle of the
closed/deep NAM/CMC and the shallow/progressive UKMET. As this
feature lifts quickly northeast, it will be accompanied by a 250mb
jet streak which will begin to arc poleward in response to the
amplifying longwave trough across the west, driving increasing LFQ
diffluence atop the Midwest, and working in tandem with height
falls to drive surface low pressure development over the Central
Plains. This low will lift E/NE quickly Sunday into Monday, moving
from Kansas Sunday morning to Michigan Monday morning, and then
progressing as an open wave to off the New England coast by
Tuesday morning. In addition to the deep layer ascent noted above,
increasing WAA ahead of the low and associated warm front will
help expand precipitation northeastward as low-level winds advect
Gulf moisture northward.

On Day 1 /Sunday/ as the low deepens and moves through the
Midwest, deep layer synoptic ascent combined with at least modest
frontogenesis will produce moderate to at times heavy snowfall.
The heavy snowfall will likely be confined to any banding that my
develop N/NW of the surface low as the 700mb low and 500mb trough
interact to the NW. The stronger guidance (NAM CMC) has weakened
today leading to less banding potential, which combined with the
transient nature of the trough suggests total snowfall should be
modest, and WPC probabilities indicate a 20-30% chance for 4
inches across northern and central IA with lower probabilities
extending as far east as the L.P. of MI. However, locally higher
snowfall is possible in any banded structures. Additionally, some
very light freezing rain is possible D1 as precip onset occurs
through WAA overrunning a weak and retreating surface high
pressure.

Further east and on D2, WAA will spread precipitation northward
through the Mid-Atlantic and into the Northeast. The column will
initially be cold enough for snow everywhere, but warming
low-level temperatures in response to WAA, mid-level divergence,
and a retreating high pressure producing return flow across the
east, should quickly change p-type to rain for the Mid-Atlantic
and Southern New England. This suggests that after a brief period
of snow everywhere, any significant accumulations should be
confined to northern PA, including the Poconos, and northward into
the Catskills, Adirondacks, and central New England where WPC
probabilities for 4 inches are 10-30%.

As the wave pulls away late D2, broad cyclonic flow across the
Great Lakes will be reinforced by a shortwave dropping across Lake
Superior Monday evening and then towards New England by Tuesday
morning. This will enhance the LES potential in the favored W/NW
snow belts despite modest 850mb temps falling to around -10C and
mostly ice covered lakes. WPC probabilities on D3 are high for 4
inches downwind of lake Ontario on D3, with maximum amounts
exceeding 8 inches possible.


...Pacific Northwest through the Northern Rockies...
Days 1-3...

Broad cyclonic flow across the CONUS due to a broad but amplified
trough will keep confluent mid-level NW flow angled into the
Pacific Northwest through early next week. Within this mid-level
flow, at least one shortwave trough will drop into WA state on
Monday evening, while persistent upper level jet energy drives
moisture and ascent continuously into the region. The combination
of high Pacific moisture and continued mid-level confluence will
produce an atmospheric river (AR) that is progged by both the GEFS
and ECENS to reach 500 kg/m/ms IVT, indicating a weak 2-day AR is
likely during the period. This AR combined with the synoptic
ascent, aided by modest WAA as winds back ahead of the shortwave,
will drive ascent and precipitation across the terrain from the
Olympics through the Northern Rockies, extending into the Tetons
by D3. D1 and D2 are likely to feature the most QPF and strongest
ascent as the core of the AR moves onshore. During this time, snow
levels are likely to rise on the moist warm flow, starting D1
around 3000-5000 ft, and climbing through D2 to more widespread
4000-6000 ft. Above these levels, snow will be plentiful, and WPC
probabilities are high for 8 inches int he WA Cascades and parts
of the Northern Rockies. Locally, snowfall could be much higher
than that, exceeding 2 ft each day. By D3, the best ascent and
moisture begins to shift to the southeast, so while heavy snow
will likely continue in the WA Cascades and Northern Rockies,
although with slightly less intensity, it will also shift into the
NW WY ranges. WPC probabilities on D3 are high for 8 inches once
again across all these ranges.


The probability of a quarter inch of freezing rain is less than 10
percent Days 1-3.

Weiss

$$
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FOUS11 KWBC 210831
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
330 AM EST Sun Feb 21 2021

Valid 12Z Sun Feb 21 2021 - 12Z Wed Feb 24 2021

...Midwest through the Mid-Atlantic and Northeast...
Days 1-3...

A closed 850 mb low traversing across Iowa to WI guidance will
cause downstream warm advection with precip continuing to increase
in coverage today. Low level frontogenesis along the
circulation's path is expected to lead to a band of enhanced
precip. The high res guidance has nudged amounts up this cycle
with several inches of snow expected in western Iowa, with a
general 4-6 inches extending into northeast Iowa. Amounts are
slightly lower in WI and MI as the circulation moves steadily east
northeast, limiting the duration of snow and thus amounts.

Further east on Monday/D2, low-mid level warm advection will
spread precipitation northward through the Mid-Atlantic and into
the Northeast. The column will initially be cold enough for snow
in the northern mid Atlantic, but warming low-level temperatures
should quickly change p-type to rain for the Mid-Atlantic and
Southern New England. This suggests that any significant
accumulations should be confined to interior locations like
northeast PA, northward into the Catskills, and central New
England where WPC probabilities for 4 inches are around 30%.
Higher probabilities exist int he southern Adirondacks as south to
southwest boundary layer winds turn upslope in windward terrain,
enhancing QPF and thus snow amounts. The GFS has higher amounts
and looks overdone based on the short duration of ascent and prior
performance.

Following the wave departing New England, the 00z ECMWF shows a
secondary 850 mb wave crossing the Great Lakes Monday night and
into the lower Lakes early Tue. A period of enhanced snow is
expected in the Tug Hill Tue., but shifting wind directions leads
to shifting locations of bands of snow.


...Pacific Northwest through the Northern Rockies...
Days 1-3...

Low-mid level confluent flow channels moisture onshore across the
Pacific northwest and into the northern Rockies. At least one
shortwave trough will drop into WA state on Monday evening, while
persistent upper level jet energy and divergence maxima drives
ascent across the region. The combination of high Pacific moisture
and the strong jet will produce an atmospheric river (AR) that
supports heavy precipitation where moisture and lift occur. A 700
mb jet peaking near 50 kt provides strong bands of convergence
across WA, ID, and western MT, where the flow leads to strong
upslope flow in windward terrain. The couplets of upper
divergence/lower convergence, mid level frontogenesis maxima, and
upslope flow lead to high rates of snow, with the long duration
leading to several feet of snow expected across the WA Cascades
into the ranges of northern ID.

The strongest 300 mb jet moves south from British Columbia across
WA, ID, and MT Monday night, then departing east across the
northern Plains on Tue. Consequently, QPF and snow amounts are
not as high on Tue., with aid provided in the Pacific northwest as
the trough passage is accompanied by drying aloft. As the upper
trough shifts east onto the Plains, snow develops in the ranges of
eastern WY and the Black Hills Tue-Wed.

The probability of a quarter inch of freezing rain is less than 10
percent Days 1-3.

Petersen


$$
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FOUS11 KWBC 211945
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
245 PM EST Sun Feb 21 2021

Valid 00Z Mon Feb 22 2021 - 00Z Thu Feb 25 2021

...Eastern Great Lakes and Northeast...
Days 1-2...

A shortwave moving through the Midwest and into the Great Lakes
will drape a vorticity lobe southward swinging across the
Mid-Atlantic Monday aftn. A wave of low pressure beneath this
feature will weaken and shift across the Great Lakes, while a warm
front draped to its east will lift northward. The primary surface
low will occlude to a triple point, and this feature, along with
the warm front, will move across the region Monday. Ahead of this
system, cold high pressure will be in place, but pronounced ascent
through strong WAA and mid-level divergence will weaken the high
and cause it to retreat quickly eastward.

Intense WAA noted by 60+ kts of wind at 850mb from the S/SW will
drive robust ascent forecast to exceed 20 micro-bars within the
DGZ Monday. While this warm advection will gradually erode the
cold air and cause a changeover from snow to rain as far north as
PA, initially the entire area should see snowfall. Some of this
could be heavy, with the HREF mean showing high probabilities for
1"/hr across PA/MD and into NY/NJ. The temporal duration of heavy
snow should be limited, especially where rain changeover occurs,
but some light accumulations of 1" or more are likely even in the
I-95 corridor Monday. Further north and into the terrain of PA,
NY, WV, and New England, WPC probabilities are moderate for 4".
This system pulls away early Tuesday, and cold advection in its
wake moving atop the Great Lakes will produce LES in the favored
W/NW snow belts. The heaviest snow D1-2 is likely SE of Lake
Ontario where a few inches of WAA synoptic snow Monday will be
followed by heavy LES Monday night and Tuesday. WPC probabilities
for 6" are low to moderate in the vicinity of the Tug Hill Plateau
both Monday and Tuesday.


...Pacific Northwest through the Northern Rockies...
Days 1-3...

An atmospheric river (AR) ongoing this afternoon will persist
through Monday and into Tuesday across the Pacific Northwest, with
moisture spillover into the Northern Rockies likely as mid-level
confluence drives moisture robustly to the east. Embedded within
this flow, two distinct shortwaves will rotate southeast through a
broad trough to enhance ascent, and drive periods of WAA during
the greatest height falls. Snow levels will climb through the AR
as the moisture becomes more tropically sourced, and this will
limit significant accumulations to the higher terrain. However,
within the high terrain of the Olympics, Cascades, Northern
Rockies, and into the Tetons (D2), heavy snow is likely with WPC
probabilities for 8 inches high on both days. Two-day
accumulations could exceed 3 ft in the Cascades where the best
overlap of mid-level fgen and upslope flow combine with the deep
layer synoptic lift. By D3 /Wednesday/ the second shortwave will
dive into the Four Corners region leaving broad shortwave ridging
across the area. This should cut off the moisture and shunt the
ascent southeastward, bringing an end to the heavy snow, although
lingering moderate snow may persist across Idaho during Wednesday.


The probability of a quarter inch of freezing rain is less than 10
percent Days 1-3.


Weiss


$$
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FOUS11 KWBC 222104
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
404 PM EST Mon Feb 22 2021

Valid 00Z Tue Feb 23 2021 - 00Z Fri Feb 26 2021


...Northern Plains to the Northeast...
Days 1-3...
A well-defined shortwave dropping southeast into the Pacific
Northwest Monday evening is forecast to move quickly east --
reaching the northern High Plains by mid Tuesday. Models have
been trended upward with QPF/snow amounts associated with this
progressive, but dynamic system as it moves across the northern
Plains, Upper Midwest and Great Lakes on Tuesday and Wednesday.
In addition to favorable upper forcing, strong low-mid-level
frontogenesis will likely be supportive of a period of moderate to
heavy snow shifting east across North Dakota into northern
Minnesota on Tuesday, and then across northern Minnesota into
northern Wisconsin and northern Michigan Tuesday night into
Wednesday morning. Latest WPC PWPF shows a narrow stripe of 2-4
inches, with locally heavier amounts, likely across this region.
As the system moves to the east late Wednesday, there is some
indication that northwesterly flow on the backside of the system
may contribute to some lake-enhanced totals across portions of the
eastern U.P. and northern Lower Michigan. This system is expected
to move across southern Ontario and Quebec Wednesday evening,
before brushing northern New York and New England with some light
amounts Wednesday night into early Thursday.

...Pacific Northwest to the Northern Rockies...
Days 1-3...
A trio of systems is expected to bring periods of heavy mountain
snow as they move across the region over the next few days. The
first system, (noted previously), is forecast to produce heavy
snows as it moves across the region Monday night. Areas impacted
are expected to include the northern Cascades, the Blue Mountains
and the Rockies from northern Idaho and western Montana to western
and central Wyoming. This system will be quickly followed by a
second shortwave dropping southeast into the Northwest on Tuesday
-- bringing additional heavy snows to portions of the region late
Tuesday into early Wednesday. Shortwave ridging behind this
system will provide short period of dry weather before the third
system drops into the Northeast on Thursday, with heavy snows
likely across the Olympics and northern Cascades.

...Central Rockies/High Plains...
Day 3...
Snows are expected to develop across the central Rockies as the
shortwave trough digging across the Northwest on Wednesday
continues to dig southeast and approaches the region Wednesday
evening into the overnight. Low-level, convergent, upslope flow
is expected to support snow developing over the Colorado High
Plains and foothills Wednesday night into early Thursday, with WPC
PWPF showing 50 percent or greater probabilities for accumulations
of 4-inches or more along a good portions of the I-25 corridor.


For Days 1-3, the probability of significant icing (0.25-inch or
more) is less than 10 percent.

Pereira

$$
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FOUS11 KWBC 232136
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
435 PM EST Tue Feb 23 2021

Valid 00Z Wed Feb 24 2021 - 00Z Sat Feb 27 2021


...Northern Plains to the Northeast...
Day 1...
A progressive, but dynamic shortwave will continue to move through
the northern Plains and Upper Midwest this evening, reaching the
northern Great Lakes Wednesday morning. Strong low-to-mid level
frontogenesis combined with upper level forcing will continue to
support the potential for a narrow band of snow developing and
moving steadily east from eastern North Dakota through central
Minnesota, into northern Wisconsin and northern Michigan Wednesday
evening and overnight. As the system moves east into southern
Ontario on Wednesday, northerly winds on the backside of the
system may offer a brief window for lake effect snow showers --
bolstering totals across portions of the U.P. and northern Lower
Michigan. Highest totals through the Day 1 period (ending 00Z
Thu) are expected to center over the eastern U.P. and the Tip of
the Mitt -- with WPC PWPF showing 50 percent or greater
probabilities for accumulations of 4-inches or more across the
region.

After moving across southern Ontario and Quebec during the day on
Wednesday, this system is expected to impact northern New York and
New England with generally light accumulations as it moves across
the region late Wednesday and early Thursday.

...Pacific Northwest to the Northern Rockies...
Days 1-3...
A shortwave trough dropping across the Northwest this evening may
produce some additional locally heavy snows early in the period
across portions of the northern Cascades, Blue Mountains, and the
northern Idaho ranges before dropping farther southeast during the
evening and overnight. This will be followed by a brief period of
dry weather as a shortwave ridge moves across the region on
Wednesday into early Thursday. Unsettled weather is likely to
return during the day on Thursday as another shortwave trough and
associated frontal band approach the region, with heavy snow
accumulations likely across the Olympics and northern Cascades by
late Thursday. Heavy snow is likely to continue across the
Olympics and Cascades while expanding east into the Blue Mountains
and portions of the northern Rockies late Thursday through Friday.
Guidance shows strong northwesterly flow ushering a string of
shortwaves across the region during that time. Two day totals
ending late Friday are likely to exceed two feet across a large
portions of the northern Cascades, as well as parts of the Blue
Mountains and Bitterroot Range.

...Central Rockies/High Plains...
Day 2...
A shortwave trough moving across the Northwest Wednesday night is
forecast to continue to dig southeast through the Great Basin,
with snows developing across the central Rockies Wednesday night.
As the system approaches, low level easterly flow will support
developing snows across the High Plains and along the foothills as
well. While some areas along the Colorado I-25 corridor may see
local accumulations of 4-inches or more, widespread heavy amounts
along and east of the interstate are not expected.

For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Pereira



$$
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FOUS11 KWBC 242102
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
402 PM EST Wed Feb 24 2021

Valid 00Z Thu Feb 25 2021 - 00Z Sun Feb 28 2021


...Pacific Northwest to the Northern Rockies...
Days 1-3...
A passing shortwave ridge will continue to promote dry weather
across much of the region through the early part of the period.
However, a shortwave trough and associated frontal boundary that
is forecast to move into the region on Thursday is expected to
mark the onset of a period of unsettled weather, resulting in very
heavy snow accumulations across portions of the high terrain. A
well-defined shortwave and associated frontal band dropping
southeast into the region on Thursday is expected to produce heavy
snow accumulations across the Olympics and northern Cascades by
the late afternoon hours. As heavy snows continue across these
areas, heavy accumulations are also forecast to develop farther to
the east late Thursday into the early Friday, with impacts across
the Blue Mountains and along the northern and central Idaho
ranges. As this leading shortwave continues to dig to the
southeast, upstream energy embedded within strong northwest flow
will drop across the Northwest, supporting additional periods of
heavy snow through Friday into Saturday before an upper ridge
begins to build east into the region. Storm accumulations of
several feet are likely across the higher elevations of the
northern Cascades. Accumulations of 2 feet or more are likely for
portions of the Blue Mountains, as well as the the Bitterroot
Range.

...Central Rockies and High Plains...
Day 1...
As an upper level shortwave approaches from the west, snows are
expected to develop across the Colorado Rockies Wednesday night.
Low level easterly flow developing east of the mountains is
expected to support developing snows over the High Plains into the
foothills as well. Guidance has shown a slight northerly shift
with the wave, edging the potential for heavier snows farther to
the north along the I-25 corridor and Front Range. Models have
also trended more amplified with the wave over the past day as
well, signaling greater potential for significant snowfall
accumulations across a broader area. Latest WPC PWPF shows snow
accumulations of 4-inches or along much of the northern Front
Range and across the Denver Metro onto the Palmer Divide.

...Maine...
Day 1...
A progressive, but dynamic shortwave moving east of the northern
Great Lakes this afternoon is expected to generate a period of
heavy snow as it moves across northern Maine Wednesday evening and
overnight. Latest guidance has trended upward with amounts, with
strong low-to-mid level frontogenesis expected to support a period
of moderate to heavy snows moving east across the region. Several
HREF members are showing the potential for snowfall rates of 0.5-1
inch/hr within developing bands moving across the region beginning
late this evening.

For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Pereira

$$
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FOUS11 KWBC 252146
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
445 PM EST Mon Jan 25 2021

Valid 00Z Tue Jan 26 2021 - 00Z Fri Jan 29 2021

...Central Plains to the Northeast and Mid-Atlantic...
Days 1-2...
Ongoing moderate to heavy snow centered across eastern Nebraska
and Iowa is expected to diminish during the evening into the
overnight hours as the favorable forcing gradually wanes.
However, hi-res guidance indicates that banded snow, producing
snowfall rates of 0.5 in/hr, may persist for a few hours across
Iowa into northern Illinois and far southern Wisconsin early in
the period -- promoting high probabilities for additional snow
accumulations of 4-inches or more. Easterly to northeasterly flow
developing early across southern Lake Michigan and continuing
through the overnight into early Tuesday is expected to support
lake-enhanced heavy totals along the southwestern lakeshores.

Meanwhile, moisture spreading north along a low-to-mid level
baroclinic zone lifting across the Appalachians and Mid-Atlantic,
will encounter a wedge of cold air to the north -- setting the
stage for snow, followed by a wintry mix across the central
Appalachians and Mid Atlantic. While there remains a strong
signal for significant ice accumulations across portions of
western Virginia, the Eastern West Virginia Panhandle, western
Maryland and western Pennsylvania, probabilities for at least
measurable snow have increased farther east, with the 12Z NAM and
hi-res model suite indicating a period of moderate snow developing
this evening before changing over to mixed precipitation across
the I-95 corridor into the Delmarva.

As the the low-to-mid level frontal band continues to translate
northward, axis of heavier, organized precipitation is forecast to
lift through the northern Mid-Atlantic into the Northeast on
Tuesday, with thermal profiles supporting mostly snow across
Upstate New York and New England. Models show a period of strong southwesterly-southerly flow/frontogenesis ahead of the
approaching low level center supporting a period of heavier
snowfall developing across Upstate New York into western New
England, with some orographically enhanced totals from the
southern Adirondacks into the southern Green and Berkshire
mountains on Tuesday.

...Southwest...
Day 1...
A deep upper trough/developing closed low moving across Southern
California into the Southwest is expected to produce widespread
high elevation snow across the region through Monday night into
Tuesday across the region. This includes areas along the Mogollon
Rim into the mountains of southeastern Arizona and southwestern
New Mexico -- where widespread totals exceeding 8-inches are
expected. Locally heavy totals are also forecast for the
mountains of southern Nevada and southern Utah.

...Western U.S....
Days 2-3...
Beginning Tuesday and continuing through the remainder of the
period, a major winter storm is expected to produce widespread
snow across portions of the Northwest, the northern Rockies and
California, with multiple feet likely across the Sierra and
mountains of Northern California. Heavy snows are expected to
develop across the mountains of Northern California as a deep low
dropping south from the Gulf of Alaska approaches the coast late
Tuesday. While the heaviest amounts are expected to fall across
the higher elevations of the Klamath mountains into the southern
Cascades, snow levels are expected begin low, supporting
accumulating snows near the coast. Although the initial low is
expected to weaken off the coast, energy digging south into the
base of the trough is expected to amplifying the trough --
focusing the strongest inflow farther to the south, with heavy
snows moving south along the Sierra Tuesday night into Wednesday.
Heavy snows will likely continue along the central and southern
Sierra through Thursday as the upper trough slowly pivots east.


Pereira


$$
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FOUS11 KWBC 262111
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
410 PM EST Tue Jan 26 2021

Valid 00Z Wed Jan 27 2021 - 00Z Sat Jan 30 2021


...Western U.S....
Days 1-3...
A major winter storm is expected to produce multiple feet of heavy
snow and very strong winds across the Sierra and mountains of
Northern California through Friday morning. Blizzard conditions
are expected, especially across portions of the Sierra.

An occluded front, associated with a deep low dropping southeast
along the Washington and Oregon coasts, is expected to move
inland, bringing heavy precipitation into portions of Northern
California this evening. Snow levels are expected to be low at
the onset -- dropping below 1000 ft in some locations. Therefore,
in addition to heavy amounts across areas that will likely include
the Klamath Mountains and the Shasta Cascade region, significant
accumulations are possible closer to the coast and across the
northern Sacramento Valley -- where several inches are possible
Tuesday night. Models continue to show a band of intense
precipitation, fueled by highly anomalous moisture, developing
along the associated cold front as it drops south Tuesday night
into Wednesday -- supporting very intense snowfall rates along the
Sierra. Despite gradual weakening of the parent low along the
Washington and Oregon coasts on Wednesday, energy digging south
through the eastern Pacific will continue to amplify the trough
and support a prolonged period of deep southwesterly flow across
central California into the Sierra. Deep southwesterly flow is
expected to persist across California as a new low develops and
drops south from the British Columbia to the Washington and Oregon
coasts on Thursday. The threat for heavy snow is forecast to
diminish on Friday as the upper trough finally begins to move
inland. In addition to the very heavy totals that are likely
across the Northern California mountains and the Sierra, locally
heavy storm total accumulations on the order of a foot or more are
also likely along the Southern California Transverse and northern
Peninsular ranges. Widespread heavy amounts of a foot or more are
also likely from the eastern Oregon mountains and along the
central to eastern Idaho ranges into western Wyoming.

...Northeast...
Day 1...
Light to moderate precipitation associated with a low level front
lifting out ahead of a wave moving through the Great Lakes, will
continue to move east across the Northeast Tuesday evening. While
mostly light snow accumulations are expected, some areas of the
Adirondacks, Green, Berkshire and White mountains could see
additional snow accumulations of 4-inches or more before
precipitation ends.

...Southern and Central Appalachians...
Day 2...
A shortwave trough, associated with the remnants of an upper low
currently moving out of the Southwest, is expected to move from
the Ohio and Tennessee valleys into the Appalachians Wednesday
night. This system is expected to support widespread light snows
across the region, with some potential for locally heavier amounts
falling across the southern and central Appalachians Wednesday
night.

For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Pereira

$$
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FOUS11 KWBC 270823
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
323 AM EST Wed Jan 27 2021

Valid 12Z Wed Jan 27 2021 - 12Z Sat Jan 30 2021


...Western U.S....
Days 1-3...
A major winter storm is expected to produce multiple feet of heavy
snow and very strong winds across the Sierra and mountains of
Northern California through Friday morning. Blizzard conditions
are expected, especially across portions of the Sierra. Snowfall
rates of 4 inches per hour were reported tonight near Mt. Shasta,
CA.

Heavy amounts are forecast the Klamath Mountains and the Shasta
Cascade region. Models continue to show a band of intense
precipitation, fueled by highly anomalous moisture,
and upper jet intensifying to 110-130 kt that extends onshore
across central CA to the NV border through tonight that has
embedded persistent upper divergence maxima.
This results in several feet of snow in the central CA Sierra
Nevada Mountains on Day 1 today.

As the upper trough approaches the coast on Thursday, the upper
jet begins to weaken, so the strong upper divergence maxima
gradually wane with time.
An additional 3 to 4 feet are possible in the central to southern
CA Sierra Nevada range. In addition to the very heavy totals that
are likely across the Northern California mountains and the
Sierra, locally heavy storm total accumulations on the order of a
foot or more are also likely along the Southern California
Transverse and northern Peninsular ranges.
As the upper jet persists across eastern OR into ID and northwest
WY, continuing moisture transport and lift along the jet axis
supports widespread heavy amounts of a foot or more are also
likely from the eastern Oregon mountains and along the central to
eastern Idaho ranges into western Wyoming.

The threat for heavy snow in CA is forecast to diminish on Friday
as the upper trough finally begins to move inland. However, the
upper jet crossing southern CA into AZ reintroduces moisture
transport into the Mogollon Rim of AZ and also areas from Mt.
Charleston NV across the southern Wasatch of UT, so several inches
of snow are possible Fri in these areas. The snow
coverage/intensity wanes as the upper trough departs the southwest
and crosses the southern Rockies.

...Great Lakes...
Day 1...
Light snow are expected in the lee of Lake MI Day 1 as winds back
from northeast to north, favoring snow ending on the IL side of
the lake and developing on the IN side. Long duration northerly
trajectories should favor moistening the air mass to support
accumulating snow showers.

Days 2-3...
The models indicate northwest flow with trajectories crossing Lake
Huron and then Lake Erie, with moistening of the airmass crossing
the lakes favoring snow showers in the lee of Lake Erie in
northwest PA to southwest NY. Further northeast, a Georgian Bay
connection to Lake Ontario supports snow showers in the lee of
Lake Ontario along the southern shore and areas downstream, such
as the Syracuse area. Several inches are possible south of Lake
Ontario until drying aloft early on Day 3 reduces snow shower
coverage and intensity.


...Oh Valley to Southern and Central Appalachians...
Day 1...
A low-mid level trough is expected to move from the Mid MS Valley
to Ohio and Tennessee valleys into the Appalachians Wednesday
night. This system is expected to support widespread light snows
across the region, Due to orographic enhancements, locally heavy
amounts are expected across the southern and central Appalachians
Wednesday night, focused near the NC/TN border and southwest VA.
As the low pressure develops off the NC coast, cold air arrives on
the back edge of the precip in southern VA to northern North
Carolina, with rain changing to snow. Light accumulations are
possible here.

For Days 1-3, the probability of significant icing (0.25 inch or
greater) is less than 10 percent.

Petersen

$$
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FOUS11 KWBC 280911
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
411 AM EST Thu Jan 28 2021

Valid 12Z Thu Jan 28 2021 - 12Z Sun Jan 31 2021

...Western U.S....
Days 1-3...

Ongoing major winter storm due to a prolonged Atmospheric River
(AR) will continue at least one more day across CA and parts of
the Southwest before slowly waning Friday. Persistent moisture
spreading onshore within a Pacific jet streak of 110-130kts
combined with unidirectional SW 850-500mb flow will drive a narrow
corridor of IVT approaching 750 kg/m/s into CA on D1. This will be
accompanied by slowly rising snow levels, reaching as high as 5000
ft, but with the moisture source slowly sinking southward. Late D1
into D2, the pronounced mid-level trough driving this confluent
moisture stream will drop southeast and come onshore CA late
Friday before opening and advecting through the Four Corners into
Saturday. This will finally bring a slow end to the persistent
overlap of forcing and moisture into CA, with less intense and
shorter duration forcing pushing snowfall eastward.

On D1, exceptional snow is once again likely in the Sierra where
WPC probabilities for 12" are greater than 95%, and some locations
will likely receive in excess of 4 ft of snow above 5000 ft due to
snowfall rates which could reach 4"/hr at times. As the moisture
shunts slowly southward, heavy snow will develop in the transverse
ranges of Southern CA as well, and WPC probabilities are moderate
for 12 inches in the San Gabriels and San Bernadinos. Waves of
moderate to heavy snow due to impulses embedded within the moist
flow will also lead to heavy accumulations of 12 inches or more in
the Sawtooth of Idaho.

Forcing and moisture finally advect eastward D2, but lingering
heavy snow is likely the first half of Friday before snowfall
winds down. WPC probabilities are moderate for 8 inches in the San
Bernadinos and southern Wasatch, with some lower probabilities
along the Mogollon Rim and White Mountains of AZ.

By D3, another shortwave will approach the Pacific Northwest,
preceded by a backing of the mid-level flow to drive warm/moist
advection onshore ahead of subsequent height falls. This will
produce a new round of heavy snow, focused in the Olympics and WA
Cascades where WPC probabilities are moderate for 8 inches,
generally above 3000 ft.


...Eastern New England...
Day 2...

A low pressure system deepening well offshore will leave an
inverted trough to its west. This trough is progged to rotate
cyclonically around to the west then south, scraping the immediate
coasts of Maine and Massachusetts. This trough will be accompanied
by strong 925-850mb fgen and 0-2km theta-e lapse rates that are
less than 0. At the same time, a mid-level closed low moving
overhead will help to steeping lapse rates through a deeper
portion of the column, while Atlantic moisture spreads inland
behind the departing system. All of these features together will
likely create a band of heavy snow rotating along or just off the
coast on Friday. Guidance has come into good agreement with this
feature, and while there is some uncertainty as to whether the
heaviest snow will reach the coast, there has been a westward
trend overnight which has caused an uptick in snowfall
probabilities. The highest probabilities for 4" are along the
mid-coast of Maine and then again across Outer Cape Cod where
snowfall rates could exceed 1"/hr at times. There is potential for
higher snow amounts across Cape Cod, but at this time the heaviest
snow is forecast to remain just offshore.


...Midwest to Central Appalachians...
Day 3...

A negatively tilted shortwave trough lifting out of the Four
Corners will move through the Central Plains and into the Midwest
Saturday night into Sunday. Atop this trough, a modest zonal jet
streak will race eastward, placing the favorable LFQ with a
divergence maxima atop the region, and the combination of these
features will drive an intensifying area of surface low pressure
moving northeast out of Oklahoma. Increasing mid-level divergence
downstream of the trough and intensifying WAA funneling Gulf of
Mexico moisture northward will drive pronounced ascent northward,
leading to an expanding area of precipitation from the Central
Plains into the Midwest, with secondary precipitation spreading
into the Central Appalachians. The column will initially be
marginally cold for snow, so there is likely to be a zone of mixed sleet/freezing rain, although current WPC probabilities for any
significant freezing rain accretions are low. However, the latter
half of D3 /Saturday night/, heavy snow is likely to develop as
the isentropic lift maximizes and the WCB begins to wrap
cyclonically around the low to the NW. There remains considerable
spread in timing of this system as well as the strength of the
warm nose, but there is increasing potential for a swath of heavy
snowfall from Iowa to Ohio. WPC probabilities are high for 4
inches from eastern IA through NW Indiana, including the Chicago
metro area. An additional area of moderate to heavy snow is likely
very late D3, developing across the Central Appalachians where WPC probabilities are moderate for 4 inches.


Weiss

$$
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FOUS11 KWBC 292055
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
355 PM EST Fri Jan 29 2021

Valid 00Z Sat Jan 30 2021 - 00Z Tue Feb 02 2021


...Midwest...
Days 1/2...

The trough ejecting east from CA becomes negatively tilted as it
crosses the southern Rockies tonight and closes into a mid-level
low by the time it reaches MO Saturday. This primary low slows
over the Midwest through Sunday due to a blocking pattern
downstream over eastern Canada. A reinforcing shortwave trough
catches up to the main low Sunday and refocuses the surface low to
a coastal low by Sunday evening (more on this eastern portion in
the next section).

Rain ahead of the lee trough/low is pretty anemic right now along
the NM/TX border, but this will blossom overnight as the surface
low develops and Gulf moisture is introduced. Plain rain will
expand and lift northward in warm air advection ahead of the
surface low. A developing warm nose and cold surface conditions by
Saturday morning will allow some freezing rain to develop on the
leading edge of the precipitation shield over both central KS/NE
and eastern IA/MO/western IL Saturday morning. The highest
probabilities for this freezing rain remain over east-central Iowa
with about a 20 percent chance of a tenth inch. Deepening moisture
and antecedent cold air along with a developing TROWAL north and
west of the low center allows locally heavy snow to develop late
Saturday afternoon over northern IL/IN and northeast IA as well as
southern WI. The low shifts east Saturday night, keeping areas
such as the Chicago metro in the cold conveyor belt through the
event with increasing easterly flow allowing enhancement from Lake
Michigan over the Chicago to Milwaukee metro areas through Sunday.
Day 2 snow probabilities for 6 or more inches are 40 to 70 percent
for the Chicago-Milwaukee metro corridor west to northeast IA and
also over northern IN into northwest OH. Further local
enhancements are possible north of the 700mb low center which is
forecast to track just south of Chicago Sunday morning.


...Southern and Central Appalachians through the Mid-Atlantic and
into New England...
Days 2/3...

As mentioned in the previous section, the main system over the
Midwest Saturday night translates energy to a coastal low in the
Carolinas through Sunday with further development expected just
offshore through Monday. Flow between these two features will be
over a cold air damming wedge extending down the eastern slopes of
the Appalachians from a 1034mb surface high centered over northern
Quebec Saturday night into Monday. This sets up a classic case of
a front end wave of mostly snow with the warm nose extending
across the surface boundary over the NC/VA Piedmont causing mixed
precip with some ice accumulation and snow farther north. The
erosion of this cold air damming wedge is likely to be rather slow
as it will be reinforced by coastal low development Sunday with
precipitation expanding up the Mid-Atlantic coast to southern New
England Sunday night through Monday. Day 2 snow probabilities for
6 or more inches are moderately high over the south-central
Appalachians of VA/WV with decreasing probabilities through the
Potomac Highlands (with Day 2.5 probabilities capturing the whole
front end precip with moderately high probabilities for 4 or more
inches across the central Mid-Atlantic from central VA to
south-central PA. As the low develops and intensifies off the
Mid-Atlantic coast Sunday night/Monday a dry slot lifts through
the southern Mid-Atlantic as an intense band of precipitation
develops along a coastal front resulting in a second heavy snow
threat area from the Baltimore/Washington metro area northeast to
southern New England. Day 3 snow probabilities for 6 or more
inches are moderate from south-central PA across the northern half
of NJ and the NYC metro with low probabilities into southern New
England (though the bulk of the storm will occur in New England
Monday night into Tuesday).

A typical wedge pattern to the freezing rain threat area sets up
with Day 2 ice probabilities for more than a tenth inch across
south-central VA, particularly along the western NC/VA border,
into the foothills of western NC and south onto the northern NC
Piedmont then north up the western slopes of the Allegheny
Mountains through east-central WV toward Pittsburgh. Probabilities
for a quarter or more of an inch of ice are low with areas of 30
percent along the western NC/VA border and in southern
east-central WV, also for Day 2.


...Western U.S....
Days 1-3...

The full-latitude trough is ejecting east from CA bringing an end
to the 3+ day heavy precip event across CA with ongoing precip
quickly tapering off by this evening in CA as the focus quickly
moves through the Desert SW this evening and the southern Rockies
overnight. Day 1 snow probabilities for 6 or more inches are
moderately high for the White Mtns of AZ, the southern Wasatch in
UT, and the San Juans of CO as well as western CO ranges.

The next wave is a frontal system wrapping around a Gulf of Alaska
low and precip arrives into the WA to northern CA coast this
evening though amplification of the offshore trough will keep
inland progression to a minimum through Monday before another
full-latitude trough pushes through CA Tuesday night. Day 1 snow
probabilities for 6 or more inches are moderate to high over the Shasta/Siskiyou Mtns as well the highest OR and WA Cascades and
the Olympics. Increasing moisture and lowering snow levels expands
snow over these same areas on Day 2 with snow probabilities
generally moderate to high for 8 or more inches. Then this pattern
continues for northern CA Day 3 with moderate to high
probabilities for 12 or more inches over the CA and southern OR
Cascades as lighter precip occurs farther north in OR and WA.


Jackson

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FOUS11 KWBC 300858
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
358 AM EST Sat Jan 30 2021

Valid 12Z Sat Jan 30 2021 - 12Z Tue Feb 02 2021


...Midwest/Ohio Valley
Days 1-2...

Shortwave energy ejecting out of the Four Corners will sharpen
into a negatively tilted trough and close off over Missouri this
evening before lifting towards lower Michigan by Sunday aftn. This
feature will be accompanied by modest but coupled jet streaks, and
the combination of these features will drive surface cyclogenesis
from the TX Panhandle northeast towards the lower Ohio Valley
through Sunday. Downstream of this trough, mid-level divergence
will rapidly increase, which combined with low-level WAA will
drive an expanding precipitation shield across the Mid-MS Valley
northeastward into the Great Lakes and Ohio Valley. While the
southern half of this region will be warm enough for all rain,
there is likely to be a stripe of heavy snow on the northern edge
of this precip shield north of the 700mb low track, with a narrow
band of modest freezing rain in between.

As the precip lifts northward, it will encounter slowly retreating
cold high pressure. The isentropic ascent of the moist air atop
this high will spread snowfall from eastern IA into southern WI,
northern IL, northern IN, and into OH. The lift at 290-300K is
quite moist noted by mixing ratios of 3g/kg, which will ascent
into the DGZ to drive heavy snow rates of 1"/hr or greater as
noted by HREF probabilities. While the developing TROWAL may
remain modest and somewhat south of the axis of heaviest snow, a
prolonged period of moderate to heavy snow north of the eastward
shifting 700mb low will likely produce heavy accumulations,
although some moderation is likely due to the heavy wet nature of
the snow with SLRs less than 10:1. WPC probabilities on D1 are
high for 6 inches in northeast IL into northwest IN, shifting east
across northern IN into western OH D1.5. There is likely to be
some isolated totals approaching 10", with the highest amounts
possibly in the Chicago vicinity due to some lake enhancement as
easterly flow moves across Lake Michigan.

Further to the southwest, a zone of moderate freezing rain is
possible where the warm nose reaches above 0C but surface
temperatures remain cold within the surface high pressure regime.
Most of the freezing rain is likely to be light as p-type
transitions from rain to snow due to dynamic cooling, but WPC
probabilities indicate a 40-50% chance for 0.1" of ice across
parts of Iowa.


...Southern and Central Appalachians through the Mid-Atlantic and
into New England...
Days 1-3...

The low pressure mentioned above affecting the Midwest will become
vertically stacked and occlude on Sunday, with secondary low
pressure development occurring off the VA coast Sunday night.
There remains some spread in the placement and timing of this
secondary low development, but it is likely that this low will
become the dominant feature by Monday and then move very slowly
off the coast as it interacts with the broader upper low and lobes
of PVA rotating through the main trough. As this secondary low
meanders northeast through D3, it is likely to strengthen as upper
divergence intensifies within an increasingly coupled jet
structure.

Before secondary low development begins, broad WAA from the Gulf
of Mexico will overspread the region from the southwest bringing
periods of rain, snow, sleet, and freezing rain to the region. A
1030mb surface high over Quebec will wedge down along the eastern
slopes of the Appalachians as far as SC, and as WAA commences, it
will lead to an overrunning situation with wet-bulb surface
temperatures below freezing. Guidance has slightly sped up the
timing of precipitation for the Appalachians and adjacent
foothills, while also showing a subtly stronger warm nose. This
has led to an increase in freezing rain across western and central
NC and into southern VA, where WPC probabilities are now as high
as 30% for 0.25" of accretion. However, slowly warming sfc temps
and expected periods of heavier rain rates suggests accretion will
be limited at times by runoff, so widespread warning criteria
freezing rain is not expected. Lighter freezing rain amounting to
0.1" or less is likely further northeast into WV and the I-95
corridor from DC to Philadelphia where a dry slot moving overhead
Sunday night will lead to a loss of cloud ice and a prolonged
period of light freezing rain or freezing drizzle.

Further north, the leading WAA/isentropic ascent will lead to a
thump of snowfall late Saturday into Sunday from the terrain of VA
northward into PA and NJ. This WAA appears to be modest such that
snowfall rates of greater than 1"/hr should be the exception
rather than the rule, but very moist ascent at 300K noted by
mixing ratios of 4g/kg lifting into the DGZ should support
widespread moderate to at times heavy snowfall producing several
inches of accumulation. WPC probabilities through D2 of 4" are as
high as 60%, including the I-95 corridor from DC to PHL.
Uncertainty increases considerably on D3 due to the secondary low
development and its impact. As the low deepens offshore, a robust
CCB is likely to develop just north of the dry conveyor/slot,
which will be aligned with intensifying NE to SW deformation.
Exactly where this occurs is still very much in question, and
there has been a notable shift northward in the 00z suite of
guidance overnight. While easterly low-level flow overrunning a
coastal boundary will lead to enhanced snowfall just NW of that
boundary, likely from Boston southwest to NYC and then just inland
of PHL, the location of this CCB and deformation band could
produce local maxima of snowfall much greater than the areal mean.
This will become better resolved with time, but for D2.5-D3, WPC
probabilities suggest a greater than 50% chance of 6" from
south-central PA northeast to extreme southeast NH, with local
amounts more than double this likely.


...Western U.S....
Days 1-3...

A slow moving trough will drop out of the Gulf of Alaska beginning
this evening, moving onshore the Pacific Northwest only by Tuesday
morning. Ahead of this trough, mid-level winds become confluent
and backed to the SW, driving warm and moist advection from
northern CA into the Pacific Northwest and Northern Rockies all 3
days of the forecast period. Within this flow, weak impulses will
repeatedly advect onshore, driving enhanced ascent. This suggests
that periods of moderate to heavy snow will occur through early
next week, with the heaviest snow in the favored high terrain of
the Sierra and northern CA ranges which will be somewhat
orthogonal to the 700mb flow for upslope. Snow levels will rise on
this warm advection, becoming 6000 ft or more in CA, and 3000-4000
ft further north. WPC probabilities are high for 8" all 3 days in
the Trinity/Shasta/Siskiyou ranges, where 3-day snowfall may
exceed 5 feet in some places. Further north, WPC probabilities in
the Cascades and Olympics are moderate all 3 days for 8 inches,
with event total snowfall likely reaching 2-3 ft in the higher
terrain.


Weiss

$$
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FOUS11 KWBC 302103
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
402 PM EST Sat Jan 30 2021

Valid 00Z Sun Jan 31 2021 - 00Z Wed Feb 03 2021


...Midwest/Ohio Valley
Day 1...

A closed low is apparent along the eastern KS/OK border this
afternoon in mid-level water vapor from GOES-East. This low will
shift east-northeast across the Midwest through Sunday when a
reinforcing shortwave trough (currently over western WY) will
shift south of the low, opening it and amplifying the trough over
the Southeast US which in turn promoted translation of energy to
the Carolina coast Sunday/Sunday night (more on this in the next
section). This low is already vertically stacked with the surface
low slowly filling as it moves into the Midwest. Plain Rain is
mainly seen in the precip shield with some freezing rain in
east-central IA. This ice area on the northern periphery should
expand a bit into the evening as air temperatures drop to the
subfreezing wet bulb temp with saturation. Farther northeast, over
eastern IA, northern IL, southern WI and northern IN, snow is
expected to begin soon as marginal antecedent conditions are
further dynamically cooled as the TROWAL further develops. There
is likely to be a stripe of heavy snow on the northern edge of
this precip shield north of the 700mb low track, with a narrow
band of modest freezing rain in between. While the developing
TROWAL may remain modest and somewhat south of the axis of
heaviest snow, a prolonged period of moderate to heavy snow north
of the eastward shifting 700mb low will likely produce heavy
accumulations, although some moderation is likely due to the heavy
wet nature of the snow with SLRs less than 10:1. WPC probabilities
for Day 1 are 70 to 80 percent for 6 or more inches in northeast
IL across northern IN into northern OH with moderate probabilities
now across OH through Day 1.5. Day 1 probabilities for 8 or more
inches are up the Chicago/Milwaukee metro corridor due to some
lake Michigan enhancement in easterly flow.

On the south edge of the snow line, over north-central IL/IL into
south-central OH a narrow stripe of light freezing rain is likely,
though expected to result in less than 0.1" ice to the southwest.


...Southern and Central Appalachians through the Mid-Atlantic and
through New England...
Days 1-3...

The reinforcing shortwave trough mentioned above allows surface
low development to begin late Sunday over coastal Carolina, east
of a cold air damming wedge extending from a 1034mb surface high
over northern Quebec. This low will become the dominant feature
Sunday night and then move very slowly off the Mid-Atlantic coast
or even stall Monday/Monday night as the parent upper trough
closes again with further reinforcing waves making the surrounding
trough negatively tilted. The low then shifts north past Maine
Tuesday/Tuesday night.

As the cold air damming wedge sets up tonight, broad WAA from the
Gulf of Mexico will overspread the region ahead of the main trough
bringing a gradient of rain south then mix then snow to the north.
As usual WAA precip is outpacing most guidance for onset timing
with greater confidence in freezing rain over southwest VA and the
northern NC Piedmont and Blue Ridge as well as up east-central WV,
a typical freezing rain pattern for a CAD wedge that extends into
northern GA. Day 1 ice probabilities for a quarter inch or more
are 40 to 50 percent along the west-central NC/VA border through
the northern NC Blue Ridge.

Farther north, the leading WAA/isentropic ascent will lead to a
thump of snowfall late Saturday into Sunday from the terrain of VA
northward into PA and NJ. This WAA still appears to be modest such
that snowfall rates of greater than 1"/hr should be the exception
rather than the rule with 12Z HREF 1"/hr probabilities limited to
the south-central Appalachians in two rounds late tonight and
Sunday morning. WPC snow probabilities for 6 or more inches on Day
1 are 70 to 80 percent for the south-central Appalachians of WV/VA
up through the north-central VA Blue Ridge (Shenandoah NP) with
moderately high probabilities for 4 or more inches through the
Washington DC metro area.

Most of the front end precip then shifts offshore ahead of the
developing low Sunday night, returning onshore with a vengeance
across the northern Mid-Atlantic (well west into PA) Monday as the
TROWAL really gets going. The main shift with QPF/snow is on Day 2
where the 12Z consensus is for a pivoting stripe of moderate to
heavy snow across PA/NJ than then shifts up the coast through
Tuesday. Day 2/3 snow probabilities focus this second wave of the
storm with Day 2 moderate probabilities for 8 inches from central
PA to northern NJ/the NYC metro and Day 3 over the Adirondacks and
across south-central New England then up the Maine coast. Given
the closing off of the upper low and associated axis', certainty
with Day 3 is less at this time, particularly with where the
coastal front/surface low track sets up in Maine.


...Western U.S....
Days 1-3...

Low pressure lingers just south of the Gulf of Alaska as troughing
amplifies off the West Coast. Persistent precip occurs over far
northern CA and up the OR/WA Cascades and west through Monday
night before the focus shifts south through CA through Wednesday.
Periods of moderate to heavy mountain snow snow will occur in this
precip axis with snow levels will remaining around 5000ft in
northern CA, decreasing to about 4000ft in WA. WPC probabilities
are moderate to high for 12" on Day 1/2 in the
Trinity/Shasta/Siskiyou ranges, where 3-day snowfall may reach 5
feet on mountain peaks. Further north, WPC probabilities in the
Cascades and Olympics are moderate all 3 days for 8 inches, with
event total snowfall likely reaching 2-3 ft in the higher terrain.
Moderate to high Day 3 probabilities for 8 or more inches extend
down the CA Cascades and northern Sierra Nevada.


Jackson

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FOUS11 KWBC 310856
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
355 AM EST Sun Jan 31 2021

Valid 12Z Sun Jan 31 2021 - 12Z Wed Feb 03 2021

...Southern and Central Appalachians through the Mid-Atlantic and
New England...
Days 1-3...

...Major nor'easter developing this morning will bring heavy snow
from Virginia to Maine...

A potent closed mid-level low evident on GOES-E WV imagery this
morning is rotating over Indiana with a significant moisture
shield blossoming to the east. This closed low will waver eastward
slowly before diving southward in response to vort energy swinging
through the associated longwave trough to its south. At the same
time, this longwave trough amplification will drive downstream jet
streak strengthening and potentially a modest coupled jet
structure to drive intense upper diffluence, and guidance indicate
a robust divergence maxima will move off the coast of VA tonight
before lifting northward. The interaction of these features will
drive secondary surface cyclogenesis off the Delmarva, and this
low will meander slowly northeast through Monday, before
elongating and lifting towards the Gulf of Maine on Tuesday.

Today through tonight, most of the precipitation will be driven by
intense WAA on southerly flow ahead of the main upper low, with
moisture being advected northward from the Gulf of Mexico. A cold
and dry high pressure centered over Canada will maintain a wedge
down through the Appalachians, and as precipitation overruns this
air mass, snowfall will overspread the region from SW to NE. This
first burst of snow could be moderate to heavy at times as
isentropic ascent maximizes into the moistening DGZ, and WPC
probabilities are high for 4 inches from western MD through
central PA and into central NJ. One caveat to this snowfall is a
rapidly accelerating dry slot noted in RH fields and the DGZ which
will result in a loss of cloud ice for much of northern VA,
eastern MD, and into DE/southern NJ. The guidance has become more
aggressive with this dry slot, and in response there is likely to
be a prolonged period of light freezing rain or freezing drizzle
which could accrete to more than 0.1" along this I-95 corridor
Sunday night into Monday morning. Further south into the Foothills
and Piedmont of NC/VA, more significant freezing rain is likely as
the warm nose surges above 0C, and WPC probabilities are high for
0.1", and as high as 40% for 0.25" in a few locations.

On Monday, the secondary low is likely to deepen rapidly, and
guidance has continued to show a more northern track of this low
tonight. As this low deepens, it will enhance the precipitation
shield through WAA, but also develop into an environment favorable
for intense banding. The setup looks favorable for a
laterally-quasi stationary deformation band Monday into Tuesday,
before pivoting northeastward on the edge of the dry slot. Where
this band sets up is still somewhat uncertain, but the most likely
location at this time is from central PA northeastward to just
north of NYC where the CCB is persistent. While much of the
northeast on Monday is likely to see heavy snow, and WPC
probabilities for 6" are high from south-central PA through
Albany, NY, Boston, MA and towards Portland, ME, the heaviest snow
is likely in this band where local maxima in excess of 18" are
possible. There is some uncertainty for the immediate I-95
corridor due to potential warm air wrapping into the coast causing
a changeover to a mix or rain, but just inland from the major
cities there could be widespread 12" of snowfall. Additionally, as
the low pulls away, the pivoting deformation band may try to surge
southward as the 700mb trough digs to the southeast. While
guidance has significantly backed off on the southward extent of
this banding, it is possible some snow could rotate back as far
south as Washington, D.C. for additional light accumulations, but
confidence is increasing that the heaviest snow on Monday will
remain NW of the I-95 corridor from D.C. to PHL.

By D3, the low finally begins to kick out into the Gulf of Maine,
and may get pulled northward very close to the coast. This could
produce some light freezing rain in parts of coastal Maine, but
again the predominant p-type D3 should be snow from upstate New
York through Northern New England. WAA snow at onset followed by
upslope snow as the low departs has caused an increase in WPC
probabilities for upstate NY, VT, and NH where there is a high
likelihood for at least 6 inches of snowfall. Across Maine,
persistent and robust ascent could produce an expansive area of
more than 12" of snow on D3 just inland from the coast.

...Ohio Valley...
Day 1...

Lingering moderate to heavy snow will persist across IN and OH as
an upper low moves overhead and a surface low weakens due to
energy transfer to the coast. A band of deformation north of the
surface low is likely to enhance snowfall at least for several
hours in OH before the forcing wanes to the east. Otherwise, snow
will gradually lessen from west to east through the day. WPC
probabilities on D1 are high for 4 inches in eastern OH and parts
of western PA.

...Western U.S....
Days 1-3...

A slow moving mid-level low dropping out of the Gulf of Alaska
will move onshore WA and OR Tuesday night with height falls and
PVA. Ahead of this feature, however, prolonged confluent and moist
flow will drive moisture onshore from northern CA into WA state
and the Northern Rockies, aided by a modest but persistent Pacific
jet streak. The persistent warm/moist flow will keep snow levels
elevated to 4000-5000 ft, but above these levels heavy snow is
likely all 3 days of the forecast period. WPC probabilities on D1
and 2 are each high for 8 inches in the Olympics and WA Cascades,
as well as the Shasta/Trinity/Siskiyou ranges of northern CA. By
D3, as the trough finally shifts onshore, the best forcing and
moisture begin to shunt southward such that probabilities for 8
inches become high in the Sierra while remaining high in the
Cascades. 3-day totals may exceed 3 ft in the highest terrain.


Weiss

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FOUS11 KWBC 012058
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
357 PM EST Mon Feb 01 2021

Valid 00Z Tue Feb 02 2021 - 00Z Fri Feb 05 2021


...Mid-Atlantic through the Northeast...
Days 1-2...

...Major nor'easter will continue to spread heavy snow from
eastern Pennsylvania through Maine through Tuesday...

Upper level low pressure will reclose over the central
Mid-Atlantic coast this evening and merely drift north through
Maine through Wednesday. In the low levels, strong frontogenesis
has allowed a TROWAL to develop as the WCB wraps northward around
the low, and a potent CCB drives westward now north of New York
City. The heavy snow associated with this TROWAL will continue to
pivot northeast across increasingly interior New England and
stretch southeast back to the central Mid-Atlantic tonight before
the new low forms/shifts into the Gulf of Alaska Tuesday morning,
refocusing heavy snow in Maine (and pivots into interior northern
Maine Tuesday evening). Within this potent band, snowfall rates in
excess of 2"/hr are likely, with moderate to at times heavy snow
spreading as far northward as the Canadian Border. Additionally,
as the low pivots northeast slowly, the deformation/comma snows
will try to pivot southward as far as the Mason-Dixon line with
uncertainty exactly how far south this band will stretch.

Day 1 snow probabilities (starting at 00Z) favor terrain and
particularly northeast facing terrain which is windward in this
Nor'easter with moderate to high probabilities for an additional 8
or more inches in the Poconos, Catskills, Worcester Hills, Green
Mtns, and White Mtns with a continuous swath of low probabilities
from interior southeast PA up to northern Maine. Day 1.5 snow
probabilities are moderate to high for 8 or more inches farther
inland and include the Adirondacks and northwestern and far
northern Maine. Low pressure finally lifts away from Maine
Wednesday night.

Light freezing rain and sleet is likely across eastern ME as the
surface low gets pulled inland allowing warm air to wrap northward
atop the continued cold surface temperatures within Canadian high
pressure. Day 1.5 WPC probabilities for 0.1" of ice accumulation
are 20 to 30 percent in eastern Maine.


...Southern Appalachians...
Day 1...

Low level northerly flow around the deep low along the
Mid-Atlantic coast with streamlines in this flow back to Lakes
Erie and Huron. Aloft, a series of reinforcing shortwave troughs
will enhance lift in this somewhat moistened continental air
tonight, promoting periodically heavy upslope snow for the Great
Smokies and highest southern Appalachians. Day 1 WPC probabilities
are high for 6 inches in a narrow corridor of the Smokies
primarily on the west /upwind/ side, with local storm total maxima
of more than 12 inches likely.


...Western U.S and Northern Plains....
Days 1-3...

A potent and positively tilted mid-level trough will persist off
the Pac NW coast through Tuesday before a reinforcing trough
arrives Tuesday night and ejects the long wave trough southeast
across the Intermountain West, crossing the central and northern
Rockies Wednesday night. Ahead of the trough through Tuesday,
persistent backed but confluent mid-level flow overlapped with a
Pacific jet streak will continue to drive a modest atmospheric
river (AR) into northern CA and the Pacific Northwest, with
residual moisture spilling over into the Northern Rockies. This
moist airmass will be warm, driving snow levels above 5000 ft. WPC probabilities on Days 1 and 2 are moderate for 12 or more in the
Olympics, higher Cascades, the northern/central Sierra Nevada, and
the Tetons. As the trough axis spills onto the northern Plains
Wednesday night and Thursday, the associated sweeping cold front
brings a risk of snow. Day 3 snow probabilities are high for 6 or
more inches in the CO Rockies and low over northern MN. Southerly
flow over cold antecedent conditions brings risk of mainly light
freezing rain to the Upper Midwest (MN/WI) on Day 3.

Jackson

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FOUS11 KWBC 022119
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
418 PM EST Tue Feb 02 2021

Valid 00Z Wed Feb 03 2021 - 00Z Sat Feb 06 2021

...Northeast/Eastern Great Lakes...
Day 1...
The strong coastal system currently impacting the Northeast and
northern Mid-Atlantic will continue to gradually move northeast --
with its primary surface low moving from the Gulf of Maine into
the Canadian Maritimes overnight. Additional snow accumulations
of 4-inches or more can be expected during the evening and
overnight hours across northern Maine and along the U.S.-Canada
border back into northern New York. Northerly flow across Lake
Ontario is expected to support lake enhanced snows, generating
locally heavy totals across the Finger Lakes region of New York.

...Pacific Northwest and Northern California to the Northern and
Central Rockies...
Day 1...
A mid-to-upper level trough centered along the Northwest coast is
expected to translate east into the Intermountain Region and
Rockies as a low drops along the coast of British Columbia and
into the Pacific Northwest Tuesday night into Wednesday.
Southwesterly flow ahead of the low is expected to support
moderate to locally heavy snows across the central and southern
Cascades, Klamath Mountains, as well as the northern Sierra.
Meanwhile, light to moderate snows are forecast to accompany a
low-to-mid level frontal band advancing southeast across the
Intermountain Region and Rockies Tuesday night and Wednesday.
Some locally heavy snow accumulations are possible across the
northern Rockies and central Rockies.

...Central Rockies...
Day 2...
Locally heavy snows are expected to continue through late
Wednesday across portions of the central Rockies, with an axis of
strong frontogenesis and favorable upper jet forcing bolstering
snowfall rates as it moves across the western to central Colorado
ranges.

...Central Plains to Upper Midwest...
Days 2-3...
As the previously noted trough moves east, surface low pressure
developing over the central High Plains late Wednesday is forecast
to move into the mid Mississippi valley on Thursday. As energy
begins to be phase aloft and the upper trough begins to assume a
negative tilt, the surface low is expected to rapidly intensify
and lift north into the western Great Lakes Thursday evening.
While widespread heavy snow accumulations are not expected, strong
low-to-mid level frontogenesis, along with favorable upper
forcing, will likely support at least a brief period of heavy snow
from eastern Nebraska to Wisconsin on Thursday, while strong gusty
winds develop around the rapidly strengthening system. A wintry
mix at the onset of precipitation may produce some light ice
accumulations, before changing over snow across portions of the
Upper Midwest and western Great Lakes regions.

As the low tracks through the upper Great Lakes on Friday, strong
northerly winds on the backside of the system will support
developing lake effect snows, with heavy accumulations becoming
likely in the Upper Michigan snowbelts, as well as along the
eastern shores of Lake Michigan.

...Pacific Northwest to the Northern and Central Rockies...
Day 3...
A low-amplitude shortwave embedded within fast northwest flow is
expected to bring another round of organized snows across the
northern Cascades Thursday night. This energy along with a strong
front settling southwest will support periods of snow from the
northern to the central Rockies on Friday.

Pereira


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FOUS11 KWBC 032056
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
355 PM EST Wed Feb 03 2021

Valid 00Z Thu Feb 04 2021 - 00Z Sun Feb 07 2021

...Central Plains to the Great Lakes...
Days 1-3...
An amplified upper trough will move into the Plains overnight -
driving a strong cold front south and east through the northern
and central Plains into the Mississippi valley Thursday morning.
A brief period of rain or a wintry mix, followed by a changeover
to snow is expected across the mid Missouri into the mid and upper
Mississippi valleys as the front pushes east Thursday morning.
While widespread heavy snowfall accumulations are not expected,
there remains a good signal that strong low-to-mid level
frontogenesis along with favorable upper jet forcing will likely
produce at least a brief period of heavy snowfall, especially
across portions of Iowa and southern Minnesota, into Wisconsin,
northern Illinois and the western U.P. of Michigan - where WPC
PWPF shows high probabilities for accumulations of 4-inches or
more during the Day 1 period (ending 00Z Friday). The wintry mix
expected on the onset is expected to produce minor ice
accumulations across portions of the upper Mississippi valley into
the upper Great Lakes before precipitation changes over to the
snow. By late Thursday, phasing energy aloft will support an
intensifying low that will translate northeast across the upper
Great Lakes. Northwesterly flow is expected contribute to some
lake-enhanced snows across the western U.P. of Michigan by late
Friday. Then as the low continues north of the Lakes, deep
northwesterly to westerly flow is expected to support the
development lake effect snows along the U.P. snow belts, as well
as along the eastern shores of Lake Michigan. Deep cyclonic flow
will continue to support periods of lake effect snow shows in the
lee of the upper Great Lakes through Saturday. WPC PWPF indicates
that three day totals of a foot or more are likely across portions
of the U.P. and along the northeastern shores of Lake Michigan.
Meanwhile, much colder air spreading east, along with low level
southwesterly to westerly flow will support the development of
lake effect snows east of Lake Erie and Ontario on Saturday.

...Central Rockies...
Day 1...
A strong cold front, along with a well-defined shortwave moving
into the base of the longer wave trough, will support heavy snows
moving south across the central Rockies Wednesday evening, with
locally heavy accumulations possible across the western to central
Colorado and northern New Mexico ranges.

...Northern and Central Rockies...
Days 2-3...
A series of shortwaves embedded within strong northwesterly flow,
along with a strong cold front settling southwest, will support
periods of snow, with locally heavy accumulations expected across
portions of the northern into the central Rockies on Friday and
Saturday. Areas impacted are expected to include the northern
Idaho, the western to central Montana, the western and southern
Wyoming ranges and the northwestern Colorado ranges. WPC PWPF
indicates that two day totals of 8-inches or more are likely
across these areas.

Pereira

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FOUS11 KWBC 040842
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
341 AM EST Thu Feb 04 2021

Valid 12Z Thu Feb 04 2021 - 12Z Sun Feb 07 2021

...Upper Midwest across the Great Lakes...
Days 1-3...
An amplified upper trough swings east from the northern Great
Plains today with the associated strong cold front crossing the
Upper Midwest. A brief period of rain or a wintry mix in
prefrontal precip will changeover to snow with most of the QPF
going toward accumulating snow is expected across the mid Missouri
into the mid and upper Mississippi valleys as the front pushes
east Thursday morning. Locally heavy snow is expected,
particularly with strong low-to-mid level frontogenesis along with
favorable upper jet forcing as the trough takes on a negative tilt
from northeast IA, southeast MN, across much of WI and the U.P. of
MI as well as into northern IL where Day 1 WPC PWPF are moderate
to high (higher farther north) for accumulations of 6 or more
inches. The wintry mix expected on the onset is expected to
produce minor ice accumulations (less than 1 tenth inch) across
central and eastern WI, northern IL and much of lower MI and
central OH before precipitation changes over to the snow. By late
today, phasing energy aloft will support an intensifying low that
will translate northeast across the upper Great Lakes.
Northwesterly flow will contribute to lake-enhanced snows across
the western U.P. of Michigan and the western L.P. tonight
continuing in earnest through Friday night. Day 1.5 snow
probabilities for 6 or more inches are moderately high along the
western shore of the L.P. Deep cyclonic flow will continue to
support periods of lake effect snow shows in the lee of the upper
Great Lakes through Saturday. WPC PWPF indicates that three day
totals of a foot or more are likely across portions of the U.P.
and along the northeastern shores of Lake Michigan. Meanwhile,
much colder air spreading east, along with low level southwesterly
to westerly flow will support the development of single-band lake
effect snows east of Lake Erie and Ontario on Saturday with low to
moderate probabilities for 6 or more inches over localized areas
east of Erie and Ontario on both Days 2 and 3.


...Pacific Northwest...Northern and Central Rockies...
Days 1-3...
A series of shortwaves embedded within strong northwesterly flow
will arrive this afternoon into interior sections of the Pacific
Northwest. These waves, along with a strong cold front settling
southwest to the crest of the northern Rockies, will support
periods of snow, with locally heavy accumulations expected across
portions of the WA Cascades northern into the central Rockies on
tonight through Saturday night. Areas impacted are expected to
include the WA Cascades, northern Idaho ranges, western to central
Montana ranges, the western and southern Wyoming ranges and the
northwestern Colorado ranges. WPC PWPF indicates that three day
totals of 24 or more inches are likely across the WA Cascades and
northern Rockies. Cooling temperatures behind the cold front and
expanding precip in the northwesterly flow also bring about
moderate to high probabilities for 4 or more inches over the
Montana High Plains and foothills on Day 3.

Jackson

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FOUS11 KWBC 060904
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
403 AM EST Sat Feb 06 2021

Valid 12Z Sat Feb 06 2021 - 12Z Tue Feb 09 2021

...Great Lakes...
Day 1...
The center of trough sprawling across the entirety of the CONUS
(and nearly all of North America) is a gyre over northern/western
Ontario where several lobes of vorticity are apparent in water
vapor imagery early this morning. The next reinforcing shortwave
trough is over the northern MT/ND border and will shift southeast,
crossing the Upper Midwest tonight. The ongoing lake effect event
off Lakes Superior and Michigan will wane this morning on approach
of this trough. Westerly flow and strong cold air advection will
continue to support single band LES development across Lakes Erie
and Ontario, with locally heavy accumulations continuing through
this afternoon across the Buffalo and Tug Hill regions of New York
where there are moderate Day 1 probabilities for 6 or more
additional inches. Flow backs south-southwesterly by this evening,
cutting off the lake fetch.

...Pacific Northwest to the Northern and Central Rockies and High
Plains...
Days 1/2...
Shortwave impulses in a strong NWly get extending from the
northern BC coast to the northern Rockies will shift over the
PacNW and northern Rockies through Sunday, supporting additional
heavy mountain snows from the WA Cascades across the northern
Rockies, with lighter snows spreading east into the northern High
Plains to the central Plains/Neb into Sunday night. Day 1 has the
greatest heavy snow threat with moderate probabilities for 18 or
more inches for the WA Cascades/Bitterroots of ID/MT to the Big
Belt Mtns just east of the Bitterroots as well as the Tetons. The
Day 1 probability for 4 or more inches is moderate to high for the
central MT High Plains and moderate for 4 additional inches in
central Neb on Day 1.5.

...Central Plains to the Mid Mississippi Valley...
Days 1-3...
The confluence of low level flow spreading up the Plains and the
reinforcing trough over the northern Plains is providing favorable
forcing aloft along with strong low-to-mid level frontogenesis and
locally moderate snow moving progressively southeast across Neb
and northern KS this morning. This shifts across northern MO and
southern IA today. Strong forcing along with high snow-to-liquid
ratios in this cold sector activity have helped to bolster the
potential for locally heavy totals, with Day 1 probabilities for 4
or more inches moderate in central to southeast Neb, and low
across northeast KS east over central IL. As the previously noted
upstream energy in the Northwest makes its way into the region, a
second round is expected to develop tonight and continue into
Sunday with low Day 2 snow probabilities for 4 or more inches over
southeast Neb.

...Southern Appalachians to the Mid-Atlantic and Northeast...
Days 1-2...
Model preference is for the 00Z ECMWF/NAM which is slower than the
00Z GFS. This focuses the progressive wave to be a little slower
than the previous forecast and turning closer to the Northeastern
Seaboard due to influence with the northern stream trough crossing
the Great Lakes tonight/Sunday. This makes for more QPF farther
inland/north in NY and New England through Sunday, continuing into
Sunday night for eastern Maine. The shortwave trough is currently
moving into the northern TX Panhandle and will swing east to the
Mid-South states today and the southern Appalachians tonight.
Surface low development along the Carolinas Coast begins tonight
with the low continuing to develop as it tracks northeast from
Cape Hatteras past the Mid-Atlantic and New England coasts through
Sunday evening. The key will be low level frontogenesis allowing
dynamics to overcome marginal antecedent conditions in the central
Mid-Atlantic with cold air located farther inland (where less
moisture is available). Day 1 snow probabilities for 4 or more
inches for the southern Appalachians then across the interior
central Mid-Atlantic (almost entirely northwest of I-95 - but that
can be deceiving as mesoscale forcing is greater south of I-95
where QPF is higher) into eastern PA for Day 1.5.

Farther north, colder antecedent conditions and the offshore track
allows all snow along the Long Island and southern New England
shores, so it's a QPF concern with continued rapid deepening
allowing a intense low level forced bands to lift across southern
New England. Day 2 snow probabilities are moderate for 6 or more
inches over RI, southeast MA, and far eastern ME with low
probabilities extending from southeast PA (back in Day 1.5 probs),
across the NYC metro and southern/central NY up interior southern
New England and the northern New England coast.

Day 1 freezing rain probabilities are about 20 percent for a tenth
inch of ice in the southern Blue Ridge of northern GA and western
NC, but for days 1-3, the probability of significant icing
(0.25-inch or greater) is less than 10 percent.

Jackson

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FOUS11 KWBC 070858
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
357 AM EST Sun Feb 07 2021

Valid 12Z Sun Feb 07 2021 - 12Z Wed Feb 10 2021


...Mid-Atlantic through eastern New England...
Day 1...
A shortwave trough rounding the sprawling trough around the deep
vortex over Ontario is apparent in GOES-16 mid-level water vapor
imagery as over northern AL with surface low pressure developing
along the Carolina Coast. Snow overnight has been contained to the
higher Piedmont and southern Appalachians with the initial wave of
precip up toward the central Mid-Atlantic and coastal plains
starting as rain until wetbulbing down toward the freezing mark.
This makes for marginal low level conditions that require being
overcome by dynamically forced banding northwest of the of the
surface low. Guidance has wavered considerably about the strength
of this banding across the central Mid-Atlantic this morning with
the 3km NAM waffling on the magnitude of low level frontogenesis
over recent runs. That said the 06Z 3km NAM seems reasonable with
the expected pattern of colder, but less dynamic/less QPF north of
DC seeing accumulating snow and snow struggling to accumulate
through the morning hours along and south of the I-95 corridor.
However, should banding setup well the associated dynamic cooling
should allow some moderate snow rates which could accumulate to a
couple inches quickly. Low probabilities for 2 or more inches are
on the VA Blue Ridge and across the Balt-Wash metro.

Farther northeast, the stronger low and offshore track allows
essentially all snow north from southeast PA. Across northern NJ,
the NYC metro, southeast New England and far eastern Maine, the
combination of colder conditions and low level frontogenesis
allows a stripe of heavy snow. Day 1 probabilities for 4 or more
inches are moderate for northern NJ up through southern CT with
moderate probabilities for 6 or more inches for RI and southeast
Mass and moderate probabilities for 8 or more inches for Downeast
Maine.

...Northern Cascades to the Central Rockies...
Day 1...
Shortwave energy embedded with strong northwest flow aloft is
expected to bring another surge of deeper moisture while nudging
the arctic air mass farther south. This will support additional
periods of heavy snow, with heavy accumulations into tonight
across portions of the region. Impacted areas are expected to
include the northern Cascades, the Bitterroot Range and the
western Montana ranges west of the Divide. In these areas, expect
additional accumulations of a foot or more. Rates decrease tonight
with less moisture available and heavy snow should end by Monday.

...Central Plains to the Mid Mississippi Valley...
Days 1-2...
A shortwave trough shifting into southeastern Nebraska will
continue to move east into the lower Missouri and mid Mississippi
valleys through today. While widespread heavy amounts are not
expected, strong forcing along with high snow-to-liquid ratios may
support some totals of 4-inches or more (Day 1 probabilities for
that are 10 to 20 percent), especially across portions of central
to eastern Nebraska into southwestern Iowa and northwest Missouri.
Additional rounds of mainly light snow are expected late Sunday
into Monday as upstream energy moves out into the central Plains,
with some amplification of the flow and right-entrance region
upper jet forcing supporting an axis of heavier snows centered
across the Mid Mississippi valley on Monday.

...Great Lakes to the Northeast...
Days 2/3...
While a deep vortex remains centered over central Canada, models
continue to show a well-defined shortwave moving south of the
center across the Great Lakes into the Northeast on Tuesday. This
is expected to support widespread light to moderate synoptically
driven snows, followed by some lake effect activity late in the
period. A coastal low developing by late Tuesday should support
organized heavier snows along the northern New England coast into
Tuesday night.

For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Jackson

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FOUS11 KWBC 072112
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
411 PM EST Sun Feb 07 2021

Valid 00Z Mon Feb 08 2021 - 00Z Thu Feb 11 2021


...Maine...
Day 1...
System currently impacting the Northeast will continue to move
progressively northeast - tracking east of Cape Cod this evening
and then east of Nova Scotia overnight. Snow is expected spread
east into eastern Maine this evening, with heavy
accumulations(6-inches or more) likely across Downeast Maine.

...Northern Rockies...
Day 1...
A string of shortwaves embedded within fast, northwest flow, along
with an arctic boundary, will continue to support periods of
mountain snow across the Northwest into the northern Rockies.
While widespread additional heavy accumulations are not expected,
some areas, especially those in and around the Bitterroot range
could see locally heavy accumulations (8-inches or more) through
the evening and overnight.

...Great Lakes and Ohio Valley to the Northeast...
Day 2...
A fast-moving shortwave is expected to support mainly light snow
accumulations as is moves across the region Monday night and
Tuesday. While some orographically favored areas across Upstate
New York and western New England may see locally heavier totals,
widespread accumulations of 4-inches or more are not expected.

...Ozarks and Mid Mississippi Valley to the Ohio Valley and
Central Appalachians...
Day 3...
A subtle shortwave moving through the base of a broad upper trough
will support isentropic lift and the development of light
precipitation across the Ozarks and mid Mississippi valley by late
Tuesday. Then by early Wednesday, right entrance upper jet
forcing along with a low-level boundary is expected to support the
development of better organized precipitation from the mid
Mississippi through the Ohio valleys. Shallow arctic air in place
will likely support a wintry mix, with the 12Z models showing a
good signal for at least minor ice accumulations from southern
Missouri and northern Arkansas eastward along the main stem of the
Ohio River beginning late Tuesday and continuing into Wednesday.
More snow than freezing rain is expected farther to the north,
with portions of central Illinois, Indiana, Ohio and central
Appalachians likely to see at least a few inches of snow Tuesday
night and Wednesday.

Pereira

$$
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FOUS11 KWBC 082020
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
319 PM EST Mon Feb 08 2021

Valid 00Z Tue Feb 09 2021 - 00Z Fri Feb 12 2021

...Ohio Valley, Great Lakes, to Northeast...
Days 1-2...

A fast moving shortwave trough embedded within broad cyclonic flow
will race from the Ohio Valley tonight through the Northeast on
Tuesday and then off into the Canadian Martimes Tuesday night.
Weak surface low development is likely off southern New England
late Tuesday, but additional ascent from that development is
likely to be confined to New England late. Primary forcing for
precipitation will be WAA ahead of the shortwave combined with
modest mid-level divergence, along with a band of mid-level
frontogenesis stretching west to east from Ohio into New England.
It is likely that this fgen, which has shown an intensifying trend
in recent guidance, will produce the heaviest snow rates. However,
the most robust omega looks to be centered just below the DGZ, so
snow rates may remain modest and less than 1"/hr. However, a
several hour period of moderate snow across the Ohio Valley into
the Mid-Atlantic, combined with surface low development off New
England, and some upslope enhancement into the terrain of the
Northeast has led to an increase in WPC snowfall probabilities.
There exists a low chance for 4" across parts of IN, OH, and
western PA within the best band of fgen, and a high chance in the
terrain of the Catskills, Poconos, and Berkshires which will have
the support of the additional ascent. Local maxima above 6" are
possible in these latter locations.

Behind the system, continued CAA will produce some LES in the
favored NW snow belts. However, due to the exceedingly cold air,
the DGZ is actually at the surface (or below) so efficient crystal
growth is unlikely. Still, there exists a high chance for 4" in
the Keweenaw Peninsula of the U.P. of Michigan, with 10" possible
in isolated locations.

...Mid-Atlantic...
Day 3...

A weakening shortwave moving across the Southeast will interact
with strengthening upper diffluence in the RRQ of a departing jet
streak to produce a wave of low pressure along a stalled boundary
over the Deep South. Prolonged and robust WAA ahead of this
feature will spread precipitation northeastward into the
Mid-Atlantic late Wednesday night and Thursday. There remains
considerable spread into the thermal structure of this event as
precipitation overspreads the region, and there is likely to be a
rain, mix, snow event on Thursday. At this point the best
potential for significant snow (4" or more) looks to be in the
higher terrain of WV/MD, and also into Pennsylvania which will be
colder. There has been a noted shift southward in the axis of
heaviest snowfall with the guidance today, and WPC probabilities
have accordingly shifted such that there is now a moderate risk
for 4" in far western MD and adjacent WV, stretching eastward
along the PA/MD border towards Philadelphia.

Further south and west, there is likely to be a transition zone
with some light freezing rain accretions in the Central
Appalachians and adjacent foothills into central/SW Virginia. WPC
probabilities here are as high as 30% for 0.1" of ice.

...The West...
Days 1-3...

The shortwave trough ejecting out of the Southwest D1-2 leads to
confluent and moist mid-level flow converging across the CO
Rockies, atop a cold front that will become stationary and banked
against the terrain. This will lead to several rounds of moderate
to heavy snow tonight through Wednesday. Snow levels east of the
Front Range will be at the surface behind this front, but snowfall
is expected to remain confined to the terrain where snow levels
will generally hold steady at 4000-6000 ft. WPC probabilities on
D1 and D2 are moderate for 6 inches, with 2-day maxima exceeding
12 inches likely.

Wednesday night into Thursday, the above shortwave lifts east
shutting off the forcing across CO, but another shortwave will
round the trough and dig out of the Gulf of Alaska moving into the
Pacific Northwest. This will bring height falls and renewed
Pacific Jet energy, while also driving snow levels rapidly
downward. The heaviest snow is likely D3 in the WA and OR
Cascades, the Olympics, and the ranges of Idaho where WPC
probabilities are moderate for 6 inches. More notably, snow levels
crashing to the surface could bring some light snowfall to the
Columbia Gorge and lowlands around Portland, OR and Seattle WA by
Thursday.

...Southern Plains eastward through the Mid-Atlantic...
Days 1-3...

...Significant freezing rain event likely Wednesday through
Thursday...

Broad cyclonic flow across the CONUS will persist through
late-week, while shortwave energy moves through the flow to
produce periods of wintry precipitation. The first of these is a
weak impulse progged to race across Missouri and through the Ohio
Valley tonight into Tuesday with ascent aided by RRQ diffluence
and modest WAA ahead of the impulse. Cold high pressure will hold
in place keeping surface temperatures below freezing, and a period
of overrunning precipitation as snow or sleet/freezing rain is
likely. The total duration of forcing and intensity of moist
advection is modest so accumulations are likely to be light.
However, WPC probabilities indicate a low chance for 0.1 inches of
freezing rain across southern Missouri and far northern Arkansas.

A much more significant system will begin Wednesday, and guidance
is beginning to show more agreement in a significant freezing rain
event Wednesday and Thursday. A shortwave moving onshore CA
Tuesday will eject into the Southwest Wednesday and then weaken as
it becomes embedded in the westerlies into Thursday. Ahead of this
feature, confluent mid-level flow will help to reinforce very cold
high pressure centered over Canada behind a cold front which will
be sagged across the Southern Plains and Deep South. As this
shortwave interacts with the low-level baroclinic zone, an
intensifying jet streak across the Great Lakes will place the
increasingly favorable diffluent RRQ atop the region, and the
combination of these features is likely to produce surface
cyclogenesis moving along the boundary to the south. As this low
moves eastward, WAA from the south will intensify downstream, with
robust 290K isentropic ascent lifting atop the reinforced cold
high pressure. The guidance is in generally good agreement in the
overall setup, however, the NAM appears a bit too warm with the
warm nose, while the CMC is on the cold edge of the envelope. The ECMWF/ECmean/GFS all are in close agreement and used heavily for
the forecast - which suggest a narrow but impressive band of
freezing rain will develop Wednesday and then expand northeast
from Missouri, through the lower Ohio Valley, and into the
Mid-Atlantic.
There has been a southern push in the guidance this aftn,
suggesting some areas on the north side may get more sleet, and
the axis of heaviest freezing rain may shift a little further
south.

Using the preferred blend, the heaviest freezing rain is currently
most likely in the Ohio Valley where a long duration of modest
rates due to condensation pressure deficits falling towards 0C
combine with efficient accretion as surface wet bulbs remain in
the 20s. WPC probabilities for 0.25" are as high as 40% from
northern Arkansas through far western Tennessee, and much of
Kentucky. While guidance continues to differ on the SW extent of
the heaviest icing potential, some locally backed flow as a wave
of low pressure develops should isentropically ascend atop the
cold front back into AR/OK, leading to some higher probabilities
there as well. However, the most significant freezing rain
accretions, which could exceed 0.5" in places, appear most likely
across Kentucky.

Weiss

$$
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FOUS11 KWBC 092110
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
409 PM EST Tue Feb 09 2021

Valid 00Z Wed Feb 10 2021 - 00Z Sat Feb 13 2021

...Central Appalachians and Mid-Atlantic...
Days 2-3...

Two waves of wintry precipitation are expected to produce a
multitude of precip types through the forecast period.

The first is associated with a modest shortwave ejecting through
confluent mid-level flow and moving atop the Mid-Atlantic
Wednesday night into Thursday. This feature will be accompanied by
modest PVA and upper diffluence within the RRQ of a departing jet
streak over New England. More impressively, WAA and isentropic
ascent will cause precipitation to expand northeastward while the
weak wave moves east through Thursday morning. Within the cold
sector of this system, a period of moderate snow is likely, with
the heaviest accumulations expected in the upslope W/SW terrain of
WV and western MD into the Laurel Highlands of PA. WPC
probabilities for 4 inches are as high as 50% in these areas, with
lesser probabilities extending to near I-95 in MD and VA. South of
there, some light freezing rain is likely as the WAA drives a
modest warm nose atop the cold surface wedge, with some light
sleet in between the two zones. WPC probabilities for 0.1" of
freezing rain on D2 are generally 10-30%, and highest in the VA
foothills.

After a respite much of Thursday, a secondary wave will lift
northeast, this one with an associated surface low moving off the
NC coast and away to the northeast. This will produce another
round of WAA precipitation, with snow north and sleet/freezing
rain to the south. The guidance today has trended south with this
feature, likely due to a stronger response of the high pressure
wedged down from the north. While there still exists a lot of
spread in the model solutions, it appears the heaviest snowfall
with this second wave will occur in a west to east oriented band
from the Panhandle of WV eastward towards the DE coast where WPC
probabilities range from 30-60% for 4 inches, highest in the
terrain of WV. This second wave is likely to have more significant precipitation with it as the WAA and isentropic ascent are
accompanied by stronger synoptic lift as a divergence maxima moves
overhead. There could locally be snowfall amounts much higher than
4 inches, but confidence in placement of these is too low at this
timescale to mention, especially with uncertainty in how the
system will track over the next few days. Like the first system,
there is also likely to be a narrow corridor of moderate freezing
rain where the warm nose tops the cold surface wedge and sub
freezing low-level temperatures. WPC probabilities for 0.1" of
freezing rain on D3 have increased to 30-50%, highest in far
northern NC into central VA.


...Southern Plains, Mississippi Valley, Ohio Valley...
Days 1-3...

...Damaging freezing rain event likely Wednesday and Thursday...

A shortwave moving slowly south of the Four Corners will shed
pieces of energy northeastward and into confluent moist flow
through the Southern Plains and into the Mississippi and Ohio
Valleys Wednesday through Friday. At the same time, an expansive
jet streak centered over the Great Lakes will leave a persistent
RRQ atop the region, and this favorable diffluence will interact
with a low-level baroclinic zone and the mid-level impulses to
spawn waves of low pressure moving northeast along the stalled
surface front. Ahead of the main trough axis and downwind of the
low pressure waves, low-level WAA will intensify, with isentropic
lift on the 290K-300K surfaces increasing atop an arctic high
pressure. The combination of deep synoptic ascent atop this
increasing isentropic upglide will lead to expanding
precipitation, with all p-types expected. For this area, the most
significant impactful wintry precip is likely to be freezing rain
as a warm nose lifts northward atop still very cold surface
temperatures. While the guidance has continued to trend southward
with its axis of freezing rain, due to a stronger ridge to the
north counteracting the warm nose, there is still likely to be a
corridor of heavy freezing rain. Forecast soundings for parts of
AR, TN, and KY indicate warm nose intensity and depth nearing the
90th percentile for long duration freezing rain events, and the
WSE means have shown an increase in point max freezing rain
accretion. While some runoff is expected in the heavier rates,
there is a high probability for 0.25" of accretion from near
Little Rock, AR northeastward to Jackson, KY. It is likely that
some areas will receive more than 0.5", creating a damaging icing
event for parts of the area. Surrounding this axis of heavy
freezing rain, light accretions are likely, including far western
WV, northern TN, and back into OK/TX where locally backed flow
will lead to better isentropic ascent around a secondary wave of
low pressure and prolonged light freezing rain or freezing drizzle
is likely.

North of this axis, some moderate to heavy sleet and light to
moderate snow is expected where the warm nose is either weaker or
non-existent. Where the most sleet accumulates, snowfall amounts
will be tempered, but there are some modest WPC probabilities for
2" of snow/sleet, highest in northern KY. By Thursday night the
best forcing expands eastward and moisture gets shunted to the
south, bringing an end to this prolonged and damaging freezing
rain event.


...Western United States...
Days 1-3...

Lingering snowfall across the central CO Rockies is expected on D1
as residual moist mid-level flow and a weak shortwave ejecting to
the east combine to drive ascent. WPC probabilities on D1 for 6
inches are moderate in some of the higher peaks.

Much more significant snowfall is likely to spread across the
Pacific Northwest and then down through much of the West beginning
late D2 and especially on D3. As broad cyclonic flow envelops much
of the CONUS, confluent mid-level NW flow will drive moisture
southeastward from the Pacific, while an amplified shortwave drops
from British Columbia to the OR/CA coast Friday morning. This
feature will be accompanied by an upper level divergence maxima in
the LFQ of a sinking Pacific jet streak to drive ascent, while a
surface reflection wave of low pressure advects towards the OR
coast on D3. The guidance has become slightly more aggressive with
moisture, and subtly further north with the low track today, and
while it is a near certainty that heavy snow will spread into the
terrain of the Sierra, as well as the ranges from the Olympics of
WA southeast as far as the CO Rockies, there is also potential for
lowland snow and ice with this event as snow levels crash. WPC
probabilities in the mountains are high for 8 inches, with more
than 12" likely in the more favorable terrain of the Sierra,
Cascades, and down into the CO Rockies.

With the northern push of the low today, moisture is more likely
to spread into the Portland and Seattle areas, with snowfall
likely even into the cities. The precipitation may start as some
freezing rain/sleet as warm advection tops cold surface
temperatures as a wedge of high pressure builds east of the
Cascades and funnels through the passes, but should change to all
snow outside of the OR Coast where WPC probabilities for freezing
rain are as high as 30% for 0.1". For the lowlands south of
Seattle, WPC probabilities have increased for 4 inches and now
show a moderate risk for this accumulation, with higher amounts
likely in the Columbia Gorge. The biggest uncertainty involves the
cities themselves. While Seattle should be cold enough for all
snow, they may be on the edge of the heaviest precip, although WSE
plumes continue to show an increase in snowfall and it is likely
that more than 2" accumulates in Seattle. For Portland, the
snowfall is less certain as some mixing is likely early and the
city may try to warm above freezing with the further north low
track, but probabilities are high for 1" there as well.


...Great Lakes...
Day 1...

A strengthening area of low pressure moving away from New England
this evening will leave robust CAA in its wake, embedded within
broad troughing across the eastern CONUS. The air mass is
exceedingly cold such that the favorable DGZ is at or below the
surface, leading to inefficient ice crystal growth. However,
enough latent heat off the Lakes and modest lapse rates should
provide enough instability for small regions of LES in the
Keweenaw Peninsula and downwind of Lake Ontario. WPC probabilities
are high for 4 inches in these narrow corridors.


Weiss

$$
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FOUS11 KWBC 102104
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
404 PM EST Wed Feb 10 2021

Valid 00Z Thu Feb 11 2021 - 00Z Sun Feb 14 2021

...Central Appalachians and Mid-Atlantic...
Days 1-3...

Multiple systems will impact this region during the next 3 days,
each with a distinct impact.

The first is a wave of low pressure which will race from KY to off
the NJ coast tonight through early Thursday. This wave will move
along a decaying baroclinic gradient, driven by a weak shortwave
and modest RRQ diffluence within an ejecting jet streak. While
synoptic ascent will be modest, increasing WAA ahead of the system
will spread expanding precipitation northward, and there has been
an increase in QPF/snowfall with today's model runs. A cold column
favoring slightly above climo SLRs will help increase
accumulations north of the Potomac River as well, and WPC
probabilities feature a 40-60% chance for 4" from far western MD,
along the MD/PA line, into southern NJ. In addition, some light
freezing rain may occur across northern VA in the foothills of the
terrain, where WPC probabilities indicate up to a 30% chance for
0.1" of accretion.

System #2 follows quickly on the heels of the first wave, as yet
another weak low pressure moves along the stalled front to spread
precipitation northeastward from the Ohio Valley. The guidance has
trended south with this impulse today, as well as weaker, likely
due to a strong surface high pressure and dry air advecting down
into the wedge from the north. However, strong upper diffluence
within the RRQ of a strengthening jet streak, modest WAA ahead of
the low, and persistent moist confluent mid-level flow will
produce an axis of heavier wintry precipitation generally south of
Washington D.C. WPC probabilities for 4" of snow are very low
across VA due to the weaker system, but intensifying WAA atop the
cold wedge could produce a swath of moderate freezing rain from
far northern NC into central VA. WPC probabilities for 0.1" of
accretion are as high as 40% in this area.

Finally, as this second system moves away, and even more
substantial low pressure is likely to develop in the Deep South
and then move offshore the Carolinas by Saturday night. Increasing
height falls, and potent upper diffluence as a secondary jet
streak strengthens downwind of a longwave trough axis should lead
to deepening of this wave of low pressure. At the same time,
isentropic ascent transporting moisture from the Gulf of Mexico
will drive expanding precipitation northward, with a significant
overrunning event becoming more likely. A swath of sleet and
freezing rain is likely to overspread central NC northeastward
towards NJ on Saturday, and while there remains considerable
uncertainty into the exact placement of sleet/freezing rain, WPC
probabilities indicate more than 50% chance for 0.1" of accretion
across much of VA and northern NC.

...Southern Plains, Mississippi Valley, Tennessee Valley...
Days 1-2...

...Damaging freezing rain event continues through Thursday...

A stalled boundary draped from TX northeast to KY will waver in
place through Thursday as waves of low pressure move along it. At
the same time, a 130kt jet streak remains angled across the
Northeast leaving the favorable RRQ over the front, with a
longwave positively tilted trough continuing to drive moist
confluent flow to the NE and into the region. Low-level southerly
flow out of the Gulf of Mexico will continue to lift
isentropically atop the front and into the arctic high pressure,
creating a prolonged and intense overrunning event with freezing
rain and sleet the primary weather type. A long duration freezing
rain event will continue most of Thursday before gradually winding
down to the northeast as forcing wanes and shifts eastward.
However, favorable surface wet bulbs temps with moderate rain
rates suggests freezing rain will accrete efficiently to produce
significant accumulations of ice. The exception to this may be
across TX and into OK/AR where there is a signal for some negative
theta-e lapse rates indicative of elevated convection, and it is
possible some thunder sleet or thunder freezing rain will occur
overnight into Thursday morning there. During this time, rain
could runoff more than accrete, but this should be the exception
rather than the rule. Additionally, the southern trend has
continued once again today such that the heaviest axis of freezing
rain has shifted slightly south/southwest, with more sleet
possible further north. The heaviest freezing rain is likely to
exceed 0.25" and may exceed 0.5" in places, producing scattered to
widespread damage. WPC probabilities indicate the best chance for
this will be in a stripe from south central Arkansas northeast
into eastern Kentucky, including western Tennessee. Lighter but
still significant icing of up to 0.25" could occur as far west as
central Texas near San Antonio. By Thursday night, most of the
freezing rain should be winding down, with just limited additional accumulations possible in Kentucky.

...The West onto the Central Plains...
Days 1-3...

Persistent longwave troughing across the CONUS around a central
gyre positioned near Minnesota will drive periods of shortwave
energy southeast from the Gulf of Alaska and into the Pacific
Northwest, then advecting southeast into the Central Rockies. The
first of these will drop southeast into northern CA early Friday
morning, followed by a more impressive vorticity lobe stretching
into WA/OR Saturday morning. Each of these waves will be
accompanied by a surface low and complex frontal structure, as
well as Pacific jet streaks arcing across the West with associated
divergence maxima.

On D1, snowfall across the West will be somewhat limited as
shortwave ridging early in the period takes some time to get
displaced. However, by late D1, precipitation is likely to spread
into the Cascades and WPC probabilities for 6 inches of snow are
low to moderate, highest along the OR/WA border. During D2, the
more robust deep layer ascent drops onshore with mid-level
divergence and WAA spreading precipitation across the Sierra,
Cascades, and points southeast as far as the CO Rockies. Snow
levels will vary considerably D2 as an arctic front lingers banked
against the N/E sides of the terrain, leading to additional
localized ascent as well. WPC probabilities for 8 inches are high
in the Sierra, CO Rockies, and ranges of NW Wyoming, generally
above 4000-5000 ft. Further north WPC probabilities are moderate
for 8 inches in the Sawtooth, Absarokas, and Cascades but with
snow levels down to 1000 ft or less. By D3, the first wave ejects
quickly to the east to be replaced almost immediately by an event
stronger wave, jet, and surface low with associated deep layer
forcing. 700-500mb RH and omega rise dramatically late Friday, and
intense moist advection is likely to produce heavy snow from
Sierra northward into the Olympics and Cascades, and east to the
Blue Mtns and Sawtooth. WPC probabilities are high for 12 inches
in the Cascades on D3.

With snow levels crashing to the surface D2 and D3, and a wedge of
high pressure banking against the Cascades, cold easterly flow
into the lowlands of WA and OR should allow precipitation to fall
as ice or snow. The models continue to shuffle their low
placements both with the Thursday-Friday system, and the stronger Friday-Saturday low. However, it is becoming more likely that
moderate to heavy snow will impact both the Seattle and Portland
metro areas, with the heaviest snow likely late Friday into
Saturday as some enhanced low-level fgen collocated with intense
overrunning WAA occurs. WPC probabilities for 4 inches are low on
D2 in the lowlands, but high on D3 and an impactful snow event is
likely. Southwest of Portland, OR, a significant icing event is
becoming more likely as well, and WPC probabilities are as high as
30% for 0.25" on D2, lingering with additional freezing rain
likely on D3.

Weiss

$$
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FOUS11 KWBC 120933
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
433 AM EST Fri Feb 12 2021

Valid 12Z Fri Feb 12 2021 - 12Z Mon Feb 15 2021

...Pacific Northwest...
Days 1-3...
The upper low associated with the ongoing initial round of
precipitation across the region is already dropping southeast of
the region, and is expected to continue to dig progressively
southeast across the western U.S. on Friday. However, continued
onshore flow interacting with arctic high pressure will continue
to support precipitation, including lowland snow and freezing rain
across portions of the Pacific Northwest early Friday. A strong
surface ridge centered east of the Cascades will support easterly
flow and cold air spreading east across western Washington and
Oregon. Amounts west of the Cascades are expected to be
relatively light before the next system begins to impact the
region Friday night. This second system is expected to bring
heavier accumulations into the region, with significant snowfall
accumulations expected across both the Portland and Seattle metro
regions. Ongoing freezing rain across portions of northwestern
Oregon into southwestern Washington is expected to continue, with
locally heavy ice accumulations likely. Meanwhile, heavy snow is
likely across the Cascades, with several feet expected along the
southern Washington and Oregon Cascades. Similar to the first
system, this second shortwave is expected to move quickly
southeast of the region on Saturday. However, unsettled weather
is forecast to continue through the remainder of the weekend, with
another system expected to impact the region by early Monday.
Snow levels however will begin to increase, with the potential for
lowland snow decreasing late Sunday into early Monday.

...Great Basin/Southwest/Central and Southern Rockies...
Day 2...
The shortwave impacting the Northwest Friday night into Saturday
is forecast to dig southeast into the Great Basin on Saturday and
then into the Southwest by Sunday morning. Large scale ascent
supported by left-exit region upper jet forcing will help to
produce widespread precipitation across the region, including
areas of locally heavy snow across the mountains. Areas impacted
are forecast to include the northern Nevada mountains, the Utah
ranges, as well as the Arizona Rim Country. Southwesterly flow
ahead of the digging trough will draw increasing moisture into the
southern Colorado and northern New Mexico ranges, where orographic
effects along with a low-to-mid level front sliding southwest are
expected to help support heavy snows.

...Central to Southern Plains/Lower Mississippi to the Ohio
Valley...
Days 2-3...
Favorable upper jet forcing, along with low level easterly flow
will support snow developing east of the central Rockies into the
Plains Saturday night into Sunday, with at least a few inches
likely across portions of eastern Colorado, western Nebraska and
western Kansas. As the upper trough continues to dig across the
Southwest and southern Rockies. Snows are expected to develop
farther to south across the southern Plains, with left-exit region
upper jet forcing along with low-to-mid level frontogenesis
supporting the potential for heavy snowfall rates and
accumulations across the Texas Panhandle region into central
Oklahoma and south-central Kansas. As cold air continues to
extend farther south, accumulating snow may reach as far south as
interior portions of South Texas Monday morning, with perhaps a
wintry mix and accumulating ice reaching the coast. Periods of
freezing rain, with at least minor ice accumulations are expected
to extend from eastern Texas across the lower Mississippi and into
the Ohio valleys late Sunday into early Monday.

...Mid Atlantic...
Day 2...
Low amplitude energy and a weak coastal wave interacting with a
cold air wedge are expected to produce the next round of wintry
weather for the Mid Atlantic on Saturday. Overall trend in the
models showed lighter QPF across areas with the greater potential
for freezing rain -- lowering probabilities for significant ice
accumulations across North Carolina and Virginia. However
impactful ice accumulations are still likely, especially across
portions of southern and central Virginia, extending northeast
into the Northern Virginia and Maryland I-95 corridor.

Pereira



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FOUS11 KWBC 122105
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
404 PM EST Fri Feb 12 2021

Valid 00Z Sat Feb 13 2021 - 00Z Tue Feb 16 2021

...Pacific Northwest...
Days 1-3...
As a large cyclonic gyre persists across the Northern Plains
driving broad cyclonic flow across most of the CONUS, a shortwave
will drop out of the Gulf of Alaska through the mid-level NW flow
to move ashore the Pacific Northwest early Saturday. This feature
will be accompanied by a surface low and frontal structure which
will weaken as it approaches the coast, mid-level height falls and
divergence, and LFQ diffluence within an approaching jet streak.
This deep layer ascent will be enhanced by broad WAA as mid-level
flow backs to the SW ahead of the trough axis, and a prolonged
robust precipitation event is likely tonight through Saturday.
Heavy snow is likely in the favored terrain of the Cascades and
other ranges east into Idaho as moisture spillover takes place.
Snow amounts in excess of 12" are almost certain in the Cascades,
with lesser probabilities extending eastward. Potentially more
impactful is the snow and ice that should impact the lowlands,
including the metro areas of Seattle and Portland. For Seattle,
WPC probabilities are high for 4", with more than 6" possible as
snow levels crash to the ground. For Portland it is trickier as
east winds out of the Gorge will try to keep the low-levels cold,
but WAA aloft, especially with a more northern low track shown in
guidance, will try to push a warm nose above 0C. A mixture of snow
and ice is likely in Portland, and WPC probabilities are moderate
for 4" of snow and 0.1" of ice. If the column is just a little
colder (more snow) or a little warmer (more ice) a significant
winter event of either type is still possible. However, SW/W of
Portland, a major freezing rain event is likely where WPC
probabilities for 0.5" are as high as 20% through Sunday morning.

Continued confluent mid-level flow and yet another elongated
shortwave will approach the coast and move onshore Sunday,
bringing renewed precipitation after just a brief respite. Snow
levels remain low in this arctic air mass, and WPC probabilities
are high on D3 for 8 inches in the Cascades, with light snow
accums possible into the lowlands once again.

...Great Basin/Southwest/Central and Southern Rockies...
Days 1-2...

The shortwave impacting the Northwest tonight into Saturday is
forecast to dig southeast into the Great Basin on Saturday and
then into the Southwest by Sunday morning. Large scale ascent
supported by left-exit region upper jet forcing will help to
produce widespread precipitation across the region, including
areas of locally heavy snow across the mountains. Areas impacted
are forecast to include the northern Nevada mountains, the Utah
ranges, as well as the Arizona Rim Country. Southwesterly flow
ahead of the digging trough will draw increasing moisture into the
southern Colorado and northern New Mexico ranges, where orographic
effects along with a low-to-mid level front sliding southwest are
expected to help support heavy snows, and the heaviest snow of the
period is likely D2 in the San Juans where WPC probabilities are
high for 12 inches.

...Central Plains to the Gulf Coast into the Ohio Valley...
Days 2-3...

...Large winter storm to bring heavy wintry mix of precipitation
to the South...

A potent arctic front digging down through the middle of the
country will sink into Texas coincident with an impressive
shortwave digging into the Southwest. This shortwave will
gradually weaken as it moves into the Plains Sunday, but deep
layer forcing will remain intense as a coupled jet structure
develops downstream of the main longwave trough axis to drive
ascent atop mid-level divergence. These features will work
concurrently atop the low-level baroclinic zone to produce a
surface low over the Gulf of Mexico by early Monday which will
then race northeast towards the TN VLY by the end of the forecast
period. This evolution is likely to produce two distinct areas of
heavy precipitation.

For snow, intensifying WAA/isentropic ascent ahead of the
shortwave will overrun the surface boundary and interact with the
arctic high to produce heavy snowfall beginning Sunday morning
over Kansas, dropping south into TX Sunday night, and then lifting
northeast towards TN on Monday. An extremely cold column will
support SLRs that may approach or exceed 20:1, and this "fluff
factor" will lead to rapid snowfall accumulations despite modest
total QPF. WPC probabilities for 6" on D2 are above 50% for the TX
Panhandle into western OK, and then shift into north TX and
central OK D3. The heaviest snow accumulations may approach 12" in
Oklahoma. As the surface low starts to develop the main isentropic
ascent will get shunted east, but increasing frontogenesis and the
synoptic ascent will drive a continuation of heavy snowfall from
east TX through MO and IL on D3, where WPC probabilities for 4
inches are as high as 40%.

To the south of the heavy snow, and primarily on D3 as the surface
low develops, an area of freezing rain is likely. As WAA spreads
moisture northward it will lift atop the arctic front, and
although surface temps will remain well below freezing, the warm
nose should rise above 0C supporting a mix of sleet and freezing
rain. The guidance has trended a little further SE with this axis
today, and at this time the heaviest freezing rain is expected
from near Houston, TX northeast through central LA, MS, and into
TN/KY, with more than 0.25" possible in some locations. This could
become a damaging event for areas that do not frequently receive
significant freezing rain.

Between the snow and sleet a narrow corridor of mixed precip,
including heavy sleet, is likely. At this time it is difficult to
discern exactly where this will occur, however, any sleet could
either cut down on snowfall accumulations or freezing rain from
eastern TX into Missouri and points southeast.

...Mid Atlantic...
Days 1-2...

A wave of low pressure moving up the coast will spread northward
into the Mid-Atlantic as mid-level SW flow spreads moisture atop a
residual cold surface high pressure wedging down east of the
Appalachians. The warm ascent will drive a warm nose >0C quickly
northward, but surface temperatures are likely to remain below
freezing as noted by forecast wet bulb temperatures. This suggests
a period of moderate freezing rain and sleet beginning early
Saturday and persisting into Sunday morning. The guidance today
has backed off a bit on total moisture due to a relatively low
amplitude pattern, but there is still likely to be significant
freezing rain from northern NC through VA and into the I-95
corridor from Washington, D.C. to Philadelphia. WPC probabilities
for 0.25" are as high as 40% in central VA, with generally less
than 0.25" expected elsewhere. Travel along the I-95 corridor late
Saturday into Sunday could become hazardous.


Weiss


$$
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FOUS11 KWBC 130900
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
359 AM EST Sat Feb 13 2021

Valid 12Z Sat Feb 13 2021 - 12Z Tue Feb 16 2021

...Mid Atlantic...
Day 1...
A series of weak disturbances embedded within moist, southwesterly
flow aloft and interacting with shallow cold air east of the
mountains, will encourage freezing rain, with significant ice
accumulations likely across portions of central Virginia into
southern Maryland on Saturday.

...Pacific Northwest...
Day 1...
As arctic air remains entrenched, another shortwave trough will
slide southeast across the region later today. More snow is
expected across the lowlands, including the Seattle and Portland
metro regions, where an additional accumulations of at least an
inch or two is likely. Heavy accumulations are likely for
portions of the Cascades, with additional accumulations of 1-2
feet expected across the higher elevations of the southern
Washington and Oregon Cascades.

...Intermountain West and Rockies...
Day 1...
The previously noted shortwave is forecast to move progressively
southeast on Saturday -- supporting precipitation spreading quicky
east of the Cascades through the northern Intermountain West into
the northern and central Rockies Saturday morning, before reaching
into northern and central Arizona and the southern Rockies by late
in the day. Large-scale ascent supported in part by left-exit
region upper jet forcing, will contribute to widespread
precpitation across the region, including locally heavy mountain
snows. A low level wave developing over Utah and moving southeast
into Colorado along the left-exit region of the upper jet, is
expected to support a period of enhanced upslope flow and
convergence -- raising the potential for heavy snowfall rates
across the central and southern Utah, into the southwestern
Colorado and northern New Mexico ranges late Saturday into Sunday.
By early Sunday, local snowfall accumulations of a foot or more
are likely across those areas.

...Central and Southern Plains to the Lower Mississippi Valley...
Days 1-2...
Supported by low level easterly flow and an upper jet couplet,
snow will begin to spread east of the central Rockies into the
High Plains Saturday night, with accumulations of 4-inches or more
likely along the Kansas-Colorado border by midday Sunday. As the
upper trough begins to move east across the southern Rockies,
favorable upper jet forcing, along with a low level baroclinic
zone will support heavy snows developing farther across portions
of Oklahoma and northern Texas on Sunday. As the upper trough
continues farther east, drawing colder air across the southern
Plains, a wintry mix, including accumulating snow is forecast to
extend as far south as South Texas Sunday night. Sleet and
freezing rain is likely to develop along the Texas coast and
across eastern Texas into the lower Mississippi valley Monday
morning, with significant ice accumulations possible across
eastern Texas into northern Louisiana and western Mississippi.

...Southern Plains to the Northeast...
Day 3...
Beginning late in the Day 2 period and continuing into Monday,
areas of light to moderate snow and freezing rain are expected to
move northeast along a low-to-mid level baroclinic zone extending
form the Ohio valley through the northern Mid Atlantic and
Northeast.

Meanwhile, organized moderate to heavy precipitation developing
late in the Day 2 period along the right-entrance region of the
upper jet and near a low-to-mid level frontal zone will continue
to move east from eastern Texas and across the lower Mississippi
valley, with significant ice accumulations possible from
southeastern Texas, through Lousisana into Mississippi. A stripe
of heavier snow is expected to develop farther to the north from
northeastern Texas to western Tenneesse and Kentucky Monday
morning. This area of organized heavier snow is expected to
translate northeast through the Ohio valley and into the Lower
Great Lakes region Monday night into early Tuesday. South of the
heavier snow, a stripe of significant ice accumulations is
expected to spread from the Tennessee valley through the central
Appalachians and into the northern Mid Atlantic, southern New York
and New England Monday night into Tuesday morning.

Pereira

$$
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FOUS11 KWBC 132203
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
503 PM EST Sat Feb 13 2021

Valid 00Z Sun Feb 14 2021 - 00Z Wed Feb 17 2021

...Mid Atlantic to Long Island NY and Coastal Southern New
England...
Day 1...
A wave of low pressure at the sfc is forecast to move east
northeast south of Long Island NY and southern New England.
Surface cold air that is below freezing is in place for the
northern mid Atlantic to southern New England. The layer near 850
mb rises above freezing, so a mixture of sleet and freezing rain
moves north across the northern Mid Atlantic to Long Island NY and
coastal southern New England. The precip tapers Sunday as the
wave departs and drier air aloft advects across the region. The
models are in good agreement on the storm track/intensity, so the
models were equally weighted in the probability forecasts.

...Pacific Northwest...
Day 1...
The models indicate confluent flow aloft results in a strong west
northwest to east southeast jet stream across the Pacific
Northwest.
A few surges of 700 mb ascent cross the OR Cascades, weakening as
you go north across the WA Cascades.
Heavy accumulations are likely for portions of the OR Cascades,
with additional accumulations of 1-2 feet expected across the
higher elevations of the Oregon Cascades.

...UT/CO/AZ/NM...
Day 1...
The upper trough this evening across UT and AZ drifts east with
time across the ranges of CO/NM. Upper divergence maxima along
the path of the upper trough helps focus snow across the San Juan
Mountains of northern NM/southern CO, and Sacramento Mountains of
NM.
Secondary upper divergence maxima in northwest flow behind the
trough focus continuing ascent in southeast CO to northwest NM on
Sun.
Locally heavy snow are expected with a foot likely in the San Juan
Mountains and portions of the central UT Wasatch.

...Central and Southern Plains to the Lower Mississippi Valley/TN
Valley...
Days 1-2...

...A major freezing rain event is forecast from southeast Texas
across central Louisiana, and western to northern Mississippi...

Supported by low level easterly flow and an upper jet couplet,
snow will begin to spread east of the central Rockies into the
High Plains tonight. As the upper trough begins to move east
across the southern Rockies, favorable upper jet forcing, along
with a low level baroclinic zone will support heavy snows
developing across the TX/OK panhandles to central Oklahoma.
Northern Texas will be in a precip type transition zone, with a
period of sleet before changing to snow.

As the upper trough continues farther east, drawing colder air
across the southern Plains, a wintry mix, including accumulating
snow is forecast to extend as far south as South Texas Sunday
night. Sleet and freezing rain is likely to develop along the
central to eastern Texas Gulf coast and across eastern Texas into
the lower Mississippi valley Monday morning, with significant ice
accumulations possible across eastern Texas into northern
Louisiana and northern Mississippi, northern Alabama, continuing
to central Tennessee, eastern Kentucky, and southwest West
Virginia.

A stripe of 0.25 inches of freezing rain is possible across
central LA to western/northern MS. A tenth of an inch is possible
down to the Gulf coast of southwest LA and eastern Texas. The
probability of icing greater than 0.10 inches if 50-75 percent
across central TN to eastern KY.

The event winds down on Day 3 as the cyclone moves steadily
northeast up into the northeastern part of the country.

...Ohio Valley/Great Lakes to the Northeast...
Day 2/3...

A widespread mixed winter precipitation event is expected on Day 2
in the upper Ohio Valley and central Appalachians, and continuing
across the lower Great lakes, New York, and New England Day 3.
Phasing northern and southern streams shortwaves leas to an
expansion of the precipitation shield across the Ohio Valley
Monday, with snow in the northern Ohio Valley to the lower Great
Lakes. Further south, the precip changes to sleet and then
freezing rain across central to eastern KY and southern WV.

On Tue, the low may reform off the northern Mid Atlantic to
southern New England region.
Those models that advect warmer air 850-700 mb north across Y/New
England create a change over from snow to sleet and freezing rain.
The further north/west you go in PA/NY/New England, the greater
the chances of snow are. There is potential for up to a foot of
snow in portions of northwest PA to western and northern NY and
VT. Enhancement near the southern end of Lake Michigan leads to
several inches of snow in northern IN, and enhancement off Lake
Superior likewise leads to several inches of snow in the eastern
UP of Michigan.

Further south from New York City to coastal southern New England,
the greater the chances are of a change over to rain.
In between is a likely areas of multiple precip types with
uncertainty on the duration of each.
The highest probability of significant icing is from southern WV
across south central PA/northern NJ, southeast NY, and southern
New England.


Petersen

$$
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FOUS11 KWBC 140945
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
444 AM EST Sun Feb 14 2021

Valid 12Z Sun Feb 14 2021 - 12Z Wed Feb 17 2021


...Southern Rockies to the Northeast...
Days 1-3...
A major winter storm will continue to impact a large portion of
the U.S. into early next week, with significant impacts reaching
as far south as the western and central Gulf Coast.

A well-defined shortwave trough digging into the southwestern U.S.
overnight is forecast to swing east across the southern Rockies
later today. As it does, models show an area of strong ascent
supported by left-exit region upper jet dynamics, along with low
level easterly flow, producing locally heavy precipitation across
southern New Mexico, with heavy snow accumulations of up to a foot
or more likely along the upslope regions of the Sacramento
Mountains Sunday morning.

Meanwhile, favorable mid-to-upper level forcing and modest low
level frontogenesis/convergence will begin to support some
organized areas of light to moderate snow moving east across
northern Texas and Oklahoma into southeastern Kansas and
southwestern Missouri on Sunday. WPC PWPF shows high
probabilities for snow accumulations of 4-inches or more extending
from the western portion of North Texas across central into
northeastern Oklahoma. By late Sunday and continuing into early
Monday, models continue to show a strong signal for organized
snows developing farther to the southeast across East Texas into
the ArkLaTex region. Low-to-mid level frontogenesis along with
favorable upper jet forcing is expected to help support the
potential for moderate to heavy snowfall bands developing across
the region, with WPC PWPF indicating high probabilities for
snowfall accumulations of 4-inches or more. As this area
develops, lighter snows are forecast to spread farther south into
South Texas, while a wintry mix with accumulating ice develops
along the Texas coast.

Heavy snows are expected to move northeast across northern
Mississippi into western Tennessee and the lower Ohio valley on
Monday. Meanwhile, freezing rain with significant icing is
forecast to develop farther to the southeast from the central Gulf
Coast to Middle Tennessee. The consensus of the forecast
guidance, which was already cold, has trended colder overnight --
shifting the axis of heavier snow and ice farther to the southeast
than previous runs. This is reflected in the latest WPC PWPF,
which shows high probabilities for ice accumulations of 0.25-inch
or more extending now covering a large portion of central
Mississippi and southern Louisiana.

Moderate to heavy snows will continue to move northeast through
the Ohio valley into the Lower Great Lakes late Monday into
Tuesday, while a wintry mix, including accumulating ice extends
from the Tennessee valley to southern New England. Light to
moderate snows are expected to quickly move from Update New York
across northern New England on Tuesday, diminishing as the system
moves into the Canadian Maritimes late in the day.

...Pacific Northwest to the Southern Plains...
Unsettled weather is expected to continue across the Northwest as
a series of shortwaves drop southeast across the region. Snow
levels are expected to remain low intialy -- supporting additional
lowland snow early before snow levels begin to gradually increase
on Sunday. Additional heavy accumulations are likely across the
Olympics and Cascades, with lighter accumulations spilling east
across the Intermountain West through early Monday. On Monday and
continuing into Tuesday, heavier amounts are expected to develop
farther to the east across the southeastern Washington and
northeastern Oregon mountains into the northern Rockies. By early
Tuesday, energy dropping southeast will begin to re-amplify the
trough over the Great Basin and Southwest once again, with areas
of heavy snow developing across the higher elevations of the Great
Basin into the central Rockies. Similar to the previous
shortwave, this system is expected to dig across the Southwest
into the southern Rockies. By early Wednesday, the threat
organized snow is forecast to the return to portions of the
southern Plains, with WPC PWPF indicating increasing probabilities
for significant accumulations from the eastern Texas Panhandle
into western Oklahoma by Wednesday morning.

Pereira

$$
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FOUS11 KWBC 142215
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
514 PM EST Sun Feb 14 2021

Valid 00Z Mon Feb 15 2021 - 00Z Thu Feb 18 2021


...Southern Plains to the Mississippi Valley, Great Lakes, and
Northeast Days 1-2...

A major winter storm is expected across the southern Plains to the
MS Valley, Great Lakes, and Northeast.
The models show a 700 mb trough crossing east from the Southern
Rockies onto the plains will amplify overnight into Monday, with a
closed low developing. Low-mid level convergence east of the
wave/low is expect to produce an area of snow from central TX/OK
east across southern MO/AR, western KY/TN, then southern IL/IN
Monday afternoon. WPC PWPF shows high probabilities for snow
accumulations of 4 inches or more extending from Northeast Texas
across eastern AR into southeast MO, western KY, and southern
IL/IN.

Downstream from the snow area will be a precip type transition
zone consisting of sleet and freezing rain across southeast TX to
LA, eastern AR, western TN, and central KY. The models forecast a
band of significant freezing rain from southeast LA across MS to
central TN and eastern KY. A quarter inch of icing will be common
in these areas. The probabilities for a quarter inch of icing are
highest in southeast LA across central to northeast MS, northwest
LA, central TN, and southeast KY.

The stronger/more amplified 700 mb wave/closed low heads towards
the Great Lakes on Day 2.
The stronger southerly flow east of the low advects more moisture
north across the Ohio Valley and Great Lakes.
This has lead to a significant shift of the snowfall north into
southern Lower MI and off the southern shore of Lake MI from
Chicago to northern IN.

Moderate to heavy snows will continue to move northeast through
the northern Ohio Valley into the Lower Great Lakes, and
western/northern New York and northern New England into Tuesday.
The highest probability for 4 inches of snow is across
western/northern NY, northern VT.NH/interior ME.

A wintry mix, including accumulating ice extends eastern KY across
WV, western MD, much of southern PA, northern NJ, southeast NY,
and southern New England. The highest probability for a quarter
inch of icing on Day 2 is across western MD, south central PA,
northern NJ, the lower Hudson Valley of NY, and southern New
England.

The event tapers as the system moves into the Canadian Maritimes
late Tuesday.

...Pacific Northwest to the Southern Plains...
On Day 1, a quickly moving 700 mb shortwave in northeast Pacific
moves onshore, with an influx of moisture advection and ascent
that progresses across the Pacific northwest and northern Rockies.
A secondary surge arrive slate Monday, so the 2 rounds will likely
lead to heavy snow in the WA Cascades, and secondary lower maxima
are expected across the OR Cascades inland to the Blue Mountains
and ranges of ID, northern
NV and UT to western WY.

On Day 2, the continuation of moist confluent flow at 700 mb with
enhanced moisture and ascent streams from northwest to southeast
across WA/OR/ID down into western WY/UT/CO, reaching NM by the end
of Day 2. Another 1-2 feet of snow are possible in the WA/OR
Cascades.
The snow should wind down on Day 3 in the northwest as a 700 mb
ridge approaches form the Pacific and drier air overspreads the
region.

On Day 3 Tue night-Wed., the 700 mb trough crosses from the
Rockies on to the High Plains. Snow extends form the ranges of NM
and then develops across the TX panhandle and across OK to the
ArklaTex. A mixture of sleet and freezing rain is expect south of
the snow area in eastern TX to northern LA and eastern AR. The
WPC PWPF indicates increasing probabilities for the Sangre
deCristo Mountains of northern NM/adjacent southern CO and then
from the Texas Panhandle across Oklahoma to western AR on Wed. the
models have better than average agreement and continuity for this
system, so the models are equally weighted for QPF and temperature
profiles.
The highest probabilities for 0.25 inches of icing are in
northeast TX to northwest LA and adjaceent southwest AR.

Petersen


$$
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FOUS11 KWBC 150909
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
408 AM EST Mon Feb 15 2021

Valid 12Z Mon Feb 15 2021 - 12Z Thu Feb 18 2021


...Central Gulf Coast to the Northeast...
Days 1-2...
Heavy snow and significant icing are expected to extend from the
central Gulf Coast to New England as a fast-moving, major winter
storm moves from the central through the eastern U.S.

As a well-defined shortwave moving through the base of a deep
longwave trough centered over the central U.S. begins to pivot
from the southern Plains into the lower Mississippi valley,
moderate to heavy precipitation will continue to organize along an
axis of strong ascent supported by favorable upper jet forcing and
strong low-to-mid level frontogenesis that extends north from the
lower Mississippi valley. Heavy snow will continue to develop and
spread northeastward from the ArkLaTex region into the lower Ohio
valley during the day today, with widespread accumulations of
4-inches likely, with locally heavier amounts expected.
Meanwhile, freezing rain with significant ice accumulations is
expected to develop farther to the southeast of the area of
heavier snow -- with ice accumulations of 0.10-inch or more likely
from as far south as southeastern Louisiana, extending
northeastward into eastern Kentucky and West Virginia on Tuesday.
Locally heavier ice accumulations are expected, with some portions
of Mississippi, northern Alabama and Middle Tennessee likely to
see accumulations exceeding 0.25-inch during the day on Monday.

As the previously noted shortwave continues to lift steadily to
the northeast, the axis of heavy snow is forecast to shift farther
to the north, extending across the lower Great Lakes region into
the Northeast by early Tuesday. Strong low-mid-level
frontogenesis will continue to contribute to the generation of
heavy snows as it lifts from the Ohio valley into the Lower Great
Lakes and Northeast Monday night into early Tuesday. During the
overnight and morning hours, snow accumulations of 4-inches or
more likely to the extend from Indiana to northern New England,
with heavier totals likely near the Lower Great Lakes into
northern New York. Northeasterly flow is expected to support lake
effect snows and locally heavy totals along the southwestern
shores of Michigan as the surface-low level wave passes to the
southeast of the region late Monday into Tuesday. Meanwhile,
accumulating ice, with locally heavy amounts will likely extend
from Tennessee valley northeastward across the central
Appalachians into the the northern Mid Atlantic, southern New York
and southern New England. This will likely raise total ice
accumulations up over 0.25-inch across portions of eastern
Kentucky.

During the day on Tuesday, moderate to heavy snows will continue
to shift east across northern New England, with an icy mix farther
to the south across central New England and coastal Maine.
Precipitation is expected to diminish as the system moves into
Atlantic Canada Tuesday evening.

...Pacific Northwest to the Lower Mississippi Valley...
Days 1-3...
A second far-reaching major winter storm will continue to develop
over the Northwest as another upper level shortwave drops
southeast across the region today. Contrasting with the previous
system, snow levels are on the rise, with areas across the
Washington lowlands already transitioned over to rain. However,
icing will remain a concern early in the period farther to the
south along the Columbia Gorge into the Portland metro before
temperatures increase later today. Heavy snows are likely across
the Olympics and along the Cascades early on, spreading south and
east across the Intermountain West into the northern and central
Rockies on Monday. As the leading shortwave continues to dig
southeast, re-amplifying the upper trough over the Southwest, the
threat for heavy snow is expected to reach once again into the
higher elevations of northern and central Arizona as well as
portions of the southern Rockies Monday night into early Tuesday.
Meanwhile, deep northwesterly flow with embedded energy aloft will
continue to support unsettled weather, including periods of heavy
mountain snow from the Cascades to the northern Rockies through
Tuesday. By late Tuesday, snow is expected to develop once again
across portions of the southern Plains, with some potential for
heavy accumulations developing from the eastern Texas Panhandle
across Oklahoma. A more significant threat is expected to develop
farther to the east on Wednesday. As the upper trough moves east
and amplifies over the southern Plains on Wednesday, models once
again show a strong signal for heavy precipitation developing
within an area of strong ascent supported by favorable upper jet
dynamics, along with low-to-mid level frontogenesis. Overnight
guidance suggests heavy snows likely to develop from southeastern
Oklahoma and northeastern Texas northeastward across Arkansas into
southern Missouri and the lower Ohio valley on Wednesday.
Meanwhile, freezing rain with heavy ice accumulations is expected
from eastern Texas through the ArkLaTex region into southeastern
Arkansas and northern Mississippi.

Pereira

$$
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FOUS11 KWBC 092131
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
430 PM EST Sat Jan 09 2021

Valid 00Z Sun Jan 10 2021 - 00Z Wed Jan 13 2021

...Southern Rockies to the Lower Mississippi Valley...
Beginning this evening and continuing through Sunday, a winter
storm is likely to produce significant snowfall accumulations
across portions of the southern Rockies and southern Plains. Then
by late Sunday and continuing into early Monday, the focus is
forecast to shift farther east, with the potential for heavy snow
impacting portions of the lower Mississippi valley.

A shortwave trough digging into the Four Corner region this
afternoon is forecast to swing east into the southern Rockies
later this evening. Left-exit region upper jet forcing is
expected to help support large-scale lift across the southern
Rockies and High Plains this evening into the overnight. This
lift along with deepening moisture, supported by low level
southeasterly flow across Texas, will foster an expanding area of
precipitation from the Colorado and northern New Mexico mountains
eastward into the High Plains. For the evening into the overnight
hours, models are in generally good agreement, showing a weak
mid-level circulation moving east from northeastern New Mexico
into the Panhandle region, supporting some organized heavier rates
across portions of southeastern Colorado and northeastern New
Mexico into the northern Panhandle region and southwestern Kansas.

During the overnight hours, the focus for organized heavier
amounts is expected to shift farther to the south. As a
developing low level center moves across West Texas, models have
remained overall consistent in showing heavier rates developing
along a low level boundary setting up across the South Plains,
northern Permian Basin, the northern Concho valley and the Big
Country. With thermal profiles suggesting snowfall from the onset
across most areas, sustained lift interacting with ample moisture
is expected to support several hours of moderate to locally heavy
snowfall across the region Sunday morning. Several current and
time-lagged members of the HREF indicate areas of snowfall rates
in excess of 0.5 in/hr moving east across the region Sunday
morning. WPC PWPF probabilities indicate that snowfall
accumulations of 4-inches or more are likely across this region.
Low level forcing is expected to support areas of snow moving east
across portions of central to eastern Texas during the day on
Sunday, before reaching into the ArkLaTex region by late in the
day. Marginal boundary layer temperature are expected to limit
accumulations on the ground, however areas where heavier rates do
occur will likely be sufficient for at least an inch or two of
accumulating snow. Greatest potential for significant
accumulations is expected across portions of Central Texas and
western North Texas. By late Sunday and continuing into the
overnight, there remains a good signal for significant snowfall
developing near the southern ArkLaTex region into northern
Louisiana. Favorable upper jet forcing along with low level
frontogenesis is expected to support a period of moderate to heavy
snow, with at least an inch or two likely extending from the
ArkLaTex to as far east as central Mississippi.

For Days 1-3, the probability of significant icing (0.25-inch or
more) is less than 10 percent.

Pereira


$$
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FOUS11 KWBC 100846
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
346 AM EST Sun Jan 10 2021

Valid 12Z Sun Jan 10 2021 - 12Z Wed Jan 13 2021


...Southern Plains to the Lower Mississippi Valley...
Day 1...

A low pressure system, currently centered over southern NM and far
western TX will shift east across west TX today and North TX
tonight before weakening as it crosses the Mid-South Monday.
Left-exit region upper jet forcing is supporting large-scale lift
across the southern High Plains of eastern NM and the TX Panhandle
this morning. This lift along with deep moisture, supported by low
level southeasterly flow across Texas is producing organized
heavier snow rates across eastern NM, the TX Panhandle south to
Midland, TX/the Permian Basin. With thermal profiles maintaining
snow from the onset across the Concho Valley and NW TX, sustained
lift interacting with ample moisture is expected to support
several hours of moderate to locally heavy snowfall across this
region and east to the I-35 corridor south of DFW into this
afternoon.
Then, this evening as surface low pressure refocuses on the TX
coast, a secondary area of moderate to locally heavy snow is
expected to develop in east TX and across northern LA to the MS
Delta tonight. Marginal boundary layer temperatures tonight in
this area may limit notable accumulations on the ground except
where dynamics are best with heavier rates repeating in bands
allowing 2 to perhaps 5 inches accumulating near the TX/LA border
south of I-20. Day 1 snow probabilities which begin at 12Z are
moderately high for 6 or more inches across Northwest TX with 20
to 30 percent probabilities for 4 or more inches extending from
the southern TX Panhandle all the way east across TX into northern
LA.
The system then weakens/fills and outpaces cold air limiting heavy
snow to the Day 1 period with some notable light snow up to an
inch Monday morning in northern MS.


...Northwest...
Days 2/3...

A potent shortwave trough rounding a Gulf of Alaska low pushes
into BC Monday night with an Atmospheric River pushing into the WA
coast. Snow levels generally rise from 4500ft to 6000ft in the
Olympics and WA Cascades Monday night with particularly heavy snow
above these levels. Day 2 snow probabilities are moderately high
for 8 or more inches in the northern WA Cascades. Zonal flow on
Tuesday focuses the Atmospheric River into the OR/CA border into
Wednesday with very high snow levels (around 8000ft) in the very
moist air. Deep Pacific moisture will make it past the Cascades
with Day 3 snow probabilities for 8 or more inches likely for the
Blue Mtns of OR and the Bitterroots on the ID/MT border and high
probabilities for the northern WA Cascades.


For Days 1-3, the probability of significant icing (0.25-inch or
more) is less than 10 percent.

Jackson


$$
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FOUS11 KWBC 111955
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
254 PM EST Mon Jan 11 2021

Valid 00Z Tue Jan 12 2021 - 00Z Fri Jan 15 2021

...Upper Midwest...
Day 3...

A closed low will deepen over central Canada Wednesday before
plunging into the Northern Plains and Upper Midwest Thursday.
While guidance still varies considerably in the strength and
evolution of this feature, a blend of the ECENS/GEFS means which
were near the center of the guidance cluster were preferred for
this iteration. This suggests an anomalously deep closed mid-level
low will drive a cold front and arctic intrusion into the region,
with a wave of low pressure developing along the baroclinic
boundary. This low is likely to become vertically stacked and
intense across MN/WI by the end of D3, with precipitation
spreading across much of the Upper Midwest. Ahead of the front,
WAA will drive expanding precipitation, some of which is likely to
be freezing rain into MN/IA/WI, although the duration and
intensity of freezing rain should be light. As the low wraps up,
the combination of WAA and spokes of vorticity rotating around the
primary mid-level gyre will produce periods of moderate snowfall.
WPC probabilities for 4 inches are as high as 20% in northern MN
for D3, with additional snow possible beyond this forecast period.

...Pacific Northwest and Northern Rockies...
Days 1-3...

A strong Atmospheric River (AR) is beginning tonight and will
persist into Wednesday before shunting to the east. Strong
850-500mb confluent flow will drive IVT as high as 800 kg/m/s
onshore, focused into OR, for a duration of at least 24 hours.
Impulses embedded within this flow will enhance ascent already
robust through the persistent LFQ of an upper jet streak
positioned along the WA/OR coast, with associated frontal
boundaries aiding in low-level convergence. QPF is likely to be
excessive in this persistent moist flow with intense lift, and at
the high elevations this will lead to exceptional snowfall.
However, WAA on subtly backed SW 700-500mb flow ahead of the
fronts will raise snow levels to as high as 7000 ft Tuesday,
crashing back to as low as 3000 ft behind the cold front late D2
into D3, but by this time most of the precipitation will have
ended. This implies that the exceptional snowfall, which could
exceed 5 ft in places, will be confined above 7000 ft, and in the
ranges most orthogonal to the most intense moisture/forcing
overlap which would be the WA Cascades.

The heaviest snow on D1 is likely in the WA Cascades where WPC
probabilities are high for 12 inches, and locally 3 ft is
possible. In the other ranges including the Olympics, Northern
Rockies, and Blue Mountains of OR, WPC probabilities are high for
6 inches. On D2, the pattern remains mostly unchanged with a
continuation of the strong AR and another impulse moving inland.
This continues the heavy snow risk across these same ranges with
high 12 inch probabilities persisting in the WA Cascades, and 6"
probabilities of greater than 40% extending into the Tetons of WY.
By D3 the jet streak shifts eastward and longwave ridging bulges
up from off the coast of CA. This will bring an end to the heavy
snowfall. During this event, snowfall at pass levels in the WA
Cascades could reach 6-12".

...Eastern Great Lakes...
Days 1-2...

A series of weak shortwaves will move across the Great Lakes
through Wednesday leaving broad cyclonic flow in their wake. This
will produce modest but favorable conditions for LES downwind of
Lake Erie and Ontario Monday night through Wednesday morning
before forcing wanes the latter half of D2. Briefly heavy snow
rates are possible, especially into the Tug Hill Plateau east of
Lake Ontario. However, low EL's at least modest directional wind
shear, and a DGZ which is lacking deep moisture should preclude
significant snow amounts. There are some low probabilities for
1"/hr snowfall rates during this time, but WPC probabilities
indicate an extremely low chance for 4" of snow on either D1 or
D2, with total accumulation remaining below 6" for the 2 days.

For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Weiss


$$
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FOUS11 KWBC 132100
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
307 PM EST Wed Jan 13 2021

Valid 00Z Thu Jan 14 2021 - 00Z Sun Jan 17 2021

...Upper Midwest...
Days 1-2...

A potent trough digging out of western Canada will evolve into an
anomalously deep upper low across the Midwest Thursday night and
then stall briefly, before ejecting to the east/northeast into
Saturday. As this upper low deepens, a potent jet streak will
rotate through the base of the amplifying longwave trough, and the
combination of the diffluent LFQ of this jet streak with height
falls and PVA will produce a surface low over WI Thursday night,
which will then pivot nearly in place, possibly retrograding SW
briefly into Friday before lifting away through D3. Two rounds of
precipitation will accompany this feature, with moderate to heavy
snow likely, as well as some light freezing rain.

Round 1 will involve a period of WAA precipitation spreading
eastward ahead of a cold front as mid-level flow backs to the SW
and divergence supplies ascent. As the precipitation expands
northeastward, the WAA aloft will drive a warm nose atop cold
surface temperatures. This suggests, and there exists good model
consensus on this, that a narrow band of freezing rain is likely
to move eastward which may accrete a few hundredths of an inch
from central MN through eastern IA and much of WI. As the cold
front pushes eastward and forcing begins to intensify, precip may
mix with or transition to snow, but accumulations on D1 are likely
to be primarily light, with WPC probabilities for 4 inches less
than 30%.

Round 2, which will lag significantly from Round 1 in some places,
but may just evolve in place across eastern MN and WI, is expected
to be more significant. As the upper low closes off and stalls,
spokes of vorticity will rotate cyclonically around it. The
interaction of these features will lead to some enhanced mid-level
deformation, which is evident on most of the guidance which has
come into better positional agreement this aftn. At the same time,
a ridge of higher theta-e air (TROWAL) will rap all the way around
the upper low and drop southward across MN, coincident with some
WAA from the north (an indicator of how wrapped up this system
becomes). While the overall column cools, the combination of the
TROWAL, deformation axis, and mid-level WAA will produce briefly
intense ascent, but also deepen the DGZ while driving an
isothermal layer within the DGZ. This could enhance SLRs across
parts of MN and into northern IA, and snowfall forecasts have
increased this aftn. While most of the snow will likely be light
to moderate, a long duration of snow combined with at least
periods of heavier snow has led to WPC probabilities increasing
for the area. There is now a high risk for 6" from just north of
the Twin Cities southwest into the Coteau des Prairies and the
Buffalo Ridge. It is this area that has the best potential for
some heavier snowfall, and a longer duration of pivoting snowfall,
and local amounts to 10" are possible. Elsewhere, WPC
probabilities are moderate for 4 inches surrounding the higher 6"
probs, and extending into the U.P. of Michigan.


...Central Appalachians and Northeast...
Day 3...

The closed low from the Midwest will broaden and begin to lift
east/northeast Friday into Saturday, spreading waves of
precipitation eastward both along the front and associated with
vorticity impulses rotating around the gyre. The primary surface
low is expected to occlude over Michigan, with a secondary wave
developing along the triple point to the east near New England.
Height falls to steepen lapse rates beneath the mid-level vortex
will combine with waves of energy to produce periods of snow
showers over the Central Appalachians, with increasing NW flow
driving upslope flow into the favored windward slopes of WV and
PA. WPC probabilities are low for 4" on D3. Further east,
precipitation spreading northward on WAA ahead of the secondary
low will move across upstate NY and New England. Much of this
precipitation is expected to be rain, or briefly snow changing to
rain, except in the terrain of the Adirondacks, Greens, and
Whites, where snow levels may rise above 3000 ft. Above these
levels, especially in the White Mountains, heavy snow is likely.
WPC probabilities are high for 4 inches in the Adirondacks and
White Mountains of NH.


For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Weiss


$$
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FOUS11 KWBC 252025
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
325 PM EST Thu Feb 25 2021

Valid 00Z Fri Feb 26 2021 - 00Z Mon Mar 01 2021


...Pacific Northwest to the Northern Rockies...
Days 1-2...
Strong northwesterly flow, with embedded shortwave energy aloft,
will continue to support periods of heavy snow across the
Northwest into the northern Rockies through Friday and into early
Saturday. Through late Saturday, widespread additional
accumulations of 8-12 inches are likely across the Olympics, the
Washington and Oregon Cascades, the Blue Mountains, as well as the
Rockies from northern Idaho and western Montana to eastern Idaho
and western Wyoming. Within these areas, local accumulations of
2-feet or more are likely across the higher elevations of the
Cascades, the Blue Mountains and the Bitterroot Range. Drier
weather is expected by late Saturday and continuing into Sunday as
a strong ridge shifts east across the region.

For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Pereira

$$
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FOUS11 KWBC 262124
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
424 PM EST Fri Feb 26 2021

Valid 00Z Sat Feb 27 2021 - 00Z Tue Mar 02 2021


...Pacific Northwest to the Northern and Central Rockies...
Day 1...
Mid-level energy embedded within northwest flow will support
additional periods of locally heavy snow from the Cascades to the
northern Rockies Friday evening, with WPC PWPF indicating
additional accumulations of 8-12 inches likely, and locally higher
totals possible, for portions of the Cascades, Blue Mountains and
Bitterroot Range. Meanwhile, mid-level energy moving through the
base of an amplifying trough will support the potential for
periods of heavy snow farther to the southeast from the
northeastern Nevada mountains to the central Rockies beginning
this evening. Periods of snow are then expected to continuing
into Saturday as the previously noted upstream energy digs to the
southeast. Areas impacted are expected to include the eastern
Idaho to western Wyoming ranges, the northern and central Utah
mountains, and south-central Wyoming to north-central Colorado
mountains. WPC PWPF shows accumulations of 6-12 inches, with
locally heavier possible, for those areas.

As the upper trough begins to split, with the northern stream
segment moving east of the Rockies into the Plains, while the
southern stream wave digs farther to the south, the potential for
heavy snow is expected to decrease by early Sunday. From Sunday
and continuing through Monday, a well-defined ridge is expected to
support dry weather as it moves east across the region.

...Upper Midwest/Great Lakes...
Day 2...
A northern stream shortwave emanating from the trough in the West
is expected to track east-northeast from the central Plains late
Saturday into the northern Great Lakes by late Sunday. This will
likely bring a stripe of mainly light snows from Nebraska and
South Dakota into southern Minnesota and western Wisconsin
Saturday, with snows shifting northeast across northern Wisconsin
into the U.P. of Michigan on Sunday. While widespread heavy
amounts are still not expected, models have been trending upward
with amounts, especially from southern Minnesota northeastward.
Guidance has trended more amplified with the wave as it moves
across the region, along with showing an increasing signal for
mesoscale banding, supporting a narrow stripe of heavier amounts
developing across the region on Sunday.

...Northeast...
Day 1...
Weak southern stream energy lifting from the southeast, along with
a more defined northern stream trough lifting across the Great
Lakes will support precipitation spreading north from the central
Appalachians and Mid-Atlantic into the Northeast Friday night into
Saturday, with precipitation continuing across northern New
England into late in the day. As high pressure retreats, a wintry
mix at the onset is expected to change over to rain across most
locations. Exceptions may include the higher elevations of
northern New Hampshire into central and northern Maine, where
precipitation may remain mostly snow and the probabilities for
accumulations of 4-inches of snow or more are highest.

For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Pereira

$$
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FOUS11 KWBC 270857
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
357 AM EST Sat Feb 27 2021

Valid 12Z Sat Feb 27 2021 - 12Z Tue Mar 02 2021

...Northern Plains to the Western Great Lakes...
Days 1-2...

A compact and fast moving but potent low pressure system will race
from eastern Nebraska Saturday evening to Ontario, Canada Sunday
evening while deepening. This low will deepen in response to
height falls as a shortwave embedded within an otherwise
positively tilted longwave trough lifts northeast, coincident with
intensifying jet-level diffluence as a poleward arcing jet streak
reached 170kts. This deep layer synoptic ascent will be aided by
locally intense mesoscale forcing as strong mid-level fgen
develops on the NW side of this surface low. Guidance has
continued its trend towards a stronger low again overnight, and
this has led to an increase in snowfall. Additionally, there is
increasing evidence that some CSI banding with possible upright
convection /thundersnow/ is possible especially from near the
Buffalo Ridge of MN northeast into the western U.P. of Michigan
due to negative theta-e lapse rates and -EPV intersecting the
saturated DGZ. HREF snowfall probabilities for 1"/hr have
increased, and there is likely to be a narrow band of 1-2"/hr
snowfall rates late Saturday night through Sunday afternoon. WPC
probabilities for 4 inches are highest in the Black Hills due to
elevation and some upslope enhancement, and the western U.P. of MI
which will experience the most intense banding due to the more
thoroughly developed low. However, anywhere in between these areas
WPC probabilities for 4 inches are as high as 30%, and locally 6+
inches of snowfall is possible from central SD through southern MN
and into WI and the U.P. of MI.


...Northeast...
Days 1 & 3...

Weak southern stream energy lifting from the southeast, along with
a more defined northern stream trough lifting across the Great
Lakes will support precipitation spreading north from the central
Appalachians and Mid-Atlantic into the Northeast on Saturday, with precipitation continuing across northern New England into late in
the day. As high pressure retreats, a wintry mix at the onset is
expected to change over to rain across most locations. Exceptions
are likely to include the higher elevations of northern New
Hampshire into central and northern Maine, where precipitation may
remain mostly snow and the probabilities for accumulations of
4-inches of snow or more are as high as 60%, generally above 3000
ft.

On Monday night, a potent trough will dig across the Great Lakes
reinforcing the broad cyclonic flow across the eastern CONUS. This
trough will be accompanied by rapid and potent height falls along
with PVA to drive an area of precipitation, mostly as snowfall. A
surface low will pass east beneath the shortwave, driving a strong
cold front off the coast late Monday night. While snowfall
accumulations with this feature are expected to generally be
modest, WPC probabilities for 4 inches are as high as 30% in the
favored upslope terrain of the Adirondacks and Greens.
Additionally, while accumulation outside of the terrain should be
light, there appears to be an increasing potential for snow
squalls as low-level CAPE climbs to 50-150 J/kg coincident with
negative 0-2km theta-e lapse rates. These squalls could race
across New England and Upstate New York/Pennsylvania producing
briefly intense snowfall rates and lowered visibility late Monday
and Monday night.


...Pacific Northwest...
Day 3...

A closed low off the coast of CA will extend a lobe of vorticity
into Canada, with embedded weak height falls dropping into WA
state Monday night. Ahead of this feature, briefly backed
mid-level flow will drive some WAA precipitation into the Olympics
and Cascades of WA producing periods of moderate snowfall. This
feature will drop southeast quickly leaving generally zonal flow
in its wake, so forcing for ascent is likely to be modest and of
short duration. WPC probabilities for 6 inches are generally less
than 40%, highest in the far northern Cascades.


For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Weiss

$$
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FOUS11 KWBC 272055
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
354 PM EST Sat Feb 27 2021

Valid 00Z Sun Feb 28 2021 - 00Z Wed Mar 03 2021

...Northern Plains to the Upper Peninsula of Michigan...
Day 1...

A trough over the northern Rockies becomes increasingly positively
tilted as a reinforcing trough allows the northern stream to
separate and eject east across the northern Plains tonight and the
northern Great Lakes Sunday. Lee side surface cyclogenesis
currently over southwest KS lifts northeast this evening tracking
across central IA tonight and central WI Sunday morning. This low
will deepen in the left exit region of a jet streak currently
intensifying over the southern Plains. This deep layer synoptic
ascent will be aided by locally intense mesoscale forcing as
strong low to mid-level fgen develops NW of this surface low.
Continued evidence that some CSI banding with possible upright
convection (and possible thundersnow) is possible especially from
near KFSD northeast into southwest MN, then again for northern WI
across the U.P. of Michigan due to negative theta-e lapse rates
and -EPV intersecting the saturated DGZ. 12Z HREF snowfall
probabilities for 1"/hr increased again, and there is likely to
be a narrow band of 1-2"/hr snowfall rates this evening into the
overnight over southeast SD and southwest MN then again Sunday
morning through midday from northern WI to the western U.P. Day 1
WPC probabilities for 4 or more inches are moderately high under
the mid-level trough over southwest SD and the Black Hills, near
the Buffalo Ridge in southwest MN, and then over northern WI to
the central U.P. where there are 30 to 40 percent for 6 or more
inches. From southeast SD to northwest WI, including the MSP
metro, are continued Day 1 WPC probabilities or at least 30
percent.


...Northeast...
Days 1-3...

The shortwave trough lifting northeast over the northern Great
Lakes Sunday afternoon will direct warm air advection across the
Northeast Sunday/Sunday night. Locally trapped cold air should
allow for some light icing over north-central PA Sunday morning
and the northern Appalachian Valleys over interior New England
Sunday night. Day 1/2 ice probabilities are generally 20 to 30
percent for a tenth inch or more of ice in north-central PA, in
the Adirondacks, and the White Mtns of NH.

A potent trough with associated Arctic cold front will dig across
the Great Lakes Sunday night and the Northeast/New England through
Monday. This will reinforce broad cyclonic flow across the eastern
CONUS and be accompanied by rapid and potent height falls along
with PVA to drive an area of snow across the eastern Great Lakes
and New England Monday afternoon through Monday evening with lake
effect snow continuing through at least Monday night. While
snowfall with this quick moving feature are expected to generally
be modest, WPC probabilities for 4 inches are as high as 30% in
the favored upslope terrain of the Adirondacks and Greens on Day
2.5 with local snow belt areas also likely to see a few to several
inches of snow where the bands set up. Additionally, while
accumulation outside of the terrain should be light, there
continues to be potential for snow squalls as low-level CAPE
climbs to 50-150 J/kg coincident with negative 0-2km theta-e lapse
rates. These squalls could race across New England and Upstate New York/Pennsylvania producing briefly intense snowfall rates and
lowered visibility late Monday and Monday night.


...Southern High Plains...
Day 2...

As the trough over the northern Rockies becomes more positively
tilted and amplified tonight a closed low spins off the end and
tracks east from AZ over NM Sunday into Monday and across the
southern Plains Monday night/Tuesday. Gulf-sourced moisture
currently spread over the eastern half of TX is drawn west to the
Panhandle Sunday night which is is lifted topographically and from
the approaching low, resulting in locally moderate snow over
eastern NM into the TX Panhandle late Sunday night/Monday morning.
Day 2 snow probabilities are 10 to 20 percent for 2 or more inches
along the central NM/TX Panhandle border.


...Pacific Northwest...
Days 1 & 3...

A weak shortwave trough overtopping the Northeast Pacific ridge
crosses BC tonight and provides some lift in continued onshore
NWly flow into western WA and makes for mainly moderate northern
Cascades and Olympic snows into Sunday morning as snow levels rise
from 2000 to 3000ft overnight. Day 1 snow probabilities are 20 to
30 percent for 6 or more inches for the higher Cascades.

The northern portion of an amplifying long wave trough approaching
the West Coast Monday weakens as it reaching the Pacific Northwest
coast Monday night as a low closes off CA. Some additional
moderate precip can be expected Monday night with Day 3 snow
probabilities generally 20 to 30 percent with snow levels around
2000ft.


For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Jackson

$$
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FOUS11 KWBC 280936
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
435 AM EST Sun Feb 28 2021

Valid 12Z Sun Feb 28 2021 - 12Z Wed Mar 03 2021

...Upper Midwest into the U.P. of Michigan...
Day 1...

A fast moving but rapidly deepening surface low pressure will lift
northeast from MN this morning into Ontario by Monday morning.
This low will be driven by a potent shortwave and the LFQ of a
strengthening upper jet streak progged to reach 170kts. This
robust deep layer ascent allows for rapid surface pressure falls,
and this ascent will be aided by intense mesoscale forcing as
well. As the low moves northeast, an intense band of frontogenesis
is likely to stream northeast, coincident with the saturating DGZ,
and beneath a developing TROWAL, albeit with modest intensity.
This suggests that within a broader footprint of light to moderate
snow, a heavy but narrow snow band will develop and lift
northeast. Within this band, snowfall rates of 1-2"/hr are
possible as shown by recent HREF guidance and suggested by CSI
potential shown in cross sections. These rates should overcome the
progressive nature of the surface low, so despite a short
duration, WPC probabilities are high for 4 inches across northern
WI and into the U.P. of MI, where local amounts near 8" are
possible.


...Northeast...
Days 1-2...

Low pressure and its associated mid-level trough will move
northeast through Ontario, leaving SW flow and WAA into the
Northeast. Embedded within this flow, a weak shortwave is progged
to lift across PA and into New England Sunday night, which will
enhance precipitation across the region. The column is marginally
cold enough for wintry precipitation, and WAA is expected to
overwhelm most of the area leading to primarily rainfall. However,
far northern ME is likely to remain cold enough for some modest
snow accumulations Sunday night and WPC probabilities are as high
as 30% near the Canadian border. Additionally, some lingering cold
air in the terrain of the Catskills, Adirondacks, and Whites could
lead to some modest accretions of freezing rain with WPC
probabilities indicating less than 20% chance for 0.1".

After this system moves away, a more potent shortwave/closed low
will race across Northern New England Monday night driving an
arctic front southward across New England. An area of snow will
likely accompany this feature as ascent is provided through rapid
height falls and PVA. However, somewhat limited moisture and fast
progression should limit snowfall accumulations, with WPC
probabilities for 4 inches less than 20%, and confined to the
terrain of the Adirondacks and Greens which should benefit from
some upslope enhancement. Additionally, some lake effect snow is
likely E/SE of Erie and Ontario, but forecast soundings indicate
the DGZ struggles to saturate so LES accumulations are forecast to
be modest. Behind the arctic front, the environment looks primed
for snow squalls across Upstate New York, parts of New England,
and potentially into northern PA. While accumulations from these
squalls should be limited, briefly intense snowfall rates and
gusty winds are likely Monday night.


For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Weiss

$$
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FOUS11 KWBC 282028
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
327 PM EST Sun Feb 28 2021

Valid 00Z Mon Mar 01 2021 - 00Z Thu Mar 04 2021

...Northern Great Lakes through the Northeast...
Days 1-2...

A lobe of an Arctic low is currently along the northern
Manitoba/Ontario border. A southern stream shortwave is riding a
strong SWly jet ahead of this lobe and is currently over the U.P.
of MI and a reinforcing northern stream shortwave trough is
pushing southeast over southern Manitoba on a strong NNWly jet.
The leading, southern stream wave (currently producing heavy snow
in the U.P.) lifts well into Quebec tonight, allowing an initial
cold front to shift across New England Monday morning. A light
wintry mix can be expected over the Adirondacks and mountainous
New England tonight in the warm air advection ahead of this front.
Northern Maine is cold enough to allow a burst of snow with the
cold front...HREF probabilities for 1"/hr rates are moderate from
12Z to 16Z across far northern Maine where Day 1 snow
probabilities for 6 or more inches are 20 to 40 percent.

Monday afternoon the arctic cold front associated with the
reinforcing shortwave trough shifts southeast across New England.
An area of snow will likely accompany this feature as ascent is
provided through rapid height falls and PVA. However, somewhat
limited moisture and fast progression should limit snowfall
accumulations outside of lake effect snow belts. Day 1.5 WPC snow
probabilities for 2 or more inches are moderate for the northern
Adirondacks and interior mountainous New England (Greens and
Whites) from upslope enhancement. Additionally, some lake effect
snow is likely E/SE of Erie and Ontario, but forecast soundings
indicate the DGZ struggles to saturate so LES accumulations are
forecast to be modest, generally around 2 inches. Behind the
arctic front, the environment continues to look primed for snow
squalls across Upstate New York, parts of New England, and
potentially into northern PA. While accumulations from these
squalls should be limited, briefly intense snowfall rates and
gusty winds are likely Monday night with Day 1.5 snow
probabilities for 1 or more inches generally above 5 percent
across upstate NY.


For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Jackson

$$
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FOUS11 KWBC 012030
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
330 PM EST Mon Mar 01 2021

Valid 00Z Tue Mar 02 2021 - 00Z Fri Mar 05 2021

...Northeast...
Days 1-2...

Low pressure lifts north from eastern Maine rest of this afternoon
with associated rain/snow lifting out before 00Z. In the wake of
this low is a particularly cold mid-level low that is sending an
Arctic front with associated snow squalls east from the eastern
Great Lakes this afternoon, quickly crossing the rest of the
Northeast through the evening. While accumulations from these
squalls should be minimal, briefly intense snowfall rates of more
than 1"/hr, strong westerly winds, and temperatures rapidly
dropping below freezing could whiten the ground and produce
significantly reduced visibility, particularly in upstate NY.
However, the low level flow is not very perpendicular to the
front, so the ability for this activity to reach the coastal
areas/I-95 corridor is considered limited.

Behind this Arctic front, strong CAA will produce periods of LES
in the favored NW snow belts east of Lakes Erie and Ontario. While
duration and intensity is modest (mainly ending tonight), briefly
heavy snow is possible. WPC Snow probabilities for 2 or more
inches are limited to an area of 10 to 20 percent probability just
southeast of Syracuse.

A clipper pushes east-southeast from Ontario into the interior
Northeast Tuesday night. Northern slopes of the Tug Hill Plateau
and the Adirondacks look to get brief moderate snowfall rates with
Day 2 snow probabilities for 4 or more inches 40 to 60 percent
with some additional snow in the northern Greens of VT.


...Southwest to Central Rockies...
Days 2-3...

Upper level low pressure closes well off the northern CA coast
(near 130W) tonight and shifts south well offshore through Tuesday
before turning east. This low crosses southern CA late Wednesday
and tracks to the Four Corners area by Thursday morning. The
influx of Pacific moisture ahead of this low allows for moderate
higher elevation snows for the southern CA ranges (snow level
around 5000ft) as well as the higher ranges (snow levels 5500ft to
6500ft) of southern UT, the Mogollon Rim in AZ, and in particular
the San Juan Range of CO/northern NM. Snow probabilities for 4 or
more inches are currently limited to just the highest peaks of
southern CA and AZ, but moderate to high for 6 or more inches on
Day 3 in the southern UT ranges and San Juans.


For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Jackson

$$
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FOUS11 KWBC 022055
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
355 PM EST Tue Mar 02 2021

Valid 00Z Wed Mar 03 2021 - 00Z Sat Mar 06 2021

...Northern New York/Vermont...
Day 1...

A low will move across southern Ontario and then southern Quebec
to just north of Vermont Wednesday before decaying as it moves
towards Maine. The trailing front south of the low crosses
northern NY and VT. The post-frontal flow becomes westerly, with
moist upslope flow coming into both the Tug Hill and NY Adirondack
Mountains, followed by the northern green Mountains of VT.
Where ascent corresponds to the enhanced moisture in northern NY,
several inches of snow are expected.
The veering flow turns to the north tomorrow night, with drying
aloft simultaneously occurring. As a result, snow shower coverage
and intensity should quickly wane Wed night to Thu morning. WPC
probabilities for 4 inches are as high as 60% in the northern
Adirondacks.

...CO Rockies...
Days 2-3...

A low-mid level circulation moves from Southern California and
across eastern UT and C. Enhanced low level convergence/ascent
near the 700 mb circulation center crosses southern Utah, the Four
Corners and the ranges of CO, leading to a period of snow across
the mountains, with the higher QPF centered on the San Juan
Mountains, where the strongest 700 mb convergence is forecast.
Secondary snow accumulation maxima are forecast in other ranges of
western to central CO, and Wasatch of southern Utah. WPC
probabilities indicate a high likelihood for 4" in the San Juan
Mountains, and moderate in the southern Wasatch, central Mogollon,
and other CO ranges.
The event winds down Thu as the 700 mb low moves east across the
high plains, with ascent departing the mountains.

...Pacific Northwest...

Day 3...

A closed mid-level low is expected to linger off the coast of
Washington and Oregon Thursday night and Friday. East of the low a
Pacific jet streak and warm/moist mid-level SW flow extends from
the coast of northern CA across western OR and WA. The slow
eastward progression of the low will allow for slow movement of
the bands of precipitation across western WA and OR. Heavy
amounts of precipitation are possible in the mountains due to the
combination of enhanced moisture in the low level jet and ascent
from both the approaching front and also orographic lift in
windward terrain. However, the southerly component of flow leading
to warming, with snow levels likely to climb above 4000 ft,
limiting significant snowfall to the Olympics and far northern
Cascades, where 1 to 2 feet of snow is possible in the highest
terrain.
The probability of 8 inches is high in the WA Olympics. becoming
moderate in the northern WA Cascades.

For Days 1-3, the probability of significant icing (0.25 inch or
greater) is less than 10 percent.

Petersen

$$
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FOUS11 KWBC 022055
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
355 PM EST Tue Mar 02 2021

Valid 00Z Wed Mar 03 2021 - 00Z Sat Mar 06 2021

...Northern New York/Vermont...
Day 1...

A low will move across southern Ontario and then southern Quebec
to just north of Vermont Wednesday before decaying as it moves
towards Maine. The trailing front south of the low crosses
northern NY and VT. The post-frontal flow becomes westerly, with
moist upslope flow coming into both the Tug Hill and NY Adirondack
Mountains, followed by the northern green Mountains of VT.
Where ascent corresponds to the enhanced moisture in northern NY,
several inches of snow are expected.
The veering flow turns to the north tomorrow night, with drying
aloft simultaneously occurring. As a result, snow shower coverage
and intensity should quickly wane Wed night to Thu morning. WPC
probabilities for 4 inches are as high as 60% in the northern
Adirondacks.

...CO Rockies...
Days 2-3...

A low-mid level circulation moves from Southern California and
across eastern UT and C. Enhanced low level convergence/ascent
near the 700 mb circulation center crosses southern Utah, the Four
Corners and the ranges of CO, leading to a period of snow across
the mountains, with the higher QPF centered on the San Juan
Mountains, where the strongest 700 mb convergence is forecast.
Secondary snow accumulation maxima are forecast in other ranges of
western to central CO, and Wasatch of southern Utah. WPC
probabilities indicate a high likelihood for 4" in the San Juan
Mountains, and moderate in the southern Wasatch, central Mogollon,
and other CO ranges.
The event winds down Thu as the 700 mb low moves east across the
high plains, with ascent departing the mountains.

...Pacific Northwest...

Day 3...

A closed mid-level low is expected to linger off the coast of
Washington and Oregon Thursday night and Friday. East of the low a
Pacific jet streak and warm/moist mid-level SW flow extends from
the coast of northern CA across western OR and WA. The slow
eastward progression of the low will allow for slow movement of
the bands of precipitation across western WA and OR. Heavy
amounts of precipitation are possible in the mountains due to the
combination of enhanced moisture in the low level jet and ascent
from both the approaching front and also orographic lift in
windward terrain. However, the southerly component of flow leading
to warming, with snow levels likely to climb above 4000 ft,
limiting significant snowfall to the Olympics and far northern
Cascades, where 1 to 2 feet of snow is possible in the highest
terrain.
The probability of 8 inches is high in the WA Olympics. becoming
moderate in the northern WA Cascades.

For Days 1-3, the probability of significant icing (0.25 inch or
greater) is less than 10 percent.

Petersen

$$
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FOUS11 KWBC 030829
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
328 AM EST Wed Mar 03 2021

Valid 12Z Wed Mar 03 2021 - 12Z Sat Mar 06 2021

...Four Corners...
Days 1-2...

A mid-level closed low emerging from Southern California this
evening will move eastward across the Four Corners Thursday before
ejecting into the Southern Plains on Friday. Enhanced mid-level
divergence, height falls, a LFQ jet streak divergence maxima, and
intensifying southerly WAA will combine to produce moderate to
heavy snow in the terrain of the Four Corners mountains this
evening and Thursday. Snow levels are expected to climb as high as
6000-8000 ft during the heaviest precip, confining the significant
snow to these elevations. The heaviest snow through Thursday is
likely to be in the San Juans which will benefit from upslope
enhancement due to the orthogonal southerly flow, and WPC
probabilities are high for 6 inches each of the two days.
Otherwise, WPC probabilities are moderate for 6 inches on D1 in
the southern Wasatch, extending into most of the CO Rockies on D2.
Lighter snows accumulating to a few inches are likely D2 across
the Palmer Divide.


...Pacific Northwest...
Days 2-3...

A closed mid-level low is likely to move slowly west of the coast
Thursday and Friday, with pronounced southerly 700-500mb flow
drawing tropically sourced moisture northward along the coast of
northern CA and into WA/OR. As the low slowly migrates eastward, a
surface cold front will move onshore, but not until late D3
/Friday evening/, which will finally bring a slow wane to the
precipitation. However, a prolonged period of warm/moist advection
ahead of this front will drive significant precipitation across
the Coastal Ranges and into the WA/OR Cascades. Snow levels will
rise on this warm flow, and are expected to generally be above
4000 ft in WA/OR, before falling to 2500-3000 ft on D3. WPC
probabilities D2 are high for 6 inches in the Olympics, where
isolated totals approaching 2 ft are possible. By D3, there will
be enough of an eastward push in the flow to drive heavy snow
across the northern Sierra, Siskiyous/Shastas/Trinities, and into
the OR/WA Cascades, as well as continuing in the Olympics. WPC
probabilities are moderate to high for 6 inches in these ranges,
with local maxima above 12" likely.


For Days 1-3, the probability of significant icing (0.25 inch or
greater) is less than 10 percent.

Weiss

$$
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FOUS11 KWBC 040846
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
345 AM EST Thu Mar 04 2021

Valid 12Z Thu Mar 04 2021 - 12Z Sun Mar 07 2021

...Colorado...
Day 1...

A closed mid-level low over the western AZ/UT border early this
morning will track east over southern CO today before turning
southeast over the southern Great Plains tonight. Enhanced
upper-level divergence in the left exit region of a westerly jet
over AZ/NM today, low level convergence with notable fgen on the
north side of the low at 700mb and 600mb, and warm/moisture
advection with a feed of tropical Pacific and some western Gulf
moisture ahead of the low makes for snow, locally heavy in the
higher elevations of the through this evening. Thermal profiles
are rather marginal over the Denver metro area, but the consensus
low track puts that metro area in the fgen region, so a band or
two of accumulating snow in that region east of the Front Range.
Day 1 WPC probabilities (which begin at 12Z today) are moderately
high for 6 or more inches over the northern CO Rockies,
particularly the Front Range from Ft. Collins to CO Spring.


...Pacific Northwest/California...
Days 1-3...

A closed mid-level low and reinforcing trough will slowly shift
toward the OR and WA coasts today, pushing ashore Friday night. A
pronounced southerly flow will advect warm air and moisture
northward along the coast of northern CA and across WA/OR. As the
low slowly migrates eastward, a surface cold front will linger
just offshore tonight through Friday before moving onshore Friday
night. A prolonged period of low level pre-frontal convergence
combines with the warm/moist advection ahead of the front to
produce prolonged low elevation rain and high elevation snow
across the OR/WA Cascades and WA Olympics through Friday. Day 1
WPC probabilities are moderate for 8 inches in the Olympics and
Day 1.5 has these probabilities expand to the high WA Cascades.

Friday afternoon through Friday night, the cold front moves
onshore in California and progresses inland. The surge in moisture
and low level convergence with the front drives a period of snow
across the Klamath/Shasta Siskiyous/Trinities, into the OR
Cascades, and northern Sierra Nevada with initial snow levels
around 5000ft decreasing to around 3500ft as the snow tapers off
late Friday night. Day 2 WPC probabilities are moderate for 8
inches in the higher Klamath and Siskiyou ranges, with local
maxima above 12" likely. These probabilities expand down the
northern Sierra Nevada for Day 2.5. Veering flow behind the cold
front cuts off Pacific moisture on Saturday with greatly
diminished precip rates expected inland over the Great Basin.


...Northern New York/northern Vermont mountains...
Days 1/2...
Deep low pressure currently north of the Gulf of St. Lawrence
retrogrades, shifting west over eastern Quebec as troughing
increases over the Northeast. Areas where the overlap of
wrap-around moisture and boundary layer convergence/ascent where
the northwest winds turn upslope results in snow showers Thu night
across the northern Adirondack and Green mountains. High mean
layer relative humidity near 90 percent tonight supports an
extended period of snow showers. On Friday as the low pulls away,
drying aloft develops across the region along with sinking motion,
supporting the snow tapering in coverage/intensity. Day 1.5
probabilities for 4 or more inches are 20 to 30 percent over
northern stretches of these two ranges.

For Days 1-3, the probability of significant icing (0.25 inch or
greater) is less than 10 percent.

Jackson

$$
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FOUS11 KWBC 052119
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
419 PM EST Fri Mar 05 2021

Valid 00Z Sat Mar 06 2021 - 00Z Tue Mar 09 2021


...Pacific Northwest/California to the Northern Rockies...
Days 1-2...
Ongoing precipitation associated with a frontal band moving inland
across the Pacific Northwest and Northern California is expected
to continue through the evening, with additional mountain snows
likely from the Olympics and Cascades to the northern Sierra.
While accumulations are expected to be light across most areas, a
well-defined shortwave moving through the base of the broader
scale trough is expected to amplify near the Northern California
coast overnight. This will likely support some heavier
accumulations, especially across the mountains of northwestern
California into the southern Cascades. WPC PWPF suggests
widespread additional accumulations of 4-8 inches are likely
across this region during the evening and overnight hours. Snows
are expected to quickly diminish across this area as the shortwave
lifts northeast of the region on Saturday.

Shortwave energy rotating out ahead of a closed low centered west
of British Columbia will drive another front into the Northwest
Sunday morning, with heavy snow accumulations likely over the
Olympics. The potential for organized heavy amounts is expected
to diminish as the front moves east and weakens on Sunday.

...Eastern Great Lakes...
Days 1-2...
Deep northwesterly flow with embedded energy aloft will support
multi-band lake effect snow showers south of Lake Ontario and
eastern Lake Erie Friday night into Saturday. While widespread
heavy amounts are not expected, at least a couple of inches of
snow are likely, especially north of and along the Finger Lakes
region. Snows will likely diminish as a surface high pressure
shifts east into the region by late Saturday.

For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Pereira



$$
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FOUS11 KWBC 060829
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
329 AM EST Sat Mar 06 2021

Valid 12Z Sat Mar 06 2021 - 12Z Tue Mar 09 2021


...Pacific Northwest/California to the Northern Rockies...
Days 1-3...
Frontal band precipitation moving inland across Northern
California is just reaching the eastern side of the San Joaquin
Valley, but will be winding down over for the northern Sierra
Nevada by 12Z (when Day 1 snow probabilities begin) as the
mid-level shortwave trough axis lifts northeast into the Great
Basin with veering low level flow in the approaching ridge cuts
off Pacific moisture.

A cold front from a low pressure system drifting south well off
the BC coast reaches the WA/OR/far northern CA coast late tonight
with snow levels around 2000ft. Organized precip/mountain snows
diminish as the front moves east and weakens on Sunday and onshore
flow backs southerly, cutting off moisture influx. The next
trough/frontal boundary from this low that should be off the WA
coast by Sunday night reaches the PacNW coast Monday with a decent
fetch of Pacific moisture pushing across far northern CA. Day 1.5
snow probabilities are high for 6 or more inches for the Olympics
and higher WA Cascades. Day 3 snow probabilities for 6 or more
inches are moderate for the Klamath/CA Cascades and
northern/central Sierra Nevada as well as some low probabilities
for the Wallowa Mtns of OR and the Salmon River/Clearwater Mtns of
ID.

...Eastern Great Lakes...
Day 1...
Deep northwesterly flow with embedded energy aloft and a saturated
DGZ will continue to support multi-band lake effect snow showers
south of Lake Ontario and eastern Lake Erie today. At least an
additional couple of inches of snow are likely, especially north
of and along the Finger Lakes region. Snows will diminish as a
surface high pressure shifts east into the region this evening.


For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Jackson



$$
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FOUS11 KWBC 061945
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
245 PM EST Sat Mar 06 2021

Valid 00Z Sun Mar 07 2021 - 00Z Wed Mar 10 2021


...Pacific Northwest/California to the Northern Rockies...
Day 1-3...
Shortwave energy moving through the base of an upper trough
associated with a closed low centered west of British Columbia is
expected to drive another another frontal band into the Northwest
as it lifts toward Vancouver Island Sunday morning. This will
bring another round of locally heavy mountain snow into the
Olympics and northern Cascades Saturday night and Sunday.
Shortwave ridging will then likely bring a brief period of drier
weather across the Northwest Sunday evening. Then Sunday night
into Monday, the latest guidance disagrees on the timing and
extent to which wet weather will return to portions of the
Northwest and northern Rockies. Overall, models are in generally
good agreement with the synoptic setup -- showing a well-defined
shortwave lifting northeast from Northern California to the
northern Rockies, ahead of the previously noted low as it drops
farther to the south off of the Washington and Oregon coasts.
Left-exit region upper jet forcing along with a low-to-mid level
baroclinic zone dropping southeast is expected to interact with a
modest increase in moisture to support increasing precipitation
coverage from Northern California to western Montana. However,
models differ on the extent of the more organized heavier
precipitation, with the NCEP guidance generally wetter than the
non-NCEP consensus. While forecast confidence is generally
limited, probabilities for heavier snow accumulations have
increased some, especially over northeastern Oregon into northern
Idaho during the Day 2 period. As this leading shortwave lifts
northeast across the northern Rockies and High Plains into central
Canada on Tuesday, upstream energy moving through the base of the
trough will draw it farther south across California, with
increasing potential for heavier mountain snow developing across
the Klamath Mountains into the southern Cascades and northern
Sierra.

For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Pereira



$$
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FOUS11 KWBC 070833
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
333 AM EST Sun Mar 07 2021

Valid 12Z Sun Mar 07 2021 - 12Z Wed Mar 10 2021


...Pacific Northwest/California to the Northern Rockies...
Day 1-3...

Three shortwave troughs round a low pressure system shifting south
from off the BC coast and down the West Coast, tracking into The
West through midweek.

The first wave is reaching the WA/OR coast early this morning and
will lift northeast over WA today. 0.5" PW air pushing to the WA
Cascades will make for moderate mountain snow today with moderate
Day 1 snow probabilities for 6 or more inches for the Olympics and
high WA Cascades with snow levels dropping to around 1500ft this
morning.

The second shortwave trough is currently just reaching the low
pressure system from the central Gulf of Alaska, it amplifies the
surrounding trough through tonight before swinging east-northeast
across northern CA Monday, the northern Great Basin to MT Monday
night. Left-exit region upper jet forcing along with a low-to-mid
level baroclinic zone dropping southeast is expected to interact
with a modest increase in moisture to support increasing
precipitation coverage from Northern California to western
Montana. Day 2 snow probabilities for 6 or more inches are
moderate in the Klamath and CA Cascades, the Wallowa Mtns of
northeast OR, the Salmon River and Clearwater Mtns of ID as well
as ranges in central MT.

The third shortwave trough ejects south from the eastern Gulf of
Alaska tonight, amplifying the trough even further south before
ejecting east-northeast across the central CA coast Tuesday,
crossing UT Tuesday night. High level Pacific moisture wraps
around the lee-side surface cyclone that develops over the central
High Plains Tuesday night, aiding a swath of higher elevation snow
(snow levels around 5000ft) to develop across WY to the Black
Hills that continues through Wednesday. Day 3 snow probabilities
for 6 or more inches are moderately high for the Klamath Mtns,
down the full extent of the Sierra Nevada and low for ranges in
east-central NV, the northern Wasatch in UT, and around the
Bighorn Mtns in WY.


For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Jackson



$$
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