FOUS30 KWBC 081950
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
249 PM EST Mon Feb 08 2021

Day 1
Valid 16Z Mon Feb 08 2021 - 12Z Tue Feb 09 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Pagano


Day 2
Valid 12Z Tue Feb 09 2021 - 12Z Wed Feb 10 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Gallina



Day 3
Valid 12Z Wed Feb 10 2021 - 12Z Thu Feb 11 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Gallina

$$
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FOUS30 KWBC 100831
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
331 AM EST Sun Jan 10 2021

Day 1
Valid 12Z Sun Jan 10 2021 - 12Z Mon Jan 11 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ALONG THE
CENTRAL TEXAS GULF COAST...

A small marginal risk area was maintained along the central Texas
Gulf coast for a period of heavy rainfall Sunday morning from
Corpus Christi to Matagorda. The low level southeasterly flow is
expected to strengthen quickly Sunday morning along the Texas Gulf
coast ahead of height falls moving east from northeast Mexico into
coastal Texas. This will result in anomalous 850-700 mb moisture
flux values, 3.5 to 4 standard deviations above the mean/strong
boundary layer convergence into the developing coastal front
expected to lie parallel to the coast Sunday morning, resulting in
the heavy rain potential. Consensus from the latest hi res
guidance is for heavy rain possibly moving across this area in the
1500 to 2000 utc time frame. However, with the bias of the hi res
simulated radars to be too slow to develop convection, would not
be surprised if the activity fires prior to this. Instability of
150-250 j/kg is expected along the immediate coastal areas Sunday
morning and not extending very far inland, keeping the heavy rain
threat confined to coastal locations. With the greatest
instability expected to remain offshore, heaviest totals also
likely offshore, with the western edge of this heavy precip area
possibly affecting the above mentioned coastal regions. Hourly
rainfall rates of up to 1.5" may fall across areas that have
received heavy rains over the past 2 weeks resulting in isolated
runoff issues where soil saturation is high as per the National
Water Model.

Oravec


Day 2
Valid 12Z Mon Jan 11 2021 - 12Z Tue Jan 12 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE OLYMPIC PENINSULA AND WESTERN WASHINGTON...

A large Gulf of Alaska cyclone will start to shift further south
and east with a tight southern gradient that becomes regionally
zonal supporting a tightening upper-level jet. At the start of
the forecast period, 11.12z, a solid shortwave will be progressing
along the leading edge of this developing flow regime crossing
Vancouver Island. The trailing cold front and moisture
flux/atmospheric river will be reducing in influence as it crosses
the Olympic Peninsula and toward the mouth of the Columbia River
toward 12.00z. This will be a preparatory shot of moderate
moisture flux with 30-45kts of southwesterly flow and .75" of
Total PWats across saturated soils. This is not likely to cause
flooding concerns given the slight break in heavy rainfall over
the last week and the short duration of moderate rainfall
expected. However, given NWM deep (40cm) saturation ratios are
nearly 1 at the coast while only the highest peaks of the Olympics
are average, combined with AHPS precip anomalies are well above
normal in the last 2 week period across western WA into NW Oregon,
expectation is for higher than normal run-off and recharging
streams to elevated levels prior to next wave.

The upper-level pattern at the nose of this developing 150+kt 25H
upper-level jet will be oriented favorably for cyclogenesis. As
such after 00z, a broad warm advection pattern develops over the
Northeast Pacific stretching from NW California toward the Olympic
Peninsula. This plume will support again southwesterly flow in
excess of 75kts at 85H and given its fetch from the subtropical
moisture plume, the flux is high and total PWats increase to
1.25-1.4" with 1" values along the coast by 06z. There remains
solid model uncertainty in the magnitude of the strength of the
approaching shortwave. The operational ECMWF is significantly on
the stronger side of the guidance which supports strong moisture
flux and Integrated Vapor Transport (IVT) values of 1000 kg/ms
which is over 4 StdDev from the norm with a recurrence interval
around 15-30 years. Though a slight northward shift of the 00z run
(while still clearly on the over-amplified side of the guidance)
has tilted the orientation of the front more southerly than prior
runs and directs increased heavy rainfall into northwest WA versus
prior runs, increasing the concern for the most saturated areas.

The remaining guidance anchored by the 00z GFS/UKMET/CMC and
ensemble suite suggest a more subtle wave though were also
trending a bit faster and stronger as well, toward a common
solution. This should provide some confidence overall for a
heavier rainfall threat across Western Washington. Similar solid
moisture flux will support .5"+/hr rain-rates given IVT values in
the 750 kg/ms range and increase the rates. The precise
timing/orientation of the strongest flux is unfortunately
straddles the transition time in the Excessive Rainfall Outlook
from day 2 to day 3, or 12.12z. As a result of all the factors
and coordination with the local forecast offices in Seattle and
Portland, a Slight Risk was introduced across the southwest flanks
of the Olympics where the hydrological situation is a bit worse
but the Marginal Risk was placed further south across much of
Northwest Oregon to account for the slightly faster southward
slide of the moisture axis along the Northwest coast by 12.12z
seen in nearly all guidance members. Rapid-rises along streams
and rivers and the potential for mudslides continues to exist
within this threat area.

Gallina

Day 3
Valid 12Z Tue Jan 12 2021 - 12Z Wed Jan 13 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
WESTERN WASHINGTON AND NORTHWEST OREGON PARTICULARLY THE
OLYMPIC/COASTAL AND CASCADE RANGES...

...Western Washington/Oregon into far Northwest California...

Very strong moisture flux, near 10-20 year return intervals will
be ongoing at the start of the day 3 forecast period, 12.12z (see
day 2 discussion for additional details). Favorable left exit
dynamics will be further deepening a northeast Pacific Cyclone to
the northwest of Vancouver Island. Strong warm advection will be
occurring with 750-1000 kg/ms of Integrated Vapor Transport (IVT)
with 1-1.25" of total PWats advected 65-85kts of southwesterly
85mb flow across northwest Oregon into the Olympic Peninsula.
This will support an additional 1-2" across the Olympic Range
which is enough to support a continuation of the Slight Risk from
day 2 across this area, while expanding the Marginal Risk into the
Northern Washington Cascades, given highly saturated and therefore
hydrophobic soils, resulting in mudslides and rapid stream rises.

As the cyclone occludes and slides away, the cold front and
precursory moisture flux axis will slide southward along the
coastal range, though flow will start to reduce slightly and IVT
values will dip toward 600-800 range. By 13.00z, the flow should
broaden again with the approach of another much weaker but still
important shortwave. This will once again back low level flow for
additional warm advection across northwest California into the
central OR. The operational ECMWF, which had been more aggressive
in pressing the cold front south has backed off given the further
northward shift in the initial occluded cyclone and therefore
reduced the focus of extreme rainfall amounts along the southwest
Oregon coast. Magnitudes still remain in the 3-5" total range,
and given much drier ground conditions, will only necessitate a
Marginal Risk at this time. The greater concern for the mid to
late Day 3 period, will be the mean orientation of the
IVT/moisture flux plume across central Oregon into the northern
Oregon Cascades, while perhaps not as affected as further north,
grounds still remain saturated and NWM shows anomalous
river/stream run off out of the Central Coastal Range and Northern
Cascades. Guidance Combine that with solid warm advection raising
snow levels and potential snowmelt, as well as the large burn scar
areas from this summer, a Slight Risk of excessive rainfall is
introduced across the Western WA/Northwest OR Coastal Range as
well as SW WA/Northern OR Cascade Ranges. Rapid stream rises and
potential for rock/mudslides are becoming increasingly likely.

...Eastern Washington/Northern Idaho Panhandle...

Given the depth/strength of the moisture flux axis, increased
moisture in a modest warm advection scheme will bleed through the
Columbia River Basin and even potentially over-top the Oregon
Cascades. Ensemble IVT values across the region will be 400-500
kg/ms which is generally with PWat values nearing .75" both of
which are within the 97-99th percentile of climatology and 3
StdDev from normal. Solid upslope flow across northeast WA into
the northern Idaho Panhandle will be available but there is fairly
sizable model spread, in timing both of the moisture return as
well as snow level rises across the region. NWM depicts much of
the Eastern Washington Columbia Plateau remains fairly dry at
depth, values over the last column of counties in WA and into N
Idaho are running about .7 to .8 saturated at 40, due to 300-400%
of normal precip over the last week or so. At this time, in
coordination with Spokane WA forecast office, an Excessive
Rainfall category was not delineated due to some uncertainty, but
if trends continue a Marginal Risk may be needed across this area
to account for potential flooding and mudslides.

Gallina

$$
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FOUS30 KWBC 110800
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
259 AM EST Mon Jan 11 2021

Day 1
Valid 01Z Mon Jan 11 2021 - 12Z Mon Jan 11 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Bann


Day 2
Valid 12Z Tue Jan 12 2021 - 12Z Wed Jan 13 2021

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
NORTHWEST OREGON...

...Western Washington/Oregon...
At the start of the forecast period, 12.12z, the second of three
shortwaves progressing along a strong gradient along the southeast
side of a global scale cyclone in the Gulf of Alaska. This will
result in the very strong, very moist atmospheric river to be
directed along and southeast of the cold front that will be
crossing the Olympic Peninsula. Nearly all short-range global
guidance is coming into alignment with 50-75kt 85H flow and 1-1.2"
Total PWats directed at the mouth of the Columbia river and
southward. This puts Integrated Vapor Transport (IVT) magnitudes
near 4 StdDev and at the threshold of 1000 kg/ms, which is
indicative of a strong to extreme AR. The wave will continue to
flatten and the height-falls will stagnate throughout the
remainder of Tuesday (13.00z) while continuing to direct the axis
of the plume across the Northwest Coastal Range of
Oregon...bleeding over toward the Cascade Range.

Upstream, guidance has come into much more solid agreement in the timing/placement of the 3rd of the significant shortwave/surface
waves. Favored orientation to strong Upper-level jet, supports
solid amplification after 13.00z around 42N135W and with another
slug of deep subtropical moisture and accelerating southwesterly
flow, IVT values once again surge to around 1000 kg/ms with the
00z GFS suggesting near 1250 kg/ms at landfall along the
North-Central OR coast by 06-12z. This will likely yield
rain-rates of .75"/hr even without any appreciable instability.
The combination of rounds should allow for areal totals to near
3-5" across both the Coastal and Cascade Ranges, with the bulk of
the 3rd wave's moisture falling as rainfall as snow levels rise
even across the all but the highest peaks of the Cascades.
Hydrologically, the Northwest Oregon Coastal Range has been
running about 150-300% of normal with National Water Model (NWM)
saturation ratio very near 1" suggesting very high run-off, likely
to result in rapid-rises along streams and flooding along large
main-stem rivers. Additionally, mud/rock slides are possible. As
such have maintained the Moderate Risk of excessive rainfall
across the OR Coastal Range where the available moisture flux will
be greatest.

...Southwest Oregon/Far Northwest California...
Further south, boreal rain-forest of Curry county, will see
increased low level confluent flow with greater coastal surface
flow between 12-00z on the 12th, as mid-level forcing slides
south, greater orographic ascent due to favored directional flow
will provide more efficient rainfall rates across this area, with
areal rainfall totals of 6-8" possible by 13.12z. This area has
been much drier than locations further north, as well as, being
generally less susceptible to such rates, due to higher
climatological frequency. Still, as stated above, the strength and
duration of the Atmospheric River still demands a Slight Risk of
Excessive Rainfall for the potential of rapid-rise response on
streams/rivers in the area.

...Eastern Washington/Northern Idaho Panhandle...
As the guidance has come into better agreement in placement/timing
of the shortwaves and the moisture flux axis, as well as a
continued upward trend, this places locations east of the higher
terrain of the Cascades at increasing risk. Additionally, the
orientation of the plume and placement may be ideally oriented for
greater moisture depth to bleed through the terrain across the the
Columbia River Valley into the Columbia Plateau. NAM/GFS and
ECMWF IVT plumes remain persistent with mid to upper 40s surface
Tds and even total PWat values reaching near 1" by late in the
forecast period 13.06z. As the third strong shortwave approaches,
winds will accelerate to 50-65kts generally angled from Southwest
component through the Columbian Plateau toward the Northern
Rockies of E Washington and northern Idaho. AHPS precipiation
anomalies still remain well above normal and NWM shows deeper
saturation rations in the .8 range from Spokean to Lewiston and
points east on the lower flanks of the terrain. Higher than
normal stream flows and mudslides experienced last week were due
to a combination of snow melt and moderate flux. However, the
flux and moisture availability is very strong and with rising snow
levels by early Wednesday, areal QPF values of 1-2" are possible
and may result in similar higher stream flows and mudslides. In
coordination with WFO Spokane, a Marginal Risk was introduced
across this area of overlapping hydrological conerns and deeply
saturated orthogonal upslope flow

Gallina


$$
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FOUS30 KWBC 130751
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
250 AM EST Wed Jan 13 2021

Day 1
Valid 01Z Wed Jan 13 2021 - 12Z Wed Jan 13 2021

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE PACIFIC NORTHWEST FROM THE CASCADES WESTWARD TO THE COAST
RANGE...

0100 UTC Update...The forecast below remains on track, therefore
no changes to the outlook area were made. Earlier observed
rainfall rates along the Coastal Ranges of Washington and Oregon
were in the 0.5 to 0.7 in/hr range and similar rates are expected
throughout the next 6-12 hours for much of western Oregon into
southern Washington and northwestern California. Blended TPW
imagery showed a relative max in precipitable water surging toward
the southern OR/northern CA coast as of 00Z (around 1.25" at
EKA),which will likely account for a brief surge in rainfall
intensity in the short term. Please reference WPC Mesoscale
Precipitation Discussions (MPDs) for more details on this event.

0830 UTC Discussion below...

The latest qpf totals from the global and hi res models remain
very consistent over the past few runs, resulting in only some
minor tweaks being made to the previous slight and moderate risk
areas on the Excessive Rainfall Outlook for areas of the Pacific
Northwest from the Cascades westward to the Olympic Range and
Oregon/northwest California coast ranges. The well advertised
atmospheric river event will continue through most of the upcoming
day 1 time period, weakening during the last 6 hours, 0600-1200
UTC Wed. The latest blended TPW loops continue to show a plume of
anomalous pw values stretching across all of the north Pacific
with western origins near the Philippines. Broad west
southwesterly mid to upper level flow currently across much of the
north Pacific will begin to buckle toward the end of the day 1
period, resulting in an end to the tap of the anomalous pw plume
affecting the Pacific Northwest. Prior to this, however, pw
values 2.5 to 3 standard deviations above the mean will continue
to impact the Pacific Northwest. The strong west southwest low
level flow in this anomalous pw axis will support 850-700 mb
moisture flux anomalies of 3-4+ standard deviations above the mean
and IVT values of 600-800 kg/ms for most of the day 1 period, with
a weakening in the 0600-1200 utc Wed time period as the onshore
flow begins to become cutoff from the aforementioned buckling of
the large scale north Pacific flow. Max hourly rainfall rates of
.50-1" likely through most of this period through the
Washington/Oregon Cascades, Olympic Range and south through the
Oregon Coast Range into far northwest California, with areal
average precip totals of 2-5" and max totals of 6-10" in the
favored terrain regions. The National Water Model shows stream
flows rising quickly this period given the expected heavy totals,
with much of this area reaching high flow by the end of day 1.
Please see WPC's mesoscale precipitation discussion #007 valid
until 1545 UTC for additional information across this area.

Hurley/Otto/Oravec


Day 2
Valid 12Z Thu Jan 14 2021 - 12Z Fri Jan 15 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Gallina

Day 3

The Day 3 outlook will be updated by 0830Z.


$$
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FOUS30 KWBC 272000
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
259 PM EST Sat Feb 27 2021

Day 1
Valid 16Z Sat Feb 27 2021 - 12Z Sun Feb 28 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE MIDDLE MISSISSIPPI VALLEY INTO THE OHIO VALLEY...

A storm system will emerge from the Rockies and move across the
Central Plains, eventually lifting toward the Great Lakes region
through the forecast period. As a result, the trough axis across
the central U.S. will strengthen paving the way for mid-level
shortwaves to move atop a residual surface boundary draped across
the region. In addition, strong return flow ahead of the digging
trough will promote deep moisture transport which is evident by
precipitable water values climbing over 1.5 inches which is around
2.5 standard deviations above the mean. This moisture feed will be
aided by 40-50+ knot low level southwesterly winds into a
strengthening frontogenetic zone within the right entrance region
of an upper jet streak. As the aforementioned residual boundary
lifts north as a warm front, isentropic ascent will become
maximized later this evening (28/06Z) with elevated convection
likely. Therefore, expect multiple rounds of showers and isolated
thunderstorms to focus along and north of the boundary which will
be positioned from the Mid-MS Valley into the OH Valley. While
hourly rain rates may struggle to reach more than 1 inch (12Z HREF
neighborhood probabilities >1"/hr were no higher than about 30%,
and mostly around 10-20%), it is the potential for training that
could result in 3 hourly rainfall totals approaching flash flood
guidance (2 inches) in some locations.

Overnight rainfall (1-3") generally fell within the southern
portion of the Marginal Risk area (southwestern TN to southeastern
KY) but farther north within the Slight Risk area recent 7-day
rainfall has only been around 0.1" (with recent snow melt as
well). Rain rates may overwhelm the antecedent dry conditions
resulting in excess runoff and thus scattered flash flooding in
some locations exacerbated by high soil moisture. 12Z CAM guidance
still shows a north-south (and even east-west) displacement in the
heavy rain axis (or axes), but the inherited Slight Risk accounts
for this uncertainty to focus on the areas of best agreement along
with where better forcing/instability will reside. Did nudge the
eastern end a bit farther east based on the newer models and lower
FFG.

Fracasso/Pagano


Day 2
Valid 12Z Sun Feb 28 2021 - 12Z Mon Mar 01 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE LOWER MISSISSIPPI VALLEY, THE TENNESSEE VALLEY, AND INTO THE
ALLEGHENY MOUNTAINS...


20z update:
As alluded to in this morning's discussion below, there was a
trend toward a slightly faster solution with respect to the
northern stream shortwave. This allowed for slightly faster
progression with convection along the front across the Ohio River
Valley, reducing totals slightly. As such very small adjustments
eastward were made, including better/deeper moisture flux bleeding
through the Cumberland Plateau into Western Virginia.

Further south, the trailing cold front will slow across Middle
Tennessee a bit, increasing the potential for flooding across
saturated areas from recent rainfall, and additional 2-3" totals
are expected, however in very narrow swaths associated with
stronger embedded cores along the front. With flattening
mid-level flow but continued isentropic ascent/upstream SSW 850mb
moisture flux, backbuilding/redeveloping cells are likely, posing
the greatest risk for isolated rainfall streaks with flash
flooding potential through Saturday evening before upstream flow
weakens and propagation vector toward the east increases by
overnight Monday. As such, the trend continued for slightly
northward but also more scattered/streaky heavy rainfall
footprints across the Tennessee Valley into Lower MS Valley.

Gallina

---Earlier discussion---
Troughing across the central U.S. will maintain deep southerly
flow across the region through the forecast period. Mid-level
shortwaves will continue to round this trough, moving atop two
distinct surface boundaries which will act as the focus for
multiple rounds of showers and thunderstorms. Early Sunday
morning, a warm front will be draped across the OH Valley/Central
Appalachians bringing the first round of heavy precipitation to
the region. As the cold front advances east from the MS Valley
through the day, heavy precipitation will develop along or just
ahead of the boundary.

Strong moisture feed and return flow from the Gulf of Mexico will
mean a surge in precipitation water values to over 1.5 inches
which is around 2.5 standard deviations above the mean. This
moisture transport will be aided by 35-45 knot low level
southwesterly winds into a strengthening frontogenetic zone within
the right entrance region of an upper level jet streak. Therefore,
large scale lift and deep layer moisture will not be limited.
Though, it should be noted that instability may be fairly modest
given only marginal convergence, some cloudiness and weak
mid-level lapse rates.

So, while most of the key ingredients for heavy rainfall clearly
exists, it is the multiple rounds of heavy rain across the OH
Valley/Allegheny Mountains and training/back building along the
aforementioned cold front that could lead to rain rates exceeding
flash flood guidance in some locations. High resolution models are
indicating rain rates could exceed 1.5 inches/hour, mainly
associated with the cold front across the TN Valley and into the
Deep South. The front is expected to be fairly progressive which
should help limit the extent of flash flooding. Regardless, based
on the 00Z guidance, the areas of greatest concern for flash
flooding will extend from the Lower MS Valley (where some
locations received upwards of 4 inches of rain within the last 24
hours) across the TN Valley and into the OH Valley/Allegheny
Mountains (where FFG is as low as 0.75 inches in 3 hours). This
is also supported by ensemble exceedance probabilities.

Given some lingering model uncertainty, anticipate there will be
modifications made to the extent and position of the Slight Risk
area. We will continue to monitor the latest model trends and
modify accordingly.

Pagano

$$
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FOUS30 KWBC 281956
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
255 PM EST Sun Feb 28 2021

Day 1
Valid 16Z Sun Feb 28 2021 - 12Z Mon Mar 01 2021

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE TENNESSEE VALLEY, LOWER OHIO VALLEY AND INTO SOUTHERN WEST
VIRGINIA...

Ongoing heavy rain and flooding event will continue through the
day, focused most over Kentucky into West Virginia. Two-day
rainfall totals of 1-4" across KY into West and Middle TN (esp
around Nashville and points N/NW). 12Z soundings show precipitable
water values 1.25-1.50" from LA to the Ohio River which is both
1.5 to 3 sigma above normal and near record values for the time of
year (large area of 90th-95th percentile). Surface warm front over
the region will continue to support locally heavy rain (rates
1-2"/hr) in the warm sector via embedded convection with
additional convective elements much farther southwest over AR.
With 850-500mb flow within a narrow range (230-240 degrees, 12Z
BNA sounding) and continued moisture transport ahead of the cold
front back through Missouri, flash flood threat will continue in
the Moderate Risk area through at least the late afternoon/early
evening. See WPC MPD #031 for additional synopsis. Flash flood
guidance has lowered in response to recent rains and is below
1"/3h over southeastern KY into much of WV. There, instability
will be lower (elevated) on the cool side of the boundary but
guidance shows a swath of 0.75-1.25" rainfall which will
exacerbate any flooding.

Farther southwest into warm sector, convection will increase later
today around the ArkLaTex through AR within the SPC Slight Risk
area where instability is highest (1000-2000 J/kg CAPE, LI < -4
and Tds in the upper 60s). FFG values were higher here as recent
rainfall has been under about 0.5" southwest/south/southeast of
LIT. Maintained the Slight Risk area here into MS and northern AL
as convection will move eastward this evening and rainfall rates
may be high enough to cause flooding issues. Activity should
diminish after 06Z. Northern part of afternoon convection is
forecast to lift northeastward back through West TN this evening
coincident with (or ahead of) a weak area of low pressure along
the cold front, providing additional rainfall over repeating
areas. Training convection through the day will lead to rainfall
totals over 3" to perhaps 5" in local spots which will cause
runoff on already saturated soils. Cold front will finally push
into western KY around 00Z which will start to guide the rainfall
eastward.


Fracasso


Day 2
Valid 12Z Mon Mar 01 2021 - 12Z Tue Mar 02 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF EASTERN TEXAS THROUGH CENTRAL MISSISSIPPI...

20z update:

Guidance trends including 12z Hi-Res CAMs continue to support the
stationary inflection point and associated downstream 850-7H,
further tightening the overall forecast placement and magnitudes
from 01.18z through 02.06z before the upstream shortwave deepens
the 850mb low and starts the eastward progression. Still deep
layered steering flow supports a downstream area of rainfall
across N LA into Central MS. Stronger early morning instability
from the western Gulf overnight isentropically ascends supporting
some increased rainfall efficiency as the 850mb low tracks
eastward along this preconditioned footprint along/south of I-20.
This remains generally south of the lowest FFG axis, so becoming
more confident in tightening the margins of the Marginal Risk area
but with 2-3" areal totals expected generally at or slightly below
the higher FFG values, thinking flash flooding risk is still
generally isolated and the Marginal category is appropriate at
this time.

Gallina

Prior Discussion
~~~~~~~~~~~~~~~~~~~~~
A cold front dropping south from the Tennessee Valley will leave a
frontal zone draped across the Deep South to start the forecast
period (Monday morning). Meanwhile, a strengthening closed
mid-level shortwave over the High Plains of TX advances east.
This will promote deep southwesterly flow atop the seemingly
shallow surface boundary. Isentropic ascent is expected to
continue ahead of the approaching low through Monday which should
provide continual precipitation along and north of the surface
boundary from east TX into LA/AR. Better forcing for ascent will
arrive later Monday as the mid-level low approaches with a
resurgence of anomalous deep layer moisture.

Multi-model signal exists for periods of moderate to heavy
rainfall to occur across the Lower MS Valley region. The key
ingredients are present including precipitable water values
(1.5-1.75 inches) that are 2.5 standard deviations above the mean,
modest instability (MUCAPE averaging around 500 J/kg) and
strengthening synoptic scale lift ahead of the closed mid-level
low. However, there is a bit of uncertainty with respect to the
placement of the aforementioned surface boundary, the interaction
with the approaching shortwave and residual cloudiness which will
heavily influence the QPF amounts and placement. Regardless, most
00Z guidance is illustrating 2-3+ inches of rain that extends from
eastern TX into portions of northern LA/southern AR. While
instability is not all too impressive, training could result in
higher hourly rainfall totals (potentially approaching 1 inch) in
some locations, especially through Monday afternoon/evening.

An additional concern is that the NAM and RGEM still depict a
stronger shortwave ridging between the exiting shortwave and the
approaching one in TX. This deflects the moisture stream and
returns the warm front further north providing a potential for
stronger overrunning elevated thunderstorms with higher rates
nearer the Arklatex, where hydrological conditions are much lower,
posing a slightly higher risk for excessive rainfall.

At this time, taking into account all available model guidance,
the latest WPC QPF footprint shows the highest values just south
of where the heaviest precipitation occurred over the past 48
hours. Therefore, after careful review of the antecedent
conditions and in coordination with the local offices, a Marginal
Risk was retained/modified with the assumption that soils could
withstand the expected precipitation and rates. However, it
should be noted that with dormant vegetation, runoff could be a
flooding concern no matter where the heavy precipitation occurs.

Based on precisely where the heaviest
rainfall/training/backbuilding signal sets up closer to Day 1, an
upgrade to a Slight Risk may be warranted.

Gallina/Pagano


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FOUS30 KWBC 121947
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
345 PM EDT Wed May 12 2021

Day 1
Valid 16Z Wed May 12 2021 - 12Z Thu May 13 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE CENTRAL GULF COAST...

1600 UTC update

Only some minor changes made to the marginal risk area along the
central Gulf coast. The previous area was trimmed slightly to the
south and slightly on the eastern end over the Florida panhandle
based on latest hi res guidance and where heaviest rains fell over
the past 24 hours.

Oravec

0900 UTC discussion

In general not expecting any widespread heavy rainfall today, and
the flash flood risk should stay rather low. The ongoing
convection is expected to remain progressive as it tracks across
the Gulf Coast this morning, and the upstream environment behind
this activity is pretty worked over. Thus not anticipating any
development too organized behind this ongoing line. However, the
actual surface front is hanging back, so the central Gulf Coast
will remain south of the main frontal boundary for much of the
day. Can not rule out some development closer to this front today
into this evening, and given the wet antecedent conditions,
localized heavy rainfall with any cells may result in an isolated
flash flood risk. If it were not for the antecedent conditions, no
risk of excessive rainfall would be needed today. But given the
increasingly saturated ground and front hanging back...the safe
option is to maintain the Marginal for now, in case some
redevelopment is able to occur over any more sensitive locations.

Chenard


Day 2
Valid 12Z Thu May 13 2021 - 12Z Fri May 14 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF EXTREME SOUTHEAST FLORIDA...

At the start of the forecast period (13.12z), shortwave currently
crossing the central Gulf Coast will be exiting well off the NE
Florida coast but supporting slow southward building of the high
and sag of the frontal zone across central FL. By midday,
filtered insolation and weak southerly surface flow should support
increasing instability along/ahead of the front but also mild
capping to build up instability. Strengthening northeasterly flow
will sharpen the front and increase sfc to boundary layer moisture
flux convergence to develop strong thunderstorms. Additionally,
the tail end of 3H jet will be oriented favorably for right
entrance divergence and larger scale ascent/outflow to enhance
clustered updrafts.

GFS/ECMWF soundings and TPW forecast depicts west to east deep
moisture axis ideally oriented for mid-cloud entrainment to
accompany low-level flux suggesting 1.75-2" values and strong flux
convergence to support very efficient rainfall production for
thunderstorms after 21z, likely focused along the eastern coast
where sfc flow is enhanced due to sea-breeze/onshore flow. As
such rainfall rates in excess of 2-3"/hr are possible. Weak
shear/mean layer flow may support greater than desired pulse
nature but there are suggestions of weak organized clusters that
would support very slow motions for high rainfall totals in 1-2
hours. 12z HREF probabilities support this with 2"/hr rates in
the 30% range but also 6hr totals of 3" and 5" at 60-70% and
25-30% respectively (mostly between 21-06z).

While, ground conditions are dry and FFG is in typical very high
range, Hi-res CAMs and other global guidance are honing in on best
forcing to near enough to Southeast Florida urban centers. Given
the magnitude of rates and potential 3-5" localized totals, there
is sufficient confidence to support a Marginal Risk of Excessive
Rainfall across the urban corridor of southeast Florida.
Potential for thunderstorms over current Day 1 period could even
increase localized rapid inundation flooding risk and will be
monitored for tonight's update moving into the Day one period.

Gallina


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FOUS30 KWBC 222000
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
359 PM EDT Sat May 22 2021

Day 1
Valid 16Z Sat May 22 2021 - 12Z Sun May 23 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
TEXAS, THE HIGH PLAINS, AND THE SOUTHERN/CENTRAL ROCKIES...

...Central/Eastern Texas...
A weak surface low pressure system will continue to drift
northwest between the weakening trough in the West and ridge in
the East. Still anticipate convection to become more widespread
through the afternoon with localized multiple rounds of heavy rain
and isolated flash flooding.

The retrograding eastern ridge will continue to shift the best
moisture transport west from western LA to east TX through the
early afternoon. Precipitable water values will increase to around
2 inches (aided by 20-30 knot southeasterly low level flow) which
is over 2.5 standard deviations above the mean. Coincident with
this transition will be diurnal heating leading to better
instability through the afternoon. MUCAPE values will range
between 500-1000 J/kg. Thus, anticipate convection to blossom with
rain rates exceeding 1.5 inches/hour. Areal average precipitation
will range from 0.5-1.5+ inches with locally higher amounts in 12Z
guidance centered in north-central TX. Therefore, the Marginal
Risk area was expanded north a bit.


...Southern/Central Rockies and adjacent High Plains...
The trough in the west will continue to weaken as it shifts north
and east through the forecast period. Mid-level impulses will
round the trough and cross existing surface boundaries with pooled
moisture and instability helping to focus convection, especially
on the High Plains. This activity will likely continue through the
evening hours with the strengthening low level jet. Thus, multiple
rounds of heavy rain could lead to localized flash flooding.

Precipitable water values will increase across the region to
around 0.75-1.25 inches aided by southern flow (both from the
Pacific and Gulf of Mexico). Values will be 2+ standard deviations
above the mean. Instability climbing to above 2000 J/kg MUCAPE by
the afternoon with convection locally enhanced by low level
upslope flow. In addition, subtle mid-level impulses aloft will
interact with lingering surface boundaries which will likely act
as a focus for convection from the afternoon into the
evening/overnight hours. Rain rates may exceed 1 inch/hour with
multiple rounds of precipitation resulting in localized higher
hourly storm totals. Areal average precipitation will vary quite a
bit with some locations observing 3+ inches. 12Z guidance
consensus has a main QPF axis farther west over the northern
Plains, so the Marginal Risk was shifted north up to the southern
edge of the Nebraska Sandhills. Burn scars were also taken into
consideration when maintained the risk area in the CO Rockies.

Jackson/Pagano


Day 2
Valid 12Z Sun May 23 2021 - 12Z Mon May 24 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF CENTRAL/SOUTHEASTERN TEXAS...

21z Update...
...Central Texas to Middle Texas Coast...
Early morning convection today (May 22) helped to lower FFG values
exist along the Middle Texas Coast. This is generally aligned
with increasing confluence and theta-E flux across throughout the
day 2 period but significantly enhancing after 24.00z that will be
capable of intense rates over 2"/hr. This is supported by about
half of the Hi-Res CAMs and solid response out of the 12z NAM,
ECMWF and CMC solutions, providing confidence. Given this and in
coordination with local forecast offices, a Slight Risk was
introduced.

...Upper Midwest...
Hi-Res CAM and global guidance trended a tad eastward resulting in
a shift of heaviest rainfall solutions. As such, small adjustment
removing portions of western MN were taken, while bringing the
Marginal Risk Area across Southern half of WI to Lake Michigan.

...Western Dakotas...
At the start of the forecast period, 23.12z, a compact shortwave
within the parent Western closed low will be advancing northward
across W CO into the Northern High Plains. Deep moisture will be
streaming northward across the High Plains on 25-30kts of LLJ
throughout the day. Given the strong height-falls, the western
branch of the LLJ &TROWAL will develop pulling 3-4 Std. Dev of
moisture flux into the Hi-Plains of E MT. This will help to surge
the Warm Front northward.

Strong convective response to the DPVA from the shortwave should
occur by early to mid-afternoon across E MT/WY, though anomalous,
the moisture will be limited so rainfall efficiency should be
limited. As the line expands and advances east, it will intersect
the warm front and deeper moisture across W SD, where total PWats
will be nearing 1.0", so flux convergence may support rates of
1.25-1.5" toward 21-00z (and CAPEs over 1500 J/kg). Shear
profiles may support embedded rotating updrafts, which may further
enhance low level entrainment and flux convergence resulting in
sub-hourly totals of 1.5-2". Dry ground conditions are the
dominant, limiting feature across the area with NASA SPoRT 0-40cm
soil saturation ratios below 30%, these embedded streaks of higher
rainfall totals are less likely to soak into the ground with
enhanced localized run-off, perhaps resulting in small
basins/watersheds flooding in short duration. CSU First Guess ERO
suggests probabilities of exceedance for day 2 in the 15-20%
range; typically due to the methodology of training against the
GEFS across the High Plains into the West tends to yield higher
ranges of probability...it does provide sufficient confidence to
suggest a Marginal Risk of Excessive Rainfall and Flash Flood
potential exists. In agreement with local forecast offices, we
have introduced a broad Marginal Risk over the lowest FFG values
of Western North and South Dakota.

Gallina

---Prior Discussion (09z)---
...Central Texas to the Middle Texas Coast...
In the wake of a remnant low to mid-level low tracking northward
through northern Texas/Oklahoma early Sunday morning, an axis of
high precipitable water values (1.7 to 2.1 inches, or +2 to +3
standardized anomalies) will remain focused from the western Gulf
of Mexico into the middle Texas coast and portions of central
Texas. Aloft, a mid-level shear axis will remain oriented from
south to north across the region with embedded vorticity maxima.
850-300 mb mean flow is forecast to be on the weak side across
central/southern Texas, ranging between 5 and 15 kt. Meanwhile,
850 mb flow will remain roughly perpendicular to the coast and
vary between 10 and 25 kt, with the Gulf of Mexico acting as a
source region for moist and unstable air. Stronger 850 mb flow
allows for backward propagating Corfidi vectors, supportive of
backbuilding and training.

While there are differences with the degree of instability
forecast for the afternoon hours, the high moisture environment
should be able to support tall/skinny CAPE profiles as depicted by
the NAM. Forcing for ascent will be aided by smaller scale
vorticity maxima along the mid-level shear axis. The coarser
resolution models support a relative max in QPF across central
Texas to the coast with roughly 0.5 inches to 1.5 inches, but the
NAM_nest and FV3_LAM are perhaps too aggressive with smaller scale
maxima of 3-6 inches (NAM_nest locally higher). The potential for
2-4 inches on a localized basis atop wetter than average soils
supports a Marginal Risk.

...Upper Midwest...
A Marginal Risk was added to southern Minnesota, northeastern Iowa
and central/southern Wisconsin to account for the potential of
localized flash flooding. Anomalous precipitable water values will
travel northward through the Plains into the Upper Mississippi
Valley, on the west side of a ridge centered over the Tennessee
Valley. At the surface, a front will extend roughly west to east
across Minnesota into Wisconsin, with the nose of the moisture
axis intersecting the front near the Minnesota/Wisconsin border
(precipitable water values along the front of 1.5 and 1.8 inches).
Deeper-layer mean flow will be roughly parallel to the frontal
boundary which may support some repeating/localized training of
thunderstorms.

Differences in the latitude of the front remain, with the GFS and
FV3_LAM north of the remaining model consensus. 12 hour hi-res QPF
was available through 00Z/24, showing unanimous consensus for 2-4
inches locally across the Marginal Risk area. Additional heavy
rain will be possible beyond 00Z in the vicinity of the slow
moving frontal boundary. Despite the relative lack of rain over
the past two weeks, flash flood guidance values are only about 1.5
to 2.5 inches in 3 hours across a majority of the region, and
these values could be surpassed Sunday into Sunday night.

Otto

Day 3
Valid 12Z Mon May 24 2021 - 12Z Tue May 25 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE CENTRAL ALLEGHENY PLATEAU AND ALLEGHENY MOUNTAINS AND PORTIONS
OF CENTRAL TEXAS TO THE MIDDLE TEXAS COAST...

...Allegheny Plateau and Central Mountains...
At the start of the forecast period, 24.12z, a convectively
induced shortwave will be in the vicinity of the Allegheny plateau
at a distant periphery of right entrance to Polar Jet streak over
SE Canada and northern New England. The anomalous eastern U.S.
ridge will continue to retrograde west, allowing for the streamer
of deep Gulf moisture (2-3 std. dev of normal) to atop the ridge
across the central Great Lakes, intersecting with the trailing
edge of a frontal zone crossing through the Northeast. While,
this shortwave is awkwardly timed coming through the region with
respect to instability, global guidance does keep some sort of
axis aligned with the moisture axis as well. Additionally, there
is likely to be ongoing scattered convection from the day 2 period
at the start, but solid 15-20kts of WNW inflow along the moisture
axis should support ample flux and nearly orthogonal orographic
ascent. In addition, this should turn propagation vectors into
the flow suggestive of a favorable back-building environment and
given generally unidirectional steering flow to the moisture axis
could provide opportunity for back-building cells to repeat.

A limiting factor would be generally dry to very dry ground
conditions in the vicinity per 0-40cm soil saturation ratios well
below 10% according to NASA SPoRT derived product. Still, complex
terrain and potential for sub-hourly rain totals at 1.5-1.75"/hr
rates could approach the naturally lower FFG values in the area.
As such, and in coordination with local forecast offices,
introduced a Marginal Risk of Excessive Rainfall across Northeast
WV, Southwest PA and adjoining portions of Eastern OH, and Western
MD.


...Central Texas to Middle Texas Coast...
At the start of the forecast period, there is likely to be ongoing
convection within the deep moisture plume oriented SE to NNW
across the central Texas Coast toward the Dallas-Fort Worth Metro.
Deep tropically rich moisture over 2-2.25" through the column and narrow/skinny unstable profiles will provide ample moisture
loading for efficient rainfall production. Scattered 2-3" totals
in coarser global guidance is more than sufficient to result in
flash flooding concerns given the saturated ground conditions. As
such a Marginal Risk is placed along the mean moisture flux axis
across Texas, once again for the Day 3 period. An upgrade to a
higher category is once again contingent on the placement of the
heavy rainfall over the next few days and so is too uncertain to
delineate at this time.

Gallina

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FOUS30 KWBC 280805
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
405 AM EDT Fri May 28 2021

Day 1
Valid 12Z Fri May 28 2021 - 12Z Sat May 29 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS CENTRAL TO
SOUTH CENTRAL TEXAS...

...Central to south central Texas...
The surface cold front currently sinking southward into the
Southern Plains is expected to become stationary and oriented west
northwest to east southeast from eastern New Mexico into Central
Texas. Upstream shortwave energy pushing eastward from the
Southern High Plains will ride along this front and support
additional organized convection pushing southeast in the axis of
pw values 1.5-2+ standard deviations above the mean in the
vicinity of this boundary. There are some timing differences with
this convection, but general model agreement for potential of
organized activity moving southeastward late afternoon Friday into
Friday night/early Saturday. A slight risk area was added from
the previous outlook where the latest HREF neighborhood
probabilities are showing values of 50-80%+ for 2"+ totals,
40-60%+ for 3"+ totals and 5-20% for 5"+ totals this period.

...Southern to Central Appalachians into the Mid-Atlantic...
No significant changes to the broad marginal risk area depicted in
the previous outlook stretching across the Southern to Central
Appalachians into the Mid-Atlantic. The amplifying mid to upper
level trof across the Mississippi Valley will help surge a broad
region of pw values 1.5 to 2+ standard deviations above the mean
northeastward across these areas beginning this morning.
Convection likely to increase in this high pw axis along and ahead
of the associated cold front where very favorable right entrance
jet dynamics will accentuate lift. Simulated hi res radars are
showing the potential for more than one round of convection to
push eastward ahead of the front across the Southern to Central
Appalachians and into the Mid-Atlantic with .50-1"+ hourly totals
possible with each round. FFG values are lowest across the
Central Appalachians from southwest Pennsylvania, far western
Maryland, central West Virginia into portions of southwest
Virginia, with higher values south into the Southern Appalachians.
Given the strong upper dynamics and anomalous pw values, didn't
want to exclude any regions in the Appalachians where heavy rains
are possible. HREF neighborhood probabilities are very high
across the entire marginal risk area for 1"+ totals 80-90%+ and
30-70%+ for 2"+ totals this period. The marginal risk area was
maintained to the north of the west to east oriented frontal zone
expected in the vicinity of the Mason-Dixon line, across the urban
areas from Philadelphia to northern New Jersey. Rainfall rates
are not expected to be as great as across areas south of this
front, but with relatively low FFG values isolated urban runoff
issues possible from pockets of heavy overrunning rains pushing
northeast across this region.


Oravec


Day 2
Valid 12Z Sat May 29 2021 - 12Z Sun May 30 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE CENTRAL & SOUTHERN HIGH PLAINS AND SOUTHEAST TEXAS...

...Southeast Texas into far Southwest Louisiana...
Global guidance suggests there may be a weakening ongoing
convective complex in the vicinity of Central Texas at the start
of the forecast period, 29.12z. The GFS and ECMWF continue to
support an MCV traveling southeast along the trailing trof/frontal
zone that is draped across the Northwest Gulf and coastal
southeast Texas/southwest Louisiana. Ample remaining instability
and moisture pooled along the boundary may help to feed any
ongoing thunderstorms, but even the strongest solutions suggest
weakening (5-15kts) of southerly component inflow from the south
for isentropic ascent along any outflow induced boundaries from
the complex. Hi-Res CAMs are nearly vacant of convective
activity, providing reduced overall confidence in the global
solution's manifesting. However, if there are cells, they are
likely to be efficient rainfall producers with 1.75" of total PWAT
but given weaker flux convergence, they may not be all that
excessive; however, they will be moving across compromised soils
where 2-week precip anomalies per AHPS remain 300-500% of normal.
Even though there has been some recent improvement per NASA SPoRT
0-40cm soil saturation, all the factors suggest maintaining a
Marginal Risk mainly for the earliest time periods on Saturday
morning into the afternoon.

...Central and Southern High Plains...
The base of a northern stream trof axis will be crossing out of
the northern High Plains, though weak lingering positive tilt
troughing will exist over the Four Corners. This will support
southerly low level flow across the Central Plains, while also
increasing return moisture across the Southern High Plains by the
end of the forecast period. As the northern shortwave progresses
northward, the trailing frontal zone will drop south along the
intersection of the High Plains, maximizing moisture flux
convergence with its slow southward progression; combine this with
diurnal upslope components and insolation to support a ribbon of
increased instability across the Central High Plains. Convection
will develop in the mid-afternoon/early evening. Clusters nearest
the frontal zone may slow a tad and build up-scale for localized
heavy rainfall given slower mean flow and cell motions. While some
Hi-Res CAMs are particularly aggressive and result in areal 2-4"
totals, there is typical placement uncertainty. Lower FFG values
across the Southern Platte River Valley and generally well above
normal soil saturation, suggest a Slight Risk may be needed
further north.

However, strong low level flow, may reduce overall totals as
clusters may propagate further south, faster with general eastward
march off the higher terrain after night-fall. Additionally,
stronger moisture return that was easterly through the day will
veer slowly over the southern High Plains of NM/and the TX/OK
panhandles. As such, the focus for stronger upscale enhancement
continues to shift south into early Sunday morning. Again, some
solutions such as the NAM-Conest and FV3CAM show extreme rain
totals, but this is typical bias, though UKMET/ECMWF and recent
NAM solutions suggest better clustering further south too with
2-3" areal average QPF values. Overall, these model solutions
should result in a Slight Risk, particularly given the higher than
normal soil saturation; however, the precise placement and pace of
southward propagation provides too much uncertainty to place a
Slight Risk category at this time.

Gallina


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FOUS30 KWBC 310740
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
339 AM EDT Mon May 31 2021

Day 1
Valid 12Z Mon May 31 2021 - 12Z Tue Jun 01 2021

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
NORTH CENTRAL TEXAS...

...North Central Texas into Central to Eastern Oklahoma...
A well defined MCV and associated surfaced low expected to push
eastward today from northwest Texas across southern to central
Oklahoma. There is potential for widespread heavy rains to the
east and northeast of the MCV across much of Oklahoma and along
and ahead of the trailing cold front extending to the southwest of
the MCV/surface low across north central Texas. Heavy rains just
ahead of and to the northeast of the MCV should be progressive to
the east this period. Concerns for slower moving and training of
convection will be along the trailing frontal boundary where
southerly to south southwesterly low level flow in an axis of PW
values 1.5-2+ standard deviations above the mean will impact the
frontal boundary. A moderate risk area was added from the
previous outlook for this period to portions of north central
Texas where the greatest potential exists for slow moving or
training of cells along this front. The moderate risk encompasses
the area where the latest HREF neighborhood probabilities are high
for 3"+ totals this period (40-80%) and 10-50% for 5"+ totals.
This is approximately along the I-20 corridor from Abilene to
Dallas and northeast of that to the OK/TX border. WPC qpf leaned
toward the HREF blended mean and HRRR from 0000 UTC, trending
faster to the southeast than the previous forecast. This has
resulted in the slight risk area also extending farther to the
southeast from the previous outlook.

...Middle Rio Grande of Texas...
A slight risk area was added across the middle Rio Grande Valley
region of Texas where much of the latest hi res guidance is
showing the potential for an axis of heavy rains pushing
southeastward parallel to the middle Rio Grande Valley in the 0000
to 1200 UTC time period Tuesday. This convection is on the
southwest side of the frontal convection pushing across north
central Texas. Moist south southeasterly flow ahead of this
convection will support it pushing fairly quickly to the
southeast. HREF neighborhood probabilities, however,are fairly
high for 2 and 3"+ totals, 40-60% and 30-50% respectively,
warranting a slight risk across this area.

Oravec



Day 2
Valid 12Z Tue Jun 01 2021 - 12Z Wed Jun 02 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
ARKANSAS AND SOUTHERN MISSOURI...

...Southern Missouri to northeast Texas/western Louisiana...
It is expected that a mature and slowly decaying MCV will be
crossing central OK at the start of the forecast period with
ongoing moderate to locally heavy showers extending northeast
along the best convergence with the deformation zone, supported by
right entrance ascent pattern across the lower Missouri River
Valley. Low level flow response will be increasingly confluent as
main Pacific stream of deeper moisture flow across Northeast
Mexico into central TX will be converging with a surge of return
moisture flow out of the central Gulf. Eventually, this
confluence will support deeper unidirectional southwesterly
steering flow supporting WAA across Arkansas ahead of a subtle
developing surface wave along the Red River. This WAA will help
to support solid moisture flux across Arkansas and into south
central MO, and allow for modest instability to build through the
late morning to midday generally about 1000-1500 j/kg. While,
flux is strong, low level flow is likely to be fairly oblique to
the frontal zone yielding scattered pockets of convective
initiation. Still, deep unidirectional flow with less than 10kts
for propagation vectors and solid low level jet may support
backbuilding and training convective cores.

Slow eastward motion of the frontal zone is expected as the MCV
shears into the northern flow, but by late evening, upstream
northern shortwave will be digging across the Central Plains and
in combination with subtropical speed max, could support further
enhancement of the 85H wave and increase overall moisture
convergence for increasing coverage of fairly efficient rainfall
producing cells. Hourly rate/totals of 1.5-2" are possible.
Combine that with saturated ground conditions particularly across
the western half of AR into SW MO/E OK: 80-90% 0-40cm ratios per
NASA SPoRT soil saturation product and 150-300% of normal
precipitation anomalies from AHPS supports an upgrade to Slight
Risk across the intersection of saturated grounds and favorable
atmospheric parameters. Greater uncertainty remains across
North-central Texas, where model signals suggest best
convergence/confluence may have ended and there would be a general
lull relative to areas described above. Still stationary front,
ample instability may result in more widely scattered activity to
currently keep in a narrow Marginal Risk area. 00z HREF in the
last two periods denote very strong probabilities of 50% for
3"/3hrs across the Northern Sabine River Valley, though this is
the first hint of such a strong signal, so will have to watch
closely and depending on how Day 1 rainfall unfolds and results in
compromised FFG values, a Slight may need to be expanded further
south and west.


...Big Bend/Rio Grande Valley...
Deep moisture will remain in place across the Pecos River Valley
and intersecting Rio Grande Valley with 1.75" total PWats and
clear skies will result in high instability with CAPE values
nearing 2500-3000 J/kg per GFS/ECMWF. Hi-Res CAMs are a bit more
aggressive in dropping overnight convective complex deeper into
the Rio Grande Valley. However, remaining moisture and day-time
heating is likely to result in scattered redevelopment by midday
from the Davis Mtns south through the Serranias del Burro and
adjacent Rio Grande Valley in Southwest Texas. Rainfall rates up
to 2"/hr and slow cell motions may result in widely scattered
flash flooding concerns enough to maintain the Marginal Risk area.
The Marginal Risk was expanded south along the Rio Grande a bit
further given the Hi-Res trends mainly at the start of the
forecast period.

Gallina

Day 3
Valid 12Z Tue Jun 01 2021 - 12Z Wed Jun 02 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL STRETCHING FROM
THE OHIO VALLEY TO THE LOWER MISSISSIPPI VALLEY AS WELL AS WESTERN
EDWARDS PLATEAU/RIO GRANDE VALLEY IN TEXAS...

...Ohio River Valley through lower Mississippi River Valley...
At the start of the forecast period, 02.12z Wednesday, a
sharpening mid-level trof across the mid-Mississippi Valley will
be bottoming out before lifting into the central Great Lakes by
the end of the period. The associated surface low will leave the
Tri-Rivers area moving north with an attendant frontal zone that
extends southwest into the lower Mississippi Valley into Eastern
Texas. Deep and fairly unidirectional flow parallel to the
boundary will continue to transport anomalous moisture (1.5-2
StdDev) nearing 1.5" into the OH/IND/KY Tri-state with solid flux
on 30-35kt 85H SWly LLJ. Given deep flow regime being parallel to
the boundary provides a solid potential for back-building and
training environment; however, the negative to this is limited
convergence along the boundary. The greatest of which is likely
to be very near the surface wave as it lifts north. Rainfall
rates could be efficient at 1.5"/hr but given recent dry ground
conditions and higher FFG values, it is more likely to yield
highly localized and scattered flash flooding concerns. As such a
Marginal Risk is placed at this time.

Further south into the Lower Mississippi Valley, the moisture is
higher and perhaps relatively increased instability may support
increased localized rain-rates but due to proximity to the exiting
shortwave, winds are expected to be much weaker at cloud base to
limit deeper moisture entrainment. However, rates would be higher
and given weaker flow, duration could support similar highly
localized flash flooding concerns, particularly over most
saturated ground conditions over LA. Still, too many
contingencies are in place at day 3 to highlight location higher
than a Marginal Risk of Excessive Rainfall at this time.


...Western Edwards Plateau into Rio Grande Valley...
It is probable that a convective complex and associated boundaries
will be placed across southwest Texas from the Day 2 period, well
in advance of the stalling/tail-end of the frontal zone as it sags
southward across the Permian Basin/Edward Plateau. While the
initial complex is likely to overturn some of the more
unstable/deeper available moisture in the Rio Grande Valley, there
is solid model agreement in return response through the
morning/early afternoon to support 1.6-1.75" of total PWats and up
to 2000 j/kg CAPEs along/south of the frontal zone across the
lower Pecos Valley and Edwards Plateau. Solid moisture
convergence from 15-20kts of flow back up the valley should be
sufficient to break out scattered convection. Weak steering flow,
should allow for southeast propagation across areas that have
received well above normal rainfall over the last few days, with
more to come in the short-term forecast. Though not all guidance
suggests a stronger secondary complex late Wednesday into
Thursday, the 00z NAM/GFS and Gem-Regional all suggest potentially
solid totals of 3-4" possible, which is concerning. Still, there
is much to unfold and fairly large uncertainty for this to occur
though the environment parameters may support it. As such, in
coordination with local forecast offices, have placed a Marginal
Risk across the areas of lowest FFG values, along/south of the
frontal zone.

Gallina

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