FOUS30 KWBC 142030
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
329 PM EST Thu Jan 14 2021

Day 1
Valid 16Z Thu Jan 14 2021 - 12Z Fri Jan 15 2021

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

Fracasso


Day 2
Valid 12Z Fri Jan 15 2021 - 12Z Sat Jan 16 2021

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

12Z model guidance came in a bit slower than previous runs with
the low pressure system in the Northeast, and localized rainfall
amounts of 2-3 inches appear possible given strong dynamics and
weak instability. Current QPF shows max rainfall amounts east of
New York City into Connecticut. If the instability axis and QPF
shift west toward the more urban corridor in and around NYC, this
could result in localized flood issues and a Marginal Risk may be
needed. However, limiting factors include that the QPF and its
associated front are progressive, and antecedent conditions should
not contribute to flooding as the last couple of weeks have been
dry. So coordination with the OKX office led to no issuance at
this time.

Tate


Day 3
Valid 12Z Sat Jan 16 2021 - 12Z Sun Jan 17 2021

...A Marginal Risk of Excessive Rainfall Across Portions of
Coastal New England...

...2030Z Update...

The daytime update took Massachusetts out of the Marginal Risk in
coordination with BOX. As the previous discussion indicated,
antecedent conditions have been dry and streamflow is about
normal. Rainfall amounts of less than 2 inches should not cause
any concerns there. Elsewhere, the Marginal Risk was placed from
the coast of New Hamshire and Maine back toward lower elevations
of the White Mountains, where the highest QPF is and some
precipitation should fall as rain rather than snow (higher
elevations are likely to remain all or mostly snow). Some low
snowpack is in place, which could contribute to flooding concerns
with flashier streams.

Tate

...Previous Discussion...

Large/deep closed low over the Ohio Valley during the period will
promote a plume of deeper moisture sourced from the Atlantic and
Gulf across southern New England Saturday morning. As a surface
low develops/deepens over interior NY, 850 mb flow is expected to
increase in the 06Z-12Z time frame with the greatest onshore flow
arriving around 12Z-18Z between 50-60 kts. Precipitable water
values will be high for mid-January in the region, roughly between
1-1.25" and about 1.5 standard deviations above normal. IVT values
are above the 90th percentile as well. The WPC QPF was based on a
blend of the UKMET/GFS/ECMWF (GFS was still a bit faster but
trended better this cycle) which showed areal average totals of 1
to locally 1.5 inches in a relatively short period (6-12 hours).
The moisture transport is expected to move through the area
quickly, which may limit overall flooding concerns despite the
impressive/high environmental parameters.

Antecedent conditions have been drier than normal over the past 14
days and for some areas, the non-frozen soils will allow more of
the rainfall to be absorbed versus total runoff. However, the
environmental conditions and a slight uptick in QPF suggests
potential for runoff/urban flooding problems and in coordination
with BOX/GYX, a Marginal Risk was introduced for mainly the
coastal areas of New England.

Taylor

$$
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FOUS30 KWBC 152027
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
326 PM EST Fri Jan 15 2021

Day 1
Valid 16Z Fri Jan 15 2021 - 12Z Sat Jan 16 2021

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

Localized rainfall amounts of 2-3" appear possible across portions
of Long Island and CT given strong dynamics and weak instability
within an atmospheric river moving through the region. Inflow at
850 hPa is 50-60 kts, while precipitable water values are around
1" and MU CAPE values are forecast to be ~100 J/kg. This should
lead to hourly rain totals ~0.4", which is supported by the 00z
HREF probabilities of 0.5"/hour rising into the 40-50% range.
Since guidance has maintained good continuity with day shift, the
max rainfall amounts remain expected east of New York City into
Connecticut. If the instability increases a bit and higher
amounts shift west toward the more urban corridor in and around
NYC, this could result in localized flood issues and a Marginal
Risk may be needed. Limiting factors are the progressive front,
only a couple hours of heavy rainfall anticipated over urban areas
of northeast NJ/NYC with the lowest flash flood guidance values,
and a dry two week period. Coordination with OKX/the Upton NY
forecast office led to the continuation of no Marginal Risk
assigned at this time.

Roth/Tate


Day 2
Valid 12Z Sat Jan 16 2021 - 12Z Sun Jan 17 2021

...A Marginal Risk of Excessive Rainfall Exists Across Portions of
Coastal New England...

...Parts of New Hampshire into Maine...
Maintained the Marginal Risk area over parts of coastal New
England as low pressure over New York begins to occlude and spread
widespread precipitation across the region on Saturday.
Circulation around the low will draw deep-layered moisture across
the region, and southeasterly low level flow up to 70 kts at 850
mb will direct that moisture inland across the coastal areas into
the foothills and upslope region of the White Mountains.
Precipitable water values above an inch would be near record
values for mid-January according to the sounding climatology at
Gray, Maine. The strong low-level flow and high precipitable water
values will combine to yield IVT values above the 90th percentile.
This is a synoptic setup conducive for moderate to heavy rainfall
along the coast and areas as far inland as the foothills and
upslope region of the interior mountains. However, there are
several factors mitigating the risk of excessive rainfall. WPC
deterministic QPF values were generally in the 1 to 1.75 inch
range, with only part of the area near the coast and nearby
foothills expected to receive all of the precipitation in the form
of rain. It will also help that rain will not be falling on a deep
snowpack and that the system will be progressive. Rainfall rates
according to the high res models should generally remain under
half an inch an hour. As a result, the risk of flooding should be
limited to minor flooding of smaller streams and tributaries of
the mountains or flooding/ponding in regions of poor drainage or
low lying areas.

Bann/Tate


Day 3

The Day 3 outlook will be updated by 2030Z.

$$
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FOUS30 KWBC 161542
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1041 AM EST Sat Jan 16 2021

Day 1
Valid 16Z Sat Jan 16 2021 - 12Z Sun Jan 17 2021

...A Marginal Risk of Excessive Rainfall Exists Across Portions of
Coastal New England...

16z Update: Forecast looks on track. Total rainfall of 1.5"-2"
seems likely from southeast NH into portions of southern Maine.

Chenard

...Previous Discussion...

...Parts of New Hampshire into Maine...
Maintained the Marginal Risk area over parts of coastal New
England as an approaching area of low pressure spreads widespread
precipitation across the region beginning early Saturday morning.
The circulation around the low will draw deep-layered moisture/an
atmospheric river across the region, and southeasterly low level
flow of 50-80 kts at 850 mb will direct that moisture inland
across the coastal areas into the foothills and upslope region of
the White Mountains; MU CAPE will be hard to come by, 0-100 J/kg
through the period. Precipitable water values above an inch would
be near record values for mid-January according to the sounding
climatology at Gray, Maine. This is a synoptic setup conducive
for moderate to heavy rainfall along the coast and areas as far
inland as the foothills and upslope region of the interior
mountains. However, there are factors mitigating the risk of
excessive rainfall. Only part of the area near the coast and
nearby foothills expected to receive all of the precipitation in
the form of rain as interior sections are expecting heavy
snowfall. While the rain may not be falling on a deep snow pack,
it is falling on cold ground, but the system will be progressive
with heavy rainfall expected to be around four hours in duration
per the 00z HREF output. Hourly rain totals should peak around
0.4" an hour range based on the available ingredients, which was
supported by 00z HREF probabilities of 0.25" and 0.50" in an hour.
As a result, the risk of flooding should be limited to minor
flooding of smaller streams and tributaries of the mountains or flooding/ponding in regions of poor drainage or low lying areas.

Roth


Day 2
Valid 12Z Sun Jan 17 2021 - 12Z Mon Jan 18 2021

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

Bann


Day 3
Valid 12Z Mon Jan 18 2021 - 12Z Tue Jan 19 2021

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

Bann



$$
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FOUS30 KWBC 162254
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
553 PM EST Sat Jan 16 2021

Day 1
Valid 2250Z Sat Jan 16 2021 - 12Z Sun Jan 17 2021

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

Chenard


Day 2
Valid 12Z Sun Jan 17 2021 - 12Z Mon Jan 18 2021

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

Campbell


Day 3
Valid 12Z Mon Jan 18 2021 - 12Z Tue Jan 19 2021

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

Campbell

$$
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FOUS30 KWBC 170844
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
343 AM EST Sun Jan 17 2021

Day 1
Valid 12Z Sun Jan 17 2021 - 12Z Mon Jan 18 2021

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

Roth


Day 2
Valid 12Z Mon Jan 18 2021 - 12Z Tue Jan 19 2021

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

Bann


Day 3
Valid 12Z Tue Jan 19 2021 - 12Z Wed Jan 20 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER A PORTION
OF THE DESERT SOUTHWEST...

...California/Arizona Desert...
Moisture will be drawn northward across portions of the California
and Arizona desert as a closed mid-level low forms on Tuesday and
then moves off the southern California coast late Tuesday night or
early Wednesday morning. The difluent flow aloft which forms east
of the low will help set up a region where synoptic-scale lift is
favored over the desert while at the same time the flow taps
deep-layer moisture and transports it northward. The 16/12Z
Canadian and some of the blended models are very aggressive in
producing widespread moderate to heavy rainfall across the
Southwest U.S...with broad areas of rainfall in excess of half an
inch and maximum amounts in excess of 2 inches. The GFS, on the
other hand, was less bullish on amounts but still focused on the
same area. Thinking is that the atmosphere will initially need
some time to moisten up sufficiently in order to produce locally
heavy rainfall, favoring the less aggressive solutions, but model
QPF and forecast soundings suggest an increasingly favorable
environment for locally heavy rainfall late in the period. In
particular, the core of the moisture plume is forecast to have
precipitable water values between 0.75 inches and 0.9
inches...which would be roughly 1.5 standardized anomalies greater
than climatology for mid-January. And there are signs that some
modest instability develops in the low level during the afternoon
and evening...so rainfall rates could be locally enhanced enough
to produce localized run-off problems or ponding in regions of
poor drainage.

Bann

$$
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FOUS30 KWBC 172015
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
314 PM EST Sun Jan 17 2021

Day 1
Valid 16Z Sun Jan 17 2021 - 12Z Mon Jan 18 2021

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

Chenard


Day 2
Valid 12Z Mon Jan 18 2021 - 12Z Tue Jan 19 2021

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

Campbell


Day 3
Valid 12Z Tue Jan 19 2021 - 12Z Wed Jan 20 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER A PORTION
OF THE DESERT SOUTHWEST...

21Z update... The latest model guidance continues to depict low
and mid-level moisture being drawn northward as a closed low
tracks away from the southern California coast. While this is the
general consensus, the QPF associated with this feature/developing
system has noticeable spread on amounts although the general
placements are similar. The WPC amounts trended a little drier
than the previous forecast, however the setup will remain
favorable for some localized areas of moderate to possibly heavy
rainfall. The Marginal Risk area that was already in effect still
adequately encompasses the region where excessive rainfall may
occur.

Campbell

Previous discussion...

...California/Arizona Desert...
Moisture will be drawn northward across portions of the California
and Arizona desert as a closed mid-level low forms on Tuesday and
then moves off the southern California coast late Tuesday night or
early Wednesday morning. The difluent flow aloft which forms east
of the low will help set up a region where synoptic-scale lift is
favored over the desert while at the same time the flow taps
deep-layer moisture and transports it northward. The 16/12Z
Canadian and some of the blended models are very aggressive in
producing widespread moderate to heavy rainfall across the
Southwest U.S...with broad areas of rainfall in excess of half an
inch and maximum amounts in excess of 2 inches. The GFS, on the
other hand, was less bullish on amounts but still focused on the
same area. Thinking is that the atmosphere will initially need
some time to moisten up sufficiently in order to produce locally
heavy rainfall, favoring the less aggressive solutions, but model
QPF and forecast soundings suggest an increasingly favorable
environment for locally heavy rainfall late in the period. In
particular, the core of the moisture plume is forecast to have
precipitable water values between 0.75 inches and 0.9
inches...which would be roughly 1.5 standardized anomalies greater
than climatology for mid-January. And there are signs that some
modest instability develops in the low level during the afternoon
and evening...so rainfall rates could be locally enhanced enough
to produce localized run-off problems or ponding in regions of
poor drainage.

Bann

$$
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FOUS30 KWBC 181532
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1031 AM EST Mon Jan 18 2021

Day 1
Valid 16Z Mon Jan 18 2021 - 12Z Tue Jan 19 2021

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

Jackson


Day 2
Valid 12Z Tue Jan 19 2021 - 12Z Wed Jan 20 2021

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

Bann


Day 3
Valid 12Z Wed Jan 20 2021 - 12Z Thu Jan 21 2021

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

Bann


$$
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FOUS30 KWBC 202015
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
314 PM EST Wed Jan 20 2021

Day 1
Valid 16Z Wed Jan 20 2021 - 12Z Thu Jan 21 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
SOUTHERN CALIFORNIA AND SOUTHERN ARIZONA...

...Southern California/Arizona...

Upper low west of the northern Baja will continue its eastward
movement today along with its associated surface boundaries still
offshore. Low-level moisture will continue to increase through the
day from the Peninsular Ranges and especially into southern
Arizona. Recent RAP analysis showed precipitable water values of
around 0.75-1.00 inches there which will level off over southern
California but increase over southern and especially southeastern
Arizona late this afternoon into the evening as the moisture
transport on 15-25kt winds shifts focus eastward. These PW values
are about 1.5-3 standard deviations above normal (from west to
east) through the Risk area. With daytime heating and steepening
lapse rates, some marginal instability is forecast to
develop--upwards of 500 J/kg near the lower Colorado River/Yuma,
AZ area. Rounds of scattered to numerous showers and isolated
thunderstorms are forecast with maximum hourly totals between
0.5-1" expected, supported by the latest HREF probabilities. Areal
average precipitation during the period is forecast around 1-2
inches with some local amounts near 3 inches along or just south
of the Mexican border. Rain rates and areal averages could result
in flash flooding, particularly across the more susceptible
Peninsular Range this afternoon. Focus will shift to southeastern
Arizona this evening and overnight as the upper low moves across
the northern Baja and weakens.

Fracasso/Taylor




Day 2
Valid 12Z Thu Jan 21 2021 - 12Z Fri Jan 22 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE LOWER MISSISSIPPI VALLEY...

...Lower Mississippi Valley...
...20Z Update...
The axis of heavier precipitation along with the resultant higher
QPF footprint and Marginal Risk area was shifted south at this
forecast update as models are coming into better agreement on the
front position and surface low track. While models are
converging, the detail of the boundary placement could mean the
difference between one location getting over 3 inches and another
(lets say 50 miles away) seeing less than a quarter of an inch of
rain. So, ironing out these details will continue into the near
term. A lot of this stems from a weak system moving through the
South on Day 1 (through Thursday morning) ahead of a cold front
dropping south which will eventually stall across the Lower MS
Valley into the Southeast. This will be the axis by which the
next system (on Day 2) will track. Therefore, it may take some
time to best determine where the heaviest rain will occur.
Wherever this may be, multiple rounds of moderate to heavy rain
will track west to east as mid-level impulses move atop the
boundary, increasing in intensity Thursday afternoon through the
overnight hours. Localized flash flooding is possible along this
narrow corridor.

Pagano

...Previous Discussion...
The dynamics remain less than ideally aligned with a closed low
over the Desert Southwest filling and getting absorbed into the
southern stream flow...and that energy reaches the Lower
Mississippi Valley late Thursday into Friday. While the dynamics
associated with this system are not ideal, the alignment of deep
layered flow parallel to a low level boundary should be enough to
allow for a period of training that potentially results in
localized flash flooding. In addition, the backing and increasing
of low level inflow shown by the models on Thursday evening could
result in increased rainfall rates by enhancing moisture flux
convergence along the front. It is the increased hourly
precipitation that would act to whittle down the high flash flood
guidance that is currently in place. A key feature that may limit
rain rates will be marginal instability.

Areal average precipitation will range between 1-3+ inches, with a
narrow corridor of 2.5-3.5 inches likely. The exact placement of
the heavier footprint of QPF will depend on the position of the
surface front and progression of the surface low along this
boundary. While the antecedent conditions are dry, the training
as mentioned above could be enough to produce nuisance/localized
flash flooding issues. Therefore, maintained the Marginal Risk
area.

Bann


Day 3
Valid 12Z Fri Jan 22 2021 - 12Z Sat Jan 23 2021

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

Pagano

$$
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FOUS30 KWBC 212023
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
322 PM EST Thu Jan 21 2021

Day 1
Valid 16Z Thu Jan 21 2021 - 12Z Fri Jan 22 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE LOWER MISSISSIPPI VALLEY...

...Louisiana into Mississippi and southwestern Alabama...

Rainfall over the Lower Mississippi Valley will increase and
expand especially after 00Z tonight as an in situ stationary front
weakens across Louisiana. Shortwave exiting Texas will skirt
across the region as another cold front drops in from the north,
increasing moisture convergence in a west-to-east band generally
between I-10 to the south and I-20 to the north. Precipitable
water values are forecast to increase to about 1.6-1.8 inches
which is about 2.5 standard deviations above normal and near
climatological max values for late January. However, limiting
factors include a lack of instability and rather dry antecedent
conditions (7-day rainfall totals are near zero and soil
saturation only about 50% per the HRRR). 12Z HREF probabilities
show ~0% chance of 1"/hr rates but ~50% chance of >0.50"hr
suggesting a rather low threat of flash flooding (taken at face
value). Guidance still varied in N-S axis displacement and timing
from west to east, but consensus still lied near the 13Z NBM and
WPC preference with perhaps a very slight shift south (within the
noise). 12-hr QPF (00Z-12Z) may total 1.5-2.5" in/near the Risk
area with local amounts near 3" possible. Maintained the Marginal
Risk area despite the limiting factors as any underforecast
instability could contribute to higher rainfall rates than
currently advertised by the guidance.


Fracasso


Day 2
Valid 12Z Fri Jan 22 2021 - 12Z Sat Jan 23 2021

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

Pagano


Day 3
Valid 12Z Sat Jan 23 2021 - 12Z Sun Jan 24 2021

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

Pagano


$$
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FOUS30 KWBC 222004
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
303 PM EST Fri Jan 22 2021

Day 1
Valid 16Z Fri Jan 22 2021 - 12Z Sat Jan 23 2021

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

Fracasso


Day 2
Valid 12Z Sat Jan 23 2021 - 12Z Sun Jan 24 2021

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

Pagano


Day 3
Valid 12Z Sun Jan 24 2021 - 12Z Mon Jan 25 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHERN PLAINS AND LOWER/MIDDLE MISSISSIPPI VALLEY...

...20Z Update...
Based on the latest 12Z guidance, models are starting to come into
better agreement with respect to a storm system developing in the
Southern Plains by the second half of the forecast period (Sunday
night). Despite this, there is still some spread in terms of the
axis of heaviest rain as models are struggling with the amplitude
and speed of the associated trough which will inevitably impact
the surface low evolution and boundary positions. The heavy rain
that develops Sunday night will be largely dependent on the warm
frontal placement. It is clear that plenty of large scale lift
will exist (divergence aloft, mid-level vorticity and MUCAPE over
1000 J/kg) as well as deep layer moisture (precipitable water
surging to over 1.50 inches) within this region. At this point,
it is a question of where these ingredients will overlap with
training of convective cells likely. It should also be noted that
while the flash flood guidance is fairly high within this region
(2-4 inches in 3 hours) and the discrete hourly rain rates should
not exceed 1.25 inches, the training of precipitation will likely
enhance hourly totals helping to influence the soil moisture. So
with these factors in mind, shifted and extended the Marginal Risk
area from the Southern Plains toward the TN/OH Valley. Areal
average precipitation through the forecast period ranges between
1-2 inches, but fully expect these values to increase at future
updates. Also, a bulk of the precipitation could fall within a 6
hour period (Sunday night into early Monday) increasing the
potential for localized flash flooding. Given the aforementioned
model spread, expect more refinement to the risk area as models
continue to get a better handle on both the synoptic and mesoscale
details.

Pagano

...Previous Discussion...
A surface low pressure/frontal system is expected to pivot across
the Plains Sun into Mon, ahead of a potent mid-upper trough moving
across the Four Corners region and ejecting into the Southern
Plains Sun night. Developing divergence in the upper levels will
also provide forcing for potentially heavy rainfall in
south-central portions of the CONUS Sun evening/night. Moisture
should be abundant, with precipitable water anomalies 2-3 standard
deviations above normal. PW values of around 1.5" could be close
to record levels for January according to sounding climatology at
nearby sites of LZK and SHV. Additionally, instability levels are
forecast to be reasonably high for January, with CAPE amounts of
1000+ J/kg, especially across Texas farther south and west of the
Marginal Risk, where faster-moving and potentially severe
thunderstorms are more of a threat. The Marginal Risk was drawn at
the crest of the instability plume and where Corfidi vectors are
generally parallel to the 850mb moisture convergence axis, which
could lead to some training of rain over the same areas. Limiting
factors to flash flooding include the system as a whole being
reasonably progressive as the trough ejects fairly quickly, so the
duration of the heavy rain may not be terribly long. Furthermore,
streamflow and soil moisture are around normal after a drier than
average last couple of weeks. Thus while some ingredients for
flash flooding are in place, stayed with a Marginal Risk for now
with the possibility of upgrading to a Slight in later issuances
especially as we get into the timeframe of hi-res model guidance.

Tate


$$
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FOUS30 KWBC 230849
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
348 AM EST Sat Jan 23 2021

Day 1
Valid 12Z Sat Jan 23 2021 - 12Z Sun Jan 24 2021

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

Chenard


Day 2
Valid 12Z Sun Jan 24 2021 - 12Z Mon Jan 25 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHERN PLAINS AND LOWER/MIDDLE MISSISSIPPI VALLEY...

Model guidance continues to indicate a mid/upper level trough and
vorticity max will track fairly quickly across the Four Corners
region Sunday with the impulse moving into the Southern Plains by
Monday morning, causing a good amount of divergence aloft. At the
surface, a low pressure system is forecast to pivot over Oklahoma
Sunday and Sunday night, while abundant deep layer moisture
streams in ahead of a cold front tracking across the Southern
Plains. Precipitable water anomalies of 3+ with actual values of
around 1.5 inches are expected over portions of the Southern
Plains and Lower Mississippi Valley, along with instability as
MUCAPE is forecast to be over 1000 J/kg. While instability will be
greater farther south across much of Texas, convection there
should be faster moving and not produce a flash flood threat. The
Marginal Risk area was drawn along the track of the low in
Oklahoma and eastward as an 925-850 mb front sets up generally
along the MO/AR border. Training of convection as cells develop
along that boundary and track eastward could cause flash flood
issues as rain falls in a reasonably short amount of time, mainly
Sunday night. Flash flood guidance is relatively high (2-3 inches
in 3 hours with even some embedded higher values), and streamflow
and soil moisture are around normal, which should limit widespread
flash flooding, but areas that see training of storms could begin
to saturate soils and cause issues. The exact axis of heaviest
rain/training will depend on where the east-west oriented boundary
sets up, so the area of greatest threat will continue to be
monitored for future refinements.

Tate


Day 3
Valid 12Z Mon Jan 25 2021 - 12Z Tue Jan 26 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE LOWER OHIO VALLEY...

The 500 mb trough/vorticity max mentioned in the day 2 ERO
discussion is forecast to continue tracking eastward Monday into
the Midwest/Ohio Valley as the surface low moves eastward as well.
Heavy rain will continue to be a threat Monday during the day as
anomalously high moisture values (precipitable water anomalies of
3+ standard deviations) stream in ahead of the cold front into the
Middle Mississippi and Lower Ohio Valleys, along with at least
some weak instability (around 100-300 J/kg of MUCAPE). A low to
mid-level east-west oriented front should focus this rainfall and
could cause training of convection along the boundary. However,
flash flood guidance is high in this region as well, and
streamflow values are normal to much below normal after a dry last
few weeks. Thus widespread flash flooding issues are not expected,
but localized areas could see instances of flash flooding where
training aligns.

Tate

$$
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FOUS30 KWBC 232007
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
306 PM EST Sat Jan 23 2021

Day 1
Valid 16Z Sat Jan 23 2021 - 12Z Sun Jan 24 2021

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

Fracasso


Day 2
Valid 12Z Sun Jan 24 2021 - 12Z Mon Jan 25 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHERN PLAINS AND THE TENNESSEE RIVER VALLEY...

20Z Update: The northern portion of the existing Marginal Risk
area over Missouri was expanded some to account for a slight
northward trend in the 12Z guidance, and the majority of the
heaviest rainfall is expected during the 00Z-12Z Monday time
period. Some of the CAM guidance members are indicating a narrow
swath of 2 inch rainfall totals during the 6Z-12Z time, and this
would be the most likely time for any flooding concerns. In
addition, a new Marginal Risk area was added for much of southern
Tennessee for an axis of heavy rainfall associated with the nose
of the low level jet and low level warm frontogenesis early Monday
morning, with the 12Z CAM guidance suite indicating the potential
for 1 to 2 inches of rain possible in narrow swaths with the
potential for some convective training to develop. The previous
discussion is appended below for reference. /Hamrick

Model guidance continues to indicate a mid/upper level trough and
vorticity max will track fairly quickly across the Four Corners
region Sunday with the impulse moving into the Southern Plains by
Monday morning, causing a good amount of divergence aloft. At the
surface, a low pressure system is forecast to pivot over Oklahoma
Sunday and Sunday night, while abundant deep layer moisture
streams in ahead of a cold front tracking across the Southern
Plains. Precipitable water anomalies of 3+ with actual values of
around 1.5 inches are expected over portions of the Southern
Plains and Lower Mississippi Valley, along with instability as
MUCAPE is forecast to be over 1000 J/kg. While instability will be
greater farther south across much of Texas, convection there
should be faster moving and not produce a flash flood threat. The
Marginal Risk area was drawn along the track of the low in
Oklahoma and eastward as an 925-850 mb front sets up generally
along the MO/AR border. Training of convection as cells develop
along that boundary and track eastward could cause flash flood
issues as rain falls in a reasonably short amount of time, mainly
Sunday night. Flash flood guidance is relatively high (2-3 inches
in 3 hours with even some embedded higher values), and streamflow
and soil moisture are around normal, which should limit widespread
flash flooding, but areas that see training of storms could begin
to saturate soils and cause issues. The exact axis of heaviest
rain/training will depend on where the east-west oriented boundary
sets up, so the area of greatest threat will continue to be
monitored for future refinements.

Tate/Hamrick


Day 3
Valid 12Z Mon Jan 25 2021 - 12Z Tue Jan 26 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE LOWER OHIO VALLEY...

20Z Update: No major changes were needed for the existing Day 3
outlook area, except for a modest northward and eastward expansion
to account for a slight northward trend in the 12Z guidance suite.
/Hamrick

The 500 mb trough/vorticity max mentioned in the day 2 ERO
discussion is forecast to continue tracking eastward Monday into
the Midwest/Ohio Valley as the surface low moves eastward as well.
Heavy rain will continue to be a threat Monday during the day as
anomalously high moisture values (precipitable water anomalies of
3+ standard deviations) stream in ahead of the cold front into the
Middle Mississippi and Lower Ohio Valleys, along with at least
some weak instability (around 100-300 J/kg of MUCAPE). A low to
mid-level east-west oriented front should focus this rainfall and
could cause training of convection along the boundary. However,
flash flood guidance is high in this region as well, and
streamflow values are normal to much below normal after a dry last
few weeks. Thus widespread flash flooding issues are not expected,
but localized areas could see instances of flash flooding where
training aligns.

Tate/Hamrick

$$
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FOUS30 KWBC 240911
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
411 AM EST Sun Jan 24 2021

Day 1
Valid 12Z Sun Jan 24 2021 - 12Z Mon Jan 25 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE SOUTHERN PLAINS, LOWER TO MID MISSISSIPPI VALLEY INTO
CENTRAL TENNESSEE...

A quick moving system will bring locally heavy rainfall to
portions of the southern Plains into the lower to mid MS Valley
tonight into early Monday. Forcing is pretty good with this
system, with a compact shortwave ejecting northeastward, upper
level divergence in the right entrance region of the 250mb jet,
and strong low level moisture transport/convergence. In Fact, note
very good overlap in the upper level divergence maxima and low
level convergence axis over the region from 06z-12z Monday.
Moisture will also be impressive with this system, with PWs
forecast to be around record levels for late January. And while
not extreme by any means, instability certainly looks sufficient
for convection, with values over 1000 J/KG upstream over TX.
Capping within the warm sector may limit the extent of convection
over a good chunk of this instability pool, however closer to the
apex of the instability axis is where the strongest overlap of low
level convergence and upper level forcing will exist, which will
be more than enough to get convection going from far north TX into
OK after 00z.

Cells will be pretty quick moving, which combined with the
progressive nature of the system as a whole, will likely cap the
extent of any flash flood risk. However the 850mb moisture
transport and convergence axis does have some west/east broadness
to it...suggesting that as the better forcing over runs the
convergence axis we could see some brief repeat/training cells
along that corridor...probably from far north TX into
central/eastern OK, western AR and far southwest MO. All in all,
when considering the forcing, moisture and upstream instability in
place...it would seem like some heavy rainfall is likely across
this corridor tonight. In fact thought many of the 00z CAMs were
more than likely too light and too small with the areal extent of
rain over this region. Thus WPC did nudge the QPF closer to what
we were seeing in recent EC runs, along with the CMC and CMC
regional. Would not be surprised to see QPF amounts end up closer
to the higher end of the model spectrum as opposed to right in the middle...thus these QPF adjustments did bump up the
magnitude/extent of amounts.

With all that said, the flash flood risk still only appears to be
pretty localized with this event. Antecedent conditions overall
appear pretty neutral. Past two weeks have seen below normal rain,
but stream flows and soil moisture values still appear around
typical values for late January. So would think antecedent
conditions won't add to or reduce the flood risk...more of a
neutral factor for the most part. The progressive nature of the
system should generally cap the upper bound of rainfall magnitudes
in the 2-3" range, with most areas seeing 1-2". Thus for most of
the area would not expect flash flooding to be much of a
concern...but where localized training occurs we could see
isolated issues evolve...mainly in the 00z-12z time frame. Thus
this event seems to solidly fit into the Marginal risk category at
this point...indicating more of a lower end localized flash flood
risk.

We also will maintain an extension of the Marginal risk eastward
into portions of northeast AR into central and western TN. More
uncertainty here, and the magnitude of forcing and instability are
certainly not as great as what is forecast further west. However
there does appear to be a corridor of strong low level moisture
convergence that moves across this corridor on the nose of the
increasing moisture plume. Several 00z CAMs, including the 00z and
06z HRRR indicate the potential for a narrow swath of 2"+ amounts
here overnight as convective elements briefly train along the
convergence axis. Thus can not rule out a few localized flash
flood issues here, and will thus maintain the Marginal with some
adjustments based on latest guidance.

Chenard


Day 2
Valid 12Z Mon Jan 25 2021 - 12Z Tue Jan 26 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE LOWER OHIO VALLEY...

A mid-upper level trough/vorticity max is forecast to track from
the Plains Monday into the Midwest Monday night as it shears out a
bit. At the surface, a low pressure system should move eastward
across the Middle Mississippi Valley on Monday. Ahead of the
surface low will be an east-west oriented low to mid-level frontal
boundary that could provide a convergent axis to focus heavy
rainfall across portions of the Middle Mississippi and Lower Ohio
Valleys. Ample moisture is forecast to be in place streaming ahead
of a cold front, with precipitable water values reaching over an
inch, which are anomalously high by 3+ standard deviations for
this time of year. Additionally, some weak, mainly elevated
instability (around 100-400 J/kg of MUCAPE) could be in place
especially across the western part of the Marginal Risk area. All
this could lead to potential for heavy rainfall. However, flash
flood guidance is high in this region, and streamflow values are
normal to much below normal after a dry last few weeks. Thus
widespread flash flooding issues are not expected, but localized
areas could see instances of flash flooding where training of
convection aligns.

Tate


Day 3
Valid 12Z Tue Jan 26 2021 - 12Z Wed Jan 27 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR COASTAL
PORTIONS OF NORTHERN CALIFORNIA...

An atmospheric river is expected to take aim at California
beginning Tuesday night (in the day 3 period) and lasting through
much of the week (medium range period), causing the potential for
heavy rain as well as large higher elevation snow totals. This
will occur as a mid/upper-level low approaches from the Pacific
and spreads ample upper-level divergence to the northern half of
California Tuesday night. Additionally, a surface low is forecast
to move southward near the West Coast Tuesday into Wednesday, and
fronts associated with the low could cause a convergent focus for precipitation. Moisture parameters will be above normal, with
precipitable water standardized anomalies reaching 1.5-2 Tuesday
night and integrated water vapor transport above the 90th
percentile, even 97.5th percentile at times, according to the
Ensemble Situational Awareness Table. Through Tuesday night,
instability does not look to spread onshore much at all, which
could limit higher rain rates. However, moisture and mass field
parameters could be sufficient to cause flooding/flash flooding
issues across lower elevations of the northern half of California
beginning Tuesday evening. Flash flood risk will be heightened
across burn scar areas. One of the hazardous issues with this
event will be its persistence over California as vorticity reloads
within the Pacific upper low, keeping the moisture inflow going in
the medium range period, so expect additional Excessive Rainfall
Outlooks for this event as it phases more into the short range.

Tate

$$
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FOUS30 KWBC 242100
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
359 PM EST Sun Jan 24 2021

Day 1
Valid 16Z Sun Jan 24 2021 - 12Z Mon Jan 25 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE SOUTHERN PLAINS, LOWER TO MID MISSISSIPPI VALLEY INTO
MIDDLE TENNESSEE...

A progressive system will bring locally heavy rainfall to portions
of the southern Plains into the lower to mid MS Valley tonight
into early Monday. Supportive large-scale forcing (shortwave
ejecting northeastward, upper level divergence in the right
entrance region of the 250mb jet, and strong low level moisture transport/convergence) will maximize overnight as low pressure
deepens over central Oklahoma. Good overlap in the upper level
divergence maxima and low level convergence axis over the region
from 06z-12z Monday will coincide with well above normal moisture
levels (PWs forecast to be around record levels for late January),
though this is at the "driest" part of the year climatologically.
Instability looks sufficient for convection with values over 1000
J/KG upstream over TX aligning with the SPC Slight Risk area of
severe storms. Capping within the warm sector may limit the extent
of convection over a good chunk of this instability pool, however
closer to the apex of the instability axis is where the strongest
overlap of low level convergence and upper level forcing will
exist, which will be more than enough to get convection going from
far north TX into OK after 00z.

Cells will be pretty quick-moving which, combined with the
progressive nature of the system as a whole, will likely cap the
extent of any flash flood risk. However the 850mb moisture
transport and convergence axis does have some west/east broadness
to it...suggesting that as the better forcing over runs the
convergence axis we could see some brief repeat/training cells
along that corridor...probably from far north TX into
central/eastern OK, western AR and far southwest MO. All in all,
when considering the forcing, moisture and upstream instability in
place...it would seem like some heavy rainfall is likely across
this corridor tonight. 12Z CAMs suggest at least two separate,
perhaps competing, areas of embedded higher rainfall and rain
rates within the broad Marginal area on par with the global
guidance even given the inherent resolution differences. With the
overall favorable setup, preferred to lean toward the higher end
of the guidance overall.

However, the flash flood risk still only appears to be pretty
localized with this event. Antecedent conditions overall appear
pretty neutral. Past two weeks have seen below normal rain, but
stream flows and soil moisture values still appear around typical
values or a bit below for late January. The progressive nature of
the system should generally cap the upper bound of rainfall
magnitudes in the 2-3" range, with most areas seeing 1-2" in the
heavier sub-areas within the Marginal Risk outline. Highest 12Z
HREF prob > 1"/3h generally do not overlap in successive periods,
highlighting the progressive nature. This event seems to solidly
fit into the Marginal Risk category with a lower-end and localized
flash flood risk.

Into Middle Tennessee, still some uncertainty with heavier
rainfall as the magnitude of forcing and instability will be less
than farther west. However there does appear to be a corridor of
strong low level moisture convergence that moves across this
corridor on the nose of the increasing moisture plume. CAM
guidance has wavered but convective elements could briefly train
along the convergence axis and cannot rule out a few localized
flash flood issues there with 3-h FFG around 2"/hr and 12Z HREF
probs non-zero.

Fracasso/Chenard


Day 2
Valid 12Z Mon Jan 25 2021 - 12Z Tue Jan 26 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM PORTIONS OF
THE LOWER OHIO VALLEY INTO THE CUMBERLAND PLATEAU AND NORTHERN
TENNESSEE VALLEY...

2030 UTC Update -- For the most part, the Day 3 ERO remains on
track. Have expanded the Marginal Risk a bit farther south, to
include northern TN. This region will get into the warm sector as
the surface warm front lifts north by 18Z, thus additional
convective activity thereafter will likely hold off until the cold
front approaches. While progressive and likely narrow any line of
convection (pre-frontal and/or along the front) will have the
potential for more robust hourly rainfall rates given pockets of
500-750+ j/kg MUCAPEs per the guidance. Moreover, the latest 1
hour FFGs are now around 1" over much of northeast TN and
southwest VA, which may be come down a bit more from the more
widespread, elevated convection ahead of the warm front.

0830 UTC Discussion below..

A mid-upper level trough/vorticity max is forecast to track from
the Plains Monday into the Midwest Monday night as it shears out a
bit. At the surface, a low pressure system should move eastward
across the Middle Mississippi Valley on Monday. Ahead of the
surface low will be an east-west oriented low to mid-level frontal
boundary that could provide a convergent axis to focus heavy
rainfall across portions of the Middle Mississippi and Lower Ohio
Valleys. Ample moisture is forecast to be in place streaming ahead
of a cold front, with precipitable water values reaching over an
inch, which are anomalously high by 3+ standard deviations for
this time of year. Additionally, some weak, mainly elevated
instability (around 100-400 J/kg of MUCAPE) could be in place
especially across the western part of the Marginal Risk area. All
this could lead to potential for heavy rainfall. However, flash
flood guidance is high in this region, and streamflow values are
normal to much below normal after a dry last few weeks. Thus
widespread flash flooding issues are not expected, but localized
areas could see instances of flash flooding where training of
convection aligns.

Hurley/Tate

Day 3
Valid 12Z Tue Jan 26 2021 - 12Z Wed Jan 27 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR COASTAL
PORTIONS OF NORTHERN CALIFORNIA...

2030 UTC Update -- Expanded the Marginal Risk area farther south
to include the River and Dolan Burn Scar areas south of Salinas,
based on the trends with some of the global QPFs. The ECMWF,
UKMet, and the NBM all reflect a more progressive southern
migration of the Atmospheric River into the central CA coast
between 06-12Z Wed.

0830 UTC Discussion below...

An atmospheric river is expected to take aim at California
beginning Tuesday night (in the day 3 period) and lasting through
much of the week (medium range period), causing the potential for
heavy rain as well as large higher elevation snow totals. This
will occur as a mid/upper-level low approaches from the Pacific
and spreads ample upper-level divergence to the northern half of
California Tuesday night. Additionally, a surface low is forecast
to move southward near the West Coast Tuesday into Wednesday, and
fronts associated with the low could cause a convergent focus for precipitation. Moisture parameters will be above normal, with
precipitable water standardized anomalies reaching 1.5-2 Tuesday
night and integrated water vapor transport above the 90th
percentile, even 97.5th percentile at times, according to the
Ensemble Situational Awareness Table. Through Tuesday night,
instability does not look to spread onshore much at all, which
could limit higher rain rates. However, moisture and mass field
parameters could be sufficient to cause flooding/flash flooding
issues across lower elevations of the northern half of California
beginning Tuesday evening. Flash flood risk will be heightened
across burn scar areas. One of the hazardous issues with this
event will be its persistence over California as vorticity reloads
within the Pacific upper low, keeping the moisture inflow going in
the medium range period, so expect additional Excessive Rainfall
Outlooks for this event as it phases more into the short range.

Hurley/Tate


$$
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FOUS30 KWBC 161551
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1050 AM EST Tue Feb 16 2021

Day 1
Valid 16Z Tue Feb 16 2021 - 12Z Wed Feb 17 2021

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

...16Z Update...
No changes needed for the Marginal Risk in southeast Florida as
showers and thunderstorms sink south ahead of a cold front moving
through the region.

Pagano

...Previous Discussion...

A marginal risk area was introduced over coastal southeast Florida
from approximately PBI to HST. An axis of above average pw
values, 2+ standard deviations above the mean,expected along and
ahead of the surface cold front expected to push across southeast
Florida Tuesday. The hi res consensus is for active convection
along and ahead of this front Tuesday morning into possibly early
afternoon. Precip totals across southeast Florida have been
mostly below average over the past week or two. However, the
latest HREF probabilities show 60-80% chance for 2"+, 40-50%
chance for 3"+ and 10-15% chance for 4"+ totals this period across
southeast Florida. Concerns are primarily for urban runoff issues
where slow moving or training cells may occur for a short period
Tuesday morning into early afternoon.

Oravec


Day 2
Valid 12Z Wed Feb 17 2021 - 12Z Thu Feb 18 2021

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

Bann


Day 3
Valid 12Z Thu Feb 18 2021 - 12Z Fri Feb 19 2021

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

Bann


$$
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FOUS30 KWBC 172034
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
333 PM EST Wed Feb 17 2021

Day 1
Valid 16Z Wed Feb 17 2021 - 12Z Thu Feb 18 2021

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

Pagano


Day 2
Valid 12Z Thu Feb 18 2021 - 12Z Fri Feb 19 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE SOUTHEAST AND CAROLINAS...

No wholesale changes were needed to the Marginal Risk of excessive
rainfall located across parts of the Southeast and Carolinas
during the Thursday/Thursday night timeframe, as the forecast
remains on track. The synoptic pattern shows mid to upper
troughing centered over the Plains early Thursday shifting
eastward into the Mississippi/Ohio/Tennessee Valleys Thursday
night. At the surface, a low pressure system will track
northeastward from the western Florida Panhandle into southern
Georgia and toward the Mid-Atlantic coast. This pattern should
provide strong moist inflow from the Gulf of Mexico across the
Southeast. Precipitable water anomalies should be around 2.5
standard deviations above normal. Per the NAEFS Ensemble
Situational Awareness Table, specific humidity values above their
99th percentile and nearing their maximum values at 700, 850, and
925 mb are likely, with integrated water vapor transport above the
99th percentile as well. At least some instability will be in play
in terms of MUCAPE, though model guidance differs in terms of
amounts, with ECMWF runs showing higher MUCAPE values of 1000 to
nearly 2000 J/kg compared to other guidance like the GFS runs
(~500-1000 J/kg). The amount of instability will matter for the
rain rates, and one reason we opted not to upgrade to a Slight
Risk of excessive rainfall for this issuance was because of the
uncertainty in rain rates exceeding flash flood guidance. However,
antecedent conditions over the region are quite wet, with 7 day
rainfall totals over much of the area being 200-500 percent of
normal, which has resulted in elevated streamflow values across
the region. This sensitivity makes the area more prone to problems
from runoff and flash flooding from convection that does produce
moderate or greater rainfall rates/rainfall.

Tate


$$
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FOUS30 KWBC 182011
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
310 PM EST Thu Feb 18 2021

Day 1
Valid 16Z Thu Feb 18 2021 - 12Z Fri Feb 19 2021

..THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE SOUTHEAST...

...16Z Update...
Extended the Slight Risk farther southwest through a portion of
the FL panhandle and to the northeast into portions of the
sandhills of NC. Given the precipitation that is occurring/has
occurred and the expectation of additional rounds of moderate to
heavy rain ahead of the cold front, felt it was prudent to expand
the Slight Risk area. Periods of torrential rain has quickly
advanced east across portions of GA/SC/NC this morning with this
heavy rain inundating already saturated soils across the region.
Therefore, any additional rainfall could lead to rapid runoff and
thus flash flooding. While there was brief consideration for a
Moderate Risk based on quick basin responses to this heavy rain,
based on the ingredients (PWATs and instability) rain rates should
stay below 1 inch/hour overall. HREF probabilities support this as
well. There will be locations that may pick up 2 inches in an hour
due to training and also upwards of 0.50 inches within 15 minutes.
But these occurrences shouldn't be too widespread.

Pagano

...Previous Discussion...

A mid to upper troughing centered over the Plains shifts eastward
into the Mississippi/Ohio/Tennessee Valleys Thursday night. At the
surface, low pressure will track northeastward through southern
Georgia and toward the Mid-Atlantic coast. This pattern should
provide strong moist inflow from the Gulf of Mexico across the
Southeast, in the form of an atmospheric river per GFS-based
Integrated Vapor Transport. Per the 03z RAP, MU CAPE of 500-1500
J/kg are expected in and near the region, with highest amounts
across northern FL and southern GA within the cyclone's warm
sector. Inflow at 850 hPa of 30-50 knots implies enough effective
bulk shear for convective organization. Precipitable water values
at or above 1.5" -- 2.5 standard deviations above mid to late
February averages -- support hourly rain totals up to 1.75" where
convection manages to train across FL, GA, and sections of SC.
Considering the ingredients, the 00z HREF probabilities of 0.5"+
and 1"+ in an hour appear underdone, probably due to guidance
dispersion. Slow, steady progression of the low-level inflow and
the cyclone's warm front should keep amounts from getting
particularly extreme. The guidance supports local amounts in the
2-4" range, which could fall quickly. Antecedent conditions over
the region are quite wet, with two week precipitation anomalies of
200-400% of average, which has resulted in elevated streamflow
values across the region. This sensitivity makes the area more
prone to problems from runoff and flash flooding from convection
that does produce moderate or greater rainfall rates/rainfall.
Due to the above, have elevated the risk of excessive rainfall to
Slight from portions of the FL panhandle across southern GA into
central SC.

Roth


Day 2
Valid 12Z Fri Feb 19 2021 - 12Z Sat Feb 20 2021

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

Jackson


Day 3
Valid 12Z Sat Feb 20 2021 - 12Z Sun Feb 21 2021

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

Jackson

$$
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FOUS30 KWBC 192007
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
306 PM EST Fri Feb 19 2021

Day 1
Valid 16Z Fri Feb 19 2021 - 12Z Sat Feb 20 2021

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

Pagano


Day 2
Valid 12Z Sat Feb 20 2021 - 12Z Sun Feb 21 2021

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

Jackson

Day 3
Valid 12Z Sun Feb 21 2021 - 12Z Mon Feb 22 2021

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

Jackson


$$
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FOUS30 KWBC 200713
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
212 AM EST Sat Feb 20 2021

Day 1
Valid 12Z Sat Feb 20 2021 - 12Z Sun Feb 21 2021

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

Roth


Day 2
Valid 12Z Sun Feb 21 2021 - 12Z Mon Feb 22 2021

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

Campbell


Day 3
Valid 12Z Mon Feb 22 2021 - 12Z Tue Feb 23 2021

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

Campbell

$$
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FOUS30 KWBC 201944
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
243 PM EST Sat Feb 20 2021

Day 1
Valid 16Z Sat Feb 20 2021 - 12Z Sun Feb 21 2021

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

Pagano


Day 2
Valid 12Z Sun Feb 21 2021 - 12Z Mon Feb 22 2021

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

Rausch

Day 3
Valid 12Z Mon Feb 22 2021 - 12Z Tue Feb 23 2021

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

Rausch

$$
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FOUS30 KWBC 210813
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
312 AM EST Sun Feb 21 2021

Day 1
Valid 12Z Sun Feb 21 2021 - 12Z Mon Feb 22 2021

...A MARGINAL RISK OF EXCESSIVE RAINFALL EXISTS IN WESTERN
WASHINGTON...

Increasingly zonal flow off the Pacific within a warm advection
pattern ahead of a shortwave moving towards the AK Panhandle and
Haida Gwaii is expected to bring 850 hPa inflow of 35-45 knots
into the Pacific Northwest and precipitable water values up into
the 0.75-1" range. The persistence of the low-level inflow argues
for ~4" of liquid for the 24 hour period across western WA, which
is what the preferred QPF model blend yielded. At least half of
the precipitation is expected between 06z-12z on Monday as the
shortwave reaches its closest approach towards the end of the day
1 period and aids boundary layer moisture convergence into the
area. Instability should be virtually non-existant within this
regime (under 100 J/kg), which keeps the threat of 0.5" an hour
rain totals relatively low -- perhaps as high as 30-40% in the
vicinity of Port Angeles. However, a combination of slightly
elevated snowfall early on and snowfall on the ground presently
near the western OR/WA border should melt as the rainfall becomes
heavy and snow levels rise, which would increase hydrologic/soil
sensitivity. Southwest WA has been fairly wet during the past
couple of weeks, with precipitation of 100-200% of average leading
to more soil saturation than usual. In coordination with SEW/the
Seattle WA and PQR/the Portland OR offices, a Marginal Risk was
raised for the area.

Roth


Day 2
Valid 12Z Mon Feb 22 2021 - 12Z Tue Feb 23 2021

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR WESTERN
PORTIONS OF WASHINGTON AND OREGON...

Persistent onshore flow accompanying a shortwave tracking inland
over the Pacific Northwest will continue to bring rain to coastal
areas and the adjacent mountains. Upwards of 4 inches of rainfall
is possible during the Day 1 period, and when combined with snow
melt, is expected to lead to localized rapid runoff and rising
rivers. Model guidance is showing another 1 to 2+ inches falling
over many of the same areas, in addition to moving further south
into Oregon. Even though this additional precipitation is expected
to have hourly rates below 0.50 inch per hour, it will further
aggravate snowmelt thus increasing the flood risk from from
northern Washington to northwest Oregon. A Marginal Risk for
excessive rainfall was hoisted for this area in coordination with
the local forecast offices in Seattle and Portland.

Campbell


Day 3
Valid 12Z Tue Feb 23 2021 - 12Z Wed Feb 24 2021

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

Campbell

$$
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FOUS30 KWBC 212001
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
300 PM EST Sun Feb 21 2021

Day 1
Valid 1756Z Sun Feb 21 2021 - 12Z Mon Feb 22 2021

...A MARGINAL RISK OF EXCESSIVE RAINFALL EXISTS IN WESTERN
WASHINGTON...

...18Z Update...
The Marginal Risk area was modified to better fit the updated WPC
QPF/snowfall and to account for the precipitation shadowing from
the Olympics.

...16Z Update...
Light to moderate precipitation currently impacting the region
will increase in earnest later this evening into the overnight as
a storm system approaches. Based on the 12Z hi-res guidance, the
inherited Marginal Risk area looks good with no modification
needed. Thus, the previous forecast/discussion is on track.

Pagano

...Previous Discussion...
Increasingly zonal flow off the Pacific within a warm advection
pattern ahead of a shortwave moving towards the AK Panhandle and
Haida Gwaii is expected to bring 850 hPa inflow of 35-45 knots
into the Pacific Northwest and precipitable water values up into
the 0.75-1" range. The persistence of the low-level inflow argues
for ~4" of liquid for the 24 hour period across western WA, which
is what the preferred QPF model blend yielded. At least half of
the precipitation is expected between 06z-12z on Monday as the
shortwave reaches its closest approach towards the end of the day
1 period and aids boundary layer moisture convergence into the
area. Instability should be virtually non-existent within this
regime (under 100 J/kg), which keeps the threat of 0.5" an hour
rain totals relatively low -- perhaps as high as 30-40% in the
vicinity of Port Angeles. However, a combination of slightly
elevated snowfall early on and snowfall on the ground presently
near the western OR/WA border should melt as the rainfall becomes
heavy and snow levels rise, which would increase hydrologic/soil
sensitivity. Southwest WA has been fairly wet during the past
couple of weeks, with precipitation of 100-200% of average leading
to more soil saturation than usual. In coordination with SEW/the
Seattle WA and PQR/the Portland OR offices, a Marginal Risk was
raised for the area.

Roth


Day 2
Valid 12Z Mon Feb 22 2021 - 12Z Tue Feb 23 2021

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR WESTERN
PORTIONS OF WASHINGTON AND OREGON...

Strong onshore flow ahead of a shortwave tracking into the Pacific
Northwest will continue to bring rain to coastal areas and the
adjacent mountains into Monday, with the heaviest rainfall axis
dropping southward and trending lighter as the shortwave
approaches and then progresses inland by late Monday-Monday night.
The combination of up to 1.5-4 inches of rainfall during the Day 1
period followed by an additional 1-2 inches over many of the same
areas should lead to localized rapid runoff and rising rivers over
portions of the region from northern Washington to northwest
Oregon. While guidance indicates this period's additional
precipitation will have hourly rates comfortably below 0.50 inch
per hour, it will further aggravate snowmelt thus increasing the
flood threat. Thus a Marginal Risk area was hoisted overnight in
coordination with the local forecast offices in Seattle and
Portland, and maintained for this issuance given a similar
scenario in latest guidance and confirmation from those offices.

Rausch/Campbell


Day 3
Valid 12Z Tue Feb 23 2021 - 12Z Wed Feb 24 2021

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

Rausch

$$
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FOUS30 KWBC 221951
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
250 PM EST Mon Feb 22 2021

Day 1
Valid 16Z Mon Feb 22 2021 - 12Z Tue Feb 23 2021

...A MARGINAL RISK OF EXCESSIVE RAINFALL EXISTS FOR THE COASTAL
PACIFIC NORTHWEST...

...16Z Update...
The weakening atmospheric river will continue to direct moisture
toward the Olympic Mountains, Coastal Ranges and Cascades through
the afternoon. Expect the precipitation intensity to diminish
over the next several hours which should minimize the risk for
flooding as well. However, given there may be periods of moderate
rain over lingering snowpack in the lowlands, decided to maintain
the Marginal Risk for now. There were some additional
modification to the risk area which took into account the snow
across the higher terrain in the Olympic Peninsula. Otherwise, the
forecast is on track. The Marginal Risk area will likely be
dropped later this afternoon/evening as the precipitation
subsides.

Pagano

...Previous Discussion...

Strong onshore flow ahead of a shortwave tracking into the Pacific
Northwest will continue to bring rain to coastal areas and the
adjacent mountains on Monday, with the heaviest rainfall
axis/modest atmospheric river
dropping southward and trending lighter as the shortwave
approaches and then progresses inland by late Monday-Monday night.
The combination of ongoing rainfall followed by an additional 1-2
inches over many of the same
areas should lead to localized rapid runoff and rising rivers over
portions of the region from northern Washington to northwest
Oregon. While guidance indicates this period's additional
precipitation will have hourly rain totals comfortably below
0.50", it along with some rise in snow levels early on will
further aggravate snowmelt thus increasing the flood threat. The
Marginal Risk area was maintained for this issuance given a
similar scenario in latest guidance and confirmation from those
offices.

Roth


Day 2
Valid 12Z Tue Feb 23 2021 - 12Z Wed Feb 24 2021

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

Rausch


Day 3
Valid 12Z Wed Feb 24 2021 - 12Z Thu Feb 25 2021

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

Rausch


$$
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FOUS30 KWBC 232000
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
259 PM EST Tue Feb 23 2021

Day 1
Valid 16Z Tue Feb 23 2021 - 12Z Wed Feb 24 2021

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

Oravec


Day 2
Valid 12Z Wed Feb 24 2021 - 12Z Thu Feb 25 2021

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

Hurley


Day 3
Valid 12Z Thu Feb 25 2021 - 12Z Fri Feb 26 2021

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

Hurley


$$
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FOUS30 KWBC 242010
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
308 PM EST Wed Feb 24 2021

Day 1
Valid 16Z Wed Feb 24 2021 - 12Z Thu Feb 25 2021

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

Oravec


Day 2
Valid 12Z Thu Feb 25 2021 - 12Z Fri Feb 26 2021

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

Hurley


Day 3
Valid 12Z Fri Feb 26 2021 - 12Z Sat Feb 27 2021

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

While the likelihood of reaching or exceeding flash flood guidance
is limited, it is however a non-zero chance. Periods of moderate
to locally heavy rainfall will be supported within this
environment. PW values upwards of 1-1.25 inches will be
transported north/northeast from the Gulf of Mexico into the Deep
South, Mississippi/Tennessee Valleys and toward the Southeast
within an increasing low-level jet, reaching nearly 50 knots
around 00Z Feb 27. Strong mid-upper level lift will be present
with the best lift occurring 00Z-12Z near northern
Mississippi/western and central Tennessee as the associated front
advances. WPC has a narrow band with maximum 24hr QPF of
1.25-1.50+ inches from Arkansas/Mississippi border to the central Tennessee/Kentucky border. During this time isolated locations may
experience ponding or rapid runoff but the aerial coverage of
flooding is not expected to be widespread.

No Marginal Risk was issued with this forecast but will be watched
closely in the following updates, especially should any elevated
instability be sufficient for more intense short-term rainfall
rates.

Hurley/Campbell


$$
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FOUS30 KWBC 252042
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
341 PM EST Mon Jan 25 2021

Day 1
Valid 16Z Mon Jan 25 2021 - 12Z Tue Jan 26 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE LOWER OHIO VALLEY INTO THE TENNESSEE VALLEY AND CENTRAL
APPALACHIANS...

Frontal system over northwestern Arkansas late this morning will
move northeastward today and continue to fuel the ongoing modest
to locally heavy rain event through the afternoon. Surface warm
front over northern Arkansas is forecast to lift northward and
northeastward, with rainfall focused along the 850mb front near
the Ohio River. Broad warm air advection coupled with strong
low-level frontogenesis and moisture convergence will maintain the
PW gradient over KY this afternoon and to the ENE into WV this
evening. Hourly rainfall rates currently peak around 0.5-0.75"
with 1-3" of rainfall the past 24 hours over southern MO into
western KY. Impressive PW values 1-1.4" (2-2.5 standard deviations
above normal) over KY will trend downward and refocus along the
advancing cold front to the south this evening. Large-scale
forcing will start to wane later this afternoon as the upper
divergence weakens and moves into the Appalachians, suggesting a
loss in rainfall rates and amounts by tomorrow morning.

Instability continues to be a limiting factor, especially farther
northeast. RAP analyses show generally less than 600 J/kg MUCAPE
north of the TN/KY border but as the triple point
develops/advances a short increase in instability could support
higher rates over areas that have already seen more than 2 inches
of rain. Isolated to scattered flash flooding could evolve but the
focus will be to the eastern portion of the Marginal area into WV
into the evening. 12Z CAM guidance shows narrowing areas of 1-2.5"
total QPF through 12Z from E KY into WV where FFG was lower.

In addition, soil moisture anomalies are generally near to below
normal for the time of year, stream flows are generally below to
well below normal levels, and it has been very dry over the past
two weeks. Combine these neutral, to even prohibitive, hydro
conditions and the limited instability, will maintain the Marginal
Risk from overnight.

Fracasso/Chenard


Day 2
Valid 12Z Tue Jan 26 2021 - 12Z Wed Jan 27 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR COASTAL
PORTIONS OF NORTHERN AND CENTRAL CALIFORNIA...

2030 UTC Update -- Have expanded the southern half of the Marginal
Risk to include eastern portions of the Bay Area and the northern
Diablo Range, including the Del Puerto and Mineral burn scars.

Model guidance has been persistent regarding an atmospheric river
expected to take aim at California beginning late Tuesday and
lasting through much of the week, causing the potential for heavy
rain as well as large higher elevation snow totals for the state.
This will occur as a mid/upper-level low approaches from the
Pacific and spreads ample upper-level divergence to the northern
half of California Tuesday night. At the surface, a low pressure
system and associated fronts are forecast to linger near the coast
and could provide a convergent focus for precipitation. Moisture
parameters are still expected to be above normal, particularly in
terms of moisture transport--the Ensemble Situational Awareness
Table shows integrated water vapor transport (IVT) above the 90th
percentile along the coast by Tuesday evening and to the 99th
percentile overnight Tuesday. These factors could lead to some
flooding/flash flooding, particularly over burn scar areas such as
the River and Dolan ones south of Salinas. However, limiting
factors for flash flooding through Tuesday night include that the
AR is expected to be reasonably progressive moving southward
across the state, that instability looks to be very limited
(preventing higher rain rates), and that the ground is rather
dry/in drought. Thus stuck with a Marginal Risk for
Tuesday/Tuesday night before a wetter Wednesday.

Hurley/Tate

Day 3
Valid 12Z Wed Jan 27 2021 - 12Z Thu Jan 28 2021

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR COASTAL
PORTIONS OF CENTRAL AND SOUTHERN CALIFORNIA...

2030 UTC Update -- Per collaboration with WFOs MTR and LOX, have
upgraded a portion of the Slight Risk to Moderate, which includes
much of Monterey County (along and west of U.S. 101) and the
western portions of San Luis Obispo County. The Moderate Risk
encompasses both the Dolan and River burn scars. The 12Z guidance
continues to indicate a heavy rainfall signal, with the brunt of
the Atmospheric River (in terms of intensity and duration)
directed into the coastal region between Salinas and Santa Maria.
Albeit classified as a 'moderate' AR event per the Scripps
Institution of Oceanography CW3E AR Scale, the amplifying trough
will pivot the AR axis from a sw-ne orientation to one more
ssw-nne, essentially affecting the same area. Thus a prolonged
period of favorable deep-layer forcing with PWs ~1.00" and
integrated water vapor transport (IVT) values of 500-750 kg/ms, or
the 99th to 99.5th percentile per the NAEFS. 48 hour areal-average
QPF per a global ensemble mean, ending 12Z Thursday 1/28, is
between 5-10 inches within the Moderate Risk area.

0830 UTC Discussion...


On Wednesday, still expecting a persistent atmospheric river to
affect California like on Tuesday, as a upper-level trough/low
centered over the eastern Pacific reloads with additional energy
in its southern part during the period. This reorientation in the
upper levels actually leads to the moisture plume remaining
quasi-stationary throughout much of Wednesday, leading to steady
and moderate to heavy rain accumulating to several inches over the
central California coast. Integrated water vapor transport values
to the 99th percentile are expected, and this anomalous and
tenacious AR has led to a Slight Risk issuance from the Santa Cruz
Mountains southward across the central California coast. While
again instability may be limited, so very high rain rates might
not be seen, the persistence of the moisture inflow and thus rain
falling over the same areas could lead to flash flooding issues. A
Marginal Risk was drawn into parts of the San Joaquin Valley as
well, as 1.5-2 inches of rain is predicted, which is pretty
uncommon for the region.

Hurley/Tate

$$
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FOUS30 KWBC 262021
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
320 PM EST Tue Jan 26 2021

Day 1
Valid 16Z Tue Jan 26 2021 - 12Z Wed Jan 27 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE CENTRAL CALIFORNIA COAST...

1600 UTC update

The 1200 UTC hi res guidance is in good agreement with the 0000
UTC run with the California heavy precipitation. Therefore, no
changes made to the previous Excessive Rainfall Outlook.

Oravec

0900 UTC discussion

A deep layered closed low off the Pacific Northwest coast will
help drive a front and atmospheric river (AR) south across CA by
later today into tonight. Overall looks like a moderate intensity
AR, with IVT values approaching the climatological 97-99th
percentile, but remaining a tad below more extreme values. The AR
is also pretty progressive through the day 1 period, which will
cap the upper bound of rainfall totals through 12z Wed. With that
said, the larger scale forcing is quite strong with this system.
Model forecasts show pretty good coupling of upper level
divergence and strong low level convergence as the front and AR
shifts southward. The impressive low level onshore flow and
convergence will likely help drive a band of higher intensity
rainfall as the system shifts southward, with the upper divergence
helping support the maintenance of any heavier rates.

Antecedent conditions have been quite dry over CA, with stream
flows generally running well below normal, and soil moisture also
running below average for late January. Thus, in the general, the
1-3" of rain expected today should be primarily beneficial rains.
However the expectation of a southward progressing band of higher
rainfall rates does suggest the potential for some flash flood impacts...primarily near burn scars and any other more susceptible
areas. The 00z HREF depicts high probabilities of hourly rains
exceeding 0.5", with indications that hourly magnitudes will peak
around 0.75" within narrow localized swaths. Amounts of this
magnitude will likely exceed thresholds at several area burn
scars. This is confirmed when looking at the HREF probabilities of
exceeding 1 hr FFG, which show anywhere from a 20-70% chance of
exceedance over several burn scars. Thus while impacts outside of
burn scars will likely remain localized and mostly limited in
scale given the progressive AR and dry antecedent
conditions...impacts could be more significant within/near these
burn scar areas, mainly after ~05z tonight. For this reason we
went ahead and upgraded much of the central CA coast to a Slight
risk.

The AR is forecast to stall out as we head into day 2...which may
result in even more significant impacts across portions of
Monterey and San Luis Obispo counties...see the day 2 ERO and
discussion for more details on this threat.

Chenard


Day 2
Valid 12Z Wed Jan 27 2021 - 12Z Thu Jan 28 2021

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE CENTRAL CALIFORNIA COAST...

2030 UTC Update -- Have expanded the southern portion of the
Slight Risk area across the Central CA Coast a bit to include
Santa Barbara and the southern San Rafael ranges, while also
trimming a big of the northern portion toward the Bay Area. This
was based on the subtle trends in the guidance in terms of the timing/progression of the Atmospheric River, which per the latest
model guidance would still be considered a 'moderate' AR in terms
of intensity. Model QPFs continue to cluster reasonably well --
with 24 hour rainfall totals of 5-10" still expected within the
Moderate Risk area.

0830 UTC Discussion below...

An upper-level trough/low centered over the eastern Pacific is
expected to reload with additional energy and amplify southward on Wednesday/Wednesday night. As it does so, ample moisture will be
directed into California in the form of a potent atmospheric
river. The AR is forecast to reorient its axis from approximately
WSW-ENE to more SSW-NNE as the period progresses, but affect
basically the same land area of coastal central/southern
California with deep moisture inflow and forcing. This will cause
several inches of rain for that region--widespread 5" amounts are
expected in the 24 hour period, with some areas receiving 10" of
rain for an event total. While only considered a moderate AR in
terms of intensity (by measures such as the Scripps Institution of
Oceanography Center for Western Weather and Water Extremes), and
while rain rates may not be as high as they can get due to lack of
instability, the persistence of the AR/its duration affecting the
same area will be the main issue causing heavy rain and flash
flooding. Other measures of anomalousness such as the Ensemble
Situational Awareness Table integrated water vapor transport field
do show values over the 99th percentile for this AR. The Moderate
Risk is located over parts of Monterey County (along and west of
U.S. 101) and the western portions of San Luis Obispo County, and
encompasses the Dolan and River burn scars as those will be highly
sensitive to flash flooding.

Hurley/Tate

Day 3
Valid 12Z Thu Jan 28 2021 - 12Z Fri Jan 29 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM THE CENTRAL
CALIFORNIA COAST TO PORTIONS OF SOUTHERN CALIFORNIA...

2030 UTC Update -- Based on the latest deterministic and
probabilistic QPF guidance and trends associated with the AR on
day 3, we have expanded the Slight and Marginal Risk areas farther
east across much of the Los Angeles Metro region, the San
Gabriels, and into the San Bernardinos and Jacintos. This would
include the Bobcat, El Dorado, Blue Ridge, Bond, Silverado, and
Apple Burn Scars from last year's fires.

0830 UTC Discussion Below...

On Thursday into Thursday night, the deep upper trough mentioned
in the day 2 ERO discussion is forecast to begin moving more
progressively southeastward, and will shift the aforementioned
atmospheric river slowly southward toward southern California with
it. The AR should remain strong (above the 99th percentile in
terms of integrated water vapor transport) through the day on
Thursday before weakening. There is some question how quickly the
AR and heavy precipitation will make its way south, but 00Z model
guidance came into better agreement with the location of the
highest QPF, after 12Z/18Z guidance was more split with a farther
north ECMWF and farther south GFS/CMC. A slower movement would
lead to additional issues across the areas that will see multiple
inches of rain on day 2. As it stands, the current QPF shows 3-5"
of rain for the 24 hour period across portions of Santa Barbara
and Ventura counties. Another 2" or so of rain could also fall
farther north over parts of Monterey and San Luis Obispo Counties
especially early Thursday. Thus, a Slight Risk of excessive
rainfall is in place for those areas, given the very wet
antecedent conditions Monterey and San Luis Obispo Counties will
face from Wednesday's rain and considering the River and Dolan
burn scars on the northern end, and with several inches of rain
possible farther south.

Tate

$$
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FOUS30 KWBC 271716
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1215 PM EST Wed Jan 27 2021

Day 1
Valid 1715Z Wed Jan 27 2021 - 12Z Thu Jan 28 2021

...THERE IS A HIGH RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
CENTRAL CALIFORNIA COAST...

1700 UTC update

All of the 1200 UTC hi res guidance persists the strong onshore
flow and concentrated west southwest to east northeast oriented
precipitation band into the coastal central California through the
remainder of the day 1 Excessive Rainfall Outlook period, 1200 UTC
Thursday, with hourly rates of .50 to 1"+ likely to continue into
central Santa Lucia Range. Given this, the previous moderate risk
area was upgraded to a high over central portions of the Santa
Lucia Range and the moderate risk area was extended eastward into
the northern portion of the Diablo Range.

Oravec

0900 UTC discussion

An upper level trough/low over the eastern Pacific is pushing a
pretty robust atmospheric river (AR) southward across CA early
this morning. At this latitude an AR of this magnitude (as defined
by IVT) occurs about once a year, so while strong, not necessarily
extreme. However this AR will have good mid/upper level divergence
overlapping it, which isn't always the case. This event will also
feature very strong lower level onshore flow and convergence, also
likely aiding in rainfall intensity. It's these factors that will
likely help push hourly rainfall magnitudes up over 0.5" within
the narrow core of strongest convergence, with more localized
rates possibly even touching 0.75"-1" in an hour. However, what
this event has going for it the most is persistence. The AR stalls
out over portions of Monterey and San Luis Obispo counties for
much of the day and into tonight. Rainfall rates will fluctuate
some through the day, possibly peaking this morning and then again
this evening as additional vort energy reinforces the AR, but
should generally remain over 0.3" in an hour for most of the day 1
forecast period (and frequently even exceeding 0.5"). This results
in some pretty impressive event totals from 06z this morning
through 12z Thursday...with 6-12" of rain forecast across northern
San Luis Obispo county and into a good portion of central and
southern Monterey county. This will likely result in flooding
impacts through the period. Unfortunately three burn scars are
located within this region, the Dolan, River and Carmel burn
scars. This event total rain combined with the expected rates
raises the potential of significant flash flooding and debris
flows over these burn scars. Thus a Moderate risk of excessive
rainfall will be maintained.

This event will also likely bring rare heavy rain totals further
inland into areas that are typically rain shadowed. Of course some
shadowing will still occur...but the strong convergence and
persistent nature of the AR suggests the inland extent of heavy
rainfall will be greater than typically seen. As much as 3-7" of
rain is expected in between the Coastal Range east towards the
Diablo Range, and 2-3" across portions of the central valley from
Merced to Mendota. Rainfall of this magnitude in 24 hours ranges
from the 50-100 year ARI. Thus while burn scars are not really an
issue here, and antecedent conditions have been dry with below
normal streamflows and drier than normal soil conditions, we would
still expect to some some flooding concerns evolve.

Outside of burn scars, and within the Slight and Moderate risk
areas, the main impacts look to be rapid ponding of water in urban
areas and low lying areas with poor drainage, and also the
potential of mudslides and/or wash outs in areas of steep terrain.

A Marginal risk extends across the remainder of the central valley
into the western foothills of the sierra. Impacts here should be
more localized in nature, but still looking at 1-3" of rain
through 12z Thursday. The heaviest rains through day 1 are now
forecast to generally remain north of Santa Barbara and Ventura
counties, although 1-3" of rain is possible here within the
favored terrain. A Marginal risk appears sufficient to cover this
area through 12z Thursday.

Chenard


Day 2
Valid 12Z Thu Jan 28 2021 - 12Z Fri Jan 29 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM THE CENTRAL
CALIFORNIA COAST TO PORTIONS OF SOUTHERN CALIFORNIA...

On Thursday into Thursday night, the atmospheric river that is
effectively stalled today is forecast to finally press
southeastward as the mid-upper trough/low over the eastern Pacific
continues to amplify and then move eastward. The AR is expected to
retain its strength early Thursday (above the 99th percentile in
terms of integrated water vapor transport per the Ensemble
Situational Awareness Table) before weakening Sunday night. Given
the AR movement, heavy precipitation will generally move from
north to south during the period from Monterey and San Luis Obispo
Counties toward Santa Barbara, Ventura, and Los Angeles counties.
The Slight Risk was drawn on the northern side across portions of
Monterey and San Luis Obispo Counties, where another 1-2" of rain
could fall on very wet antecedent conditions from today's rain and
considering the River and Dolan burn scars. On the southern end,
the Slight Risk encompasses several burn scars in SGX's and LOX's
areas, though with consideration that the highest elevations will
see mainly snow as the precipitation type. In between, the highest
QPF of 3-6" is expected over portions of Santa Barbara and Ventura
Counties, but fortunately in an area generally devoid of burn
scars. The average recurrence interval for the 24 hour QPF was
only about 1-5 years, indicating by Thursday this amount of rain
is not terribly rare (though today's event will be, given the
persistence of the AR over the same area over the central
California coast). Thus did not feel any Moderate Risk outlook
area was necessary for the day 2 ERO.

Tate

Day 3
Valid 12Z Fri Jan 29 2021 - 12Z Sat Jan 30 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTHERN CALIFORNIA...

On Friday, the upper low causing the persistent atmospheric river
to be aimed at California today into Thursday is forecast to track
much more quickly eastward across California and the Four Corners
region, reaching the High Plains by Saturday morning. The
progressiveness in the upper levels should preclude any terribly
heavy QPF over those areas. However, during the day on Friday,
some QPF over an inch or so could fall across the San Gabriels/San Bernadinos/San Jacintos, which could cause isolated issues where
precipitation falls as rain, considering how wet the previous day
will be. Additionally, in lower elevations closer to the coast,
there could be marginal instability (potentially to 500 J/kg of
MUCAPE) coming onshore during the day before the end of the event.
This instability could lead to some higher rainfall rates.
Outlined a Marginal Risk to show these potential threats, but
today and Thursday should be the more impactful days in terms of
flash flooding.

Tate

$$
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FOUS30 KWBC 281552
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1051 AM EST Thu Jan 28 2021

Day 1
Valid 16Z Thu Jan 28 2021 - 12Z Fri Jan 29 2021

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE CENTRAL CALIFORNIA COAST...

1600 UTC update

No changes made to the previous Excessive Rainfall Outlook.

Oravec

0900 UTC discussion

An upper level trough will continue to deepen just off the West
Coast with the associated closed low dropping quickly to the
south, just off the central CA coast by the end of the forecast
period (Friday morning). This will allow an atmospheric river
(AR), which is currently transporting an abundance of moisture
into central CA along an associated front, to shift south through
the afternoon and into the overnight hours. While the intensity
of this AR is not anomalous (based on the IVT values of 600-800
kg/ms), the persistent axis of deep layer moisture is resulting in
extreme precipitation totals and ongoing impacts across the
region. Thus far, there has been rainfall amounts ranging between
6-8+ inches across southern Monterey and northern San Luis Obispo
counties with storm totals approaching 10 inches in the Santa
Lucia mountains. This area will need to be closely monitored over
the next several hours as models continue to show the plume of
moisture feeding into this region before the aforementioned
surface low and trough help to advance the cold front (and AR)
south and east.

This AR event has sufficient mid/upper level divergence, mid-level
vorticity moving atop the surface front and strong low level
moisture convergence which will continue to help enhance rainfall
rates, especially through 18Z. Precipitable water value, while
around 1 inch, will start to taper through the period. This is
also evident by the steady decrease in IVT. Therefore, it should
be noted that the best overlap in dynamics/ingredients are within
the first half of the forecast period and begin to wane as the
system starts to move east. Thus, rainfall magnitudes may surpass
1 inch/hour across the Southern Coastal Ranges through the
morning, but will likely lower closer to 0.5-0.75 inches/hour,
thereafter. This is also supported by the 00Z HREF probabilities.

Areal average precipitation will range between 2-6+ inches, with
the higher amounts corresponding to the upslope enhancement along
the Southern Coastal Range and Transverse Ranges. As a result, 3
to 5 inches of additional rain for portions of Monterey and San
Luis Obispo counties could bring the storm total rainfall close to
15 inches. While the AR is expected to sink south toward Santa
Barbara and Ventura counties by mid-day, residual impacts,
especially over the Dolan, River and Carmel burn scars in Monterey
county, is possible through the day. Therefore, after careful
coordination with the local offices (WFO MTR/LOX) a Moderate Risk
was introduced to cover the ongoing threat through at least 18Z.
Across Santa Barbara and Ventura counties, areal average
precipitation will range between 3-6+ inches, mainly along the
Transverse Ranges. Farther south toward Los Angeles and San Diego,
areal average precipitation is generally between 1-2+ inches. The
highest rainfall amounts are forecast to fall just outside of
significant burn scar areas; though rain rates above 0.50
inches/hour across the Avila, Cave, Bobcat, El Dorado, Lake,
Range2 and Bond burn scars may be enough to cause debris flows.
Therefore, the Slight Risk was retained and modified based on the
latest guidance, burn scars and snow levels. The risk area also
takes into account the potential for scattered urban flash
flooding and mudslides.

Pagano





Day 2
Valid 12Z Fri Jan 29 2021 - 12Z Sat Jan 30 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTHERN CALIFORNIA...

The impactful atmospheric river event of yesterday and today in
California will be wrapping up on Friday, as the associated
upper-level low tracks much more progressively eastward over
California, the Four Corners, and into the High Plains by Saturday
morning. However, there could be a window of locally heavy rain
across southern California during the day on Friday before the
moisture exits. Highest precipitation amounts over an inch will
likely be in terrain, but some of that will fall as snow at the
higher elevations. In lower elevations closer to the coast, some
modest instability (up to 500 J/kg of MUCAPE) is possible through
Friday afternoon as lapse rates steepen underneath the upper low,
which could enhance rainfall rates. Coverage of any
convection/showers should be sparse by that point though. A
handful of burn scars are located in southern California, and
today and tonight should be wet for the area, so Friday's rain
will be on top of that. Considering all these factors, a Marginal
Risk of excessive rainfall was maintained for southern California.

Tate

Day 3
Valid 12Z Sat Jan 30 2021 - 12Z Sun Jan 31 2021

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

Tate

$$
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FOUS30 KWBC 292041
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
340 PM EST Fri Jan 29 2021

Day 1
Valid 16Z Fri Jan 29 2021 - 12Z Sat Jan 30 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTHERN CALIFORNIA...

1600 UTC update

No changes made to the previous Excessive Rainfall Outlook. The
new 1200 UTC hi res runs keep showery conditions across Southern
California through this afternoon, tapering off after 0000 UTC
Saturday. The marginal risk area may possibly be removed with the
issuance of the 0100 UTC Saturday update given the expected drying
trend after 0000 UTC Saturday.

Oravec

0900 UTC discussion

A persistent atmospheric river (AR) brought several feet of snow
to the Sierra/northern Coastal Range and nearly 15 inches of rain
to portions of the central Coastal Range of California. Luckily
this feature is advancing east thanks to the progression of an
upper level low/trough. The associated cold front and axis of
deep Pacific moisture will continue to slide south/east through
the morning hours. The front will eventually work its way through
Southern California with the system moving into the Rockies by
late in the day.

The weakening AR will be directed toward Southern California
through 18Z before quickly tapering off. Precipitable water
values, which will max out early in the period at around 0.75
inches, should continue to diminish as the system moves inland.
This is evident by the weakening IVT, which drops below 400 kg/ms
by late morning. Therefore, anticipate the rain rates will fall
steadily throughout the day. However, it should be noted that
modest instability is present (500 J/kg of MUCAPE) through the
afternoon as the lapse rates steepen with cold air aloft moving
into the region. This will result in isolated
showers/thunderstorms capable of producing rain rates as high as
0.50 inches per hour. These values are enough to result in minor
impacts to area burn scars, especially for Apple, Bobcat and El
Dorado. Given this, the Marginal Risk inherited was retained with
refinements made based on the latest QPF. Areal average
precipitation of around 1-2+ inches are expected along the upslope
locations of the Transverse and Peninsular Ranges. Lower amounts
expected in the valleys. As the system moves inland, anticipate
the coverage of shower/isolated thunderstorms to rapidly diminish,
especially during the late afternoon/overnight hours.

Pagano


Day 2
Valid 12Z Sat Jan 30 2021 - 12Z Sun Jan 31 2021

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

Otto

Day 3
Valid 12Z Sun Jan 31 2021 - 12Z Mon Feb 01 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR COASTAL
NORTHERN CALIFORNIA AND SOUTHWEST OREGON...

A digging trough will approach the West Coast on Sunday with slow
eastward movement of the trough axis into Monday. This will help
focus an area of heavy rainfall along coastal Oregon and northern
California as southwesterly flow transports Pacific moisture
across the coast with forecast PWAT values just shy of an inch for
southern Oregon/northern California. There is good
deterministic/ensemble agreement on the synoptic scale pattern,
but minor timing differences among the latest guidance are
present. Integrated vapor transport (IVT) values are forecast to
peak in the 500-700 kg/m/s range and persist for 12-24 hours along
the southern Oregon coast into northern California, however the
latest guidance has the focus of greatest magnitude into northern
California near Cape Mendocino. Forecast soundings do show a small
amount of instability near the coast with MUCAPE around 100-200
j/kg, but the prospect of some convective banding should be
mitigated by low-mid level flow of relatively weak intensity
(compared to other atmospheric river cases over the past 1-2
months). Furthermore, the heaviest recent rainfall on the West
Coast has been focused south of this region, and USGS streamflow
in southwest Oregon and northwest California is close to average
levels for this time of year.

Therefore, while there is high confidence in heavy rainfall, any
potential flooding impacts should be fairly isolated, and would be
most likely around burn scars. Given the presence of several large
burn scars in this area, the nature of convective rain rates will
need to be evaluated over the next couple days as hi-res model
data becomes available, as a smaller Slight Risk area could be
warranted if rain rates are sufficiently high. Given the above, no
significant changes were made to the Marginal Risk area for the
21Z update.

Otto/Lamers

$$
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FOUS30 KWBC 300814
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
313 AM EST Sat Jan 30 2021

Day 1
Valid 12Z Sat Jan 30 2021 - 12Z Sun Jan 31 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
SOUTHWEST OREGON AND NORTHWEST CALIFORNIA...

A digging trough axis off the West Coast will help funnel Pacific
moisture toward the Coastal Ranges in southwest OR and northwest
CA through the forecast period (Saturday into early Sunday). It
appears the best forcing will arrive later Saturday and advance
into the region through the overnight hours as mid-level impulses
move atop a surface boundary. In response, precipitable water
values will increase to around 1 inch just offshore aided by 40+
knot southwesterly low level flow. While instability will be
minimal, the orographic enhancement may be enough to promote rain
rates in excess of 0.50 inches per hour. The areal average
precipitation trend did increase based on the 00Z model data with
the latest WPC QPF range between 1-3+ inches, notably higher
across the higher elevations and lower in the valleys. Given these
expected rain rates and rainfall totals combined with wet soils,
rising snow levels and residual burn scars, localized flash
flooding is a concern. Therefore, a Marginal Risk was introduced.

Pagano


Day 2
Valid 12Z Sun Jan 31 2021 - 12Z Mon Feb 01 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR COASTAL
NORTHERN CALIFORNIA AND FAR SOUTHWEST OREGON...

A digging trough will approach the Pacific Northwest on Saturday
Night and then slow its forward progress, keeping a relatively
stable configuration of deep south-southwesterly flow into coastal
Oregon and northern California on Sunday and Sunday Night. The
trough will tap a narrow ribbon of higher PWAT values around 1
inch offshore near 35N latitude, draw it north, and drive it into
the coast. The combination of these factors will result in a
focused heavy rainfall event around 24 hours in duration (mostly
during this forecast period) in far SW OR and the N CA coast.

With integrated vapor transport (IVT) projected to be around 500
kg/m/s, this atmospheric river would generally qualify as a weak
or moderate strength case. However, the existing Marginal Risk was
upgraded to a Slight Risk in some areas based on the following
factors. First, there may be heavy rain in some of these areas
just before the forecast period begins (12Z Sunday) on the order
of 1 inch, meaning the cumulative rainfall magnitudes may be above
4 inches by the end of the Day 2 period in some locations. Second,
there are several large burn scars in the area and marginal
instability on some forecast soundings near the coast (100-200
j/kg MUCAPE) could support higher sub-hourly rain rates in
convective bands. Finally, snow levels will be notably higher than
they were in the last storm system, meaning that there could be
runoff contributions from melting snow in some areas.

The Slight Risk generally matches Flood Watches now in effect from
WFOs Medford and Eureka, and some flooding impacts are possible
from the heavy rainfall.

Lamers

Day 3
Valid 12Z Mon Feb 01 2021 - 12Z Tue Feb 02 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR COASTAL
NORTHERN CALIFORNIA AND SOUTHWEST OREGON...

The digging trough will begin to move more steadily to the
southeast again on Monday and Monday Night, after briefly stalling
a day earlier. This should cause the atmospheric river to shift
south along the coast relatively early in the period and begin to
weaken. However, a continuation of some of the heavy rain from the
Day 2 period is likely in far southwest Oregon and into coastal
northern California. QPF magnitudes are lower on Day 3 given the
weakening onshore flow, decreasing PWATs, and increased forward
speed of the trough. The advancing trough will also bring cooler
air, and snow levels are likely to begin falling with less
efficient rainfall in the lower elevations. Given the expectation
of a decreasing threat of heavy rainfall during this forecast
period, a Marginal Risk was posted. A Slight Risk was considered
given the expected antecedent rainfall on Days 1 and 2, but the
signal for heavy rainfall is much more pronounced on Day 2 given
the factors discussed earlier.

Lamers

$$
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FOUS30 KWBC 310100
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
759 PM EST Sat Jan 30 2021

Day 1
Valid 01Z Sun Jan 31 2021 - 12Z Sun Jan 31 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
SOUTHWEST OREGON AND NORTHWEST CALIFORNIA...

0130 UTC update

Few changes made to the Day 1 ERO, mainly to pare the southern
portion of the Marginal Risk area based on the latest model
QPFs/slower trends.

Hurley

0900 UTC discussion

A digging trough axis off the West Coast will help funnel Pacific
moisture toward the Coastal Ranges in southwest OR and northwest
CA through the forecast period (Saturday into early Sunday). It
appears the best forcing will arrive later Saturday and advance
into the region through the overnight hours as mid-level impulses
move atop a surface boundary. In response, precipitable water
values will increase to around 1 inch just offshore aided by 40+
knot southwesterly low level flow. While instability will be
minimal, the orographic enhancement may be enough to promote rain
rates in excess of 0.50 inches per hour. The areal average
precipitation trend did increase based on the 00Z model data with
the latest WPC QPF range between 1-3+ inches, notably higher
across the higher elevations and lower in the valleys. Given these
expected rain rates and rainfall totals combined with wet soils,
rising snow levels and residual burn scars, localized flash
flooding is a concern. Therefore, a Marginal Risk was introduced.

Pagano


Day 2
Valid 12Z Sun Jan 31 2021 - 12Z Mon Feb 01 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR COASTAL
NORTHERN CALIFORNIA AND FAR SOUTHWEST OREGON...

A digging trough will approach the Pacific Northwest tonight and
then slow its forward progress as it elongates southward, keeping
a relatively stable configuration of deep south-southwesterly flow
into coastal Oregon and northern California on Sunday and Sunday
night. Rainfall will be ongoing at the start of the period with
PWAT values in the 0.8 to just over 0.9 inch range along the
southern OR/northern CA coast. Forecasts of PWAT remain similar
for the length of the Day 2 period (12Z Sun-Mon) as do
southwesterly 850 mb wind speeds hovering within 5 to 10 kt of 50
kt. Given little movement of the integrated vapor transport (IVT)
axis of 400 to 600 kg/m/s, throughout the 24 hour period, locally
heavy rainfall is expected with rainfall rates occasionally
exceeding 0.5 in/hr.

The outlook area remains fairly similar to the previous issuance
from 09Z including a Slight Risk from northwestern CA into
southwestern OR. Despite rainfall anomalies over the past 1-2
weeks showing below normal to normal (locally 150+ percent) and
current streamflows (via the National Water Model) showing below
normal to normal, there are reasons to keep the Slight Risk.
Antecedent rainfall leading up to the Day 2 period is expected to
range between 1 and 2 inches, combined with runoff from snow melt,
the presence of recent burn scars across the region and weak
instability may support the overlap of heavier rainfall with more
sensitive locations. Additional rainfall for the Day 2 period is
forecast to peak in the 3 to 5 inch range, although the bullish
NAM_nest (12Z cycle) indicates terrain enhancement across OR/CA
above 7 inches locally.

Otto


Day 3
Valid 12Z Mon Feb 01 2021 - 12Z Tue Feb 02 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR COASTAL
NORTHERN CALIFORNIA AND SOUTHWEST OREGON...

The atmospheric river event from Sunday morning into Monday
morning will continue through the day on Monday. However, as the
upper trough axis and associated cold front begin to push
eastward, the axis of ~500 kg/m/s integrated vapor transport (IVT)
will begin to weaken and translate south and east. Current
thinking for this timing/advancement is between 18Z Monday and 00Z
Tuesday, which should diminish the flood threat for the northern
and central CA coast. There remains some notable timing
differences with the trough axis for Day 3 (Mon-Tue), with the 12Z
NAM significantly slower to have height falls move east, resulting
in the NAM lingering the threat for heavy rain along the OR/CA
border longer than the remaining guidance. The Marginal Risk
encompasses the slower and more progressive solutions, but at this
time, confidence for a Slight Risk lingering from the Day 2 into
the Day 3 Excessive Rainfall Outlook period is not high enough to
warrant its inclusion.

Otto

$$
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FOUS30 KWBC 310755
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
254 AM EST Sun Jan 31 2021

Day 1
Valid 12Z Sun Jan 31 2021 - 12Z Mon Feb 01 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR COASTAL
NORTHERN CALIFORNIA AND FAR SOUTHWEST OREGON...

A trough off the West Coast will continue to sharpen which will
funnel Pacific moisture into the Coastal Ranges of southern OR and
northern CA through early Monday. This is evident by the continual
moisture convergence/transport as supported by the integrated
vapor transport (IVT) axis which maxes out around 400-600 kg/m/s
through the forecast period. Precipitable water will climb to over
0.75 inches aided by 40+ knot southwesterly low level flow. While
the flow is expected to be quite strong, the trajectory is not
truly orthogonal to the terrain which may reduce the orographic
enhancement component. As such, with limited instability, rain
rates may briefly top out around 0.5 inches/hour and this may only
occur for the terrain in southern Humboldt county, just south of
Eureka, CA where better upslope could occur. Areal average
precipitation will range between 2-4+ inches with lower values
within the valleys. With these factors in mind, combined with wet
soils from previous days rain, snow melt from rising snow levels
and residual burn scars, localized to scattered flooding is
possible within this region. Therefore, the Marginal and Slight
Risk areas were retained and refined based on the latest model
guidance and resultant WPC QPF.

Pagano


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

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR COASTAL
NORTHERN CALIFORNIA AND FAR SOUTHWEST OREGON...

The atmospheric river event that will be affecting the coast of
southern Oregon and northern California today and tonight (Day 1),
will persist into the morning hours on Monday. In fact, 850-700mb
winds near the coast are projected to peak in intensity right
around the beginning of the forecast period at 12Z Monday, but
will then rapidly diminish by 18-21Z. Therefore, there is a window
of about 6 hours or so where the heavy rain threat will persist
(over roughly the same rainfall footprint from the previous day)
before the atmospheric river weakens and shifts south along the
coast. After 18Z, rain intensity should begin to diminish, and the
advancing trough will bring cooler air that should cause snow
levels to gradually lower as well. Since the majority of the
rainfall, and associated impact, is expected to be on Day 1, the
Marginal Risk was maintained for the coastal areas of SW OR and N
CA. It's possible a small Slight Risk may be needed at some point
if it appears the heavy rain after 12Z Monday will coincide with
areas that receive the heaviest rain today.

Lamers


Day 3
Valid 12Z Tue Feb 02 2021 - 12Z Wed Feb 03 2021

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

Lamers

$$
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FOUS30 KWBC 011940
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
239 PM EST Mon Feb 01 2021

Day 1
Valid 16Z Mon Feb 01 2021 - 12Z Tue Feb 02 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR COASTAL
NORTHERN CALIFORNIA AND FAR SOUTHWEST OREGON...

...16Z update...

The forecast reasoning below still looks on track after reviewing
the latest observations and 12Z suite of hi-res models. Therefore,
no changes were made to the 09Z Excessive Rainfall Outlook for
this update.

Otto

...previous discussion follows...


A trough off the West Coast will continue to feed Pacific moisture
into the region through the forecast period (through early
Tuesday). Mid-level energy will round the trough axis, helping to
advance a cold front east which will help dissipate the prolonged
atmospheric river (AR). Ahead of this front and aligned with the
AR, precipitable water values will approach 1 inch, which is
nearly 2 standard deviation above the mean. This surge in
moisture will be aided by 50+ knot southwesterly low level flow
which will further enhance the orographic ascent along the
terrain. As the cold front moves through the area moisture will
dwindle from west to east through the period with the best
potential for heavy precipitation along the Coastal Range through
the day and across the the Cascade Range during the overnight
hours. Instability across the region will still be limited,
despite the lapse rates trying to steepen as cold air aloft moves
east. Given the aforementioned orographic effects and minimal
instability, rain rates will struggle to climb above 0.40
inches/hour. However, with an additional 2-4+ inches of
precipitation atop the 3-5+ inches received over the past 48
hours, flooding concerns continue. In addition to the wet soils,
the higher snow levels ahead of the cold front and residual burn
scars were all taken into account when evaluating the potential
flooding impacts. Thus, the Marginal Risk was retained and
modified with the main impacts being lowland flooding and isolated
potential for mudslides.

It should be noted that while outside the Marginal Risk area,
there is the potential for very localized flooding within the
valleys between Mount Shasta and the Sierra Nevada Mountains.
Some high resolution guidance has periods of heavy rain resulting
in upwards of of 2-3+ inches within this region. So, while the
antecedent conditions may not warrant a small Marginal Risk, there
is the potential for increasing river rises and run-off/lowland
flooding.

Pagano



Day 2
Valid 12Z Tue Feb 02 2021 - 12Z Wed Feb 03 2021

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

Orrison


Day 3

The Day 3 outlook will be updated by 2030Z.


$$
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FOUS30 KWBC 022006
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
306 PM EST Tue Feb 02 2021

Day 1
Valid 16Z Tue Feb 02 2021 - 12Z Wed Feb 03 2021

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

Pagano


Day 2
Valid 12Z Wed Feb 03 2021 - 12Z Thu Feb 04 2021

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

Bann


Day 3

The Day 3 outlook will be updated by 2030Z.


$$
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FOUS30 KWBC 032005
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
305 PM EST Wed Feb 03 2021

Day 1
Valid 16Z Wed Feb 03 2021 - 12Z Thu Feb 04 2021

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

Pagano


Day 2
Valid 12Z Thu Feb 04 2021 - 12Z Fri Feb 05 2021

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

Bann



Day 3
Valid 12Z Fri Feb 05 2021 - 12Z Sat Feb 06 2021

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

Bann

$$
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FOUS30 KWBC 041558
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1057 AM EST Thu Feb 04 2021

Day 1
Valid 16Z Thu Feb 04 2021 - 12Z Fri Feb 05 2021

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

Roth/Hurley


Day 2
Valid 12Z Fri Feb 05 2021 - 12Z Sat Feb 06 2021

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

Lamers



Day 3
Valid 12Z Sat Feb 06 2021 - 12Z Sun Feb 07 2021

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

Lamers


$$
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FOUS30 KWBC 060717
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
216 AM EST Sat Feb 06 2021

Day 1
Valid 12Z Sat Feb 06 2021 - 12Z Sun Feb 07 2021

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

Hurley



Day 2

The Day 2 outlook will be updated by 0830Z.


Day 3

The Day 3 outlook will be updated by 0830Z.

$$
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FOUS30 KWBC 061858
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
157 PM EST Sat Feb 06 2021

Day 1
Valid 16Z Sat Feb 06 2021 - 12Z Sun Feb 07 2021

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

Oravec


Day 2
Valid 12Z Sun Feb 07 2021 - 12Z Mon Feb 08 2021

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

Roth/Fracasso



Day 3
Valid 12Z Mon Feb 08 2021 - 12Z Tue Feb 09 2021

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

Roth/Fracasso


$$
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FOUS30 KWBC 070753
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
253 AM EST Sun Feb 07 2021

Day 1
Valid 12Z Sun Feb 07 2021 - 12Z Mon Feb 08 2021

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

Hurley



Day 2
Valid 12Z Mon Feb 08 2021 - 12Z Tue Feb 09 2021

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

Fracasso



Day 3
Valid 12Z Tue Feb 09 2021 - 12Z Wed Feb 10 2021

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

Fracasso



$$
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FOUS30 KWBC 071901
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
200 PM EST Sun Feb 07 2021

Day 1
Valid 16Z Sun Feb 07 2021 - 12Z Mon Feb 08 2021

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

Oravec


Day 2
Valid 12Z Mon Feb 08 2021 - 12Z Tue Feb 09 2021

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

Roth/Fracasso



Day 3

The Day 3 outlook will be updated by 2030Z.


$$
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FOUS30 KWBC 092032
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
331 PM EST Tue Feb 09 2021

Day 1
Valid 16Z Tue Feb 09 2021 - 12Z Wed Feb 10 2021

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

Chenard


Day 2
Valid 12Z Wed Feb 10 2021 - 12Z Thu Feb 11 2021

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

Campbell



Day 3
Valid 12Z Thu Feb 11 2021 - 12Z Fri Feb 12 2021

...A Marginal Risk for excessive rainfall for parts of the
Southern Appalachians and Mid-Atlantic region...

Although the system is expected to be fairly progressive as it
tracks through the Tennessee Valley to the East Coast, areas along
the trailing cold front may have periods of moderate to heavy
rainfall. Models are suggesting 1.5 to 3 inches will span from
northeast Alabama to western North Carolina where there will be
some enhanced instability and lift near the Appalachians. Parts of
this region has been somewhat drier of late, however, much of the
this region will be in the warm sector and a steady stream of PW
values of 1.25-1.5+ inches advecting northward, overrunning the
cold front. Rainfall rates of 0.50 or greater will be possible
therefore opted to raise a Marginal Risk for northern Georgia,
southeast Tennessee and western North Carolina.

Campbell

$$
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FOUS30 KWBC 102019
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
318 PM EST Wed Feb 10 2021

Day 1
Valid 16Z Wed Feb 10 2021 - 12Z Thu Feb 11 2021

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

Chenard


Day 2
Valid 12Z Thu Feb 11 2021 - 12Z Fri Feb 12 2021

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

21Z update... There remains a non-zero threat that convection may
train along the cold front, potentially resulting in moderate to
heavy rainfall across parts of Louisiana and southern Mississippi
and points northeast to parts of the Southern Appalachians. The
probability is still low enough to not warrant a Marginal Risk for
excessive rainfall.

Campbell

Previous discussion... Removed the Marginal Risk area from the
southern Appalachians as the overnight guidance continued to trend
downward in the QPF. However, ingredients remain for a narrow
window of isolated convection in the presence of PW values nearing
1.25 inches Thursday afternoon/early evening at the edge of the
instability gradient. A non-zero threat remains in the general
area, especially if convection were to train over the same area in
a short time (perhaps associated with a weak surface low), before
the front pushes through Thursday night. After coordination with
FFC/MRX/GSP, removed the Marginal Risk area given the 5%
threshold.

Fracasso



Day 3
Valid 12Z Fri Feb 12 2021 - 12Z Sat Feb 13 2021

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

Campbell


$$
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FOUS30 KWBC 120831
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
330 AM EST Fri Feb 12 2021

Day 1
Valid 12Z Fri Feb 12 2021 - 12Z Sat Feb 13 2021

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

Oravec


Day 2
Valid 12Z Sat Feb 13 2021 - 12Z Sun Feb 14 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PARTS OF
THE SOUTHEAST U.S. ON SATURDAY AND SATURDAY NIGHT...

...Florida Panhandle into Southern Georgia...
Introduced a Marginal Risk area in a region of moist on-shore flow
ahead of an approaching mid-level trough and associated surface
low and surface front. Moisture streaming northward ahead of the
front should boost the precipitable water values to between 1.5
and 1.65 inches in a channel from the central Florida panhandle
into southeast Georgia and the far northern Florida peninsula.
Precipitable water values that high are roughly 2 to 2.5
standardized anomalies above climatology for the middle of
February. With the approaching front, aided by a weak area of low
pressure riding northward along the front from the Gulf of Mexico
into Florida during the afternoon, convection capable of producing
heavy rainfall rates should become more widespread in coverage on
Saturday and gradually spread eastward with time. Operational
guidance from 12/00Z all had signals for rainfall amounts of 2 to
4 inches...implying locally heavier maximum amounts...and those
signals were supported by spaghetti plots of QPF from the ensemble
means. The placement of the Marginal Risk area was a combination
of the best overlap of QPF signals and the fact that soil
saturation/stream flow was high in the area...making the area
prone to flooding from moderate to heavy rainfall rates/amounts.
Focus will gradually shift eastward during the night and
potentially linger beyond the end of the Day 2 period at 15/12Z.

Bann


Day 3
Valid 12Z Sun Feb 14 2021 - 12Z Mon Feb 15 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF
THE SOUTHEAST U.S. ON SUNDAY...

...Far southeast Georgia into the Northern/Northeastern Portion of
the Florida Peninsula...
Convection should be on-going at the start of the Day 3 period
along and ahead of a surface cold front pushing from west to east.
Southerly low level flow during the Day 2 period will have drawn
an airmass with precipitable water values of 1.5 or 1.6 inches
northward across the area ahead of the front...with enough
synoptic and mesoscale forcing to support and focus convection
capable of producing heavy rainfall. During the day, however,
some moderate to locally heavy rainfall will still be possible as
elevated moisture transport and mesoscale processes continue after
the front becomes quasi-station and oriented from northeast to
southwest later in the day. Given that stream flows/near-surface
saturation begin higher than normal, the combined effect of
rainfall during the day 2/3 period could result in problems due to
run-off, ponding and isolated flash flooding throughout the day 3
period.

Bann


$$
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FOUS30 KWBC 130110
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
809 PM EST Fri Feb 12 2021

Day 1
Valid 01Z Sat Feb 13 2021 - 12Z Sat Feb 13 2021

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

Roth/Chenard


Day 2
Valid 12Z Sat Feb 13 2021 - 12Z Sun Feb 14 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PARTS OF
THE SOUTHEAST U.S. ON SATURDAY AND SATURDAY NIGHT...

...2030Z Update...
No significant changes were needed on this day shift to the
Marginal Risk located across portions of Florida and Georgia. A
wet Friday night into Saturday, above normal moisture parameters
such as precipitable water, and above normal streamflow conditions
as described below could contribute to isolated flash flooding as
well as areal flooding. Rainfall amounts could exceed an inch or
two farther northeast into South Carolina during the period as
well, but around normal streamflow values and less convective
nature of the rain should prohibit most flash flooding there.

Tate

...Previous Discussion...

...Florida Panhandle into Southern Georgia...
Introduced a Marginal Risk area in a region of moist on-shore flow
ahead of an approaching mid-level trough and associated surface
low and surface front. Moisture streaming northward ahead of the
front should boost the precipitable water values to between 1.5
and 1.65 inches in a channel from the central Florida panhandle
into southeast Georgia and the far northern Florida peninsula.
Precipitable water values that high are roughly 2 to 2.5
standardized anomalies above climatology for the middle of
February. With the approaching front, aided by a weak area of low
pressure riding northward along the front from the Gulf of Mexico
into Florida during the afternoon, convection capable of producing
heavy rainfall rates should become more widespread in coverage on
Saturday and gradually spread eastward with time. Operational
guidance from 12/00Z all had signals for rainfall amounts of 2 to
4 inches...implying locally heavier maximum amounts...and those
signals were supported by spaghetti plots of QPF from the ensemble
means. The placement of the Marginal Risk area was a combination
of the best overlap of QPF signals and the fact that soil
saturation/stream flow was high in the area...making the area
prone to flooding from moderate to heavy rainfall rates/amounts.
Focus will gradually shift eastward during the night and
potentially linger beyond the end of the Day 2 period at 15/12Z.

Bann


Day 3
Valid 12Z Sun Feb 14 2021 - 12Z Mon Feb 15 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF
THE SOUTHEAST U.S. ON SUNDAY...

...2030Z Update...
No significant changes were needed for the Marginal Risk area
across Florida and Georgia. As described below, convection should
be ongoing Sunday before transitioning offshore, and wet
antecedent conditions could contribute to causing flooding issues,
despite relatively low rainfall amounts of generally less than 1.5
inches forecast.

Tate

...Previous Discussion...

...Far southeast Georgia into the Northern/Northeastern Portion of
the Florida Peninsula...
Convection should be on-going at the start of the Day 3 period
along and ahead of a surface cold front pushing from west to east.
Southerly low level flow during the Day 2 period will have drawn
an airmass with precipitable water values of 1.5 or 1.6 inches
northward across the area ahead of the front...with enough
synoptic and mesoscale forcing to support and focus convection
capable of producing heavy rainfall. During the day, however,
some moderate to locally heavy rainfall will still be possible as
elevated moisture transport and mesoscale processes continue after
the front becomes quasi-stationary and oriented from northeast to
southwest later in the day. Given that stream flows/near-surface
saturation begin higher than normal, the combined effect of
rainfall during the day 2/3 period could result in problems due to
run-off, ponding and isolated flash flooding throughout the day 3
period.

Bann

$$
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FOUS30 KWBC 130825
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
324 AM EST Sat Feb 13 2021

Day 1
Valid 12Z Sat Feb 13 2021 - 12Z Sun Feb 14 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS FAR
SOUTHEAST ALABAMA, PORTIONS OF THE FLORIDA PANHANDLE, NORTHERN
FLORIDA AND SOUTHERN GEORGIA...

A marginal risk area was maintained across far southeast Alabama,
portions of the Florida Panhandle/Northern Florida into southern
Georgia in the vicinity of the stationary frontal boundary
forecast to remain across this region during the upcoming day 1
period. There has been some changes in timing, especially in the
latest hi res guidance with a faster northeast push of the current
area of heavy rains moving northeast across the Florida Panhandle
into southwest Georgia early this morning. This lead area of
organized heavy rains will be moving across areas of the Florida
Panhandle, far southeast Alabama and southwest Georgia that have
had above average precipitation over the past week, up to 300 to
400% of normal. This past rain and the lead area of heavy rains
will keep stream flow anomalies high across these region.
Additional heavy rain potential exists during the upcoming day 1
period in the vicinity of the stationary frontal boundary where pw
values will remain 2 to 2.5+ standard deviations above the mean
and additional shortwave energy in the southwest flow aloft will
enhance uvvs. While model qpfs have trended down from the
previous run, mostly because of the faster timing of the lead area
of organized precip, additional heavy precip is possible in this
high pw axis and will fall across the aforementioned regions where
stream flows are high resulting in the potential for isolated
runoff issues.

Oravec


Day 2
Valid 12Z Sun Feb 14 2021 - 12Z Mon Feb 15 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF
THE SOUTHEAST U.S. ON SUNDAY...

...Far southeast Georgia into the Northern/Northeastern Portion of
the Florida Peninsula...
Overall, only minor adjustments were made to the Marginal Risk
area over a portion of the Southeast U.S. on Sunday and Sunday
night. Convection should be on-going at the start of the forecast
period from the Florida panhandle northeastward into southern
Georgia. As the day progresses, a cold front which has been
helping to focus convection should slow its forward speed and
become oriented from northwest to southeast as a quasi stationary
front by evening. The NCEP numerical guidance shows the best
chance for heaviest rainfall rates occurring during the first half
of the outlook period, and should gradually taper off Sunday
evening and during the overnight hours as the best upper level
support and an area of low pressure along the front shift
northward and away from the area. Areal average rainfall amounts
of an inch or two are expected. Normally, these amounts of
rainfall do not normally present much of a problem...but the
combined effects of the Day 1 and the Day 2 rainfall amounts on an
area which already has elevated stream flow make the area more
susceptible to even moderate rainfall amounts.

Bann


Day 3
Valid 12Z Mon Feb 15 2021 - 12Z Tue Feb 16 2021

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

Bann

$$
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FOUS30 KWBC 132010
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
309 PM EST Sat Feb 13 2021

Day 1
Valid 16Z Sat Feb 13 2021 - 12Z Sun Feb 14 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE FLORIDA PANHANDLE, NORTHERN FLORIDA AND SOUTHERN GEORGIA...

16z Update: Overall the flood risk appears to be pretty minimal
through much of the period across northern Fl into southern GA. We
are currently in between forcing features, and instability is
lacking. However an uptick in convective activity is expected
towards 12z-18z Sunday as another wave and upper jet moves across
the region. Models were about 6 hours too slow with this past wave
of activity...thus can not rule out this additional development
moving inland a bit earlier than currently forecast. For that
reason we will maintain the Marginal risk...again mainly for the
09z and after time frame Sunday morning.

Chenard

...Previous Discussion...

A marginal risk area was maintained across far southeast Alabama,
portions of the Florida Panhandle/Northern Florida into southern
Georgia in the vicinity of the stationary frontal boundary
forecast to remain across this region during the upcoming day 1
period. There has been some changes in timing, especially in the
latest hi res guidance with a faster northeast push of the current
area of heavy rains moving northeast across the Florida Panhandle
into southwest Georgia early this morning. This lead area of
organized heavy rains will be moving across areas of the Florida
Panhandle, far southeast Alabama and southwest Georgia that have
had above average precipitation over the past week, up to 300 to
400% of normal. This past rain and the lead area of heavy rains
will keep stream flow anomalies high across these region.
Additional heavy rain potential exists during the upcoming day 1
period in the vicinity of the stationary frontal boundary where pw
values will remain 2 to 2.5+ standard deviations above the mean
and additional shortwave energy in the southwest flow aloft will
enhance uvvs. While model qpfs have trended down from the
previous run, mostly because of the faster timing of the lead area
of organized precip, additional heavy precip is possible in this
high pw axis and will fall across the aforementioned regions where
stream flows are high resulting in the potential for isolated
runoff issues.

Oravec


Day 2
Valid 12Z Sun Feb 14 2021 - 12Z Mon Feb 15 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF
THE SOUTHEAST U.S. ON SUNDAY...

...Southwest Oregon and Northwest California...

A Marginal Risk area was hoisted for this part of the region with
this forecast package. Very moist air will surge onshore as a
Pacific low pressure system advancing toward/pushing inland over
the Northwest and northern California. This moisture combined with
enhanced lift and instability along the Coastal Range will result
in several hours of moderate to possibly heavy rainfall. Several
model solutions show this part of the region could have 2 to 3
inches of rainfall, with the bulk of it falling in about a 6-12
hour period.

Campbell

...Far southeast Georgia into the Northern/Northeastern Portion of
the Florida Peninsula...

21Z update... Convection is still expected to track through the
Southeast along and ahead of the slowing front. Storm motion will
slow as the front stalls across the region which will increase the
risk for excessive rainfall and flash flooding conditions to
arise. There is support aloft to sustain higher rainfall rates
across eastern South Carolina and into North Carolina, therefore
in coordination with the WFO in Charleston, South Carolina the
northern boundary of the Marginal Risk area was expanded north to
near Wilmington.

Campbell


Previous discussion... Overall, only minor adjustments were made
to the Marginal Risk area over a portion of the Southeast U.S. on
Sunday and Sunday night. Convection should be on-going at the
start of the forecast period from the Florida panhandle
northeastward into southern Georgia. As the day progresses, a
cold front which has been helping to focus convection should slow
its forward speed and become oriented from northwest to southeast
as a quasi stationary front by evening. The NCEP numerical
guidance shows the best chance for heaviest rainfall rates
occurring during the first half of the outlook period, and should
gradually taper off Sunday evening and during the overnight hours
as the best upper level support and an area of low pressure along
the front shift northward and away from the area. Areal average
rainfall amounts of an inch or two are expected. Normally, these
amounts of rainfall do not normally present much of a
problem...but the combined effects of the Day 1 and the Day 2
rainfall amounts on an area which already has elevated stream flow
make the area more susceptible to even moderate rainfall amounts.

Bann


Day 3
Valid 12Z Mon Feb 15 2021 - 12Z Tue Feb 16 2021

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

Campbell

$$
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FOUS30 KWBC 140822
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
321 AM EST Sun Feb 14 2021

Day 1
Valid 12Z Sun Feb 14 2021 - 12Z Mon Feb 15 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS NORTHEAST
FLORIDA, FAR NORTHWEST CALIFORNIA AND FAR SOUTHWEST OREGON...

...Northeast Florida...
The previous marginal risk area was decreased in size and
repositioned farther to the south across northeast Florida. The
latest surface analysis shows the primary synoptic front
stationary, lying in a west southwest to east northeasterly
direction across north Florida. There is a secondary boundary
farther to the south, lying west to east across central Florida
from Tampa to Cape Canaveral, showing up well in the latest rap32
frontogenesis fields and latest surface observations. There is a
signal in several of the latest hi res runs. fv3cam, gem regional,
nam nest and the latest Ukmet that this boundary to the south of
the primary synoptic boundary may be the focus for convection
Sunday morning. Instability is expected to increase along and
north of this secondary boundary Sunday morning, reaching 500-1000
j/kg. Farther north, instability is lacking along the coastal
Southeast. While overrunning rain north of the northern frontal
position are possible Sunday, hourly rates likely will not be
conducive to flash flooding. Greater hourly rates, 1.00 to
1.5"+/hour, likely to the south of the north Florida stationary
front and along and north of the central Florida boundary. This
may produce isolated runoff issues, especially in urbanized
regions.

...Far southwest Oregon into far northwest California...
No changes made to the marginal risk area over far northwest
California into far southwest Oregon. Onshore westerly to west
southwesterly flow expected into this region day 1 ahead of
surface frontal boundary pushing toward the Pacific Northwest.
There continues to be a strong model consensus for max heavy rain
totals across far southwest Oregon into far northwest California.
HREF neighborhood probabilities are high for 2 and 3"+ totals this
period, 90%+ and 50-90% respectively and 10% for 5"+ totals. This
area has seen above average precip over the past week with stream
flows much above normal to high in places as per the National
Water Model, with additional heavy rains possibly producing
isolated runoff issues.

Oravec


Day 2
Valid 12Z Mon Feb 15 2021 - 12Z Tue Feb 16 2021

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

Bann


Day 3
Valid 12Z Tue Feb 16 2021 - 12Z Wed Feb 17 2021

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

Bann

$$
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FOUS30 KWBC 142358
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
658 PM EST Sun Feb 14 2021

Day 1
Valid 01Z Mon Feb 15 2021 - 12Z Mon Feb 15 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS THE
COASTAL CAROLINAS, FAR NORTHWEST CALIFORNIA AND FAR SOUTHWEST
OREGON...

...Southeast...
Limited instability has been keeping rainfall rates in check
across the Coastal Carolinas, as MU CAPE values are barely above
100 J/kg, which supports hourly rainfall totals potentially
reaching 0.75" in an hour given the precipitable water values near
1.5". Deep layer soil moisture anomalies are generally above the
climatological 75th percentile...with a good portion of eastern
SC/NC above the 98th percentile. This suggests a good deal of this
rain will be runoff even with relatively lower rates. So while
true flash flooding seems unlikely given the rates...ponding of
water and localized flooding will be possible over the next
several hours...especially in any more susceptible areas.

...Far southwest Oregon into far northwest California...
Onshore westerly to west southwesterly flow expected into this
region tonight ahead of surface frontal boundary pushing toward
the Pacific Northwest. There continues to be a strong model
consensus for max heavy rain totals across far southwest Oregon
into far northwest California. HREF neighborhood probabilities
are high for 2 and 3"+ totals this period, 90%+ and 50-90%
respectively and 10% for 5"+ totals. This area has seen above
average precip over the past week with stream flows much above
normal to high in places as per the National Water Model, with
additional heavy rains possibly producing isolated runoff issues.

Roth/Chenard/Oravec


Day 2
Valid 12Z Mon Feb 15 2021 - 12Z Tue Feb 16 2021

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

Campbell



Day 3
Valid 12Z Tue Feb 16 2021 - 12Z Wed Feb 17 2021

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

Campbell


$$
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FOUS30 KWBC 151929
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
228 PM EST Mon Feb 15 2021

Day 1
Valid 16Z Mon Feb 15 2021 - 12Z Tue Feb 16 2021

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

Hurley


Day 2
Valid 12Z Tue Feb 16 2021 - 12Z Wed Feb 17 2021

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

Campbell

$$
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FOUS30 KWBC 092016
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
315 PM EST Sat Jan 09 2021

Day 1
Valid 16Z Sat Jan 09 2021 - 12Z Sun Jan 10 2021

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

Orrison


Day 2
Valid 12Z Sun Jan 10 2021 - 12Z Mon Jan 11 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ALONG THE
CENTRAL TEXAS COASTAL PLAIN...

An incoming shortwave will cross out of S NM with broad diffluent
mid to upper-level flow ahead of the nose of the 80-90kt 3H jet
across Central TX. As the surface to boundary layer, winds will
be responding with 20-25kts of SE onshore flow across the Central
Texas Gulf coast veering to a southerly 30-35kt at 850mb. The
combination of warm moisture flux and some insolation may help a
narrow well of weak instability to develop by 18-21z as high as
500 J/kg. Total precipitable water values will be encroaching on
1.5" that evening along/just east of the coast. Strong dynamic
forcing should allow for late morning convective development,
particularly close to the coast and perhaps enhanced slightly by
frictional convergence and then further enhanced with time by the
developing instability. The uncertainty remains in the timing of
the development and how long of a window where the cells may
initially be stationary waiting for the best forcing aloft while
being fed by strong onshore flow. Most guidance members remain in
the 2" areal total range over the coast before increasing to 4-6"
over the western Gulf waters as the complex matures after 21z and
shortwave progression supports offshore convective progression.

Hydrological conditions in the region remain saturated per the
National Water Model saturation ratios (generally running about
.85-1) along the coastal counties and toward the western side of
the Houston Metro region. This is aligned with 2-week
precipitation anomalies running about 300-400% of normal in the
vicinity of best thermodynamic and dynamic intersection between
Corpus Christi and Matagorda Bay. Given the ground conditions
and the some of the trends in a very small Slight Risk was
considered early this morning given the high resolution model
guidance at that time. Since QPF came down slightly this cycle,
decided to maintain the Marginal Risk for the time being.

Roth/Gallina

Day 3
Valid 12Z Mon Jan 11 2021 - 12Z Tue Jan 12 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR THE OLYMPIC
PENINSULA OF WESTERN WASHINGTON...

A large northeast Pacific cyclone will start to shift farther
south with the passing of shortwave/cold front across Vancouver
Island into the Olympic Peninsula at the start of the forecast
period, 11/12z. This is a fairly average Atmospheric River plume
with 25-45kts of southwesterly 85-7H flow providing about 400
kg/ms of Integrated Vapor Transport (IVT) across the Peninsula in
the 12-18z period (given reducing flow and total PWats to .8-.9
range).

By 21z, the trailing front should be across the mouth of the
Columbia River before a broader swath of warm-air advection
approaches with a stronger/sharper shortwave. There are model
differences to the timing and sharpness of this approaching wave.
Strong but narrowing flow ahead of the fast moving cold front will
support about 2 standard deviations of IVT flux with 1" TPW values
pressed by strengthening southwesterly LLJ up to 75kts at 850 hPa
directed mainly at Vancouver Island by 06z. Generally guidance is
supporting areal average QPF of 3-4" through 12z enhanced by
orographic ascent along the southwest flanks of the Olympic
peninsula, with the bulk coming in less than 12 hours.

Hydrologically, there had been a few day break in rainfall to
allow for rivers to recede slowly, however, NWM continues to show
very saturated deep soil conditions to fully saturated along the
coast and only .8-.9 saturation ratios along the southwest flanks
of the Olympic range itself. AHPS precipitation anomalies remain
above average particularly in the last week where they remain
about 300% of normal (though 2 week anomalies are more 150-200%).
Given this rainfall amount and saturated ground conditions a
Marginal Risk of Excessive Rainfall remains in effect for the day
3 period (11/12z-12/12z) for the entire Olympic Peninsula.
Coordination with SEW/the Seattle WA forecast office provided
insight to the current hydrologic conditions earlier this morning
and the risk for mud/landslides with this system, which was
reinforced by the National Water Center input later in the
morning.

Considering the degree of low-level inflow already forecast, any
upward trend toward a stronger/compact evolution or a slightly
quicker progression would present additional concern for
rapid-rise flooding along the rivers accompanied by landslide
potential. As such, a Slight Risk at some point is possible in
later issuances should these trends materialize.

Roth/Gallina

$$
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FOUS30 KWBC 252055
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
354 PM EST Thu Feb 25 2021

Day 1
Valid 16Z Thu Feb 25 2021 - 12Z Fri Feb 26 2021

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

However, a swath of moderate to heavy rainfall is expected around
the Arklatex, where amounts of ~2" are expected. Good coupling of
low level moisture convergence and upper level divergence within
the right entrance region of the 250mb jet, supports the expanding
coverage of rain into tonight. A forecast of 2" areal averaged
rain is often enough to consider at least a Marginal risk. However
with only weak instability in place, the rates just do not seem to
be there with this event. Localized swaths of 1" in an hour are
likely...but overall it looks like 1hr and 3hr rainfall should
peak at only ~50% of FFG. Also, current streamflows and soil
saturation over the region are around typical levels for the time
of year...thus not giving any reason to suggest flooding is likely
below FFG levels. Thus in general it looks like this heavy rain
later today into tonight will help saturate soil conditions and
result in some ponding of water in spots, but overall is not
expected to result in much of a flash flood impact...thus will
carry no risk area at this time.

Chenard


Day 2
Valid 12Z Fri Feb 26 2021 - 12Z Sat Feb 27 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE LOWER MISSISSIPPI VALLEY...TENNESSEE VALLEY...CUMBERLAND
PLATEAU...SOUTHERN APPALACHIAN MOUNTAINS...AND THE CAROLINA
PIEDMONT...

2030 UTC Update -- Have hoisted a Marginal Risk over the area,
based on the latest guidance trends, particularly late in the
period (00-12Z Sat) as the warm front lifts slowly northward
Fri-Sat. PWs Friday night climb to 1.25+ inches, which is about 2
standard deviations above normal. This as southwesterly low-level
inflow increases to 50-55 knots, becoming nearly parallel while
approaching the magnitude of the mean 850-300 mb low -- setting
the stage for upwind propagation and cell training with elevated
convective bands north of the surface warm front Friday night.
Instability looks to be on the marginal side, with MUCAPEs
generally averaging between 250-500 j/kg, which for the most part
will cap 3 hourly rainfall rates between 1-1.5 inches.

Previous discussion..

The latest guidance continues to show Gulf moisture being
transported northward within an increasing low level flow. The
best mid-upper level lift will be present with the best lift
occurring 00Z-12Z near northern Mississippi/western and central
Tennessee as the associated front advances. Periods of moderate to
locally heavy rainfall will be supported within this environment;
with a couple of the hi-res guidance showing rain rates of 0.50
inch/hour across parts of Arkansas into Mississippi and western
Tennessee during the 12-18Z period. This is also where the swath
of higher QPF is expected to occur, with WPC forecasting 1 to 2
inches. During this time isolated locations may experience ponding
or rapid runoff but the aerial coverage of flooding is not
expected to be widespread. It will however increase soil
saturation across the region and lower the FFG from the Southern
Plains/Lower Mississippi Valley to the Tennessee Valley.

Hurley/Campbell


Day 3
Valid 12Z Sat Feb 27 2021 - 12Z Sun Feb 28 2021

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FROM THE
SOUTHERN PLAINS/LOWER MISSISSIPPI VALLEY TO THE TENNESSEE VALLEY
AND CENTRAL APPALACHIANS...

Scattered to widespread rain will persist during the day 3 period
after spreading 1 to 2 inches across the region the day prior.
This antecedent moisture will have likely lowered flash flood
guidance across the Lower Mississippi and Tennessee Valleys where
an additional 1 to 3 inches are forecast. Although the hourly
rates across this region are expected to be less than 0.50
inch/hour, precipitation of this magnitude will lead to an
increased threat for flash flooding to develop. In coordination
with the local forecast offices from far northeast Texas to
eastern Kentucky/Tennessee a Marginal Risk area was hoisted. This
area will continue to be monitored as the threat of moderate to
heavy rainfall continues into the day 4 period.

Campbell

$$
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FOUS30 KWBC 262028
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
327 PM EST Fri Feb 26 2021

Day 1
Valid 16Z Fri Feb 26 2021 - 12Z Sat Feb 27 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE TENNESSEE VALLEY AND ADJACENT AREAS...

1600 UTC update

No changes made to the previous Excessive Rainfall Outlook.

Oravec

0900 UTC discussion

We will maintain a Marginal risk across portions of northern
MS/AL/GA into central and eastern TN. Total rainfall amounts, from
earlier Thur evening through today, should end up around 2-3"
across portions of northern MA/AL/GA. Rainfall rates will never
get all that high, given limited instability and quick cell
motions. However elevated convective cells will continue to track
across the region through the morning hours, with hourly totals
locally exceeding 0.5". It is the persistence of these elevated
heavy cells moving over the same area that will eventually result
in some rainfall totals approaching 3". Overall it looks like
rainfall should stay below the 1,3 and 6 hr FFG...thus this mainly
just looks to be a soaking rain without much in the way of flood
impacts. However, can not rule out some localized, mainly minor,
flooding through the morning hours where brief training of the
aforementioned elevated convective cells overlaps the increasingly
saturated soil conditions. Thus will maintain the Marginal risk
across this region.

The focus for later tonight into early Saturday shifts northward
into central and eastern TN. Model forecasts show a corridor of
strong 850mb moisture transport pushing into this region...likely
helping expand/intensify shower activity. This intensification of
low level inflow also pushes PWs up above the climatological 90th
percentile. The main question mark here will be instability. The
current model consensus is only a couple hundred J/KG at
most...which is likely not enough to get get rates over 0.5" in an
hour. However moisture convergence will be quite strong, so it
will not take much more instability to start getting rates towards
0.75" in an hour, and the orientation of the moisture convergence
axis with the mean flow would support some training. Thus while
the most likely solution would suggest rates too low for a flash
flood risk, there is at least a small threat of the event
overachieving and beginning to approach or locally exceed
FFG...which is a bit lower over eastern TN compared to areas
upstream to the southwest. We will carry a Marginal risk over this
region to account for this threat later tonight into early
Saturday.

Chenard


Day 2
Valid 12Z 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...

2030 UTC Update...

Very little change from the previous Day 2 ERO, most notably an
ever-so-slight expansion of the Slight Risk area farther southwest
into far northeast AR and the MO Bootheel. WPC areal-average QPF
from 12Z Sat - 12Z Sun has trended down a bit over the past 3
forecast cycles, with a shrinking area of 1.5". This is in line
with the NBM trends, as the maximum rainfall axis is close to a
blend of the 12Z GFS (wetter) and 12Z ECMWF (drier). It should be
noted that some of the high-res CAMs, most notably the WRF-ARW
(and to a lesser degree the ARW2) are farther south (south of the
current Slight Risk area) with the heavy rainfall axis -- no doubt
a function of deeper elevated convection closer to the surface
warm front. Certainly, the deeper/more disruptive the convection
(in terms of the expanse of deep isentropic ascent), the more the
current ERO will undergo changes as the event gets into the day 1
period.

0830 UTC Discussion...

Shortwaves rounding a mid-level trough axis situated across the
central U.S. will interact with a residual surface boundary draped
across the region. As a result, moderate to heavy rain will
develop along and just north of the boundary which will eventually
lift north as a warm front ahead of a developing system to the
west. Anticipate this to occur during the latter half of the
forecast period as the shortwave approaches Saturday evening
coincident with the upper level jet strengthening helping to
promote strong divergence aloft. With return flow increasing
through the forecast period, precipitable water values will climb
to around 1.25-1.5 inches aided by 35-45 knot low level
southwesterly flow by 28/06Z. This is 2.5 standard deviations
above the mean. With isentropic ascent along/north of the warm
front with modest elevated instability (on average 500 J/kg
MUCAPE), rain rates will likely exceed 0.75 inches per hour in
some locations. In addition, training may occur based on the deep southwesterly mean wind aligning briefly with the propagation
vector. This could result in 3 hourly rainfall totals in
exceeding of 1.5 inches.

While antecedent conditions may illustrate fairly dry conditions
as of late, with recent snow melt, soil moisture is slightly above
average. Therefore, the aforementioned rain rates may exceed flash
flood guidance resulting runoff and thus scattered flash flooding
in some locations. As a result, a Slight Risk was introduced
along a narrow corridor where the heavy rain rates overlap with
higher soil moisture. The inherited Marginal Risk was modified
based on the latest model guidance and resultant WPC QPF.

Hurley/Pagano


$$
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FOUS30 KWBC 271550
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1049 AM 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...

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

Day 3
Valid 12Z Mon Mar 01 2021 - 12Z Tue Mar 02 2021

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

Pagano


$$
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FOUS30 KWBC 280846
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
345 AM EST Sun Feb 28 2021

Day 1
Valid 12Z 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...

A broad swath of heavy rainfall is expected today from the lower
MS Valley, across the TN and lower OH valley and into WV. Plenty
of moisture in place for heavy rainfall...with PWs generally
hovering around record values for late February. A pretty good
setup for a prolonged period of convection as well, with
persistent deep and broad isentropic ascent across the region.
Strong moisture transport will continue to advect northward today,
and in essence, will be squeezed by a southward dropping cold
front. This should result in a narrow corridor of intense moisture
convergence, and unfortunately this axis is not forecast to move
much today into this evening...setting up across KY and into
southern WV. This nearly stationary, and persistently strong,
corridor of moisture convergence is rather ideal for training
convection. Combine that with the aforementioned PWs in
place...and the only other question for excessive rainfall
potential is instability. And the expectation is that we will have
enough instability to take advantage of the impressive
PWs/moisture convergence. Thus numerous to widespread flash
flooding is becoming increasingly probable across portions of KY,
far northern TN and southern WV. The higher instability, and thus
higher rates, will generally stay over central and western KY.
Rainfall intensity will drop off the further east in KY you go and
into WV too. However, localized swaths up to 0.5"-0.75" in an hour
will be probable here as well...and FFG is much lower as you go
east. So even these lower rates should still result in numerous
flooding issues.

As of 08z watching blossoming convection over northeast AR...which
is expected to move into portions of KY this morning...and really
be the beginning of the increased flash flood risk. This activity
is associated with a core of enhanced 850mb moisture transport/convergence...which will shift northeast into KY, and
interact with the southward dropping cold front, as mentioned
above. This enhanced convergence should allow the convection to
persist and train over KY into this afternoon. By this afternoon,
instability increases ahead of another wave and uptick in low
level moisture transport. This should result in an uptick in
convective coverage from northeast TX into southern MO...which
will also track across the nearly stationary convergence axis over
KY. Given the higher instability by this point...rates later this
afternoon into the evening may be even higher over KY than what we
see this morning.

Overall, expecting to see a relatively narrow swath of 3-5" of
rain stretching across KY by the time the convection has pushed
east of the area tonight and the event ends. Given the
PWs/instability forecast, rates within this corridor should be
high enough at times to result in potentially significant flash
flooding. For that reason we have upgraded to a Moderate risk from
far northern TN into a good portion of KY and southern WV. Expect
flooding to become more widespread with time as the rain continues
and soils saturate...and the potential is there for localized
significant flash flood impacts where training ends up maximized.

Further east over WV, rates will not be as high...but the
prolonged nature of steady rain, with embedded heavier convective
cells, should still result in enough of a flood risk warrant the
Moderate risk. The southern extent of the Moderate risk, from
northern TN into far southern KY, may very well see a lull in
convection for much of the day. However activity will eventually
move back into these areas this evening as the front begins to
more rapidly push eastward. So while additional rainfall here
should not be a high...this area has low FFG due to saturated
soils from rainfall over the past 24 hours...warranting their
inclusion in the Moderate risk.

The Slight risk extends southwestward into portions of eastern AR,
the rest of TN, and northern MS/AL as well. Convection is expected
to generally be more progressive by the time it gets into these
areas...as we will have a stronger push of the front from upstream
forcing. Nonetheless, the very high PWs, and 1000-200 J/KG of
instability, supports heavy rainfall rates with the activity.
Also, much of this region is showing deep layer soil saturation
from recent rainfall events...suggesting much of these higher
rates should end up as runoff. So while the magnitude and coverage
of flash flooding may not end up as high here as areas further
north and east...we would still expect to see scattered flash
flooding over this region.

Chenard


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...

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

Day 3
Valid 12Z Tue Mar 02 2021 - 12Z Wed Mar 03 2021

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

A mid-level closed low emerging from the Lower MS Valley will
become an open wave as it traverses the Southeast through the
forecast period. This will coincide with the right entrance region
of an upper level jet streak promoting strong synoptic scale
ascent and thus surface low development along a residual surface
boundary draped across the region. In response, return flow ahead
of the aforementioned low will promote isentropic ascent along and
north of the surface boundary with periods of moderate to heavy
rain moving east.

While precipitable water values will surge to over 1.5 inches
aided by 20-30 knot southerly flow, instability will be modest
with MUCAPE struggling to approach 500 J/kg. Therefore, rain
rates should not be excessive. The main concern will be the
multiple rounds of precipitation that could lead to training along
the warm front ahead of the surface low. This could lead to
locally higher 1 and 3 hourly rainfall totals. Based on the 00Z
guidance (favoring the GFS/EC and their means), the highest QPF
footprint is forecast to reside across portions of
southern/central AL/GA into southern SC at around 1-3+ inches.

While there is clearly a signal for heavy rainfall during the
forecast period, the dry antecedent conditions, resultant high FFG
values and soil type, especially south of the Fall Line, would
suggest that this region could handle the expected precipitation
and rates. However, if models trend more aggressive with the
mid-level shortwave and thus heavier precipitation trains along
the surface boundary, then runoff could be a concern. For now,
feel the mode of flooding impacts will be areal in nature with
only isolated occurrences of flash flooding. Therefore, a
Marginal Risk was introduced.

Pagano

$$
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FOUS30 KWBC 012030
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
329 PM EST Mon Mar 01 2021

Day 1
Valid 16Z Mon Mar 01 2021 - 12Z Tue Mar 02 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF EASTERN TEXAS AND THE LOWER MISSISSIPPI VALLEY...

Persistent, broad isentropic lift over a front will result in a
continuation of steady rainfall over the region into tonight.
Better forcing arrives overnight as the closed low over the
southern Plains ejects eastward and the degree of forcing and PWs
in place would usually suggest an increased flash flood risk.
However, instability is expected to continue its slow decline as
the front settles southeast. While rainfall intensity may tick up
tonight, hourly totals should struggle to get much above 0.5".
The heaviest rain is also expected to generally remain south of
areas with the highest soil saturation. These factors combined
suggest only a rather low and localized flash flood risk.
Localized amounts of 2-3" remain possible from eastern TX into
northern LA and western MS...so even though the flash flood risk
is low...some pockets of minor poor drainage and low lying
flooding are a possibility...so the Marginal risk will be
maintained. The area was adjusted southward per trends in radar
reflectivity and the 12z HREF guidance, which shows a second
skirmish of heavy rainfall possible where some locally heavy rain
occurred this morning in south-central MS.

Roth/Chenard


Day 2
Valid 12Z Tue Mar 02 2021 - 12Z Wed Mar 03 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE SOUTH AND SOUTHEAST...

2030 UTC Update --

Very minor changes were made to the Day 2 ERO. Guidance has
trended toward a more frontogenetic look with the QPF distribution
(i.e. narrowing axis of heavier rainfall), which makes sense given
the vigorous albeit flat upper shortwave traversing through strong zonal/westerly flow. Within the Marginal Risk area, the latest
CAMs are depicting high probabilities of 1.25-1.5" 3-hourly rates
between 00-12Z Wed, though based on projected instability (CAPEs), probabilities of 2+ inches within 3 hours drops off considerably.
Would therefore continue to expect minimal/isolated runoff/flash
flood impacts with these rates, particularly across
southern-eastern GA into SC where 2 week rainfall anomalies are
currently running 200-400% of normal.

0830 UTC Discussion --

A mid-level closed low will transition from the Southern Plains
toward the southern Appalachians through the forecast period.
Ahead of this feature, deep layer return flow from the Gulf of
Mexico will surge north into the Deep South and Southeast. With
strong divergence aloft within the right entrance region of a
departing jet, surface cyclogenesis will take shape along the
northern Gulf Coast region, just off the LA coast on Tuesday. As
the surface low tracks east, periods of heavy showers and
thunderstorms will develop across portions of southern AL/GA/SC
and northern FL through the forecast period.

Early Tuesday morning, a strengthening warm conveyor along and
ahead of an associated cold front should enhanced elevated showers
and thunderstorms across a training line over central MS. As the
aforementioned shortwave approaches, warm air advection will
continue to increase helping the better moisture feed and
instability surge north. Precipitation water values will climb
above 1.5 inches aided by 30-40+ knot southerly low level flow.
This is easily over 1.5 standard deviations above the mean across
this region. The bulk of the instability (albeit weak) should
remain adjacent to the Gulf and Southeast coast associated with
the position of the warm front. However, elevated instability is
expected north of this boundary with unidirectional flow leading
to training convection across southern/central AL/GA and into
southern SC by early Wednesday morning.

Areal average precipitation increased at this forecast update to
1.5-3 inches with locally higher amounts expected. Hourly rain
rates will likely not exceed 0.75 inches given the limited
instability discussed above. However, due to training and
multiple rounds of moderate to heavy rain, 3-hourly rainfall
totals could easily surpass 1.5 inches. While these values do not
exceed FFG as it stands right now, if soils receive an abundance
of rain over a short period of time, FFG could lower quickly
allowing soils to become more vulnerable to flooding. At this
time, based on the antecedent conditions and soil type, feel areal
flooding will be the main threat with localized flash flooding
possible. Therefore, the Marginal Risk was maintained and refined
based on the latest 00Z model guidance, resultant WPC QPF forecast
and ensemble probabilities.

Hurley/Pagano

Day 3
Valid 12Z Wed Mar 03 2021 - 12Z Thu Mar 04 2021

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

Hurley

$$
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FOUS30 KWBC 021950
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
250 PM EST Tue Mar 02 2021

Day 1
Valid 16Z Tue Mar 02 2021 - 12Z Wed Mar 03 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHEAST...

A swath of 2-3" of rain remains likely from southern AL into
southern GA and far southern SC. A strong mid level closed low
moves east, bringing upper level divergence within the right
entrance region of the 300mb jet, and strong low level
frontogenesis into the area. The main ingredient missing for a
flash flood threat is instability. The most organized area of
rain should end up focused close to the intersection of the front
and the 850 hPa pressure level, and little instability gets that
far north. A long duration steady to locally heavy rain event is
expected. Some embedded heavier convective elements are
possible, which should locally peak around 0.5-0.7" in an hour,
and around 1.25" in three hours. This should generally stay well
below flash flood guidance. Portions of the area (mainly eastern
GA into SC) have elevated 0-200cm soil saturation levels, and some
above average stream/river levels, so the 2-3" of rain over these
areas may result in some localized runoff issues. The Marginal
risk has been maintained, with the impacts expected to be
localized minor areal flooding, poor drainage flooding and ponding
of water in low lying areas.

Slightly higher rates are possible over the FL Panhandle into
north FL, as the surface warm front tries to lift into these
areas. However, convection here should be pretty quick moving
(with the better rainfall persistence further north near the 850mb
front). For that reason would not expect much in the way of a
flash flood risk with this convection...although hourly rain
totals could locally approach 1.5" if robust convection does
develop. The area depicted by the Marginal was unchanged in the
midday update per input from the National Water Center this
morning and recent radar reflectivity trends and the 12z HREF
probabilities of 0.5"+ in an hour.

Roth/Chenard


Day 2
Valid 12Z Wed Mar 03 2021 - 12Z Thu Mar 04 2021

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

Campbell

Day 3
Valid 12Z Thu Mar 04 2021 - 12Z Fri Mar 05 2021

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

Campbell


$$
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FOUS30 KWBC 021950
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
250 PM EST Tue Mar 02 2021

Day 1
Valid 16Z Tue Mar 02 2021 - 12Z Wed Mar 03 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHEAST...

A swath of 2-3" of rain remains likely from southern AL into
southern GA and far southern SC. A strong mid level closed low
moves east, bringing upper level divergence within the right
entrance region of the 300mb jet, and strong low level
frontogenesis into the area. The main ingredient missing for a
flash flood threat is instability. The most organized area of
rain should end up focused close to the intersection of the front
and the 850 hPa pressure level, and little instability gets that
far north. A long duration steady to locally heavy rain event is
expected. Some embedded heavier convective elements are
possible, which should locally peak around 0.5-0.7" in an hour,
and around 1.25" in three hours. This should generally stay well
below flash flood guidance. Portions of the area (mainly eastern
GA into SC) have elevated 0-200cm soil saturation levels, and some
above average stream/river levels, so the 2-3" of rain over these
areas may result in some localized runoff issues. The Marginal
risk has been maintained, with the impacts expected to be
localized minor areal flooding, poor drainage flooding and ponding
of water in low lying areas.

Slightly higher rates are possible over the FL Panhandle into
north FL, as the surface warm front tries to lift into these
areas. However, convection here should be pretty quick moving
(with the better rainfall persistence further north near the 850mb
front). For that reason would not expect much in the way of a
flash flood risk with this convection...although hourly rain
totals could locally approach 1.5" if robust convection does
develop. The area depicted by the Marginal was unchanged in the
midday update per input from the National Water Center this
morning and recent radar reflectivity trends and the 12z HREF
probabilities of 0.5"+ in an hour.

Roth/Chenard


Day 2
Valid 12Z Wed Mar 03 2021 - 12Z Thu Mar 04 2021

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

Campbell

Day 3
Valid 12Z Thu Mar 04 2021 - 12Z Fri Mar 05 2021

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

Campbell


$$
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FOUS30 KWBC 032007
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
307 PM EST Wed Mar 03 2021

Day 1
Valid 16Z Wed Mar 03 2021 - 12Z Thu Mar 04 2021

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

Fracasso


Day 2
Valid 12Z Thu Mar 04 2021 - 12Z Fri Mar 05 2021

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

Campbell


$$
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FOUS30 KWBC 041943
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
242 PM EST Thu Mar 04 2021

Day 1
Valid 16Z Thu Mar 04 2021 - 12Z Fri Mar 05 2021

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

Fracasso



Day 2
Valid 12Z Fri Mar 05 2021 - 12Z Sat Mar 06 2021

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

Taylor

Day 3
Valid 12Z Sat Mar 06 2021 - 12Z Sun Mar 07 2021

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

Taylor

$$
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FOUS30 KWBC 052017
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
316 PM EST Fri Mar 05 2021

Day 1
Valid 16Z Fri Mar 05 2021 - 12Z Sat Mar 06 2021

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

Otto

Day 2
Valid 12Z Sat Mar 06 2021 - 12Z Sun Mar 07 2021

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

Bann


Day 3
Valid 12Z Sun Mar 07 2021 - 12Z Mon Mar 08 2021

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

Bann


$$
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FOUS30 KWBC 060654
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
153 AM EST Sat Mar 06 2021

Day 1
Valid 01Z Sat Mar 06 2021 - 12Z Sat Mar 06 2021

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

Hurley


Day 2
Valid 12Z Sun Mar 07 2021 - 12Z Mon Mar 08 2021

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

Roth


Day 3
Valid 12Z Mon Mar 08 2021 - 12Z Tue Mar 09 2021

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

Roth


$$
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FOUS30 KWBC 062013
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
312 PM EST Sat Mar 06 2021

Day 1
Valid 16Z Sat Mar 06 2021 - 12Z Sun Mar 07 2021

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

...Eastern Florida Peninsula...
There is a non-zero (but sub-Marginal Risk) threat for flash
flooding along central portions of the eastern Florida Peninsula
today and early tonight, in advance of a low-mid-level trough
approaching from the eastern Gulf Coast. Total precipitable water
values are near average for this time of year (1.0 to 1.3 inches)
but kinematic profiles support slow mean storm motions and weak
low level flow but 50 to 60 kt near 500 mb from the west. Vertical
motions may be augmented by the right entrance region of a
forecast 130 kt upper level jet streak just east of the Florida
coast. Stronger cells elevated above cold pools will have the
greatest threat to produce localized totals in excess of 3 inches,
but only weak instability is expected throughout the day/night.
Given drier than average antecedent conditions over the past 2
weeks, the small spatial threat of heavy rainfall and limited
coverage, the threat for flash flooding is less than 5 percent.

Otto


Day 2
Valid 12Z Sun Mar 07 2021 - 12Z Mon Mar 08 2021

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

Bann


Day 3
Valid 12Z Mon Mar 08 2021 - 12Z Tue Mar 09 2021

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

Bann

$$
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FOUS30 KWBC 072013
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
312 PM EST Sun Mar 07 2021

Day 1
Valid 16Z Sun Mar 07 2021 - 12Z Mon Mar 08 2021

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

Otto


Day 2
Valid 12Z Mon Mar 08 2021 - 12Z Tue Mar 09 2021

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

Bann



Day 3
Valid 12Z Tue Mar 09 2021 - 12Z Wed Mar 10 2021

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

Bann

$$
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FOUS30 KWBC 082040
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
339 PM EST Mon Mar 08 2021

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

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

Chenard


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

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

Otto


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

...A MARGINAL RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF
THE CENTRAL PLAINS AND MIDWEST...

...Eastern Kansas into western Illinois...
Thunderstorms are expected to form along a cold front moving
southeastward through the Central Plains and Middle Mississippi
Valley Wednesday evening into Thursday morning. A resurgence of
850 mb flow from the southwest, from 40-60 kt) is expected from
the Southern Plains into the Middle Mississippi Valley between
00Z/11 and 06Z/11 as subtle low-mid level height falls push out
into the Plains after 00Z/11. Thunderstorms are forecast to
develop with the increase in 850 mb flow, within a corridor of
CAPE expected to range between 1000 and 2000 J/kg ahead of the
cold front from central Kansas into central Oklahoma. Precipitable
water values are forecast by the 12Z model consensus to climb to
near 1.25 inches which should support rainfall rates of at least 1
in/hr, locally higher. The concern for training arises given mean
steering flow is parallel to 850 mb flow and of similar magnitude.
This would support an axis of heavy rain oriented from southwest
to northeast, with the greatest potential for higher amounts to
the south and west where frontal movement will be slower, and
instability will be higher.

Current flash flood guidance values are as low as 2 inches in 3
hours for a large portion of Kansas into Missouri and Illinois.
Factors against flash flooding include the dry antecedent
conditions and somewhat modest moisture values, despite the
moisture being rather anomalous for this time of year (+2 to +3
standardize anomalies of PW). The Marginal Risk area was
coordinated with area WFOs and encompasses some spatial spread as
seen in the latest deterministic/ensemble guidance, extending from
the eastern Kansas/Missouri border into central/northern Missouri,
western Illinois and far southeastern Iowa.

Otto

$$
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FOUS30 KWBC 092048
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
347 PM EST Tue Mar 09 2021

Day 1
Valid 16Z Tue Mar 09 2021 - 12Z Wed Mar 10 2021

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

Pagano


Day 2
Valid 12Z Wed Mar 10 2021 - 12Z Thu Mar 11 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
NORTHERN WISCONSIN...

East of a surface low pressure system tracking northeastward
across the Central Plains into the Upper Midwest, a front is
forecast to meander over northern Wisconsin during the day on
Wednesday, with moisture pooling along it. Precipitable water
values should be over 1", over the 95th percentile for this time
of year. Some elevated CAPE of 100-250 J/kg could enter the region
in the evening, leading to rain rates exceeding 0.5". These
marginally heavy rain rates and 1-2" of rain total could become a
problem particularly because the rain is falling on snowpack
containing over 2" of liquid equivalent. The rain could cause
rapid melting and thus runoff concerns, allowing for especially an
areal flooding threat/potential for riverine flooding, but flash
flooding cannot be ruled out with these conditions.

A Marginal Risk was removed this cycle from in and around
Missouri, as it appears the cold front should progress a little
faster through the period than previously forecast. This will lead
to a shorter time frame for any localized heavy rain to fall in
any particular area, and thus less potential for training through
12Z Thu.

Tate


Day 3
Valid 12Z Thu Mar 11 2021 - 12Z Fri Mar 12 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE MIDDLE MISSISSIPPI AND LOWER OHIO VALLEYS...

The main change to the previous cycle's Marginal Risk area was to
extend it farther west into Missouri due to the frontal system
slowing down after 12Z Thursday. Instability is forecast to make
its way into the area; the ECMWF for example has pockets of over
1000 J/kg of MUCAPE in southern Missouri, which could increase
rain rates, and the slow-moving front could lead to training
issues.

See the previous discussion below for more details.

Tate

...Previous Discussion...

A front slows down as it moves into the OH Valley as the shortwave
urging it along progresses across the Great Lakes and an upper low
digs across the Southwest. This becomes a problem late in the
period as heavy rain shifts into a region that has received above
average rainfall where streamflow remains near to above average
per National Water Model output and the flow becomes increasingly unidirectional out of the west, with low-level inflow remaining
substantial (40-50 kts). Precipitable water values rise to
1.25-1.5" and a pool of instability lies upstream across the
Southern Plains and Mid-Mississippi Valley. While there is model
dispersion in this region, the threat of organized and potentially
training convection late in the period (00-12z on the 12th)
remains sufficient to raise a Marginal Risk area for the region as
a precaution. Guidance appears to be trending south, which if it
continued would lead to an uptick in the QPF in the region on
later WPC cycles and potentially raise the flash flood threat.

Roth

$$
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FOUS30 KWBC 102024
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
323 PM EST Wed Mar 10 2021

Day 1
Valid 16Z Wed Mar 10 2021 - 12Z Thu Mar 11 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
NORTHERN WISCONSIN AND THE UPPER PENINSULA OF MICHIGAN...

...16Z Update...
Moisture continues to stream into the Upper Midwest from the
Central Plains this morning ahead of a deepening surface low that
is progged to track along a stationary boundary through the day.
There is some concern that as the surface low approaches, the once
stationary front will advance a little farther north than
previously forecast (as a warm front). This will likely result in
the axis of heavier precipitation shifting north as well; mainly
far northern WI. Therefore, expanded the Marginal Risk slightly to
account for this. Based on the current radar, estimated rain
rates are around or below 0.50 inches per hour, which is below
FFG. Anticipate this trend to continue with a dry slot possible by
mid afternoon. Regardless, with multiple rounds of precipitation
expected through the next several hours falling atop cold surface
temperatures and snow pack, there could be run-off and thus
localized flooding concerns. Apart from the aforementioned minor
adjustment to the Marginal Risk area, the forecast is on track.

Pagano

...Previous Discussion...
There will be a threat of some excess runoff and flooding today
and tonight in the warm sector of a deepening low, due to a
combination of rain and snowmelt. Warm conditions over the past
couple days have already lead to some snowmelt, but there is still
a significant amount of liquid equivalent (over 2 inches in some
places) in the snow pack over northern Wisconsin and into the
Upper Peninsula of Michigan. Those are the areas with a chance of
some flooding, as rain rates are not likely to be sufficiently
high (or sustained long enough) to lead to flash flooding on their
own. Although models are projecting around 200-500 j/kg of MUCAPE
for convective rain bands or thunderstorms just east of the
surface low, hi-res models are showing hourly rain rates mostly
remaining below 1"/hr. In fact, 40km neighborhood probabilities of
just 0.5"/hr rain rates peak below 40 percent in northwest
Wisconsin.

Therefore, the Marginal Risk was concentrated in areas where the
highest probabilities of 1+ inch of rainfall in 6-12 hours overlap
with existing snow pack and the prospect of additional runoff from
snowmelt. It's worth noting there is still a substantial amount of
spread over the risk area related to rainfall totals and the
character of the warm sector air mass. Some models more
aggressively push higher dewpoints into the area, while others
more stubbornly retain colder air. At KPBH in north-central
Wisconsin by 16Z, the NBM has dewpoints +/- 1 standard deviation
from the mean at 50F and 40F respectively. Higher dewpoints and a
warmer air mass in general would promote more efficient snowmelt
if they materialize. And with respect to rainfall, some of the
newer hi-res model runs (particularly the HRRR) are pushing rain
through quickly and then enveloping most of the Marginal Risk area
in a dry slot after about 18-21Z. The global models have a
stronger signal for heavy rain. This will need to be monitored
closely, but the probabilities of 1+ inch of rain in 12 hours seem
to be high enough even on the HREF (around 60 percent) to warrant
the Marginal Risk.

Lamers


Day 2
Valid 12Z Thu Mar 11 2021 - 12Z Fri Mar 12 2021

...A MARGINAL RISK OF EXCESSIVE RAINFALL EXISTS FOR PORTIONS OF
THE MIDDLE MISSISSIPPI AND LOWER OHIO VALLEYS...

A slow moving front that eventually stalls and washes out over
portions of mid-MS and lower-OH river valleys will become the
focus for heavy rainfall during the day 2 time frame. Along and
ahead of the front, a surge of higher than normal precipitable
water (1.25-1.5") and sufficient instability (upwards of 1000 J/kg
MUCAPE) will accompany the warm sector. Precipitation should break
out early in the period across portions of IL/IN then the focus
sags south/southwest as the primary low/shortwave quickly lifts
through the Great Lakes and the trailing front stalls over MO.
Here, the mean flow will become increasingly more parallel (westerly/southwesterly) to the expected storm motions and this
could favor some localized training convection. The heaviest QPF
axis is expected to fall along/north of the Ohio River and
westward into southeastern MO, generally north of the heaviest
rainfall footprint from a couple weeks ago in KY. Other areas in
MO and southern IL have also seen relatively drier than normal
soil conditions. Despite this, the environmental conditions and
potential training does suggest a localized flash flood threat and
given these reasons, the Marginal Risk was maintained for the
update.

Taylor


$$
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FOUS30 KWBC 112012
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
310 PM EST Thu Mar 11 2021

Day 1
Valid 16Z Thu Mar 11 2021 - 12Z Fri Mar 12 2021

...A MARGINAL RISK OF EXCESSIVE RAINFALL EXISTS FOR PORTIONS OF
THE MIDDLE MISSISSIPPI AND LOWER OHIO VALLEYS...

...16Z Update...
A line of showers and thunderstorms will continue to cross the OH
Valley (mainly IN into western OH) through the next several hours.
This activity should wane by early/mid afternoon as it gets
farther removed from the best frontogenesis and upper level
forcing. As warm air advection, moisture flux and instability
increases coupled with the approach of mid-level impulses, expect
thunderstorm activity along the front (especially across mid-MS
Valley region) to blossom through the afternoon. This
precipitation will advance east into the OH Valley along the cold
front resulting in training of moderate to heavy rain. Based on
the latest 12Z high resolution model guidance, the previous
forecast is on track with only minor changes made to the risk
area.

Pagano

...Previous Discussion...
A slow moving front that eventually stalls and washes out over
portions of mid-MS and lower-OH river valleys will become the
focus for heavy rainfall during the day 1 time frame. Along and
ahead of the front, a surge of higher than normal precipitable
water (1.25-1.5" or around +2 standard deviations above normal)
and sufficient instability (500-1000 J/kg MUCAPE) will accompany
the warm sector. Low-level frontogenesis will be increasing from southwest-northeast across the outlook area, especially overnight
as the upper level jet streak (150+ kts at 250 mb) slowly migrates
across the Upper Midwest and the Lower Great Lakes. Low-level
moisture transport into the frontal zone is at best modest;
however, as the surface frontal progression begins to slow down
(becoming more w-e oriented), the potential for cell training will
be on the increase. This would especially apply later Thursday
afternoon and overnight, coinciding with the aforementioned
increase in low-level frontogenesis and as the 30-40+ kt 925-850
mb southwesterly flow begins to align more closely to the mean
850-300 mb wind. Deep-layer instability, mainly elevated, will be
somewhat limited as MUCAPEs generally top out between 500-1000
j/kg. This would limit the rainfall rates somewhat, however given
the enhanced signal for training convection, probabilities of
1.5-2 inches within a 3 hour period have come up per the most
recent HREF 40km neighborhood probabilities. Despite drying out
somewhat over the past week, the 14-day precipitation remains
between 150-300% of normal over much of the outlook area. Given
the aforementioned thermodynamic parameters, minor/isolated runoff
issues will be possible, and as a result, the Marginal Risk from
yesterday's Day 2 ERO remains in place, though with a slight
southward adjustment based on the latest high-res CAM trends.

Hurley/Taylor


Day 2
Valid 12Z Fri Mar 12 2021 - 12Z Sat Mar 13 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE MID SOUTH TO THE CENTRAL PLAINS...

20Z Update: The majority of the threat areas for Day 2 remain
similar for this forecast cycle. However, the axis of the Slight
Risk area was adjusted slightly southward by about 25 miles to
account for better 12Z CAM guidance clustering from north-central
Oklahoma to far western Kentucky. Both versions of the ARW are
indicating maxima on the order of 3 inches in the general vicinity
of extreme southeast Kansas, and into southwestern Missouri and
far northern Arkansas. The HRRR is likely the most pronounced
with the coverage of 2-3 inch rainfall totals. The previous
discussion is appended below for reference. /Hamrick

An an upper low moves through the Southwest and an upper trough
moves through the Northeast, low-level inflow backs to become more
south to southwest across the Southern Plains. Temperatures at
700 hPa slowly rise in the warm advection pattern, with the 6C
isotherm barely shifting into southern OK. This should act as the
southern fringe for active convection this period. The guidance
seems to be having its usual warm season issue with breaking out
convective precipitation in the High Plains (TX Panhandle and
vicinity) so manually increased amounts there from the model blend
utilized (which has been coming up recently as well). MU CAPE of
up to 2000 J/kg along with enough low-level inflow converging
into/over the front in the region and precipitable water values of
1.5" should allow for hourly rain totals to 2" at the northeast
side of the instability pool where non-traditional cell training
-- movement at a decent angle to the low-level inflow but along
the lines of the mid to upper level/mean wind flow -- occurs. The
rain in MO has begun and should continue for the next few days,
beginning to add up by Saturday. Despite initial dry conditions
over the past couple of weeks, soils should have saturated
sufficiently to lead to a greater flash flood/excessive rainfall
risk across southern MO, the epicenter of the Slight Risk area.
The wettest pieces of guidance continue to indicate local 4-5"
amounts across portions of OK and MO. The Slight Risk area is
similar to continuity, with few adjustments made this cycle.


Roth/Hamrick

Day 3
Valid 12Z Sat Mar 13 2021 - 12Z Sun Mar 14 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE CENTRAL PLAINS...

20Z Update: There has been a slight northwestward adjustment in
the Slight Risk area based on trends in the 12Z model guidance
suite. The deterministic guidance generally agrees that central
Kansas to south-central Nebraska would have the greatest rainfall
totals. The GFS stands out as a particularly heavy solution with
some 3-4 inch maxima within that general area, whereas other
guidance is more on the order of 1.5 to 3 inches. In addition,
the western edge of the existing Marginal Risk was trimmed back
slightly to account for more winter weather across eastern
Colorado and west-central Nebraska. The previous discussion is
appended below for reference. /Hamrick

Sufficient inflow within a cyclone's warm conveyor belt
circulation brings 500-750 kg/m/s of IVT through TX and OK towards
KS. This leads to precipitable water values to 1.25-1.5"
generally east of the 100th meridian, with lower amounts into the
High Plains within the low's developing comma head. Guidance
indicates that MU CAPE of 500+ J/kg is possible within the
system's warm sector, which could be enough, combined with
rainfall the previous day, to lead to short duration heavy rain
issues (hourly totals up to 1-1.5") across portions of central and
eastern KS. Have raised a Slight Risk in this area to accommodate
this possibility and account for rainfall from the previous day.
A broad Marginal Risk covers areas that rained heavily on previous
days and should be more sensitive by Saturday and Saturday night.
Overall amounts in the 2-3" range, with locally higher totals, are
expected in and near the Slight Risk area.

Roth/Hamrick

$$
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FOUS30 KWBC 130828
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
327 AM EST Sat Mar 13 2021

Day 1
Valid 12Z Sat Mar 13 2021 - 12Z Sun Mar 14 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE CENTRAL PLAINS...

Based on the latest trends in the guidance, there were a few minor
adjustments to the Slight and Marginal Risk areas from yesterday's
Day 2 ERO across portions of the Central Plains. An expansive deep
and dynamic cyclone will migrate slowly east across the Southwest
and Four Corners, then into the southern Rockies by early Sunday.
During this time the warm conveyor belt will continue to
mature/amplify, leading to a deepening TROWAL extending
north-northwest across much of KS/NE Saturday night. A robust LLJ,
especially Saturday night as S-SE 850 mb flow increases to 50-60
kts, will result in equally vigorous moisture transport into the
outlook areas. 850-700 mb moisture flux standardized anomalies are
around +5 across the Plains per the SREF and GEFS. Models still
depict IVT values in the range of 500-750+ kg/m/s from portions of
Texas and Oklahoma into Kansas, with the nose of the IVT making
its way north-northwestward with time. Overall, the models still
point to PW values peaking around 1.25-1.35 inches being in place
ahead of a cold front associated with the synoptic scale cyclone,
with warm-sector instability on the order of 1000-1500 j/kg of
MUCAPE per the latest CAMs lifting across western-central OK and
KS. Deep-layer instability is expected to taper to around 500 j/kg
east of the Slight Risk area, i.e. within the Marginal Risk area
across eastern KS and southern MO. Strong dynamical forcing along
with the more impressive thermodynamical profiles will support 24
hour areal-average rainfall totals of 1.5-2.5 inches across much
of the Slight Risk area, with localized totals of 3-5 inches
expected per the latest CAMs. Short-term rainfall rates will be
somewhat limited (capped) by the relative lack of instability and
PWs under 1.5 inches. However, the slow-moving WCB and eventual
pivoting TROWAL will maintain the strong deep-layer moisture flux
over the Slight Risk area for several hours, with training of
elevated convective segments likely to enhance the flash flood
risk over time (especially overnight Saturday).

Hurley


Day 2
Valid 12Z Sun Mar 14 2021 - 12Z Mon Mar 15 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE LOWER MISSOURI VALLEY...

The potential remains for some heavy to potentially excessive
rainfall over portions of the Upper Midwest into the Lower
Missouri Valley on Sunday as a deep synoptic-scale cyclone begins
to make its way out onto the Plains. There continues to be
general agreement with respect to strong south to southeasterly
flow in the low levels drawing an airmass with PW values
approaching an inch northward ahead of the system. This results
in decent agreement among the models for an axis of heavy precip
in the 1-2"+ range in the axis of much above average pw values
along and ahead of the above mentioned front. Precipitation
should be more progressive to the northeast during day 3 compared
to day 2, which is resulting in lesser areal average amounts
compared to day 2. However, with ffg values lower across this
region than areas upstream, there will continue to be at least a
marginal risk of runoff issues. This may be especially so across
portions of southern Missouri where a narrow axis of heavy rains
occurred over the past 24 hours.

Bann

Day 3
Valid 12Z Mon Mar 15 2021 - 12Z Tue Mar 16 2021

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

Bann

$$
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FOUS30 KWBC 131934
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
233 PM EST Sat Mar 13 2021

Day 1
Valid 16Z Sat Mar 13 2021 - 12Z Sun Mar 14 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE CENTRAL PLAINS...

1530Z update: Minor adjustments made to the Slight Risk area for
this update with some southward expansion into portions of the
Texas Panhandle and western Oklahoma, where previous 24 hour
rainfall totals of 1-2" (locally 3") were observed. Latest
guidance remains locked on the idea of blossoming deep convection
later in the period as low pressure organizes and deepens over the
southern Plains. Anomalously high moisture transport and low-level
jet approaching 60 kts this evening/tonight with sufficient
instability in place should focus the strongest convection over
the TX Panhandle into western OK where the latest HREF
probabilities hint at 2"+ hour rates. This could result in a
localized flash flood threat where training develop given some
parallel orientation to the deep layer mean flow. Further north,
the WCB and TROWAL setup will pivot an area of widespread moderate
to embedded heavier elevated convection over portions of KS/NE.
While rates will be lower (generally less than 1"/hr), the longer
duration poses a flood threat too.

Taylor

previous discussion:
Based on the latest trends in the guidance, there were a few minor
adjustments to the Slight and Marginal Risk areas from yesterday's
Day 2 ERO across portions of the Central Plains. An expansive deep
and dynamic cyclone will migrate slowly east across the Southwest
and Four Corners, then into the southern Rockies by early Sunday.
During this time the warm conveyor belt will continue to
mature/amplify, leading to a deepening TROWAL extending
north-northwest across much of KS/NE Saturday night. A robust LLJ,
especially Saturday night as S-SE 850 mb flow increases to 50-60
kts, will result in equally vigorous moisture transport into the
outlook areas. 850-700 mb moisture flux standardized anomalies are
around +5 across the Plains per the SREF and GEFS. Models still
depict IVT values in the range of 500-750+ kg/m/s from portions of
Texas and Oklahoma into Kansas, with the nose of the IVT making
its way north-northwestward with time. Overall, the models still
point to PW values peaking around 1.25-1.35 inches being in place
ahead of a cold front associated with the synoptic scale cyclone,
with warm-sector instability on the order of 1000-1500 j/kg of
MUCAPE per the latest CAMs lifting across western-central OK and
KS. Deep-layer instability is expected to taper to around 500 j/kg
east of the Slight Risk area, i.e. within the Marginal Risk area
across eastern KS and southern MO. Strong dynamical forcing along
with the more impressive thermodynamical profiles will support 24
hour areal-average rainfall totals of 1.5-2.5 inches across much
of the Slight Risk area, with localized totals of 3-5 inches
expected per the latest CAMs. Short-term rainfall rates will be
somewhat limited (capped) by the relative lack of instability and
PWs under 1.5 inches. However, the slow-moving WCB and eventual
pivoting TROWAL will maintain the strong deep-layer moisture flux
over the Slight Risk area for several hours, with training of
elevated convective segments likely to enhance the flash flood
risk over time (especially overnight Saturday).

Hurley


Day 2
Valid 12Z Sun Mar 14 2021 - 12Z Mon Mar 15 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM FAR NORTHEAST
OKLAHOMA INTO SOUTHERN MISSOURI...


2100 UTC update

A slight risk area was added to the updated day 2 Excessive
Rainfall Outlook from far northeast Oklahoma into southern
Missouri. The marginal risk was also extended westward by 75-175
nm across southeast Kansas into northeast Oklahoma. These changes
were to cover the axis of where heavy rains have fallen over the
past 24 to 48 hours (2-5"), resulting in high stream flow as per
the National Water Model and lowered ffg values. Well defined
frontal convergence in an axis of above average pw values, 2 to 3
standard deviations above the mean, along and ahead of the cold
front forecast to push eastward Sunday from the eastern portions
of the Southern Plains into the lower Mississippi will support an
organized area of moderate to heavy rainfall pushing across these
areas. While the precip will be progressive overall, the lower
ffg values and high stream flows will increase the risk of runoff
issues from additional precip totals of 1"+.

Oravec



Day 3
Valid 12Z Mon Mar 15 2021 - 12Z Tue Mar 16 2021

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

Oravec

$$
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FOUS30 KWBC 140852
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
451 AM EDT Sun Mar 14 2021

Day 1
Valid 12Z Sun Mar 14 2021 - 12Z Mon Mar 15 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM FAR NORTHEAST
OKLAHOMA AND SOUTHEAST KANSAS INTO SOUTHERN MISSOURI AND NORTHERN
ARKANSAS...

The Slight Risk area that was added to yesterday's Day 2 ERO was
expanded to include more of the Ozarks in southern MO and northern
AR, while extending farther east toward the Bootheel of MO. This
was based largely on the heavy rainfall footprint this region has
observed since Thursday (3 to 7+ inches). The recent heavy
rainfall has resulted in high stream flow per the National Water
Model, along with lowered FFG values. Additional areal-average
rainfall of 1-2 inches, with localized amounts of 3+ inches per
the CAMs, are expected within the Slight Risk area. Rainfall rates
will be capped somewhat given the limited elevated instability
(MUCAPEs ~500 j/kg) along with PWs ~1.25" ahead of the surface
cold/occluded fronts. However, per the latest CAMs, spotty 1-1.25
inch/hr and 1.5 inch/3hr rates are likely, and would pose a more
enhanced flash flood threat within the Slight Risk area given the
recent heavy rains and resultant lowering FFGs (down to 1"/hr and
1.5"/3hr over most locations).

Elsewhere, the marginal risk was extended westward across eastern
KS and NE, in light of the widespread moderate-heavy rain during
the predawn hours along with an additional 1-2+ inches expected
after 12Z Sunday. Deep-layer elevated instability within the
robust Warm Conveyor Belt (WCB) and eventually pivoting TROWAL is
meager at best north of the Slight Risk area (MUCAPEs generally
below 500 j/kg); however, the persistent, widespread rainfall with
bursts of heavier rain rates (0.50-1.00"/hr) could certainly lead
to isolated runoff issues by Sunday afternoon and overnight.

Hurley






Day 2
Valid 12Z Mon Mar 15 2021 - 12Z Tue Mar 16 2021

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

Bann


Day 3
Valid 12Z Tue Mar 16 2021 - 12Z Wed Mar 17 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PARTS OF THE
PLAINS TUESDAY AND TUESDAY NIGHT OVER PARTS OF THE PLAINS...

...Kansas...
Another area of low pressure will be ejecting from the Rockies
during the day on Tuesday. Low level winds back and strengthen
ahead of the low pressure system, drawing moisture back into
Southern and Central Plains by Tuesday evening which sets the
stage for another round of moderate to heavy rainfall. The
mid-level closed low will be deepening as it moves east of the
Rockies, which results in 850 mb flow backing and accelerating
into the 30 to 45 kt range over Oklahoma late Tuesday night and
early Wednesday. It is expected that precipitation will become
more widespread over southern Kansas during the night in response
to the increasing flow of moisture interacting with a
quasi-stationary or slowly moving warm front at 850 mb. In
addition, the GFS showed a well-define divergence maximum in the
400-200 mb layer moving out of New Mexico and Colorado which
becomes even better defined over southwest and south-central
Kansas by 17/09Z. The model QPF signal is mixed in terms of
placement and amounts were generally under 1.5 inches. That may be
an artifact of the PW values not being highly anomalous or in the
way the any particular model handles the placement of the surface,
low level synoptic-scale boundaries or the magnitude of the upper
level divergence. But a Slight Risk area appears to be warranted
given the agreement of the GEFS ensemble fields fitting the mass
fields/QPF of the ECMWF and UKMET runs combined with the fact that
some of these areas in Kansas have received 1 to 2+ inches of
precipitation since late Friday.

Bann

$$
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FOUS30 KWBC 141950
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
350 PM EDT Sun Mar 14 2021

Day 1
Valid 16Z Sun Mar 14 2021 - 12Z Mon Mar 15 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM FAR NORTHEAST
OKLAHOMA AND SOUTHEAST KANSAS INTO SOUTHERN MISSOURI AND NORTHERN
ARKANSAS...

1600 UTC discussion

No changes made to the previous Excessive Rainfall Outlook. The
1200 UTC hi res guidance continue to emphasize an organized area
of heavy rains pushing eastward this period from the eastern
Portions of the Southern to Central Plains toward the Lower
Mississippi Valley with overall good continuity from the prior
forecast cycle. The forecast rationale listed below has not
changed.

Oravec

0900 UTC discussion

The Slight Risk area that was added to yesterday's Day 2 ERO was
expanded to include more of the Ozarks in southern MO and northern
AR, while extending farther east toward the Bootheel of MO. This
was based largely on the heavy rainfall footprint this region has
observed since Thursday (3 to 7+ inches). The recent heavy
rainfall has resulted in high stream flow per the National Water
Model, along with lowered FFG values. Additional areal-average
rainfall of 1-2 inches, with localized amounts of 3+ inches per
the CAMs, are expected within the Slight Risk area. Rainfall rates
will be capped somewhat given the limited elevated instability
(MUCAPEs ~500 j/kg) along with PWs ~1.25" ahead of the surface
cold/occluded fronts. However, per the latest CAMs, spotty 1-1.25
inch/hr and 1.5 inch/3hr rates are likely, and would pose a more
enhanced flash flood threat within the Slight Risk area given the
recent heavy rains and resultant lowering FFGs (down to 1"/hr and
1.5"/3hr over most locations).

Elsewhere, the marginal risk was extended westward across eastern
KS and NE, in light of the widespread moderate-heavy rain during
the predawn hours along with an additional 1-2+ inches expected
after 12Z Sunday. Deep-layer elevated instability within the
robust Warm Conveyor Belt (WCB) and eventually pivoting TROWAL is
meager at best north of the Slight Risk area (MUCAPEs generally
below 500 j/kg); however, the persistent, widespread rainfall with
bursts of heavier rain rates (0.50-1.00"/hr) could certainly lead
to isolated runoff issues by Sunday afternoon and overnight.

Hurley






Day 2
Valid 12Z Mon Mar 15 2021 - 12Z Tue Mar 16 2021

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

Taylor


Day 3
Valid 12Z Tue Mar 16 2021 - 12Z Wed Mar 17 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PARTS OF THE
PLAINS TUESDAY AND TUESDAY NIGHT OVER PARTS OF THE PLAINS...

In response to a closed mid/upper level low tracking from the Four
Corners region to the southern Plains and its associated upper
level divergence, an area of low pressure is forecast to develop
and deepen over portions of TX/OK Tuesday. Low level flow will
increase Tuesday night and bring with it anomalously higher
precipitable water values (1-2 standard deviations above normal).
This higher moisture transport nosing impinging on an east/west
boundary draped in the area will allow for convection to fire up
across southern Kansas/northern Oklahoma. The latest model
guidance suggests upwards of 1000-1500 J/kg of MUCAPE will be
available. Overall model guidance has not changed too much from
the previous cycle where 1-2 inches (locally 3") will be possible.
This activity falling on areas that are running slightly above
normal for precipitation the last 14 days (thanks to the recent
rainfall from March 13), could result in some instances of flash
flooding. The Slight Risk was maintained and adjusted minimally to
account for the latest trends.

Taylor

$$
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FOUS30 KWBC 151949
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
349 PM EDT Mon Mar 15 2021

Day 1
Valid 16Z Mon Mar 15 2021 - 12Z Tue Mar 16 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF NORTHERN ALABAMA INTO NORTHWEST GEORGIA...

16Z update:
No changes to the ERO based on the latest guidance. A nose of
higher instability and moisture is expected to stall/slow over
Alabama later today, resulting in a couple rounds of convection
that could produce hourly totals in the 1-2" range. The 12Z hi-res
models support the idea of localized 3" totals. Just some minor
changes made to the Marginal Risk area.

---previous discussion---
The deep, occluded cyclone across the central CONUS early Monday
will begin to open at mid and upper levels later this afternoon
and tonight while lifting north and encountering the deep ridge
over the Upper MS Valley. Meanwhile, the Warm Conveyor Belt (WCB)
via the southern/subtropical branch will become increasingly
displaced from the cyclone -- eventually becoming more w-e
oriented across the Southeast by Monday night. At the surface, the
approaching cold front over the Lower MS Valley will begin to
stall/weaken with time, while the weak boundary south and
southwest of the Mid Atlantic CAD wedge remains quasi-stationary
over or near the outlook area. Despite the lackluster
frontogenesis, moisture transport across the outlook area will be
rather vigorous during the period, as 850 mb moisture flux
standardized anomalies range between +2 and +2.5 per the 00Z SREF
and GEFS. The guidance, especially the CAMs, continue to depict
multiple rounds of convection this afternoon and again tonight,
when in both instances a thermodynamic profile characterized by
MUCAPEs of 1000-2000 j/kg and PWs of 1.25-1.50" would support
pockets of 1-2"/hr rainfall rates.

The dry period over the last 7-10+ days has resulted in below
normal streamflow anomalies and a gradual reduction in soil
saturation per the latest National Water Model (NWM) output. The
expectation is that the multiple rounds of convection will likely
offset the antecedent soil conditions, especially overnight as the
increasing southwest low-level inflow (30-40 kts at 850 mb)
becomes nearly parallel to the mean 850-300 mb wind, thereby
enhancing the potential for training of convective segments.
Additional activity with peak rates between 1-2"/hr between 06-12Z
Tuesday might be sufficient to cause isolated/localized flash
flooding in areas that would encounter rapid runoff.

Hurley/Taylor



Day 2
Valid 12Z Tue Mar 16 2021 - 12Z Wed Mar 17 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PARTS OF THE
PLAINS TUESDAY AND TUESDAY NIGHT OVER PARTS OF THE PLAINS...

21Z update... The latest guidance persists in targeting
central/southern Kansas and the Oklahoma panhandle with the
highest rainfall; with a majority of the global guidance with
areal averages of 1 to 2 inches with isolated maximums near 3
inches. There was an increasing trend across southwest Kansas and
the Oklahoma panhandle with this update. The WPC QPF and
adjustments to the day 2 ERO reflected this trend.

Campbell

Previous discussion... In response to a closed mid/upper level low
tracking from the Four Corners region to the southern Plains and
its associated upper level divergence, an area of low pressure is
forecast to organize and deepen over portions of Texas and
Oklahoma on Tuesday. In response to the deepening area of low
pressure, low level flow will back and strengthen Tuesday night
and bring with it anomalously higher precipitable water values
(1-2 standard deviations above normal). This higher moisture
transport nosing impinging on an east/west boundary draped in the
area will allow for convection to fire up across southern
Kansas/northern Oklahoma. Overall model guidance has not changed
too much from the previous cycle where 1-2 inches (locally 3")
will be possible. This activity falling on areas that are running
slightly above normal for precipitation the last 14 days
(especially in response to the rainfall on March 13), could result
in some instances of flash flooding. The Slight Risk was
maintained and adjusted minimally to account for the latest
trends.

Bann

$$
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FOUS30 KWBC 160846
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
446 AM EDT Tue Mar 16 2021

Day 1
Valid 12Z Tue Mar 16 2021 - 12Z Wed Mar 17 2021

...THERE IS A MARGINAL RISK AREA FOR EXCESSIVE RAINFALL OVER PARTS
OF THE PLAINS AND DEEP SOUTH...

...Much of Kansas into northern Oklahoma and eastern portions of
the TX/OK Panhandles...
In response to a closed mid/upper level low tracking from the
Southwest and Four Corners region into the southern High Plains, a
compact area of robust upper level difluence and divergence will
allow a lee-side surface low to strengthen as it crosses into the
Red River Valley by early Wednesday morning. In response to the
deepening area of low pressure, low level flow will back and
strengthen Tuesday night and bring with it modestly anomalous
precipitable water values (1-1.5 standard deviations above
normal). This higher moisture transport nosing impinging on an
east/west boundary draped in the area will allow for convection to
fire up across southern Kansas/northern Oklahoma after 00Z Wed.
The guidance overall has not changed too much from the previous
cycle, where 1-2+ inches was noted. This activity falling on areas
that are running slightly above normal for precipitation the last
14 days (especially in response to the rainfall on March 13),
could result in some instances of flash flooding. However, per the
latest HREF exceedance probabilities, any instances of rapid
runoff potentially leading to flash flooding are expected to be
isolated or localized at most, and as a result, a Marginal Risk
was noted in the Day 1 ERO.


...Portions of Mississippi into Alabama and Georgia...
Within an active subtropical jet, PWs of 1.5-1.75" will average
around 2 standard deviations above normal. The degree of low-level
inflow and thus moisture transport/flux are not overly anomalous;
however, an uptick in deep-layer forcing will take place across
the Deep South within the right-entrance region of the upper jet
streak (110-130 kts at 250 mb) across the mid Atlantic. As this
occurs, the initially weak surface frontal segments over/near the
Deep South will eventually give way to a quasi-stationary,
less-diffuse front draped WNW-ESE across the Deep South by the end
of the period. MUCAPEs of 1000-2000 j/kg along with the available
moisture will support hourly rainfall rates of 1-2+ inches, as the
latest high-res guidance continues to indicate a heavy rainfall
footprint with isolated-scattered pockets of 3-5+ inch totals
during the 24 hour period. By the same token, the models,
especially the CAMs, continue to exhibit quite a bit of areal
spread with the axes of heavier QPF, even as the event is now
within the day 1 window. As a result, the Day 1 ERO casts a fairly
wide net, at least with the Marginal Risk area, with the Slight
Risk encompassing the higher ensemble mean QPF along with more
prolonged higher HREF probabilities of >1"/hr rainfall rates.

Hurley





Day 2
Valid 12Z Wed Mar 17 2021 - 12Z Thu Mar 18 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE MID MISSISSIPPI VALLEY INTO THE TENNESSEE VALLEY...

...Portions of Iowa/Nebraska/Kansas and Northern Missouri...
Moderate to heavy rainfall is possible over parts of
Kansas/southeast Nebraska into adjacent portions of southern Iowa
and northern Missouri as system lifts northeast from the
Central/Southern Plains on Wednesday night/early Thursday morning.
The models are doinig be better job tonight in depicting a
deformation zone forming over Kansas into far southeast Nebraska
which helps enhance rainfall rates along and northwest of the main
low pressure system. Some of the guidance QPF fields suggests
that some of the models were suffering from convective
feedback...so their exact deterministric precipitation amounts
seems suspect. Even so, the arrival of an airmass with PW values
close to an inch and enhanced mesoscale forcing in and near the
deformation zone supports the idea of 1 to 2 inches with isolated
amounts exceeding 3 inches during the period...with highest
amounts from far northwest KS or northeastern Missouri
northeastward.

...Mid-Mississippi Valley into the Tennessee Valley...
A well-defined front associated with the same area of low pressure
moving northeast across the Plains will help focus convection in
areas of eastern Missouri into the Tennessee Valley during the
latter part of the day on Wednesday and persisting into Wednesday
night/early Thursday. Deeper moisture, as shown by precipitable
water values increasing to nearly 1.5 inches, and 850 mb flow of
40 to 50 kts, and a low level boundary to work with suggests some
locally intense rainfall rates are possible. Opposing that for
flash flooding will be the dry conditions across a large part of
the area recently and enough steering flow to keep cells
propagating. Suspect that there will be some potential for
short-term rainfall rates to challenge the relatively high 3-hour
flash flood guidance.

Bann


Day 3
Valid 12Z Thu Mar 18 2021 - 12Z Fri Mar 19 2021

...THERE IS MARGINAL RISK OF EXCESSIVE RAINFALL FROM PARTS OF THE
OH VALLEY EASTWARD TO PENNSYLVANIA AND PARTS OF THE MID ATLANTIC
REGION...

Rain will spread eastward across portions of the southern Great
Lakes and Midwest region into parts of West Virginia and
Pennsylvania as well as parts of the northern Mid-Atlantic during
the period. Rainfall amounts and rainfall rates should be
diminishing as the system reaching this part of the country during
the period, in part because the system will be in the
filling/weakening stage and because the better instability, deeper
moisture and best upper divergence/core of the upper jet get
shunted away from the region. The models did show some half inch
per 6 hour amounts (implying some locally heavier rates in the short-term)...but these values were within the 3-hourly flash
flood guidance even in regions where topography could be a factor
in focusing the flow. Some 1 to 2 inch amounts are still
possible...and that could result in some isolated problems but the
recent dry conditions should help mitigate any broad-scale
concerns,

Bann

$$
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FOUS30 KWBC 172029
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
428 PM EDT Wed Mar 17 2021

Day 1
Valid 16Z Wed Mar 17 2021 - 12Z Thu Mar 18 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER A LARGE AREA
FROM THE LOWER MISSOURI VALLEY AND MID MISSISSIPPI VALLEY INTO THE
TENNESSEE VALLEY AND PARTS OF THE DEEP SOUTH...

16z Update: Only minor tweaks made to the ERO with this
update...as the broad Slight risk looks in good shape with
isolated to scattered flash flooding possible over a large spatial
extent today into tonight.

The most immediate risk is from northeast AR into western/central
TN and far northern MS/AL. There is a pretty good setup for some
west to east training of convection across this corridor over the
next several hours. An additional 2-4" of rain here should led to
isolated to scattered flash flooding this afternoon.

The second area of focus is southern/central MO into southern IL.
Rainfall rates should never get as intense here given less
instability, and the environmental setup is not as conducive for
training cells. However this area will see two rounds of locally
heavy rainfall...one which is ongoing now and another this
afternoon/evening ahead of the low pressure. Soil saturation is
already well above normal over this corridor...and the two rounds
of heavy rainfall should be enough to bring an increasing flood
risk into tonight.

The third area of focus is within the warm sector over MS/AL. The
primary risk here is sever weather, as depicted by the Moderate
and High risk areas in the SPC outlook. However, isolated to
scattered flash flooding will also be a possibility...as the
supercells will be capable of dropping intense rainfall rates and
there may be some repeating of cells. Also multiple rounds are
expected here (and into northern GA), with the discrete cells this afternoon...and a more linear mode tonight. With soil conditions
saturating here as well, these multiple rounds will likely lead to
isolated to scattered instances of flash flooding.

So all in all the broad Slight risk seems like the correct
messaging at the moment, with isolated to scattered flash flooding
possible. While some localized areas of more focused flash
flooding are possible...not seeing anything at the moment that
warrants any Moderate risk upgrades. But will continue to monitor
trends through the day.

Chenard

...Previous Discussion...

...Lower Missouri Valley and Mid Mississippi Valley...
A broad area of moderate to embedded heavier rain is expected
across portions of Kansas, Missouri, and southern to central
Illinois as the system lifts northeast out of the central/southern
Plains. A deformation zone of enhanced rainfall rates is expected
with the highest amounts forecast around the Kansas City area
through Missouri into west-central Illinois. Widespread amounts of
1-2" are expected, with localized totals of 3-5" per the high-res
CAMs. This is where some wetter than normal soils exist and
streamflows are higher than normal, so additional heavy rainfall,
while lacking the intensity in the short-term as rates are limited
by the relative lack of instability, could lead to mainly
localized flash flooding.


...Lower Mississippi Valley...Tennessee Valley...and parts of the
Deep South...
Deepening cyclone tracking across the southern Plains and
lower-mid MS Valley during the period will lead to robust
deep-layer forcing and rapidly improving thermodynamic profiles in
generating numerous areas of heavy rainfall from the Lower MS
Valley into the Tennessee Valley and Deep South. The hi-res
guidance continues to indicate the potential for at least a couple
of rounds of convection, associated first along and north of the warm/stationary fronts and again ahead of the strong height falls
and approaching cold front. Strong deep-layer ascent owing to the
increasingly favorable difluent flow aloft will get a boost in the
left exit region of a 90-100 mb 250mb jet streak rounding the base
of the trough. At the same time, the thermodynamic environment
will quickly become conducive for heavy rainfall ahead of the
approaching cold front. PWs will quickly spike to 1.5-1.75+ inches
as the low-level southerly inflow increases (50-55kts at 850 mb).
The accompanying robust moisture transport (850 mb moisture flux
standardized anomalies increasing to +4 to +5) will coincide with
ample deep-layer instability (MUCAPEs 1000-2000+ j/kg) in
supporting hourly rainfall rates of at least 1-2", with certainly
some locally higher rates within well organized clusters
(especially supercells).

Fast-moving storm motions and drier-than-normal soils may preclude
a greater flash flood threat, though attention will need to be
given to areas that received quite a bit of localized heavy rain
yesterday (portions of MS into AL). Flash flood guidance in those
areas is reduced quite a bit, and later shifts will monitor for
the potential upgrade to a Moderate Risk for the Day 1 ERO. At
least for the time being, given the considerable spread among the
guidance (especially the CAMs) with respect to the axes of
heaviest rainfall, the Slight Risk was maintained.

Hurley


Day 2
Valid 12Z Thu Mar 18 2021 - 12Z Fri Mar 19 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM PARTS OF
THE SOUTHERN/CENTRAL APPALACHIANS NORTHEASTWARD INTO THE NORTHERN
MID-ATLANTIC REGION AND SOUTHERN NEW YORK/CONNECTICUT...

...2030Z Update...

The main change to the overnight Excessive Rainfall Outlook was to
expand the Marginal Risk farther northeast into southern portions
of New York and Connecticut. Model guidance is agreeable with
higher rain amounts over this area given precipitable water values
3+ standard deviations above normal per the ECMWF. Limiting
factors will be low instability as well as dry antecedent
conditions. But, the urban corridor should make for less receptive
ground conditions and enhance runoff.

No significant edits were needed to the southern part of the
Marginal Risk through the Appalachians. It appears some localized
heavy rain may linger after 12Z Thursday in the Southern
Appalachians, while another round of rain on the back end of the
system could cause some modest to locally heavy rain rates in the
Central Appalachians later Thursday with higher instability. See
the previous discussion for additional details.

Tate

...Previous Discussion...

...Upslope Region of the Central Appalachians to the
Mid-Atlantic...
Maintained a Marginal Risk area for excessive rainfall from the
upslope regions of the Central Appalachians northeastward into
southeastern Pennsylvania on Thursday as low pressure approaches
the region from the west. The 17/00Z runs of the ECMWF and UKMET
depict an area of QPF with a characteristic bulls eye max for
convective grid-scale feedback in the upslope region of the
Appalachians. While the exact QPF from those models may be
suspect, the idea of some localized convective processes can not
entirely be ruled out with correspondingly higher rainfall
rates/amounts. That, combined with the terrain, suggests there is
at least some potential for problems with runoff or ponding in
poor drainage areas. The Marginal Risk area was extended to the
northeast where the model has the best QPF signal as a result of
Precipitable Water values approaching an inch and a quasi-coupled
upper jet especially seen on the 290K/295K surfaces. Thinking is
that the risk of excessive rainfall should be confined to the
first 12-18 hours of the forecast period before the system
translates far enough away and high pressure starts to funnel
drier air into the region.

Opted to remove the Marginal Risk area from places farther to the
west in the deformation zone. While there may be some short-term
enhancements to the rainfall rates as a result of the fgen
mesoscale forcing, modest instability and Precipitable Water
values at or below 0.75 inches should hold the rainfall
rates/amounts in check. In addition, the area has not had
significant rainfall in a while and it is presumed that this will
also help to mitigate excessive rainfall concerns.

Bann

$$
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FOUS30 KWBC 181540
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1139 AM EDT Thu Mar 18 2021

Day 1
Valid 16Z Thu Mar 18 2021 - 12Z Fri Mar 19 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS THE
SOUTHERN TO CENTRAL APPALACHIANS INTO THE MID-ATLANTIC...

The strong closed low moving into the Lower Mississippi Valley
this morning will continue to push eastward this period through
the Tennessee Valley and into the Southern Appalachians. A large,
well defined precipitation area is expected to the north and
northeast of this system from the combination of a well defined
comma head/deformation precip area to the north of this closed
low, north of the Ohio River, and in a region of strong isentropic
lift in the moist south southwesterly flow from the Appalachians
into the Northeast. Precipitation across much of these regions
has been below average over the past few weeks, resulting in
stream flows beginning the event below to much below average.
Some of the northern portion of the Marginal Risk across northern
PA was removed as the 12z HREF showed very little probability of
0.25" an hour totals over the area of low flash flood guidance as
precipitable water values remain below an inch and MU CAPE is
expected to remain negligible north of the Mason-Dixon line. A
marginal risk area continues from the Southern to Central
Appalachians into the Mid-Atlantic where the probabilities of
0.5"+ in an hour are highest. Adjusted the southern portions
somewhat southeast as the recent guidance shows heavy rain along a
warm front/leading edge of a cold air dam attempting to advance
northward through the coastal plain this afternoon with a notable
impact seen within the National Water Model output in that area of
Tidewater Virginia.

Roth/Oravec



Day 2
Valid 12Z Fri Mar 19 2021 - 12Z Sat Mar 20 2021

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

Bann


Day 3
Valid 12Z Sat Mar 20 2021 - 12Z Sun Mar 21 2021

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

Bann

$$
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FOUS30 KWBC 181928
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
327 PM EDT Thu Mar 18 2021

Day 1
Valid 16Z Thu Mar 18 2021 - 12Z Fri Mar 19 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS THE
SOUTHERN TO CENTRAL APPALACHIANS INTO THE MID-ATLANTIC...

The strong closed low moving into the Lower Mississippi Valley
this morning will continue to push eastward this period through
the Tennessee Valley and into the Southern Appalachians. A large,
well defined precipitation area is expected to the north and
northeast of this system from the combination of a well defined
comma head/deformation precip area to the north of this closed
low, north of the Ohio River, and in a region of strong isentropic
lift in the moist south southwesterly flow from the Appalachians
into the Northeast. Precipitation across much of these regions
has been below average over the past few weeks, resulting in
stream flows beginning the event below to much below average.
Some of the northern portion of the Marginal Risk across northern
PA was removed as the 12z HREF showed very little probability of
0.25" an hour totals over the area of low flash flood guidance as
precipitable water values remain below an inch and MU CAPE is
expected to remain negligible north of the Mason-Dixon line. A
marginal risk area continues from the Southern to Central
Appalachians into the Mid-Atlantic where the probabilities of
0.5"+ in an hour are highest. Adjusted the southern portions
somewhat southeast as the recent guidance shows heavy rain along a
warm front/leading edge of a cold air dam attempting to advance
northward through the coastal plain this afternoon with a notable
impact seen within the National Water Model output in that area of
Tidewater Virginia.

Roth/Oravec



Day 2
Valid 12Z Fri Mar 19 2021 - 12Z Sat Mar 20 2021

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

Tate

Day 3
Valid 12Z Sat Mar 20 2021 - 12Z Sun Mar 21 2021

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

Tate


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FOUS30 KWBC 191942
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
341 PM EDT Fri Mar 19 2021

Day 1
Valid 16Z Fri Mar 19 2021 - 12Z Sat Mar 20 2021

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

Oravec


Day 2
Valid 12Z Sat Mar 20 2021 - 12Z Sun Mar 21 2021

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

Taylor


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FOUS30 KWBC 200801
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
401 AM EDT Sat Mar 20 2021

Day 1
Valid 12Z Sat Mar 20 2021 - 12Z Sun Mar 21 2021

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

Oravec


Day 2
Valid 12Z Sun Mar 21 2021 - 12Z Mon Mar 22 2021

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

Campbell

Day 3
Valid 12Z Mon Mar 22 2021 - 12Z Tue Mar 23 2021

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

Campbell

Scattered to widespread rainfall is expected across the Central
Plains and portions of the Mississippi Valley. While rain rates
will likely be less than 0.50 inch/hour, periods of heavy rainfall
over central Kansas, eastern Nebraska, western Iowa and northwest
Missouri will result in the highest QPF occurring over this
region. The latest guidance continues to show southerly low-level
flow advecting PW values near 1 inch northward along a frontal
boundary over the central U.S. Instability over the central U.S.
will be enough to produce some areas of enhanced precipitation
efficiency. A majority of the guidance did show a western shift
with the maximum QPF, averaging 1 to 2 inches with isolated higher
values. Overall, the threat for excessive rainfall and flash flood
conditions is relatively low for this period, but is a non zero.
The forecast QPF more than likely will not reach or exceed local
FFG during this period. Therefore, a Marginal Risk was not
introduced.


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FOUS30 KWBC 201931
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
331 PM EDT Sat Mar 20 2021

Day 1
Valid 16Z Sat Mar 20 2021 - 12Z Sun Mar 21 2021

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

Chenard


Day 2
Valid 12Z Sun Mar 21 2021 - 12Z Mon Mar 22 2021

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

Pagano

Day 3
Valid 12Z Mon Mar 22 2021 - 12Z Tue Mar 23 2021

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

...Central Plains...
A closed mid-level low will emerge out of the Central/Southern
Rockies early Monday and pivot through the Plains approaching the
Mid-MS Valley by Tuesday morning. Shortwave energy will round the
trough axis allowing it to become negatively tilted which will
increase synoptic scale lift and surge moisture and instability
north into the Southern/Central Plains and the Lower MS Valley.
As a result, showers and thunderstorms will become widespread
across the midsection of the country as the surface low develops
and deepens through the period.

Precipitable water values will surge to around 1.25 inches in the
Southern Plains and around 1 inch in the Central Plains. This is
1.5-2 standard deviations above the mean aided by 40+ knot low
level southern flow. While instability will be more robust ahead
of the trailing cold front in the Southern Plains and Lower MS
Valley, the large scale lift is quite impressive just to the
north. The coupled upper level jet will promote strong ascent
enhancing the moisture transport from the Gulf of Mexico. In
response, hourly rain rates could surge to over 0.50 inches and
with training/multiple rounds of precipitation activity, this
could result in some flooding concerns.

Based on the precipitation analysis over the past week, the
Central Plains have received an abundance of rain. However, with
a few drier days of late and little in the way of flooding during
the last heavy rain event, the soils will likely be able to
withstand the 1-3+ inches of rain expected. If anything, the mode
of flooding will be areal in nature with very localized flash
flooding issues. Therefore, while a Marginal Risk was considered
among the offices, it was decided to hold off for now. If rain
rates increase to around 1 inch/hour and/or a more prolonged rain
event is forecast, a Marginal Risk may be introduced at subsequent
updates.

As an aside, another factor considered was the potential for
winter precipitation. While some models are suggesting that there
may be enough dynamic cooling within the deformation axis of the
developing low, it appears eastern KS/NE will be all liquid.

...Southern Plains/Lower MS Valley...
The trailing cold front associated with the aforementioned storm
system will usher in better moisture content and instability
across this region. Therefore, anticipate more showers and
thunderstorms to blossom ahead of the front across eastern TX/OK
Monday afternoon and into AR/LA through the overnight. This
activity will be progressive and occur over higher FFG. However,
locally enhanced upslope across the Boston and Ouachita Mountains
could result in higher hourly totals. The rates do not appear to
meet the threshold for even a Marginal Risk at this time, but we
will continue to monitor this area and the latest model trends.

Pagano

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