FOUS30 KWBC 270117
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
817 PM EST Thu Dec 26 2024

Day 1
Valid 01Z Fri Dec 27 2024 - 12Z Fri Dec 27 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS SOUTHEAST=20
TEXAS INTO THE LOWER MISSISSIPPI VALLEY...

Southeastern Texas into Lower Mississippi Valley...=20
An axis of thunderstorms, located ahead of an advancing frontal=20
system over TX, will steadily translate east tonight with strongly=20
diffluent/divergent flow supporting enhancements to lift,=20
especially from northern LA into AR, just ahead of a potent mid-=20
level vorticity max. Instability will be a limiting factor for=20
northern locations with only a small region of 500-1000 J/kg=20
expected from central LA into AR. However, strong forcing combined=20
with potential for training may allow for areas of training with=20
1-2 in/hr rainfall rates.

To the south, the Slight Risk across TX/LA was trimmed on the=20
western side to account for where precipitation has ended. The=20
flash flood threat looks to be the greatest across far southeastern=20
TX into western/southwestern LA where a combination of lingering=20
warm sector instability (500-1500 J/kg MLCAPE) and slower eastward=20
progression of an ongoing axis of thunderstorms will result in=20
areas of training where line orientation matches the mean steering=20
flow from the southwest. Sufficient shear for organized cells will=20
continue into the night with supercell structures supporting=20
enhanced rainfall rates of 1-2 in/hr and localized totals of 2-4=20
inches through 12Z.=20

...Pacific Northwest through Northern California...

Another atmospheric river/surge of heavy rain will bring a renewed
threat for heavy rainfall and localized flooding to the coastal=20
ranges of western WA to roughly the San Francisco Bay region. As=20
an occluded frontal system approaches the West Coast tonight, an=20
initial warm-advection driven swath of precipitation will move=20
across the West Coast with rainfall rates increasing toward 06Z as=20
higher IVT values arrive ahead of the frontal system. The latest=20
forecast from the RAP supports IVT values increasing into the=20
500-600 kg/m/s range for the central and southern coast of OR=20
tonight and as high as 800 kg/m/s for the central CA coast near San
Francisco, though winds will become less favorable with veering
toward 12Z for the San Francisco region. 850 mb winds of 50 to 60+
kt are expected to impact a broad region of the OR and=20
northern/central CA coast through 12Z along with impressive PWAT=20
values peaking in the 1.4 to 1.5 inch range, at least for the=20
central CA coast near San Francisco. Peak rainfall rates of 0.5 to=20
0.8 in/hr will be possible but have a relatively limited window of=20
~3-5 hours with these peak rates. The southwestern OR coast into=20
the northern and central CA coast will have the best potential for=20
higher/impactful rainfall rates with 12 hour totals of 1-3 inches.=20
Higher rainfall will also fall downstream into the lower elevations
of the Sierra Nevada and for northern locations across the central
WA coast down to the northern OR coast, though rates are not
expected to be as high. Localized flooding concerns will exist=20
from the expected surge in rainfall.

Otto


Day 2
Valid 12Z Fri Dec 27 2024 - 12Z Sat Dec 28 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE PACIFIC NORTHWEST AND PORTIONS OF MISSISSIPPI, ALABAMA AND
TENNESSEE...

...1930 UTC Update...
Minor tweaks made to both Marginal Risk areas, based on the latest
guidance/trends (now within the high-res CAM windows). Still
expecting the risk areas to remain sub-Slight, based on the degree
and duration of favorable moisture transport, also supported by the
latest CSU ERO first guess fields.

Hurley

...Previous Discussion...

...Southeast...

A Marginal risk was maintained across portions of MS, AL and TN with
this update. Convection will likely be ongoing at 12z Friday along
this corridor as a front pushes eastward. The front slows and starts
to wash out during the day Friday as large scale forcing weakens.
This weakening of the front and forcing decreases confidence in
convective evolution, as these factors could result in decreased
convective coverage/organization and little to no flash flood risk.
However a corridor of modest convergence will likely still exist
near the fading front, and some chance outflow from earlier
convection could locally enhance this convergence. There will be a
supply of instability upstream, and so if convection is able to
persist then there is a potential for a few training segments to
evolve within the Marginal risk area. Given the dry antecedent soil
conditions and streamflows, and only a conditional threat of a few
training segments, this is generally considered a lower end Marginal
risk. However given this risk was only recently introduced, and the
continued non-zero risk of localized training resulting in 3"+
amounts, we will hold on to the Marginal for now and continue to
monitor trends. The risk area was shifted a bit off to the southeast
to better match where the latest models depict the best chance of
these localized higher rainfall amounts.

...Northwest...

After what should be a relative lull in rainfall to start the
period, another round of stronger IVT should move into the Pacific
Northwest Friday night. 24 hour rainfall totals through 12z Saturday
will generally not be too significant. Most areas are looking at 1-
2", with peak values around 3" possible from far northwest CA into
far southwest OR. These amounts themselves would typically not pose
much of a flood concern. However it has been a wet recent stretch
over this area, and thus soul saturation and streamflows are running
well above average. Given this increased sensitivity, the uptick in
rainfall coverage and intensity Friday night may continue to pose
some flood risk.

Chenard


Day 3
Valid 12Z Sat Dec 28 2024 - 12Z Sun Dec 29 2024

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

...1930 UTC Update...
Made adjustments to the Marginal Risk area across the TN Valley and
Deep South by extending it to the Gulf Coast region. Guidance
(model QPFs) have come up a bit along the coast, most notably the
ECMWF and UKMet. Despite being a bit farther south of the best
right-entrance region upper jet forcing (and thus low-level FGEN),
these areas closer to/along the coast will have a much better
chance of seeing more intense short-term rainfall rates with
mixed-layer CAPEs of at least 1000-1500 J/Kg within the warm
sector. For now the models continue to show a fairly progressive
upper trough and surface front, with sufficient low-mid level shear
(i.e. forward or downwind propagating Corfidi vectors) that would
limit cell training over any given area for a lengthy duration
(several hours). Given this, along with the continued spread in
the guidance with respect to the QPF maxima, have maintained no
higher than a Marginal Risk in the D3 ERO over the TN Valley, Deep
South, and adjacent Gulf Coast area.

Hurley

...Previous Discussion...

...Northwest...

A stalling front and IVT plume will allow for a prolonged period of
moderate to locally heavy rain Saturday into Saturday night across
portions of northwest CA into southwest OR. The magnitude of the
event should peak Saturday night as IVT values likely exceed 750 and
large scale forcing increases ahead of an approaching mid level
trough. Not expecting there to be any instability to work with, but
the increasing IVT and forcing should allow for hourly rainfall
around 0.5", and the stalling of the front/IVT axis will allow these
higher rates to persist. Thus the Slight risk in the ERO still looks
in good shape, with the WPC 24 hour rainfall currently peaking at 3-
5" in the terrain of far northwest CA into far southwest OR. There
are some model solutions that even suggest 5-7" of rain is a
possibility.

Even though this area is used to seeing rainfall of this magnitude
without significant impacts, in this case this rain will be falling
after what has already been a prolonged wet period. So saturated
soil conditions and elevated stream and river levels will likely
increase the susceptibility to flooding. For this reason do think we
will see a notable increase in the flood risk over this area by
Saturday night, justifying the Slight risk. The inland extent of the
heavier rain could also end up greater than normal, with 1-3"
possible even in inland area of southwest OR. This should be enough
to result in at least localized flooding warranting the Marginal
risk over these areas.

...Tennessee Valley into the Southeast...

A swath of heavy rain is likely across portions of the Southeast
into the TN valley on Saturday into Saturday night. A strong
shortwave at the base o the longwave trough will eject eastward
into the area, accompanied by a strong upper jet. These two features
support robust divergence aloft and support a widespread convective
threat. The system as a whole should remain progressive, however as
low level moisture transport increases there does appear to be a
window for some training/backbuilding of convection. Overall this
looks to be a widespread 1-2" event, however localized swaths of
heavier amounts are likely. Given the magnitude of mid/upper
forcing in place, combined with PWs increasing over the 90th
percentile for late December, and sufficient upstream instability
forecast...it seems plausible that we could see localized swaths of
3-4".

While the progressive nature of the system will likely cap the
magnitude and coverage of flood impacts...do think the event has
enough going for it to support embedded Slight risk level impacts.
However exactly where this higher threat materializes remains a bit
unclear. Model guidance has been wavering a bit with the depth of
the longwave trough and shortwave strength, which ends up playing a
large role in the placement of heaviest rain. Our forecast aligns
closer to the somewhat consistent GFS and ECMWF solutions...however
it should be noted that the 00z GEM and UKMET are quicker with the
system and focus the heaviest rainfall quite a bit southeast of the
WPC forecast. While these solutions are considered lower probability
outcomes, they can not be ruled out. The inherited Marginal risk was
expanded a bit south and east...the eastward shift to account for
the aforementioned further east solutions (and a modest eastward
trend in the 00z ECMWF as well)...and the southward shift to account
for the better upstream instability pool. Given the antecedent dry
conditions over this region and the progressive nature of the
system, this event is probably going to mostly result in a localized
flash flood risk, supporting the Marginal risk. Embedded within this
there could very well be a smaller scale area where impacts are a
bit greater due to short duration training of higher rates. Best
guess location at the moment seemingly centers around
central/northern AL, but confidence remains low. Thus will not
introduce any Slight risk area at this time and will continue to
monitor trends.

Chenard


Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/94epoints.txt__;!!DZ3fjg!-JPGZY7hWxrldnz8kWbLTh7Er5QcOxFszKsw5fAaGX89=
j_RcVvOmRLC7bFbFhcU2MBroECVEJ7xOKphX1SxQxkhB-wk$=20
Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/98epoints.txt__;!!DZ3fjg!-JPGZY7hWxrldnz8kWbLTh7Er5QcOxFszKsw5fAaGX89=
j_RcVvOmRLC7bFbFhcU2MBroECVEJ7xOKphX1SxQTH8zadU$=20
Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/99epoints.txt__;!!DZ3fjg!-JPGZY7hWxrldnz8kWbLTh7Er5QcOxFszKsw5fAaGX89=
j_RcVvOmRLC7bFbFhcU2MBroECVEJ7xOKphX1SxQCMlNLGg$=20

=3D =3D =3D
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