FOUS30 KWBC 261951
QPFERD

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

Day 1
Valid 16Z Thu Dec 26 2024 - 12Z Fri Dec 27 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS EAST TEXAS
THROUGH THE ARKLATEX AND ADJACENT AREAS OF THE LOWER MISSISSIPPI VALLEY...

...16z update...
Recent observational trends, 12z Hi-Res CAMs and HREF
probabilities further support initial Day 1 reasoning and placement
of Slight Risk area across E TX into the Arklatex and Lower MS
Valley later today. As such, only slight westward adjustments were
needed with this update, to account for slightly earlier convective
initiation observed in the observational trends.

Additionally, southward adjustment with Marginal Risk in central
CA also appears on solid track with the arrival of the next AR wave
in the later 6-12 hours of the Day 1 period.

Gallina

~~~Prior Discussion~~~

..East Texas...ArkLaTex...Lower Mississippi Valley..

GOES satellite imagery shows a strong shortwave trough digging
southeastward across the southern Rockies which will be ejecting
and amplifying out across the southern Plains this afternoon and
the Lower MS Valley tonight. The mid to upper-level trough is
expected to become negatively tilted this evening as it crosses
through the ArkLaTex and moves downstream, and there will be a
substantial amount of upper-level jet energy/forcing in place via
the arrival of a 90+ kt jet streak and associated jet couplet which
the guidance has been quite consistent in adverting. Coinciding
with this will be surface cyclogenesis and the northward advance
of a warm front into the Lower MS Valley which will foster the
northward advance of moisture and instability. The 00Z HREF
consensus favors a convergent low-level jet increasing to the
order of 50 to 70 kts this evening across areas of far eastern TX
through western and northern LA and much of southern AR and western
MS. Strong convection will be developing over areas of central and
especially eastern TX this afternoon and impacting much of the
broader ArkLaTex region and the Lower MS Valley this evening and
overnight. Multicell and supercell thunderstorm activity is likely
given the available moisture and instability transport and the
enhanced shear profiles. The convection should tend to become more
progressive in time with a QLCS evolution expected tonight, but
concerns with cell-merger activity and some periodic cell-training
will support rainfall rates reaching 1 to 2 inches/hour with some 2
to 4+ inch storm totals possible. Scattered areas of flash
flooding, especially around the more sensitive urban locations,
will be possible. This will also be aided by the fact that at least
locally there has been recent rainfall that has moistened the soil
conditions and increased the streamflows. Therefore, the Slight
Risk area has been maintained and locally expanded to account for
the heavy rainfall threat and antecedent conditions.

...Pacific Northwest through Northern California...

The arrival of a new Pacific shortwave trough and associated
atmospheric river surge will bring a new round of heavier rainfall
to the coastal ranges of the Pacific Northwest on down through
northern CA later today through Friday morning. The shortwave
energy will be quite vigorous and will be amplifying which will
drive strong cyclogenesis just offshore of western WA/OR tonight.
Enhanced IVT magnitudes around the southern flank of the low center
are expected to impact especially southwest OR and northwest CA
with values increasing to as much as 750 to 1000 kg/m/s. This
strong moisture transport and related warm air advection will
facilitate rainfall rates increasing to as much as 0.50" to
0.75"/hour at least briefly as the core of the stronger IVT arrives
and with aid from orographic ascent over the coastal ranges.
Additional rainfall amounts by Friday morning with this next
atmospheric river surge are expected to be on the order of 3 to 6
inches, with the heaviest totals likely focused over southwest OR
and northwest CA. Given the level of additional rains and
wet/saturated soil conditions, there will be increasing concerns
for more flooding impacts. The Marginal Risk area will be
maintained across the region, except it has been expanded somewhat
farther south in northern CA to include the San Francisco Bay
metropolitan area given expectations for heavy rain arriving here
by early Friday morning along with some urban flooding concerns.

Orrison


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
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