FOUS30 KWBC 250058
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
758 PM EST Tue Dec 24 2024

Day 1
Valid 01Z Wed Dec 25 2024 - 12Z Wed Dec 25 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS EAST=20
TEXAS INTO NORTHWESTERN LOUISIANA AND FAR SOUTHWESTERN ARKANSAS...

Texas into Louisiana...
The base of a mid to upper-level trough continues to amplify across
central Texas with scattered thunderstorms ongoing as of 0030Z from
just south of the DFW metroplex to near a trough axis east of I-35
and with more isolated development near a weak boundary near=20
Galveston Bay. Continued amplification and advancement of the upper
trough should allow for thunderstorms to maintain across=20
central/eastern Texas through the overnight, feeding off of MLCAPE=20
that is estimated to be 1000-2000 J/kg across east-central TX per=20
SPC mesoanalysis data (1750 J/kg at CRP/00Z sounding). Low level=20
southwesterly flow, increasing slightly into the 15-25 kt range,=20
coupled with mean southwesterly steering flow will likely allow for
some repeating/training and possible backbuilding of cells=20
overnight. High rainfall rates of 2-3 in/hr seem likely where cell
training/stalling occurs.=20

The previous Marginal Risk was removed over Oklahoma into central
Arkansas where decreasing instability and forcing has allowed for a
reduction in rainfall intensity. The Marginal Risk was expanded
slightly to include the TX coast where some isolated runoff could
be possible later in the night, although the latest CAM guidance
suggests an overall increase in forward propagation toward 12Z.

Sierra Nevada Foothills...
The previous Marginal Risk was removed due to a reduction in
moisture flux and rainfall intensity as an upper trough axis has=20
moved inland across California.

Otto


Day 2
Valid 12Z Wed Dec 25 2024 - 12Z Thu Dec 26 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR COASTAL AREAS
OF WASHINGTON, OREGON, AND NORTHWEST CALIFORNIA...

...20Z Update...

The only noteworthy change to the Day 2 ERO was to expand the
Marginal Risk down the Pacific coast to include Eureka and Cape
Mendocino. This was due in part to the recent heavy rainfall (high
relative soil moisture with the top meter soil moisture percentiles
averaging above 75%). The other factor was the uptick in QPF, with
isolated orographically-enhanced additional totals between 2-3" per
the majority of high-res CAMs. Late in the period (Wednesday
night), elevated CAPEs get close to 250+ J/Kg, which will enhance
the rainfall rates. The 12Z HREF in fact shows probabilities
between 40-60+ percent of 0.50+/hr rates between 06-12Z Thu, which
will impact several of the 2021-23 burn scars.

Hurley


...Previous Discussion...

There has been little change in the forecasted evolution of the
next atmospheric river (AR) surging off the Pacific and into the
PAC NW by Wednesday afternoon. Model guidance is keen on a surge of
moisture represented by fairly elevated IVT signatures on the
order of 600-800 kg/ms within the current global ensemble blend and
12z NAEFS. A steady stream of moisture will advect onshore of the
Pacific Northwest with the current progression indicating a wave of
heavier precipitation entering coastal WA/OR by Wednesday
afternoon and continuing through Thursday morning. The latest NBM
mean QPF is pretty substantial for the areas between the central OR
coast up into the Olympic Peninsula with the latter being the
beneficiary of the expected QPF max thanks to added orographic
enhancement from the Olympic terrain just inland of the coastal
plain. Totals of 2-4" along the coast will be common over the span
of 12-18 hrs with 3-5" likely occurring across the Olympic
Mountains and adjacent Olympic National Forest where soil moisture
anomalies are hovering ~80% leading into the event. The forecasted
precip is causing some forecast rises in rivers within the local
watersheds of the areas anticipating the heaviest rainfall,
especially within the Skokomish basin south of the Olympics. The
elevated stream flows and heavy rain will enhance local flash flood
prospects for anywhere in proxy to those watersheds, so the threat
is a bit higher than normal compared to what is usually forecast
for an AR of this strength.

The coastal OR area will be more prone to urban flooding,
especially for places like Astoria that are within the secondary
maxima of the ensemble QPF for the incoming event. The previous
forecast was generally maintained, but did trim a small area across
the Olympics that will likely maintain wintry precipitation during
the entire period leading to a mitigated flash flood threat and
more of a snow load/heavy snow concern.

Kleebauer


Day 3
Valid 12Z Thu Dec 26 2024 - 12Z Fri Dec 27 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE PACIFIC NORTHWEST AND ACROSS EAST TEXAS THROUGH THE LOWER
MISSISSIPPI VALLEY...

...20Z Update...

Based on the latest guidance (trends), have shifted the Marginal
Risk area a little farther east across eastern TX-OK and across
portions of the Lower-Mid MS Valley. Moisture (TPW) and moisture
flux anomalies (850-700 mb) peak around +2 standard deviations
early in the period, then drop after 00Z Fri as the upper shortwave
flattens and moves quickly up the western side of the Eastern
Seaboard ridge. Transient nature of this system will be the main
inhibitor for flash flooding, as will the limited deep-layer
instability (particularly over northern portions of the outlook
area). Closer to the Gulf Coast, MUCAPEs will likely reach at least
500-1000+ J/Kg, however FFG values are also higher across these
areas (particularly the 3hr FFGs, which are between 3-4 inches).
Farther north, current 3-hour FFGs are considerably lower in some
areas (1.5-2 inches), however so is the available instability and
short-term QPF potential. Global guidance current indicates the
likelihood of at least isolated areas of 3-4+ inches of rain during
Day 3 across the Lower MS Valley to the Gulf Coast; however, there
remains quite a bit of areal spread with these higher totals.
Therefore, for now at least, will hold off on including a low-end
Slight Risk within the current Marginal Risk area.

Hurley

...Previous Discussion...

...Pacific Northwest...
Secondary IVT pulse will occur on Thursday during the prolonged AR
event creating a flash flood potential extending into the D3 time
frame. The multi-day precipitation forecast is upwards of 3-6" with
locally higher for many areas extending from the Olympics down
through coastal OR with the highest centered over northwest WA and
some of the coastal ranges in OR. The second pulse will actually
protrude inland further creating a risk for flash flooding inland
compared to the previous period, so the MRGL risk is more
pronounced in areal coverage compared to D2. The AR will eventually
slide southward with more emphasis placed on southwest OR into
northern CA leading to more heavy precip over likely saturated
soils. This lead to the MRGL risk extending further south,
encompassing areas that will be at risk for back-to-back events
within the last 72 hours.

..East Texas and Lower Mississippi Valley..
A repeat of the D1 setup is progged for east TX extending into the
Lower Mississippi Valley as a shortwave disturbance will make
headway into the Southern Plains with favorable ascent aloft
coupled with surface cyclogenesis and an increasingly destabilizing
environment ahead of the trough. This setup does have a bit more
vigor within the 500mb evolution as the ensemble output has a
better defined closed height field with neutral to negative tilt
within the mean trough. This will likely lead to another period of
convection with heavy precipitation cores in-of a tongue of
enhanced theta_E's that will position over eastern TX into the
Arklatex. As of this time, the QPF output via ensembles is
generally within the 1-1.5" range, but is lacking the data input
from regional hi-res guidance and relevant CAMs. If the current
500mb evolution being forecast goes as planned, expectation is for
a general crescendo of QPF in future model output.

Considering the threat expected in the D1, it is very plausible
that there could be a higher risk area (SLGT) embedded over a
broader MRGL risk forecast. As of now, the forecast inherited was
only changed to capture the areal QPF average >0.75" which extended
the western flank of the risk a bit further west compared to
previous issuance.

Kleebauer


Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/94epoints.txt__;!!DZ3fjg!_7LBziUy-zIxyCMNm-BXRGdjoBeqa_wW-mhW1CbPjps9=
7lW5IHZOMRLKXjGXcejXORzwGgRsJTj4UDcYWDO_DVVX7e4$=20
Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/98epoints.txt__;!!DZ3fjg!_7LBziUy-zIxyCMNm-BXRGdjoBeqa_wW-mhW1CbPjps9=
7lW5IHZOMRLKXjGXcejXORzwGgRsJTj4UDcYWDO_QPZ_7ME$=20
Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/99epoints.txt__;!!DZ3fjg!_7LBziUy-zIxyCMNm-BXRGdjoBeqa_wW-mhW1CbPjps9=
7lW5IHZOMRLKXjGXcejXORzwGgRsJTj4UDcYWDO_MuDH95U$=20

=3D =3D =3D
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