FOUS30 KWBC 241538
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1038 AM EST Tue Dec 24 2024

Day 1
Valid 12Z Tue Dec 24 2024 - 12Z Wed Dec 25 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR THE NORTHERN
AND CENTRAL SIERRA NEVADA FOOTHILLS AND ACROSS EAST TEXAS INTO THE
LOWER MISSISSIPPI VALLEY...

..Sierra Nevada Foothills..

...16z update...
The cold front has started to make its stronger eastward push
through central California effected decoupling the moisture plume
with depth; but also upstream severing of the connection with the
sub-tropics had already begun earlier today, per CIRA LPW suite.=20
Rates of .25-.5"/hr and totals of up to 1.25" still are in the=20
range of naturally lower FFG in steeper lower slopes that increased
runoff and widely scattered above average stream flows and=20
isolated flooding concerns remain in the current Marginal Risk=20
placement. Please refer to MPD 1184 for additional details.

Gallina

---Prior Discussion---
Current atmospheric river will progress=20
 inland during the overnight and morning hours with the primary=20
 shortwave trough axis moving through CA by late-morning through at
 least the early evening before precip potential wanes. The recent
 forecasts for a skinny axis of MRGL risk potential remains=20
 steadfast with the best forcing likely to cause some low-end flash
 flood prospects in-of the Foothills of the northern Sierra down=20
 through the adjacent hills of the central Sierra and central=20
 Valley. Latest HREF blended mean QPF has come up a bit across the=20
 central CA areas, likely attributed to a more favorable mid-level=20
 ascent pattern as the shortwave trough pivots inland and the=20
 height falls allow for a period of elevated instability across the
 areas downstream of the greatest forcing. PWAT anomalies running=20
 up to +2 deviations also coincide with a better prospect for=20
 locally heavy rainfall as this matches the general expectation for
 places already in the previous MRGL further north. Neighborhood=20
 probabilities for >2" of rainfall are very high (70-90+%) for=20
 areas along and north of US50 that are below the expected snow=20
 level of <5500ft MSL. These same probabilities are not as high=20
 further south, however they are now upwards of 25% for the same=20
 >2" threshold and are well above 70% for >1", a precip amount more
 conducive for flooding in the central Valley when assessing the=20
 current FFG intervals. Rainfall rates will likely max out at=20
 0.5-0.75"/hr at peak, but that is right on the cusp of many=20
 locations within that hourly FFG index.

Considering the current forecast probability fields and the trends
within the upper level evolution, have decided to expand the
previous MRGL risk further south along the central Sierra Foothills
with some adjustments on the northern periphery of the MRGL to
encompass an area that may still hang on to heavy rainfall in first
few hours of the period.

Kleebauer

..East Texas/Southeast Oklahoma/Arkansas...

...16z update...
GOES-E WV suite depicts subtle mid-level shortwave shed out of E OK=20
from main digging upstream larger scale trough over Southern High
Plains is aiding short-term veered/confluent low level flow across
the Red River Valley into Northern Texas with lingering deeper
layer moisture pool and increasing instability well upstream. As
such, convection has developed along the frontal zone. Downstream
thickness ridging has allowed for veered propagation vectors to
align with the unidirectional flow in the near term to allow for
favorable orientation of convection for some training/repeating
given the upstream potential for back-building/redevelopment as
DPVA strengthens southerly flow across central TX.=20

Eventually, increasing theta-E, moisture flux from the Gulf will
increase instability and low level flow to increase convective
vigor toward mid-day. Rates over 2"/hr are probable, but cold pools
and backed propagation vectors will reduce residency of any given
storm. Average to slightly below average soil saturation may result
in widely scattered enhanced run-off conditions and isolated flash
flooding remains possible and on the scale of a Marginal Risk at
this time.=20=20=20

Later this evening into early overnight, LLJ off the Gulf will
strengthen further with increasing convection along an effective
sea breeze/FGEN line lifting northward. These cells/southerly flow
ill intersect with approaching convective line/outflow boundary=20
from northern convection. This may allow for a short (1-3hr) window
for southward delay of cell motions and increased rainfall=20
production across the northern Coastal Plain of Southeast Texas.=20
12z Hi-Res CAMs show hints of pockets of 5"+ totals possible. The=20
potential for enhanced rainfall rates/totals in the 03-09z period=20
across southeast Texas has potential for locally significant totals
that would result in localized flash flooding, perhaps even as far
south as the northern exurbs of Houston. However, there remains=20
limited convergence of signals with large gaps between those heavy=20
rainfall pockets to confidently place a Slight Risk of Excessive=20
Rainfall at this time, especially given storm scale interactions=20
required for these totals to unfold, and relatively dry soil=20
conditions. As such, will watch observational and rapid refresh=20
guidance closely for need of upgrade at 01z.=20

Gallina

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

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

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...

..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!8t5olIuvO4uX5r5uws-9mCbmGIGWBbKVAIgxs0CmjGM8=
b--sxUhjVNglnWp_xj-SKNoUDPwgHBdoN4GBWT9R8-gdUVQ$=20
Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/98epoints.txt__;!!DZ3fjg!8t5olIuvO4uX5r5uws-9mCbmGIGWBbKVAIgxs0CmjGM8=
b--sxUhjVNglnWp_xj-SKNoUDPwgHBdoN4GBWT9RTWbG0AE$=20
Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/99epoints.txt__;!!DZ3fjg!8t5olIuvO4uX5r5uws-9mCbmGIGWBbKVAIgxs0CmjGM8=
b--sxUhjVNglnWp_xj-SKNoUDPwgHBdoN4GBWT9RLC5LHDg$=20

=3D =3D =3D
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