FOUS30 KWBC 241955
QPFERD

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

Day 1
Valid 16Z 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.
Rates of .25-.5"/hr and totals of up to 1.25" still are in the
range of naturally lower FFG in steeper lower slopes that increased
runoff and widely scattered above average stream flows and
isolated flooding concerns remain in the current Marginal Risk
placement. Please refer to MPD 1184 for additional details.

Gallina

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

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.

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
from northern convection. This may allow for a short (1-3hr) window
for southward delay of cell motions and increased rainfall
production across the northern Coastal Plain of Southeast Texas.
12z Hi-Res CAMs show hints of pockets of 5"+ totals possible. The
potential for enhanced rainfall rates/totals in the 03-09z period
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
limited convergence of signals with large gaps between those heavy
rainfall pockets to confidently place a Slight Risk of Excessive
Rainfall at this time, especially given storm scale interactions
required for these totals to unfold, and relatively dry soil
conditions. As such, will watch observational and rapid refresh
guidance closely for need of upgrade at 01z.

Gallina

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=20
next atmospheric river (AR) surging off the Pacific and into the=20
PAC NW by Wednesday afternoon. Model guidance is keen on a surge of
moisture represented by fairly elevated IVT signatures on the=20
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=20
Pacific Northwest with the current progression indicating a wave of
heavier precipitation entering coastal WA/OR by Wednesday=20
afternoon and continuing through Thursday morning. The latest NBM=20
mean QPF is pretty substantial for the areas between the central OR
coast up into the Olympic Peninsula with the latter being the=20
beneficiary of the expected QPF max thanks to added orographic=20
enhancement from the Olympic terrain just inland of the coastal=20
plain. Totals of 2-4" along the coast will be common over the span=20
of 12-18 hrs with 3-5" likely occurring across the Olympic=20
Mountains and adjacent Olympic National Forest where soil moisture=20
anomalies are hovering ~80% leading into the event. The forecasted=20
precip is causing some forecast rises in rivers within the local=20
watersheds of the areas anticipating the heaviest rainfall,=20
especially within the Skokomish basin south of the Olympics. The=20
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=20
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=20
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=20
frame. The multi-day precipitation forecast is upwards of 3-6" with
locally higher for many areas extending from the Olympics down=20
through coastal OR with the highest centered over northwest WA and=20
some of the coastal ranges in OR. The second pulse will actually=20
protrude inland further creating a risk for flash flooding inland=20
compared to the previous period, so the MRGL risk is more=20
pronounced in areal coverage compared to D2. The AR will eventually
slide southward with more emphasis placed on southwest OR into=20
northern CA leading to more heavy precip over likely saturated=20
soils. This lead to the MRGL risk extending further south,=20
encompassing areas that will be at risk for back-to-back events=20
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=20
headway into the Southern Plains with favorable ascent aloft=20
coupled with surface cyclogenesis and an increasingly destabilizing
environment ahead of the trough. This setup does have a bit more=20
vigor within the 500mb evolution as the ensemble output has a=20
better defined closed height field with neutral to negative tilt=20
within the mean trough. This will likely lead to another period of=20
convection with heavy precipitation cores in-of a tongue of=20
enhanced theta_E's that will position over eastern TX into the=20
Arklatex. As of this time, the QPF output via ensembles is=20
generally within the 1-1.5" range, but is lacking the data input=20
from regional hi-res guidance and relevant CAMs. If the current=20
500mb evolution being forecast goes as planned, expectation is for=20
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!-AW23Nc-0SeWQZChnwaSnfEywxAk_iXhp50LbnzFtosW=
mcbUsfA0bFQQ6EzXTY6u67hHSDPAQMpCcnkEPcSlS1cEZbM$=20
Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/98epoints.txt__;!!DZ3fjg!-AW23Nc-0SeWQZChnwaSnfEywxAk_iXhp50LbnzFtosW=
mcbUsfA0bFQQ6EzXTY6u67hHSDPAQMpCcnkEPcSle5qM-NY$=20
Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/99epoints.txt__;!!DZ3fjg!-AW23Nc-0SeWQZChnwaSnfEywxAk_iXhp50LbnzFtosW=
mcbUsfA0bFQQ6EzXTY6u67hHSDPAQMpCcnkEPcSlMqoCsLE$=20

=3D =3D =3D
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