FOUS30 KWBC 091914
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
214 PM EST Thu Jan 9 2025

Day 1
Valid 16Z Thu Jan 09 2025 - 12Z Fri Jan 10 2025

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS THE
MIDDLE AND UPPER TEXAS COAST AND WESTERN LOUISIANA GULF COAST...

...16Z Update...

The overnight forecast remains on track with little adjustment made
to the inherited Marginal Risk along the western Gulf of Mexico
coast. The moisture advection associated with this upper trough is
exceptional as NAEFS shows IVT values topping 1,000 kg/m/s over the
Upper Texas coast around 00Z this evening, which is well above the
maximum observed IVT in the CFSR database for this time of year.
In addition, RAP soundings around 00Z this evening depict warm
cloud layers as deep as 12,000ft in some cases along the Upper
Texas coast. There is the potential for briefing training of
convection along the Upper Texas coast as low pressure offshore
organizes and the warm front lifts north this evening, and
convection should contain efficient rainfall rates given these deep
warm cloud layers. That said, instability aloft is very limited
and should keep flash flooding concerns to those in highly
urbanized locations and/or areas that have a history of draining
poorly.

Mullinax


--Previous Discussion--

Increased mid and upper level forcing is ongoing over portions of
TX thanks to the slow ejection of a potent ULL situated over
Sonora. A digging shortwave trough analyzed over the Central
Rockies will continue plunging southward with an eventual partial
phase with the primary shortwave disturbance moving east into West
TX. At the surface, a maturing surface low over the western Gulf
will slowly wander northward towards the middle TX coast with
expected moisture advection regime to be a significant player in
the prospects of locally heavy rainfall in-of the immediate TX
coast. As of this time, there is a general consensus on the
heaviest precip being focused up the coast near Port Aransas up
through the remainder of the middle and upper TX coasts, pinning a
bullseye closer to the Galveston area and points northeast. This is where
a frictional convergence regime with persistent easterly flow on
the north side of the main surface low will help initiate a period
of convection just off the immediate coast within the primary axis
of higher theta_E's located on the western flank of the surface
reflection. Convection will hug the coast with some of the heavier
precip cores moving ashore creating opportunity for rainfall rates
to reach 1-1.5"/hr at peak intensity and producing totals between
2-3" with locally as high as 4" along that immediate stretch of
coast. The longwave evolution will generate ample ascent within the
diffluent axis downstream of the potent mean trough, pulling
moisture inland and generation pockets of heavy rain just inland,
but to less of a degree of the immediate coast thanks to the local
instability maximum likely hugging the coast with near 0 CAPE just
inland.

Recent probabilities from the 00z HREF are indicative of the
locally heavy rain threat with the neighborhood probs of >3" up
between 40-70% for the stretch of the TX coast beginning from Port
Aransas up the Lower Sabine where TX/LA border along the Gulf
coast. There's non-zero 5+" probs, but a much lower factor of
probabilities comparatively (<20%) and mainly confined to a small
area near Galveston. This has been the zone of heaviest precip
potential and likely strongest low-level convergence due to the
forecast proxy of the surface low and greatest theta_E advection
regime. Considering all the above variables, there was little need
to deviate from the previous forecast. The previous MRGL risk was
maintained given the steady forecast continuity and recent
favorable probability fields.

Kleebauer

Day 2
Valid 12Z Fri Jan 10 2025 - 12Z Sat Jan 11 2025

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE CENTRAL GULF COAST...

...20Z Update...

The inherited Marginal Risk remains in good shape as the region
will struggle to see a dearth of instability aloft and any
storms/showers will be progressive in their easterly movement.=20
That said, NAEFS shows that around 12Z Friday, IVT values aimed at
the central Gulf Coast will be as high as 1250 kg/m/s, which would
be above all observed IVT values in the CFSR database for this=20
time of year. Similar to the Upper Texas Coast on the Day 1 ERO,=20
soundings are highly saturated and warm cloud layers are around=20
12,000ft deep. This should result in efficient warm rainfall that=20
may support highly localized flash flooding potential, especially=20
in urbanized areas that drain poorly along the I-10 corridor. But=20
the progressive storm motions and lack of modest instability should
keep the areal extent and severity of the flash flood threat to a=20
limited number of at-risk urbanized areas through the early=20
afternoon hours on Thursday.

Mullinax

--Previous Discussion--

Our western Gulf disturbance will migrate east-northeastward with a
focus of heavier precip across southern LA through the urban zones
of southeast LA during the morning Friday. The system will be
fairly progressive and the trends have brought down storm totals
due to the progressive nature and limited instability factoring in
the local maxima relegated <2". The expectation is for there to be
a thin line of heavier rates along a developing cold rain band,
something not too common at these latitudes. There is a textbook
surface convergence pattern right within the north and western
flank of the low center as it cross through the southeastern
Parishes of LA and this aligns within a narrow tongue of elevated
theta_E's that get advected just ahead of the surface reflection.
This will lend of a period of potentially significant rainfall
rates bordering 1"/hr at peak intensity, a factor that could be
enough to generate some localized flooding within the urban zones
centered around New Orleans and points north, west, and east.

This is very much of a lower end MRGL risk considering the trends
of the forward propagation and such a skinny axis of the heavier
precip anticipated. This also aligns with more of a morning and
early afternoon risk with a quick decay behind the low as light
stratiform takes over, then dries out as drier air advects in
behind the departing low to the east. The previous MRGL risk was
maintained in full with an emphasis on flooding potential along
and south of I-10/12 with the best prospects located around urban
zones surrounding Lake Pontchartrain.

Kleebauer

Day 3
Valid 12Z Sat Jan 11 2025 - 12Z Sun Jan 12 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Mullinax

Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/94epoints.txt__;!!DZ3fjg!4Obf_Jd9oxBk1NArpjLZcNR-rXJT0bgMm5Do-aBHMbje=
a1EHVt_QzTUrFGpTPaN9VZlbd7KO07SvlSxhyQNGTZpTpes$=20
Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/98epoints.txt__;!!DZ3fjg!4Obf_Jd9oxBk1NArpjLZcNR-rXJT0bgMm5Do-aBHMbje=
a1EHVt_QzTUrFGpTPaN9VZlbd7KO07SvlSxhyQNGlZ11xgM$=20
Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/99epoints.txt__;!!DZ3fjg!4Obf_Jd9oxBk1NArpjLZcNR-rXJT0bgMm5Do-aBHMbje=
a1EHVt_QzTUrFGpTPaN9VZlbd7KO07SvlSxhyQNGOoxU5nM$=20

=3D =3D =3D
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