FOUS30 KWBC 061533
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1033 AM EST Fri Dec 6 2024

Day 1
Valid 12Z Fri Dec 06 2024 - 12Z Sat Dec 07 2024

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

...Far Deep South Texas...
The slow sagging frontal zone is nearly flat west to east across
the Northern Gulf of Mexico and a weak surface to boundary layer
cyclone has developed along the upstream edge near the southern tip
of Texas. Enhanced deep layer moisture resides along and north of
the boundary across this section of the front with total PWat
values ranging from 1.5-1.75". Warm sea surface and proximity to
the front allow for some weak instability in proximity to that low
with values of 750-1000 J/kg available along/offshore. So there
will remain some solid potential for thunderstorm activity across
far south Texas. However, the vast majority of guidance continues
to suggest best overall convergence and strongest updrafts will
remain offshore or at the near coast. It is possible some weakening
showers or rouge thunderstorm may make an appearance onshore in
proximity to the urban corridor of the lower Rio Grande Valley,
making the risk for flash flooding non-zero, but not rising to high
enough probability/coverage to reach 5% threshold for delineating 
a Marginal Risk at this time. 

Gallina


Day 2
Valid 12Z Sat Dec 07 2024 - 12Z Sun Dec 08 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
EASTERN TEXAS AND WESTERN LOUISIANA...

A low-level jet streaming Gulf moisture northward across eastern
Texas will interact with a strong shortwave trough which will move
northeastward out of New Mexico and into the central Plains. PWATs
along the Upper Texas coast are expected to increase to 1.5"+ and
instability should remain minimal (and mostly confined to the
coast). The heaviest rain is forecast to be on Saturday night due
to the typical nocturnal strengthening of the low-level jet. Soils
have become a bit more saturated in this region due to the recent
rainfall, and some rainfall is expected on Friday. Isolated flash
flooding is possible, especially in areas of more saturated soils,
urban areas, and flood prone/low lying areas.

Churchill/Roth


Day 3
Valid 12Z Sun Dec 08 2024 - 12Z Mon Dec 09 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
LOWER MISSISSIPPI VALLEY AND CENTRAL GULF COAST...

Continued moisture flux transport via a moderately strong low-
level jet is expected to result in increasing coverage of flash
flooding on Sunday, as the bulk of the forcing and jet dynamics
via the aforementioned strong shortwave trough will not arrive
until midday (along with maximized instability). Localized training
is expected to result in 2-4" totals, suggesting a lower-end
Slight Risk based on the probability spectrum (which ranges from
15-40%). Given that most of the region is experiencing very dry
antecedent soil conditions (10th percentile or lower for much of
MS/AR/LA, per NASA SPoRT-LIS 0-100 cm soil moisture), any flooding
should be relegated to flood-prone areas (with the greatest risk
for any localized significant flash flooding over west-central LA,
where antecedent soil conditions and streamflows are locally higher).


Churchill
$$

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