Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
337 PM EST Fri Dec 6 2024

Day 1
Valid 16Z 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

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

...Middle to Upper TX Coast to the Southwest LA Coast...
...2030 UTC Update...
Have removed the Marginal Risk area from the Day 2 ERO in this
region. While still a non-zero threat, given the latest model
trends (much lighter with the QPF, areal-average amounts less than
1" with heavier QPF and more appreciable deep- layer instability
remaining offshore), feel that ERO risk is below Marginal
thresholds (40km neighborhood prob of FFG exceedance less than 5%.

Hurley


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...

...2030 UTC Update...
Based on the relative fast timing with this system (peak low-mid
level moisture transport/flux with weak, elevated instability
within the anafrontal setup), per collaboration with the WFOs have
dropped the Slight Risk in the Day 3 ERO for areas north of LCH
and LIX CWAs. Farther north, drier antecedent conditions (near to
below normal soil moisture percentiles) along with MUCAPE values
under 500 J/Kg will inhibit the more intense short-term rainfall
rates needed to fit the Slight Risk criteria. Therefore even with
areas of 2-2.5" of rain expected over parts of eastern AR into
northern MS, western TN, and northwest AL -- given the lack of
instability the rainfall rates over these areas are expected to
peak between 1.5-2.0" within a 6 hour period over isolated areas (FFGs
in this region are currently ~ 3" in 6 hours).

We did however maintain the Slight Risk closer to the Gulf Coast,
where more appreciable deep-layer instability and thus higher
probability of more intense short-term rainfall rates are more
probable. Also, much of the Slight Risk area, particularly from
Jasper TX east-northeast to Alexandria LA, have received heavy
rainfall over recent days, and as a result the antecedent soil
conditions and streamflows are a bit higher compared to areas
farther north.

Hurley

...Previous discussion...
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


Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt