FOUS30 KWBC 070820
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
320 AM EST Sat Dec 7 2024

Day 1
Valid 12Z Sat Dec 07 2024 - 12Z Sun Dec 08 2024

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

Churchill


Day 2
Valid 12Z Sun Dec 08 2024 - 12Z Mon Dec 09 2024

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

The Slight Risk has been maintained along and near the central 
Gulf Coast for Sunday and Sunday night, confined to where more 
appreciable deep-layer instability exists to support a higher 
probability of more intense short-term rainfall rates (2"+/hr).
Still, this is considered a lower-end Slight Risk (based on the
wide 15-40% probability spectrum), suggesting widely scattered 
instances of flash flooding at best (given expected localized 
totals of 2-4" are near the corresponding 3-hr FFGs of 3-4". 
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 (where
NASA SPoRT-LIS 0-100 cm soil moisture in the bottom 10th percentile
for much of MS/AR/LA). In addition to drier antecedent conditions 
farther north and east across the Mid-South, very limited elevated
instability (MU CAPE ~200 J/kg at best) should prevent rainfall 
rates from climbing too high (generally 1.5"/hr or less, driven by 
impressive forcing via the shortwave and associated jet dynamics). 
This suggests that a Marginal Risk should suffice with any flash 
flooding being highly localized, despite areal average rainfall 
amounts near 2" (expected to be spread out over 6+ hours).

Churchill/Hurley


Day 3
Valid 12Z Mon Dec 09 2024 - 12Z Tue Dec 10 2024

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

The shortwave trough responsible for heavy rainfall on Sunday over
much of the Lower MS Valley looks to eject northeastward into the
central Appalachians and Mid-Atlantic on Monday, bringing largely 
beneficial rainfall (with totals generally under 1" to much of the
Northeast. Farther south along the central Gulf Coast, there are 
hints of another (weaker) shortwave trough within the southern 
stream that may enhance heavy rainfall along a trailing cold front.
Given the slowing of the front, the opportunity for training areas
of heavy rainfall will be greater on Day 3, best depicted by the 
00z ECMWF and ECENS (depicting localized banded totals of 2-5" and
probabilities for 2" exceedance as high as 30%). This risk appears 
greatest from southeast LA to southern AL and the tip of the FL 
Panhandle, though the EC model system is the most extreme and 
farthest east of all the models. Maintained the inherited Slight 
Risk area with some adjustments, limiting the eastern extent a bit 
given the lack of agreement from the other models. Uncertainty is 
still relatively high, but subsequent outlooks should get a better 
handle on things as these details begin to enter the CAM period.

Churchill
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