Subj : HVYRAIN: Excessive Rainfa To : All From : Mike Powell Date : Sat Dec 07 2024 09:50:00 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 $$ --- SBBSecho 3.20-Linux * Origin: ILink: CCO - capitolcityonline.net (454:3/105) .