Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Dec 08 2024 00:27:34 FOUS30 KWBC 080027 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 727 PM EST Sat Dec 7 2024 Day 1 Valid 01Z Sun Dec 08 2024 - 12Z Sun Dec 08 2024 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Hurley Day 2 Valid 12Z Sun Dec 08 2024 - 12Z Mon Dec 09 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE LOWER MISSISSIPPI VALLEY... ....2030Z Update... In coordination with LCH/Lake Charles, LA and LIX/Slidell, LA forecast offices, the inherited Slight Risk was downgraded to a Marginal with this update. The latest run of both the global and hi-res guidance have brought the rainfall forecast down from previous forecasts. The heaviest rains for the period are now expected in the middle of the Marginal Risk area over northern Mississippi, where interaction with a front in that area will lead to greater forcing than areas further south along the Gulf Coast. Instability will be a major limiting factor in flooding potential area-wide by greatly limiting rainfall rates. MUCAPE values throughout the duration of the rainfall will struggle to get much above 300 J/kg, suggesting that nearly all of the rainfall will be stratiform. Where the front adds forcing, there could make local convective bursts embedded within the broader rainfall shield, but do not expect there to be many, if any instances of rates above an inch per hour. Hydrologically, most if not all of the soils in the are are at or below average for moisture. Low/dry creekbeds should easily handle most of the light to briefly moderate rainfall rates. Any isolated flooding will be in any urban areas, as well as other low-lying or flood prone areas. Wegman Day 3 Valid 12Z Mon Dec 09 2024 - 12Z Tue Dec 10 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF EASTERN LOUISIANA AND COASTAL MISSISSIPPI... ....2030Z Update... In coordination with LIX/Slidell, LA forecast office, the inherited Slight was shrunk largely to the I-10 & I-12 corridors from Baton Rouge east to Biloxi. The flooding potential in this area appears greatest in urban and flood-prone areas, so most of the more rural areas of Louisiana to the Florida Panhandle were removed from the Slight. Further, there appears to be better agreement in the guidance that the heaviest rains and potential for training will be focused in the new Slight Risk area, with areas further east including Mobile and Pensacola largely missing out on heavy rains through 12Z Tuesday. As on Day 2/Sunday, instability remains a major limiting factor supporting most of the rainfall on Monday staying stratiform and light in intensity. Latest guidance keeps most of the low-level instability over the Gulf, with relatively little of it making it inland as we are in the climatological minimum for diurnal heating. The rain won't persist nearly long enough with mostly stratiform to raise big concerns for flooding. However, given the flooding history in the New Orleans area, think there still remains some potential for flooding in those urban and flood prone areas. Wegman ....Previous Discussion... 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 Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4RIP_1ul2a4CCcr4xjFD6xRTmmc61YBJwt-OmtTlAcQi= hpaqKRCFDquWYECw-694cKS1zPQZzNAUtcWNcTt0vB9kLas$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4RIP_1ul2a4CCcr4xjFD6xRTmmc61YBJwt-OmtTlAcQi= hpaqKRCFDquWYECw-694cKS1zPQZzNAUtcWNcTt03MvKneY$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4RIP_1ul2a4CCcr4xjFD6xRTmmc61YBJwt-OmtTlAcQi= hpaqKRCFDquWYECw-694cKS1zPQZzNAUtcWNcTt03r04ZZg$=20 =3D =3D =3D To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. Otherwise email Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM. --- SBBSecho 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .