Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Dec 01 2025 22:54:33 FOUS30 KWBC 012254 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 554 PM EST Mon Dec 1 2025 Day 1 Valid 01Z Tue Dec 02 2025 - 12Z Tue Dec 02 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE GULF COAST AND DEEP SOUTH... A negatively- tilted shortwave trough moving through the Southern=20 Plains at the present time is leading to a forming stationary from=20 across the Deep South to the north of a wave of low pressure in the Gulf of Mexico. An area of low-level convergence which started as=20 a frictional convergence zone near the northwest coast of Cuba is=20 curling up towards the MS/AL coast at the present time. Moisture is sufficient for heavy rainfall, but until recently, instability was scant. However, ahead of the shortwave, MU CAPE has risen to 500=20 J/kg near the southeast tip of LA, which should further increase=20 from there and spread into nearby areas of the Central Gulf Coast=20 with time, as far east as the FL Panhandle. MU CAPE could rise towards 1000 J/kg along portions of the Central Gulf Coast through early Tuesday morning. Effective bulk shear of 25-35 kts lurks across the region, which could lead to cell organization. A band of heavy rain with embedded thunderstorms has occasionally=20 led to hourly rain amounts eclipsing 1" between southeast LA and southernmost AL thus far, but this has appeared more like a longer duration heavy rain event so far. The 18z HREF and 12z RRFS are=20 not exactly in agreement regarding the potential for 2"+ by=20 12z...their overlap is in a narrow zone between Dauphin Island AL=20 and Pensacola where downstream instability and moisture overlap=20 would be greatest. The three hour flash flood guidance values=20 regionally are quite high and appear unreachable. However, there=20 is some chance of 1.5" amounts in an hour with local totals up to=20 3", which would be problematic in urban areas, so left much of the=20 Marginal Risk area from continuity intact. The Marginal Risk area=20 has been constrained on its west side per radar reflectivity trends and trends noted in the 18z HREF/12z RRFS guidance. Roth Day 2 Valid 12Z Tue Dec 02 2025 - 12Z Wed Dec 03 2025 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Surface/low level convergence associated with a frontal boundary forecast to translate eastward into the western Atlantic after 18Z may allow for a band of SW to NE training heavy rain/thunderstorms early in the period over eastern North Carolina. Weak instability and precipitable water values increasing to over 1.5 inches along the coast could support rain rates over 2 in/hr, however, recent hires guidance maintains the potential for 3 to 6 inch totals offshore/east of the Outer Banks. While the threat is non-zero, it is low enough to not warrant a risk area at this time. Otto Day 3 Valid 12Z Wed Dec 03 2025 - 12Z Thu Dec 04 2025 The probability of rainfall exceeding flash flood guidance is less than 5 percent. The very beginning of a multi-day rainfall event will begin to unfold Wednesday night along portions of the Upper Texas and Louisiana Gulf Coasts. Mainly for the 06-12Z time frame Thursday morning, the 12Z guidance continues to suggest most of the heaviest rainfall from heavy showers/thunderstorms will remain offshore. However, at least some of that rain could make its way inland. The threat remains sub- Marginal, but the area will continue to be evaluated for a later introduction of a Marginal Risk with future updates. Otto/Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5JhtpKZY8hmMIEMjlBD-SLd-BS71DInY9-GywGAOsomn= na9IpQs3e_SxE2g0hk_DIr1l-vh-NirHC8NF4VeC9BYBeb4$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5JhtpKZY8hmMIEMjlBD-SLd-BS71DInY9-GywGAOsomn= na9IpQs3e_SxE2g0hk_DIr1l-vh-NirHC8NF4VeCRhz10QI$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5JhtpKZY8hmMIEMjlBD-SLd-BS71DInY9-GywGAOsomn= na9IpQs3e_SxE2g0hk_DIr1l-vh-NirHC8NF4VeCxPbbPEo$=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 3.28-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .