Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Apr 21 2025 08:10:32 FOUS30 KWBC 210808 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 408 AM EDT Mon Apr 21 2025 Day 1 Valid 12Z Mon Apr 21 2025 - 12Z Tue Apr 22 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS SOUTHEAST TEXAS AND PORTIONS OF THE DEEP SOUTH... Low-level confluence has aided in persistent, slow-moving convection across southeast Texas and near Shreveport overnight/early Monday. Models suggest that these storms should continue within a similar regime for at least a few hours early in the forecast period (through 18Z or so) as low-level confluence shifts southward. Moist/unstable air (1.75+ PWATs) should be maintained ahead of the activity, which should support local 2 inch/hr rainfall amounts at times given expected slow storm motions. An instance or two of flash flooding is possible where the downpours can fall over sensitive/low-lying ground conditions. Across Mississippi and Alabama, a cold front is expected to make southeastward progress during the day, but stall across northern portions of both states. Models are in agreement that scattered convective activity will develop during the afternoon/evening - supported both by surface heating and low-level warm advection. Some of the higher resolution guidance has tended to focus convection from east-central MS into central AL - a scenario that may favor training and a locally higher threat for flash flooding especially given lower FFGs across AL/near the Birmingham area. This scenario is a bit uncertain in placement, however, given the overall weak forcing. A broad Marginal risk area is maintained for this outlook to account for potential for localized 1-2 inch hourly rain amounts that threaten local FFGs. Cook Day 2 Valid 12Z Tue Apr 22 2025 - 12Z Wed Apr 23 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS NORTHWEST TEXAS AND WESTERN OKLAHOMA... A return-flow regime begins in earnest during the forecast period=20 across the southern Plains and results in areas of 60s dewpoints=20 across the OK/TX Red River Valley area. Meanwhile, weak west-=20 southwesterly mid-level flow and weak perturbations within that=20 flow should spark initially higher-based thunderstorm activity=20 across the Panhandles that should spread east-southeastward toward=20 areas of greater moisture content after 00Z. The outflow dominant=20 nature of initial storms should limit flash flood potential=20 initially, with a conditional risk materializing later in the=20 evening and overnight hours if 1) any upscale growth into one or=20 two mesoscale convective complex can promote training and boost=20 rain rates and 2) heavier rainfall can materialize across portions=20 of north-central Texas/southern Oklahoma that received 3-8 inches=20 of rain in the past 72 hours or so. These factors reflect a flash=20 flood threat that is too conditional for anything beyond=20 Marginal/5% probabilities at this time, and are highly dependent on a mesoscale evolution that remains uncertain at this time. Across North Carolina, models suggest that surface heating along and south of a cold front near the NC/VA state line could provide a focus for scattered thunderstorm activity during the afternoon hours. Zonal flow aloft (parallel to this boundary) would support some potential for convective training, although the coverage of storms is a bit uncertain given modest ascent/mid-level support. A Marginal Risk/5% area may be needed in later outlooks pending=20 greater certainly on convective evolution. Cook Day 3 Valid 12Z Wed Apr 23 2025 - 12Z Thu Apr 24 2025 Substantial uncertainty exists for this outlook. Broad, western=20 U.S. troughing aloft should result in southwesterly flow aloft=20 across the High Plains for much of the forecast period. Meanwhile,=20 southerly low-level flow should gradually moisten low-levels across Kansas and vicinity - especially in the latter half of the=20 forecast period. Subtle waves within the flow aloft should promote scattered, initially high-based thunderstorm activity during the afternoon and early evening, though coverage and location is in question. Thereafter, there are hints that upscale growth of storms could result in gradually increasing heavy rain potential as low-level warm advection results in higher moisture content/PW values (1-1.5 inches) across central Kansas into central Oklahoma. Again, this scenario is uncertain given pre-existing moisture quality concerns. Given the uncertainty and dry antecedent soil conditions,=20 probabilities for excessive rainfall are removed for this outlook=20 across the Great Plains. Cook Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-F-j_g794_h6qJIaLvk1z1HxC9H9WwOJXsapBXWwPFbC= jjpvf_-TDQNp-hVuz8DyDHk4RybMvH9BRVE5dgyzUQQN_oc$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-F-j_g794_h6qJIaLvk1z1HxC9H9WwOJXsapBXWwPFbC= jjpvf_-TDQNp-hVuz8DyDHk4RybMvH9BRVE5dgyzV-dfYd4$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-F-j_g794_h6qJIaLvk1z1HxC9H9WwOJXsapBXWwPFbC= jjpvf_-TDQNp-hVuz8DyDHk4RybMvH9BRVE5dgyzUftOE18$=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) .