Subj : MESO: Nws Weather Prediction Center College Park Md To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Aug 20 2025 19:48:45 AWUS01 KWNH 201948 FFGMPD GAZ000-TNZ000-ALZ000-MSZ000-210100- Mesoscale Precipitation Discussion 0963 NWS Weather Prediction Center College Park MD 347 PM EDT Wed Aug 20 2025 Areas affected...Portions of Tennessee, Alabama, and Mississippi Concerning...Heavy rainfall...Flash flooding possible Valid 201947Z - 210100Z Summary...Slow moving showers and thunderstorms will expand through the afternoon. Rainfall rates will likely exceed 2"/hr at times, leading to pockets of 2-4" of rainfall. This may lead to instances of flash flooding, especially over urban areas. Discussion...The regional radar mosaic this aftn indicates rapid development of showers and thunderstorms from south-central TN through central MS. This convection is developing ahead of an approaching cold front and along a low-level convergence boundary formed by the interaction of convective outflows and a surface trough. Ascent across this region is being additionally enhanced by a modest shortwave tracking E/SE near the MS/TN border. This broad ascent is acting upon a favorable thermodynamic environment to support heavy rainfall. PWs as measured by GPS are between 1.8 and 2.1 inches, around the 90th percentile for the date, overlapping MUCAPE that is between 3000-4000 J/kg south of the cold front. This is driving the rapid expansion of convection noted in the regional radar, with additional storms forming due to outflow collisions and storm mergers in the generally pulse-type environment. Rainfall rates have been estimated via local radars to be as high as 2-2.5"/hr from KHTX and KOHX, leading to MRMS hourly rainfall as much as 2.5 inches. As the aftn progresses, the continued ascent drifting southward into a ribbon of enhanced moisture confluence could support more frequent rainfall rates above 2"/hr as supported by HREF neighborhood probabilities, and the HRRR 15-min precipitation accumulation suggests brief rates in excess of 3"/hr are possible. While storms should remain generally pulse due to minimal bulk shear, and mean winds support a slow southward advance of cells at 5-10 kts, frequent mergers and collisions will lead to slower and at times chaotic motion which could enhance the duration of rainfall. Where this occurs, total rainfall through this evening could reach 2-4" as reflected by HREF 6-hr rain probabilities exceeding 40% for 3 inches. 7-day rainfall across the area has been scattered, but in some parts of the discussion area, especially in AL/TN, has been as much as 300% of normal, leading to 0-10cm soil moisture from NASA SPoRT that is above the 95th percentile. While FFG remains elevated, and FFG exceedance probabilities are modest in response, it is possible that any slow moving/lingering heavy rainfall could overwhelm soils to produce rapid runoff. However, this will be most likely across any urban areas which have the greatest potential for isolated flash flood impacts into this evening. Weiss ....Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!= DZ3fjg!_AxvMuSPWL7KSY07MFdiDt52-GqDaTutEI9deFy5LT48LMqp2BpjHDnlaG_CRK7RvFt6= qGV86FtNSPGNVxC_uEp8Wpg$ for graphic product... ATTN...WFO...BMX...FFC...HUN...JAN...MEG...MRX...OHX... ATTN...RFC...ALR...ORN...TIR...NWC... LAT...LON 35948609 35908485 35648452 35018456 34358490=20 33818568 33458724 33288747 32828818 32448880=20 32188955 32178993 32489015 33079011 33699002=20 34638895 35448751=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) .