Subj : MESO: Nws Weather Prediction Center College Park Md To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Jul 21 2024 09:39:58 AWUS01 KWNH 210939 FFGMPD LAZ000-TXZ000-211500- Mesoscale Precipitation Discussion 0678 NWS Weather Prediction Center College Park MD 539 AM EDT Sun Jul 21 2024 Areas affected...Upper Texas Coast, Southern Louisiana Concerning...Heavy rainfall...Flash flooding possible Valid 210938Z - 211500Z Summary...Showers and thunderstorms will increase over the Gulf of Mexico and lift northward onshore Louisiana and the Upper Texas Coast this morning. Rainfall rates of 2-3"/h are likely, which through slow storm motions could produce 2-4" of rain with locally higher amounts. Flash flooding is possible. Discussion...The regional radar mosaic this morning shows showers and thunderstorms slowly expanding across southern Louisiana, while the GOES-E IR imagery indicates cooling cloud tops indicative of additional convection of the northern Gulf of Mexico. These thunderstorms are expanding in response to a shortwave lifting northward from the Gulf, which is providing additional lift to an area already beneath a mid-level weakness and tail of an upper jet streak. A modest 10-15 kt 850mb LLJ measured via regional VWPs is lifting northward, drawing PWs over over 2 inches and MLCAPE of 2000-3090 J/kg onshore and into a stationary front analyzed by WPC. The overlap of ascent into these robust thermodynamics is driving the expanding convection this morning, while warm cloud depths above 14,000 ft and a deep layer of moist-adiabatic lapse rates support the efficient warm rain processes driving radar-estimated rain rates of 1.5"/hr. Although thunderstorms are likely to remain scattered through the late morning as reflected by recent CAMs including the HRRR, RRFSp1, and experimental RRFSp3/p4, there is good agreement in an expansion in coverage, especially along the immediate coast. This will be driven by persistent moisture advection into the frictional convergence boundary along the coast, and aided by the shortwave lifting northward. While there is some uncertainty as to how far inland the convection can track through the morning due to modest instability over land, both the HREF and REFS ensembles indicate 2"/hr rainfall rate probabilities rising to 30-40%, suggesting locally 3"/hr rates are possible. These rates themselves could overwhelm soils, even in locations that are typically less susceptible due to swampy soils and higher FFG. However, the setup is conducive to backbuilding and training as Corfidi vectors increasingly veer to the SW and collapse. This indicates that cells will regenerate over the Gulf into the better instability and lift northeast, producing effective cell motions of less than 5 kts in some areas. Where this net motion results in longer duration training, 2-4" of rain with locally 5" is possible as reflected by HREF and REFS probabilities for 3" (5") of 30-50% (10-20%). Additionally, the 6-hr PMM from both ensembles indicates locally 4-6" of rainfall near the coast. While this portion of the Gulf Coast, especially outside of urban areas, can generally handle heavy rain, some parts of the coast have experienced well above normal rainfall of 150-300% in the last 7 days according to AHPS. This has resulted in high soil moisture anomalies, leading to slightly more favorable conditions for rapid runoff and flash flooding. While instances of flash flooding will be more likely should convection train across urban areas, anywhere that experiences training of these intense rain rates could have at least isolated impacts this morning. Weiss ....Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!= DZ3fjg!9GaNdeCHC1QCWpDbfRJFdaXoIYsiy_LXZJVFnd6mNoQDQZYd4-akFeGATz57rxZfJFsz= PTrTT8flu6P0hMKPvDDM818$ for graphic product... ATTN...WFO...HGX...LCH...LIX... ATTN...RFC...LMRFC...WGRFC...NWC... LAT...LON 30449335 30439200 30379105 30249030 30068951=20 29808901 29358887 28968905 28828976 28949052=20 29219138 29339227 29429309 29339389 29129446=20 29069492 29269524 29779509 30209440=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) .