Subj : MESO: Nws Weather Prediction Center College Park Md To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Sep 16 2023 03:36:28 AWUS01 KWNH 160336 FFGMPD TXZ000-160900- Mesoscale Precipitation Discussion 1075 NWS Weather Prediction Center College Park MD 1135 PM EDT Fri Sep 15 2023 Areas affected...South Texas Concerning...Heavy rainfall...Flash flooding possible Valid 160334Z - 160900Z Summary...Clusters of thunderstorms organizing across South Texas will continue to drop slowly southward overnight. Rainfall rates will likely reach 3"/hr at times, which through slow motions could produce 3-5" of rainfall. Flash flooding is possible. Discussion...The regional radar mosaic this evening shows an impressive cluster of thunderstorms moving slowly southward near Laredo and into the Rio Grande Plains. These thunderstorms are developing within a region of impressive thermodynamic advection as PWs above 2.1 inches and MUCAPE higher than 3000 J/kg advect inland from the Gulf of Mexico and into the Rio Grande Valley. Forcing for ascent is being provided by a convectively enhanced shortwave ejecting eastward from the Serranias del Burro, within a region of broad RRQ upper diffluence, and along the nose of a modest but intensifying LLJ. Additionally, a stationary front is analyzed by WPC to be draped from west to east across South Texas, providing an additional focus for convergence and ascent. Together, these are producing intense thunderstorms with radar-estimated rainfall rates of 2.5-3"/hr according to KCRP, which have resulted in mesonet observed rainfall of nearly 4" in Laredo and subsequent impressive FLASH response of 900 cfs/smi unit streamflow. The high-res models are struggling with the ongoing activity, with most generally severely under-initializing the convective coverage. The exception is just the most recent HRRR and the 06Z RRFS, which are used heavily for the otherwise modest-confidence forecast of the evolution. With the shortwave moving slowly eastward, and the LLJ remaining 10-20 kts from the Gulf of Mexico, the environment should support a continuation of these storms even as they move south away from the stationary front. The LLJ is progged to slowly veer more to the south, which is reflected as well by Corfidi vectors becoming more northerly. This indicates that clusters will begin to dive more to the south, but maintain organization through 0-6km effective bulk shear of 25-30 kts. Rainfall rates as progged by the HRRR sub-hourly could reach 3-4"/hr (0.75-1"/15 min) which is supported by the extreme thermodynamics in place. As cells continue to move slowly with some backbuilding possible, rainfall could reach 3-5" in some areas as reflected by RRFS TL probabilities for 3" and 5" reaching as high as 60%, and 30%, respectively. Although the HREF members are not as supportive (but not initializing well), the HREF exceedance probabilities for these thresholds reach 40% and 20%, respectively, as well. Much of South Texas has been exceptionally dry recently, noted by AHPS 14-day rainfall that is around 0-5% of normal. This has raised FFG to 3-4"/1hr and 4-5"/3hrs, which only has a low probability of exceedance of less than 15%. However, these slow moving storms and the intense rain rates could still result in rapid runoff and flash flooding, especially in urban areas. At this time, it appears the greatest risk will remain across the Rio Grande Plains and north of the populated RGV, but if storms can move south more impressively a subsequent MPD may be needed for additional flash flooding potential overnight into these areas as well. Weiss ....Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!= DZ3fjg!86LeyyBn8FheXWpJ3vCRH77fCgr_qRPgpSFvLwNBtmZ7j1Sg0HZU_DRepi0Xqs5xLR1-= grprAqyVjTAhwx5MZ_G2JY8$ for graphic product... ATTN...WFO...BRO...CRP...EWX... ATTN...RFC...WGRFC...NWC... LAT...LON 28829886 28769762 28489667 28059681 27609711=20 27059742 26659779 26389852 26389909 27039971=20 27720009 28430050 28779950=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 3.14-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .