Subj : MESO: Nws Weather Prediction Center College Park Md To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Jul 20 2024 06:58:04 AWUS01 KWNH 200657 FFGMPD LAZ000-TXZ000-201200- Mesoscale Precipitation Discussion 0668 NWS Weather Prediction Center College Park MD 257 AM EDT Sat Jul 20 2024 Areas affected...Upper and Middle Texas Coast Concerning...Heavy rainfall...Flash flooding possible Valid 200656Z - 201200Z Summary...Slow moving showers and thunderstorms will expand and drift near the Texas Coast through morning. Rainfall rates of 2-3"/hr are likely, which could produce 3-4" of rain with locally higher amounts. Flash flooding is possible. Discussion...A narrow corridor of PWs above 1.9" measured via GPS is aligned south of a stationary front analyzed by WPC to help fuel an expansion of convection noted on the KHGX WSR-88D this morning. These thunderstorms are expanding slowly to the west along and south of this boundary, aided by a ribbon of MUCAPE above 2000 J/kg, which remains surface based just off the coast into the Gulf of Mexico, and an associated ridge of high theta-e air advecting slowly northward. The GOES-E IR imagery at this hour shows rapid cloud top cooling from Galveston Bay westward, and recent rainfall rates have been estimated to be as high as 3"/hr. Storm motions across this area have been quite weak and chaotic, and model forecast soundings indicate extremely light and variable winds from the surface through 400mb, indicating that storm speeds will remain slow into the morning. Where cells have lingered, as much as 2-3" of rain has fallen according to local mesonets. The CAMs are struggling with the exact placement and evolution of activity so far, but have the general idea of a continuation and expansion westward for the next several hours. This includes the recent 3kmNAM, ARW, and RRFSp1. Although moist inflow from the Gulf at 850mb will remain modest at just around 10 kts, this will be sufficient to draw the best thermodynamics into the front, and is also greater and opposite to the mean cloud-layer winds suggesting some enhanced convergence for ascent. This combined with some frictional convergence along the coast, a weak shortwave moving into east Texas, and at least subtle upper level jet diffluence, should allow for convection to expand and intensify through the overnight hours. This is reflected by increasing 2"/hr rain rate probabilities on both the HREF and REFS, with the slow storm motions contributing to 3"/6hr probabilities reaching 40-60% on both ensemble systems. This indicates some areas could receive 3-4" of rain with locally higher amounts. The most likely location for rapid runoff resulting in flash flooding will be in urban areas. However, relatively moist soils noted by NASA SPoRT 0-40cm percentiles reaching above 90%, and HREF 3-hr FFG exceedance peaking at 30%, suggest instances of flash flooding are possible anywhere any storm mergers or repeating rounds of these slow moving heavy rain rates occur. Weiss ....Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!= DZ3fjg!--rJ49NFL88k3j7RFqB47ij73igJZbsY7LK5i52ngFD8CZzLPCNSnOr6iu3Wn4kku9b-= e29_FLN6WumAANvmxjVkP68$ for graphic product... ATTN...WFO...CRP...EWX...HGX...LCH... ATTN...RFC...LMRFC...WGRFC...NWC... LAT...LON 30029446 30029366 29789348 29339376 29029447=20 28799508 28559575 28329630 28149676 28189711=20 28569743 29039734 29499683 29769620 29909516=20 =20=20=20=20=20=20=20=20=20=20=20=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) .