Flash Flood Guidance
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833 AWUS01 KWNH 281701 FFGMPD LAZ000-TXZ000-282200- Mesoscale Precipitation Discussion 0337 NWS Weather Prediction Center College Park MD 100 PM EDT Tue May 28 2024 Areas affected...Eastern hill country of Texas through Southwest Louisiana Concerning...Heavy rainfall...Flash flooding possible Valid 281700Z - 282200Z Summary...Clusters of thunderstorms moving across eastern Texas will drop towards the Gulf Coast while expanding/intensifying through the afternoon. Rainfall rates could reach 2-4"/hr at times, resulting in 3-4" of rainfall with locally higher amounts. Flash flooding is possible. Discussion...The GOES-E visible satellite imagery this morning shows a complex evolution of convection and associated outflow boundaries dropping across eastern Texas. These clusters of thunderstorms have persisted since this morning, and are moving along a southward advancing cold front analyzed by WPC and ahead of a shortwave noted in WV imagery. The regional radar mosaic indicates two distinct clusters with accompanying outflows, with the trailing cluster just southeast of Dallas accompanied by an MCV noted in the local radar. As these clusters continue to move south, they will encounter extreme thermodynamics characterized by SBCAPE which has rapidly destabilized in clearings to above 6000 J/kg and a plume of PWs around 1.5-1.7 inches. Although PWs are currently only around the 75th percentile due to dry mid-level air noted in regional 12Z U/A soundings, 850mb dew point temperatures above +20C are at daily record, and as low-level flow off the Gulf persists from the SE, it is likely PWs will surge to 1.8-2.0 inches by this evening. 850mb inflow is progged to remain modest today at just 10-15 kts during the aftn, but this will be sufficient to persist the extremely favorable environment for heavy rain. Acting into these robust thermodynamics, ascent will increase through convergence along the outflow and deep layer synoptic lift through PVA and RRQ diffluence aloft. Together, this will support an expansion and intensification of thunderstorms, with organization in one or multiple MCS likely thanks to bulk shear of 25-40 kts. This will allow rain rates to climb to above 2"/hr as reflected by HREF neighborhood exceedance probabilities reaching 40%, with brief 4"/hr rates possible as suggested by HRRR 15-min rainfall exceeding 1" in places. The CAMs are in generally good agreement in this evolution, and the challenge becomes the speed at which thunderstorms will traverse southeast. With the extreme CAPE and DCAPE (1200+ J/kg), this suggests clusters may continue to forward propagate rapidly and shift offshore, which is supported by SE oriented Corfidi vectors reaching 20-30 kts, minimizing the duration of heavy rainfall. However, there is some potential for storms to slow as they interact with the Gulf breeze/onshore flow, allowing propagation vectors to collapse and back sufficiently to enhance rainfall duration. Should this occur, total rainfall could exceed 5", but is otherwise forecast to be generally 3-4" as shown by HREF 3"/6hrs exceedance probabilities of 40-60%. This area remains extremely vulnerable with 40cm soil moisture above the 90th% in some areas, and 0-10cm RSM above 60%, especially from the Heart of Texas through the southern Piney Woods region. Although soil moisture dries towards the coast, these intense rain rates moving over any urban areas, especially if storms can slow, would still likely result in rapid runoff, and thus flash flooding is possible through the evening across much of this area. Weiss ...Please see www.wpc.ncep.noaa.gov for graphic product... ATTN...WFO...CRP...EWX...FWD...HGX...LCH...LIX...SHV... ATTN...RFC...LMRFC...WGRFC...NWC... LAT...LON 31879593 31799399 31169222 30449130 29989126 29579177 29639312 29549399 29279461 28929537 28919663 28869761 28999802 29679803 30919745