Flash Flood Guidance
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Issued by NWS
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045 AWUS01 KWNH 170930 FFGMPD TXZ000-171400- Mesoscale Precipitation Discussion 0280 NWS Weather Prediction Center College Park MD 529 AM EDT Fri May 17 2024 Areas affected...Lower Rio Grande Valley in Southwest Texas... Concerning...Heavy rainfall...Flash flooding possible Valid 170930Z - 171400Z SUMMARY...Slow moving thunderstorms with ample moisture flux; within a favorable upstream redevelopment environment may support locally intense thunderstorms capable of 2"/hr rates and possible flash flooding. DISCUSSION...GOES-E 3.9um SWIR and local observations denote the convectively reinforced outflow boundary/cold front has begun to slow its southward push in proximity to MFE/EBG in Deep South Texas with a surface wave across the Rio Grande into Old Mexico. From there, low stratus denotes the cold front is banked up across the Serranias del Burro though deep moist air continues to flow over it from two directions. The cold air is fairly shallow so 925-850mb return south-southeasterly flow continues to pump high theta-E air along/north of the border while moderately moist Pacific air over-tops through the 700-500mb per CIRA LPW. This supports 1.75-1.9" total PWATs but heavily loaded given upper 70s moisture is isentropically ascending over the shallow cold front as far north as NW Maverick county and points west. MUCAPEs are reaching 3000 J/kg and with eroding capping. Aloft, a strong 90+kt 3H jet streak is crossing the area with a subtle shortwave inflection just upstream just south of the Big Bend of TX starting to press eastward providing upper-level favorable ascent. As such, scattered discrete convective initiation has occurred across S Dimmitt, Webb and LaSalle counties with the strongest cell just crossing the River at this time. However, high shear environment will support some hail generation initially, KDP suggest solid mixture with heavy rainfall as well. Additionally, strong effective bulk shear over 90kts suggest favorable organization and Bunker`s Rgt moving Propagation vectors suggest slowing forward speed to about 15-20kts for any developing rotating updrafts to allow for increased duration. In addition, orientation of convection and favorable upstream back-building environment suggests flanking lines may be slow to propagate eastward allowing for additional updraft towers for repeating. Given potential for 1.75-2"/hr rates and repeating, localized spots of 2-4" are possible over the next few hours; particularly near/along the Rio Grande and points west, due to proximity to deepest moisture return/unstable mid-level environment that reduces in effectiveness/stabilizes further east into central and eastern South Texas. While area has been fairly dry and FFG values are naturally higher with 3-4"/3hr range, the potential for flash flooding will be localized to those locations of 3-4" in 1-3hrs, and so coverage should be scattered to widely scattered and considered possible. Gallina ...Please see www.wpc.ncep.noaa.gov for graphic product... ATTN...WFO...BRO...CRP...EWX... ATTN...RFC...WGRFC...NWC... LAT...LON 28959923 28659867 28079838 27609821 26999815 26629842 26579905 26809944 27159970 27689997 28040019 28490049 28909997