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Flash Flood Guidance
Issued by NWS
Issued by NWS
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365 AWUS01 KWNH 200539 FFGMPD TXZ000-200900- Mesoscale Precipitation Discussion 0471 NWS Weather Prediction Center College Park MD 139 AM EDT Thu Jun 20 2024 Areas affected...South Texas... Concerning...Heavy rainfall...Flash flooding possible Valid 200340Z - 200900Z SUMMARY...Outer band of T.S Alberto may train across sensitive areas with additional spots of 2-4" possible resulting in potential reaggravation of flash flooding across S TX. DISCUSSION...Regional RADAR mosaic depicts a subtle shortwave/mid-level vorticity center moving eastward into the Rio Grande Valley near Laredo, Texas. This feature appears to be at the north-northwest edge of the inverted low to mid-level trough of the outer circulation of T.S. Alberto. Aloft, the feature is enhanced by upper-level evacuation support at the diffluent portion of the 3H jet that becomes further defined by the transverse banding in the arched cirrus pattern across central to northwest TX. This feature continues to influence low level confluence into a trailing surface to boundary layer outer band across South Texas from the wave across from Duval to Neuces and back out across the western Gulf. Ample deep layer moisture to 2.75" and unstable air mass off the coast into the western Gulf with 3000-3500 J/kg of SBCAPE will continue to support efficient rainfall production for thunderstorms that can develop. Maintaining convergence will be the biggest uncertainty for maintaining the band, but given the northern influence of low level flow backing subtly, along with fairly solid consistency in hi-res CAMs and RAP forecasts. Cells are likely to be maintained even as winds decrease about 5-10kts over the next few hours through the 925-850mb layer. Frictional convergence is likely to be the best factor for thunderstorm redevelopment and given it is fairly orthogonal to the coast (enhanced by local effects near bays/islets/peninsulas) the risk for retaining the band seems highly likely, though may waver/vary north-south through time to likely limit any particular area receiving 2-3"/hr rates for long periods. As such, streaks within the band of 2-3" are likely, with some spots of 4" remaining possible. Given area has become saturated, especially near Corpus Christi area where FFG values are below .5" at all time periods, ongoing flooding is likely to continue through early morning hours. However, for the bulk of the area of concern, flash flooding is considered possible. Gallina ...Please see www.wpc.ncep.noaa.gov for graphic product... ATTN...WFO...CRP... ATTN...RFC...WGRFC...NWC... LAT...LON 28389835 28269756 28279684 28069681 27829703 27529721 27339740 27389856 27479909 27889952 28359907