Forecast rainfall and potential flooding, Lower RGV, August 22-24 2008
More Torrential Rains for Lower RGV
Zapata, Starr, Jim Hogg, and Hidalgo County at Risk

5 PM Update

Torrential rains now cover the western Lower RGV with numerous showers and thunderstorms in Starr, Jim Hogg, and Zapata County (right). Flood Watches are in effect for Hidalgo, Starr, Jim Hogg, and Zapata County. The threat for significant flooding will continue this evening and could re-generate late tonight and especially again Saturday morning, as the possibility for another slower moving complex of thunderstorms resumes.

Composite Reflectivity, Starr and Zapata County, 519 pm August 22 2008

Forecast and Reasoning
The combination of a vigorous, but slow moving, upper level disturbance across West Texas and the Big Bend, with continued influx of deep tropical moisture in the lower and mid levels, are the ingredients to maintain the threat of numerous showers and thunderstorms through at least Saturday (Figure, below). There remains uncertainty as to exactly how the entire system will evolve, and this will be critical in where and for how long any thunderstorm complexes - those which often produce flash floods - develop. For the eastern Lower RGV, daytime heating and perhaps interaction with a sea breeze, particularly on Saturday, could allow rapid development of more banded type activity, similar to Friday, mainly from mid morning through mid afternoon. For the western Lower RGV, while scattered to locally numerous afternoon showers are expected, the larger threat begins before daybreak on Saturday, as late afternoon and evening thunderstorms developing over the northern Sierra Madre Oriental in northern Mexico set off boundaries during the overnight, and help aid a nocturnal low level increase in convergent winds which in turn form additional complexes in the Rio Grande Plains and Rio Grande Highlands, including Starr and Zapata County but extending northward into Webb County and beyond.

While it is very difficult well in advance to predict with pin point accuracy the specific location of the heaviest rainfall in such an event, resident of the Lower Rio Grande Valley who have experienced flooding over recent weeks need to remain especially vigilant should rainfall in excess of 5 inches in a few hours materialize this weekend. These areas include western Zapata County, southwestern Starr County, Hidalgo County from Hidalgo through McAllen, and portions of eastern Hidalgo and western Cameron County who may still have remnant high water from Dolly. In addition, residents along and near Los Olmos Creek near Falfurrias (northern Brooks County) may need to remain alert should torrential rains fall upstream, including Webb and Duval Counties. Stay tuned to this website or other sources for possible flood watches, warnings, and advisories through the weekend. And remember, never cross flooded roadways of unknown depths. The life you save may be your own. Turn around, Don't DrownTM! For complete information on flood safety, click here.

Water vapor satellite image with explanation of energy flow needed for possible flooding
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