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The Austin, Texas, Flash Flood: An Examination from Two Perspectives—Forecasting and Research

Robert A. MaddoxNOAA, Environmental Research Laboratories, National Severe Storms Laboratory, Norman, OK 73069

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Gary K. GriceNOAA, NWS Forecast Office, San Antonio, TX 78209

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Abstract

During the evening of 24 May 1981, rainfall from a slow-moving, multicell thunderstorm exceeded 200 mm (8 in) in just 2 h over western sections of Austin, Texas. This intense precipitation, falling on previously saturated terrain, resulted in record flash flooding. The evolution of the large-scale meteorological setting associated with the flood is briefly examined. The presentation then focuses on the development and movement of several mesoscale convective systems (MCSs) over the southern Plains during the night preceding the flash flood. It is shown that two significant mesoscale convective outflow boundaries stalled and weakened in the Austin vicinity. The flash flood storm appears to have developed along a weak baroclinic zone left behind by these outflows. Additionally, the precursor MCSs primed the hydrologic setting in south central Texas as they produced 75–125 mm (3–5 in) rains. Upper-air soundings are considered and related to the anomalous (relative to other storms over Texas) movement of the flash flood thunderstorm.

Finally, the difficulties of dealing with this severe weather event are discussed from an operational perspective. Subtle interactions between a variety of mesoscale features appear to have occurred. The flash flood storm behaved much differently, for a brief period, than thunderstorms further to the west. How might these features and characteristics have been identified and monitored in real time? The final complication in the operational scenario was the issuance of a severe thunderstorm watch by the National Severe Storms Forecast Center. This led to a shift in emphasis in the statements being issued to the public from the flash flood danger to the apparently higher priority severe thunderstorm threat.

Abstract

During the evening of 24 May 1981, rainfall from a slow-moving, multicell thunderstorm exceeded 200 mm (8 in) in just 2 h over western sections of Austin, Texas. This intense precipitation, falling on previously saturated terrain, resulted in record flash flooding. The evolution of the large-scale meteorological setting associated with the flood is briefly examined. The presentation then focuses on the development and movement of several mesoscale convective systems (MCSs) over the southern Plains during the night preceding the flash flood. It is shown that two significant mesoscale convective outflow boundaries stalled and weakened in the Austin vicinity. The flash flood storm appears to have developed along a weak baroclinic zone left behind by these outflows. Additionally, the precursor MCSs primed the hydrologic setting in south central Texas as they produced 75–125 mm (3–5 in) rains. Upper-air soundings are considered and related to the anomalous (relative to other storms over Texas) movement of the flash flood thunderstorm.

Finally, the difficulties of dealing with this severe weather event are discussed from an operational perspective. Subtle interactions between a variety of mesoscale features appear to have occurred. The flash flood storm behaved much differently, for a brief period, than thunderstorms further to the west. How might these features and characteristics have been identified and monitored in real time? The final complication in the operational scenario was the issuance of a severe thunderstorm watch by the National Severe Storms Forecast Center. This led to a shift in emphasis in the statements being issued to the public from the flash flood danger to the apparently higher priority severe thunderstorm threat.

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