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Texas A&M University: "The left-moving thunderstorms of May 26-27 1992"

Final Report

During May 26, 1992, a weak cold front was expected to move toward Houston from the north. Because stability indices suggested the likelihood of strong thunderstorms, it was expected that severe convection would develop during the afternoon along the front in response to solar heating. In spite of these favorable conditions and the absence of a pronounced lid, no significant convection occurred during the day. After sunset, however, a 300 km-long line of convection rapidly developed along the cold front and moved into the Houston metropolitan area. As the line stalled over Houston, a succession of anticyclonically rotating thunderstorms developed. These cells were associated with the strongest reflectivity and vertically-integrated liquid water, and produced heavy rainfall and minor urban flooding.

This case presented a challenge to the forecasters on duty at the time, both because of the lack of convective development during the day when expected and because of the development which finally did occur. In addition, the thunderstorms which did develop were unusual in that anticyclonic rotation appeared to be favored. These storms merit particular attention because NEXRAD mesoscyclone detection algorithms are only triggered by cyclonic rotation. Consequently, special care must be taken by WSR-88D operators to identify and track the radar signatures of these anomalous thunderstorms.

The main research objectives of the project were to:

1) develop a method for identifying and evaluating anticyclonically rotating thunderstorms using the WSR-88D PUP display;

2) determine the cause of the delay in formation of the thunderstorms so as to improve the forecasting of such events in the future;

3) attempt to understand the environmental conditions which favored strong left-moving thunderstorms along and above the frontal boundary, and those that caused the front to stall.

The project produced two major accomplishments. The first was a description of the methods to be used by WSR-88D operators for identifying and tracking anticyclonic supercells. The capability to recognize this rare event was put into practice during a similar event in 1994. The second was an analysis and description of a false radar echo known as a flare, which can indicate large hail and provide the radar operator with a few minutes' warning of hail and microbursts.