Tuesday, 12 October 2010
Grand Mesa Ballroom ABC (Hyatt Regency Tech Center)
The National Weather Service recently raised the severe thunderstorm warning criterion for large hail from 19 mm (0.75 in) to 25 mm (1.0 in) diameter. This change requires warning forecasters to identify WSR-88D data associated with 25 mm hail or larger in order to effectively issue severe thunderstorm warnings for hail. 25 mm hail reports across the Southern Plains in 2008 were gathered from National Climatic Data Center Storm Data. Environmental data were retrieved from the National Centers for Environmental Prediction North American Regional Reanalysis dataset. The reports were matched with WSR-88D and environmental data to establish radar signatures common to storms producing severe hail. The techniques examined in this study include: radar reflectivity at the -20°C and -30°C altitudes, and the altitude of the 50 and 60 dBZ echoes compared to the melting level. These datasets were evaluated statistically, and a baseline threshold for warning criterion was set at the 25th percentile for each technique in an effort to maximize skill score on a 2x2 contingency table. This threshold was tested operationally at National Weather Service offices in Fort Worth, TX and Amarillo, TX during the spring of 2010. The 25th percentile threshold and other thresholds were tested on 2009 data in order to generate a receiver operator characteristic curve for each technique. Using these curves and the spring 2010 operational testing, thresholds that maximize skill score are chosen for each technique. These thresholds are recommended as indicators of severe hail in support of warning operations for thunderstorms in the Southern Plains.
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