Wednesday, 26 January 2011
Severe hail events over central Florida are rare but earlier warnings for these events may save lives and will most likely reduce property damage. On January 5, 2010, hail threshold criteria for National Weather Service (NWS) Severe Thunderstorm Warnings changed from 3/4 inch (191 mm) to one inch (254 mm) or larger hail. Synoptic scale weather parameters were examined for the purpose of recognizing atmospheric patterns associated with hail events of varying sizes and operational forecasting of those events. Hail reports were gathered through Storm Data for a 30 year period from 1980 to 2009 for 17 counties in central Florida. These counties are roughly within 125 nm of the NWS Ruskin, FL WSR-88D radar. Out of the 1257 hail reports, 41.4 percent were one inch (254 mm) or greater, 1.7 percent were two inches (508 mm) or greater and only 0.3 percent were three inches (762 mm) or greater. Hail one inch or greater occurs most frequently from May through July. Interestingly, one inch or greater hail was reported in every month except November and December. Hail greater than two inches was only reported during February, March, April, and May. The evolution of the atmosphere beginning 72 h prior to the day of the event for hail size increments from ¾ inch (191 mm) to over four inches (1016 mm) was examined. NCEP reanalysis data were used to produce composite averages of sea level pressure, winds at various levels, precipitable water, and lifted index for the event day (Day 0) to three days prior to the event (Day -3). The primary difference between small hail events and large hail was the evolution of atmospheric moisture and instability. Since more of the larger hail events took place during the cooler months, atmospheric moisture was typically lower preceding the events and increased as it was transported over central Florida from the western Gulf of Mexico. Instability also evolved in a similar fashion. The smaller hail was more frequent during the warm season where the evolution was more subtle. Moisture and instability were plentiful over the state but increased slightly from the south over the area of interest. These patterns were associated with the subtropical ridge over central Florida drifting south and east as mid-latitude frontal boundaries pushed south toward Florida. An operational guide for hail forecasting was developed to help forecasters recognize these events.
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