An operational assessment of the predictability of giant hail events

The occurrence of giant hail, defined as hail 4 inches (10.16 cm) in diameter or greater, is a relatively rare phenomenon, accounting for less than one percent of all hail reports in the United States. Despite the infrequent nature of these events, hail of this magnitude has the potential to cause extreme damage to property and a substantial threat to exposed life. The short-term prediction of these events has been challenging. Since 2005, only 5% of convective warnings and follow-up statements issued by the National Weather Service (NWS) accurately predicted a maximum hail size of 4 inches (10.16 cm) or greater prior to occurrence, with an average underestimated error of 2.13 inches (5.4 cm).

The motives of this study are to determine the predictability of giant hail in convective storms and to improve advanced recognition of these events, increasing lead time of accurate hail forecasts in NWS warning operations. A total of 479 unique cases were derived from 633 giant hail reports gathered over a 15 year sample period stretching from 1 January 1995 through 31 December 2009 across 17 states in the central contiguous United States. Weather Surveillance Radar, 1988, Doppler (WSR-88D) data and North American Regional Reanalysis (NARR) environmental data were collected for each case. A composite of the synoptic environment associated with giant hail events was created. Additionally, environmental parameters associated with severe storms and large hail are examined in a statistical study to determine their usefulness in predicting giant hail. Several radar signatures that have shown promise in warning for severe hail are also assessed for their utility in warning for giant hail.