Hail size forecast verification across Utah using a combination of radar reflectivity heights and derived products

Reliably forecasting hail size has traditionally been a problematic endeavor for meteorologists, particularly within the Intermountain Region where large gaps in radar coverage exist, and low level reflectivity data is significantly limited. Forecasters have developed techniques to forecast severe hail, including calculating a “VIL of the day.” Although this technique has been widely used operationally, past verification studies have shown the “VIL of the day” to have limited skill in forecasting hail size. This study examines the correlation between hail size and reflectivity heights with respect to freezing and wet-bulb zero levels across Utah from 1996 through 2006. A variety of reflectivity thresholds are examined (e.g. 50 dBz, 55 dBz, 60 dBz, etc…), along with several thermodynamic fields taken from observed soundings (wet-bulb zero level, freezing level, etc…). In addition, preliminary verification data is presented using products which have just been made available to National Weather Service forecasters within the past year. These products include VIL density and enhanced VIL density.