Anticipating large hail in WFO Tallahassee's forecast area: a baseline climatology of sounding parameters

The National Weather Service changed the hail size criterion for thunderstorm warnings from ¾ inch (penny sized) to one inch (quarter sized) in January 2010. Research indicated that significant property damage, such as to roofing materials, does not occur until hailstones reach at least one inch in diameter. Core partners in emergency management and the media had also raised concern about the numerous warnings being issued for marginal events – that is, storms that may have produced hail in the penny or nickel size range, but were not expected to pose any substantive threat to lives or property.

In order to appropriately implement the new criterion into warning operations, all severe hail reports from 1955-2009 across the NWS Tallahassee, Florida, Weather Forecast Office's area of responsibility were collected. Upper air soundings from the most representative site prior to and near each event were also collected. The sounding data were analyzed to produce a baseline climatology for one inch or greater hail. A large array of sounding parameters were computed, ranging from simple thermodynamic parameters to more complex fields such as effective shear and surface to 3 km mean-mixed Convective Available Potential Energy. These data were compared to those for severe thunderstorm events that produced damaging winds but no large hail, as well as for storms that produced maximum hail sizes from .75 to .88 inches in diameter.

The results of this study will enhance the forecasters' situational awareness regarding the potential for large hail from severe thunderstorms.