Development of forecast equations to predict the severity of hail events in New York state

Forecasters across the country routinely make subjective assessments of convective potential for their forecast area based on the values of various atmospheric parameters and indices. If convection is possible, forecasters must decide whether it will be severe or non severe; and if severe thunderstorms are possible, they must determine if the primary threat will be large hail, damaging winds, tornadoes, or all three. The specific parameter values which influence certain decisions may vary from person to person depending on a forecaster's geographic location, experience, and scientific understanding of the physical processes associated with thunderstorm development and evolution. Because of the subjective nature of the decision making process, the results may not be consistent. An equation developed in Maglaras and LaPenta (1997) provided guidance on forecasting tornadic, non-tornadic but severe and non-severe thunderstorm days. It didn't identify whether the main threat from non-tornadic severe storms was damaging winds or large hail (diameter .75 inch or larger). This paper is a follow-up study to Maglaras and LaPenta (1997) and describes the development of equations that provide objective statistical guidance for determining the overall severity of hail days by category, and the expected maximum hail size in New York State. The categorical forecast equation developed in this study successfully discriminates between major and minor hail days, and the hail-size equation showed skill at forecasting the maximum hail size for a day with thunderstorms. Although these equations should only be applied in the specific geographical area for which they were derived, the method used to develop these equations can be applied elsewhere.