A City-Based Analysis of the Likelihood of Extreme Hail Sizes over the United States

Large hail causes significant economic losses in cities throughout the United States each year. Previous research has focused on characterizing the distribution and return periods of large hail for the entire country but there has been little work done that looks at the local probability over individual cities. In the past decade increasing losses have been driven by large single city loss events exceeding USD 1 Billion, including Denver, Minneapolis, Dallas, Phoenix and Oklahoma City. This presentation will focus on describing the climatology of incidence for large hail (>1 inch) and return periods of hail in Denver, Oklahoma City, Dallas, Des Moines, St. Louis, Chicago, Indianapolis, Detroit, Nashville, Atlanta, Pittsburgh and New York City on a 0.2˚ x0.2˚ grid using extreme value theory and hail observations from 1955 to 2017. Data for the observational climatology are drawn from the observations of Storm Data, CoCoRAHS, and MPing. Common hail sizes vary across the country, with larger sizes being more common in the Great Plains and smaller sizes more common throughout the Midwest and Northeast. As a result, the distribution of hail return periods is extremely variable and is strongly influenced by the incidence of hail in different locations. Fitted extreme value models for individual cities using a Generalized Extreme Value model suggest a positive shape parameter and therefore a Type II (Freshet) extreme value distribution , contrasting earlier work which showed the distribution was best modelled by a Gumbel distribution. Cities that are more susceptible to hail storms are more likely to experience large hail at higher rates than cities without frequent exposure to hail. For example, in Oklahoma City, 5-inch hail has a return interval of approximately 100 years on this grid while in New York City, 5-inch hail has a return interval of 1000 years. These results will allow a better understanding of how often large hail can be expected in major metropolitan areas. This in turn provides the impetus for a greater appreciation of the underlying risk from a financial standpoint, and allows cities to enhance preparation for hail storms in the future, as well as develop approaches to mitigate damage and reduce losses.