A synoptic climatology of severe hail in the northeastern United States
Date
2011
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Publisher
University of Delaware
Abstract
A climatology of severe hail (3/4 inches or greater) was constructed for the Northeastern Unites States (Figure 3.1). This region was chosen because it contains highly populated urban environments adjacent to vast agricultural regions, both of which are susceptible to hail damage. This study provides a baseline climatology of severe hail throughout the Northeast United States (from 1955-2009) by investigating the spatial and temporal characteristics of severe hail storms, and the extent to which population density influences severe hail reports. Synoptic patterns and thermodynamic environments are also discussed by analyzing data from the top 85 most reported severe hail outbreaks, defined as significant severe hail event days. This data consists of daily mean composite synoptic weather maps, as well as information from nine upper air stations throughout the study region. The results show that the distribution of severe hail in the Northeastern United States is influenced by a combination of meteorological, technological, and societal factors. The daily mean composite synoptic maps show that significant severe hail outbreaks in the Northeastern United States commonly occur when the region is influenced by the ascending branch of a low amplitude, anomalous, 500 hPa trough, accompanied by an anomalous surface low pressure system, situated to the west or northwest of the study region. A weak east-west oriented cold front associated with this surface low typically provides the lifting needed to initiate severe convection. The largest reported severe hailstone throughout the duration of the study occurred in Virginia, in 1968, measuring 5 inches in diameter. Conversely, 57% of all studied severe hailstones had a reported diameter of less than one inch. A majority of all severe hail reports occurred during the three month period of May, June, and July, between 1500-1659 hours local time. Thermodynamic information obtained using 00 UTC soundings showed that the K-Index, Total-Totals index, CAPE, and Wet Bulb Zero Level showed the most skill in forecasting significant severe hail outbreaks in the Northeastern United States. Due to a statically stable bias in the thermodynamic data by 00 UTC, non-surface based severe weather indices were able to provide a more reliable hail forecast than surface based severe weather indices.