Trends in U.S. Large Hail Frequency

On average, large hail causes billions of dollars of property damage in the United States (U.S.) every year. Understanding the climatology and trends in large hail-producing environments is an important component of estimating the risk of hail hazards. In this study, we use the large hail parameter (LHP) to assess trends in LHP days, defined to be days with environments conducive for hail ≥ 5 cm (2 in). From 1979 to 2017, there has been a significant increasing trend in LHP days in central and eastern portions of the U.S., driven primarily by an increasing frequency of days with steep lapse rates in the middle troposphere. Additionally, the northeast U.S. has seen an increasing frequency of days that have favorable combinations of instability and vertical wind shear to support large hail-producing storms. The increase in coverage of LHP days is significantly correlated with an increase in coverage of large hail report days east of the Rocky Mountains, and these increases are corroborated by an increase in coverage of RADAR-inferred large hail in some U.S. regions. We hypothesize there may be a climate change fingerprint, through changes to the seasonal evolution and frequency of elevated mixed layers and more moisture-laden airmasses being advected northward ahead of weather systems, on increasing large hail risk and expanding this risk eastward.