2026/07/22 by Matthew Graber, Robert J. Trapp, Zhuo Wang
#physics.ao-ph
Hailstorms in the United States produce immense economic losses and have an occurrence frequency that appears to have a long-term positive trend. Here we contribute an updated examination of such hail activity and the relevant environmental parameters over the period 1990--2024. We define a hail outbreak as any day with \gt 6 significant hail reports (hail≥ 2'' diameter) and find a positive trend in hail outbreaks since 1990 (slope=0.8 days yr-1) with the largest increases occurring in May and June and in the Southern Great Plains. Analysis of environmental parameters shows that the long-term increase in hail outbreaks correlates with a long-term increase in the significant hail parameter (SHIP, cc=0.67, p\lt 0.01). The interannual variability and long-term trend of SHIP are driven both by thermodynamic and kinematic processes, though kinematic processes play a more important role than thermodynamic processes in the interannual variability of hail outbreaks. Using warm-season (April--July) weather regimes (WRs), we find that large-scale circulation modulates the interannual variability of hail outbreak frequency. An empirical model using WR frequency captures the interannual variability in warm-season hail outbreaks reasonably well (cc=0.38, p=0.03). Our study is the first to relate hail activity to WRs and presents a better understanding of the trend and year-to-year variability of hail outbreaks.