Long-term fluctuations in hail incidences in the United States

Hail-day occurrences during a 100-year period, 1896-1995, derived from carefully screened records of 67 first-order stations distributed across the U.S., were assessed for temporal fluctuations and trends. Temporal fluctuations based on 20-year and longer periods exhibited regional coherence reflecting the control of large-scale synoptic hail producing storms on the point distributions over broader areas. Classification of the fluctuations for stations based on 20-year values revealed five types of distributions existed at most of the U.S. stations. A majority of stations, 44 of 67 stations, achieved their lowest 20-year value of the 100-year period during the last 20 years, but 7 other stations had peaks in the most recent 20 years. The linear trends of the 67 stations defined upward hail trends in the High Plains-Rockies and Southeast, areas of no trend in the northern Midwest and along the East Coast, and down trends elsewhere. The 67 station values for each 20-year period were combined to form a national hail value, and the resulting time distribution showed a mid-century peaking of hail activity followed by a decline to the lowest in 1976-1995. This trend is similar to those found for thunder-day values and crop-hail losses. The results for hail demonstrate the importance of assessing extremes on a regional as well as a national basis. Different outcomes were found.