On the temporal and spatial characteristics of tornado days in the United States

•Low-frequency tornado days became less common between 1974 and 2015.•High-frequency tornado days became more common between 1974 and 2015.•The variability of the number of tornadoes per tornado day increased between 1974 and 2015 more than the mean number.•Tornadoes that occur on high-frequency days tend to be located further east than those that occur on lower-frequency days.

More tornadoes are produced per year in the United States than in any other country, and these tornadoes have produced tremendous losses of life and property. Understanding how tornado activity will respond to climate change is important if we wish to prepare for future changes. Trends in various tornado and tornado day characteristics, including their annual frequencies, their temporal variability, and their spatial distributions, have been reported in the past few years. This study contributes to this body of literature by further analyzing the temporal and spatial characteristics of tornado days in the United States. The analyses performed in this study support previously reported findings in addition to providing new perspectives, including that the temporal trends are observed only in low-frequency and high-frequency tornado days and that the eastward shift in tornado activity is produced, in part, by the increasing number of high-frequency tornado days, which tend to occur to the east of the traditionally depicted tornado alley in the Great Plains.