Tornado seasonality in the southeastern United States

Tornadoes are among the most destructive natural events and occur most frequently in the United States. It is difficult to ascertain if the frequency of tornadoes in the U.S. is increasing because our ability to observe and report tornado occurrence has increased over time. Previous studies have demonstrated that tornado likelihood has shifted toward earlier dates across the south-central United States over the past seven decades, the region sometimes called “Tornado Alley”, if it can be assumed that seasonal observation effort has not shifted over time. It is unclear if such shifts in tornado seasonality have also occurred elsewhere, including the region of the southeastern United States where tornado likelihood has a bimodal annual distribution. We use circular methods to demonstrate that the date of observed peak tornado occurrence during the early tornado season has not changed in the past seven decades. However, the date of peak tornado occurrence during the later tornado season has shifted toward earlier dates by more than a week. The influence of tropical storms had no effect on changes in late-season tornado seasonality. The conclusions are robust with respect to whether tornado counts or tornado days are used as the response variable. Results demonstrate the ongoing need to encourage tornado preparedness in the southeastern U.S., where tornadoes tend to have a higher impact on humans, and to understand the mechanisms that underlie trends in tornado seasonality.