Climatology and Trends of Tornado-Favorable Atmospheric Ingredients in the United States

Tornadoes cause numerous fatalities and injuries each year in the United States. Understanding the geographic distribution of tornado risk and how it changes over time is critical to our ability to communicate risk and heighten public awareness and preparation—all part of our attempt to reduce the number of tornado-related fatalities and injuries. Tornadoes are most common when certain atmospheric conditions are met, namely, in a humid, unstable atmosphere with vertical wind shear. In this study, we generate seasonal climatologies and track seasonal changes of these atmospheric conditions. Each of these atmospheric conditions is found to change in different ways, creating unique spatiotemporal combinations. For example, wind shear and moisture increased in winter in the Midwest and Mid-South along with tornado counts. In spring, instability, wind shear, and moisture increased by various amounts through the Midwest and Mid-South. In addition to changes in the mean state of atmospheric conditions, trends herein show that variability is also changing. Notable changes include increases in the variability of convective available potential energy in the western portions of the north and south Great Plains in spring and summer and an increase in the variability of wind shear in the Midwest and Mid-South in fall. This study illustrates that atmospheric ingredients interact in complex ways. When considered with other studies, this study also shows that changing atmospheric conditions could be contributing to changes in tornado activity in the United States.