Changes in the seasonality of tornado and favorable genesis conditions in the central United States

We analyze changes in the seasonality of tornado reports and related environmental conditions over the central U.S. Tornado counts are normalized by annual totals to reduce effects of changing reporting practices. Wavelet analysis of the normalized tornado counts shows significant power at semiannual and annual frequencies. An environmental index based on convective available potential energy and storm relative helicity shows power at the same frequencies. Reconstruction of the normalized tornado counts and the environmental index at these frequencies provides a time‐filtered description of their seasonal variation. Onset and peak dates of the tornado season defined from the reconstruction are significantly correlated with those of the environmental index over the period 1979–2013 and are found to have trends of −3.7 and −3.8 days/decade, respectively. The agreement between changes in tornado report seasonality and those of the environmental index is evidence that the trends have a physical origin.