Changes in Seasonal Occurrence and Interannual Variability of Tornado Outbreaks

Outbreaks of tornadoes in the United States lead to loss of life and property every year, but with significant interannual variability. The overarching objectives of this study are to investigate whether such interannual variability is changing with time, and how this might be governed by predicable large-scale circulation patterns.

First, using the NOAA/Storm Prediction Center database, it is found as in prior studies that outbreak days are becoming increasingly common in the most recent 30 years, even though the number of annual tornado days is decreasing over this period. A seasonality analysis of days with at least 10 EF-1+ tornadoes, herein considered to be outbreak days, shows that the peak occurrence of outbreak days has shifted to the early boreal spring in the most recent 30 years. Finally, an analysis of interannual variability reveals that volatility has increased in recent decades.

Using the ERA5 reanalysis data, a spatial and temporal analysis of environmental parameters conducive to tornado formation, including convective available potential energy (CAPE), 0-6km bulk wind shear (VWS), storm-relative helicity (SRH), lifting condensation level (LCL), and the multivariate significant tornado parameter (STP), are used to corroborate the storm-report analyses. The environmental parameter analysis is also informing an examination of large-scale circulation patterns. Of particular interest are spatial changes in these circulation patterns, which may provide insight into the modes of climate variability driving the interannual variability in tornado activity.