Attribution of interannual variations in tornado frequency to regional atmospheric conditions

The past several years highlight the potential range of interannual variability in tornado occurrence across the United States. In this study, we seek meteorological explanations of such variability in the form of anomalies in regional-scale atmospheric conditions.

Our approach consists first of establishing the degree to which U.S. tornado occurrence can indeed be considered anomalous. Inspection of tornado days shows, not surprisingly, that considerable geographical variation exists in the magnitude of departures from the mean. Data from 2006 and 2008 illustrate this well. Tornado-day anomalies are then compared to anomalies constructed using the North American Regional Reanalysis data from the period 1979-present. Insight is provided by some fairly basic sets of variables. We find, for example, that negative anomalies in warm-season (March-April-May) soil moisture, and positive anomalies in mean sea-level pressure and 500 hPa geopotential height (and corresponding circulation anomalies), associate to varying degrees with negative anomalies in 2006 warm-season tornado days in the southern Great Plains. The converse is true for 2008. The supporting statistical analysis will be discussed, and will lead to some comments concerning seasonal outlooks.