Toward attribution of interannual variations in tornado frequency to regional atmospheric circulations

The current year highlights 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 circulations.

Our approach consists first of establishing the degree to which 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 “normal.” Data from 2006 illustrate this well. Tornado-day anomalies are then compared to anomalies constructed using the North American Regional Reanalysis data from the period 1979-present. We find, for example, that negative anomalies in 850 hPa specific humidity, and positive anomalies in 700 hPa geopotential height, associate well with negative anomalies in 2006 warm-season tornado days in the southern Great Plains. Analysis of 2008 anomalies will commence as data become available.