Analysis of Stability Indices for Severe Thunderstorms in the Northeastern United States
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In operational forecasting, many indices are used to assess the stability of the atmosphere and predict the likelihood of severe thunderstorm development. One of the shortcomings of many of these indices is that they are mainly based on observations from the southern Plains. Severe thunderstorms can occur in the northeastern United States in conditions which significantly differ from those expected in the Plains. Few attempts have been made to modify these indices for thunderstorms in the Northeast. A new set of values specifically for use in the Northeast are computed. The stability indices examined are the Showalter index, lifted index, SWEAT, K index, total totals, CAPE, CIN, and equilibrium level pressure. Thunderstorms which occurred between the months of June through August during 1998-2007 are used in this analysis. Upper air data from radiosondes at eight sites in the Northeast are used to calculate the new index values. These data are analyzed with respect to the presence or absence of severe thunderstorms within 150 km and plus/minus 3 hours of a sounding. Using these criteria, there are 423 soundings which contained severe thunderstorms and 13,012 soundings which do not. Forecast skill is calculated for each index. The best predictor in most cases is the LI, with CAPE a close second. For most indices, the threshold which result in the best forecast indicate less instability than what is typically required in the Plains. Probability density functions and scatter plots are created to visualize the data for all soundings.
dissertation or thesis