Using areal coverage of parameters favorable for severe weather to identify major convective outbreaks

Days with widespread and significant severe weather commonly feature relatively large regions of values of severe weather parameters favorable for severe weather. This study investigates the degree to which areal coverage can be used to distinguish major severe weather outbreaks from intermediate and marginal events at the time of their occurrence. These events were identified using recent studies that introduced a technique to rank and classify severe weather outbreaks.

Using the North American Regional Reanalysis (NARR) dataset, the areal coverage of a number of severe weather parameters, individually and in combination, was computed for 840 days from 1979-2006. Areal coverage was computed in two ways: the number of grid points in the specified domain that exceeded a threshold value, and the approximate mean distances and times of hypothetical storm trajectories within the favorable region. Preliminary findings suggest that certain combinations of severe weather parameters (especially CAPE and SREH) could identify major outbreak days consistently. However, several days classified as intermediate or marginal also were diagnosed as major outbreak days, indicating a substantial false alarm problem with this technique. Subjective analysis of improperly diagnosed outbreaks indicate that a subset of these cases can be identified as false alarms relatively easily; however, many cases feature synoptic and subsynoptic environments quite similar to major outbreaks. Limitations of the technique and future work to account for these will be addressed.