For tornado outbreaks, we begin with a list of eight non-exclusive criteria by which outbreaks are commonly judged: number of tornadoes, number of violent tornadoes, number of significant (>F2 damage) tornadoes, cumulative path length, deaths, number of killer tornadoes, and Destruction Potential Index (DPI, Thompson and Vescio 1998). Tornado days from 1970-2002 are ranked in each of those eight categories. From these rankings, and relative weighting factors subjectively assigned to each category, we compute an outbreak index (O-index), where positive values (O>0) constitutes a tornado outbreak". The O-index, in turn, may be used both to rank historical outbreaks and to assess the relative importance of new ones.
Little is available, other than empirical assessment of report maps, by which to judge the density, "importance" or quality of a severe thunderstorm outbreak on a nationwide basis. In fact, no consistent standard exists for largely nontornadic severe local storm events containing multiple reports of severe hail and damaging convective wind. In developing a solution to this problem, some means must exist to judge the organization of the event (as implied by report concentration), while accounting for the spatial outliers that are far removed from the concentrated event cluster but within the same time domain (i.e., a few severe downbursts in Arizona temporally coincident with a massive derecho event across the Ohio Valley). Therefore we develop a severe thunderstorm outbreak index, and describe utilization of the kernel density estimation (KDE) technique to identify clusters of severe weather reports for each convective day (1200-1200 UTC). We then rank these events according to their respective outbreak indices propose statistically based measures for severe local storm outbreaks that are more meaningful than the simple number of reports.