On the use of areal coverage of parameters favorable for severe weather to discriminate major outbreaks

Forecaster perceptions of major convective outbreaks include the notion that these events occur within relatively large regions of meteorological conditions favorable for the development of significant severe weather, particularly tornadoes. With recent studies developing a rigorous and scientifically repeatable method of identifying these events and distinguishing them from intermediate or marginal severe weather outbreaks, the investigation of a large sample of these events now is possible. This diagnostic study aims to determine the extent to which the use of areal coverage is successful as a means of discriminating major severe weather outbreaks (primarily but not exclusively major tornado outbreaks) from the less significant outbreaks. Preliminary findings suggest that the areal coverage of severe weather parameters favorable for severe weather indeed is associated with the severity of outbreaks. However, the method produces a substantial number of less significant outbreaks that are misclassified as major severe weather outbreaks. Many of these false alarms can be identified by the presence of synoptic environments that are less favorable for the development of a large number of tornadoes. However, a substantial number of intermediate and marginal severe weather outbreaks feature synoptic patterns and mesoscale environments that are difficult to differentiate from the major events. Limitations of using areal coverage as a means of outbreak discrimination are discussed, and refinements to account for these limitations are proposed.