A Statistical Overview of Operational Storm-Based Warnings

Storm-based severe weather warnings operationally replaced county-based warnings in 2007 and now allow National Weather Service (NWS) meteorologists to provide more specific information about potential threats to those in the immediate path of a storm. Past research has shown that storm-based warnings significantly reduce the number of person-hours spent under a warning, but there are still many aspects of storm-based warnings that are not well understood. Therefore, this study provides a quantitative climatological analysis of the fundamental components of storm-based warnings and offers insight into how the NWS uses the current warning system under the established directives and policies.

From October 2007 through May 2015, the NWS has issued nearly 500,000 storm-based warnings and severe weather statements (SVS), primarily concentrated east of the Rocky Mountains. Specifically, there have been 61,582 tornado warnings and 390,867 severe thunderstorm warnings (both including SVS updates) issued. A geospatial analysis of these warning counts by county warning area (CWA) shows local maxima in the Lower Mississippi Valley, Southern Plains, Central Plains, and the Southern Appalachians. Regional uniformity exists in the patterns of average speed and direction provided by the time/motion/location tag, while the duration and polygon area of an issued product varies significantly by CWA and shows little interoffice consistency. These observed consistencies and inconsistencies may be indicative of the effectiveness of storm-based warnings to convey the forecasters' representation of severe storms and their evolution. This presentation concludes with a comparison of storm-based warnings to NWS policy and an analysis of CWAs with the highest counts of warnings issued during a convective day (12 UTC - 12 UTC).