An Investigation of Local and National NWS Warning Outbreaks for Severe Convective Events

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J. Operational Meteor






The National Weather Service (NWS) is charged with the responsibility of issuing severe weather warnings for the public whenever life and property may be in danger. During severe convective events, the NWS issues severe thunderstorm, tornado, and flash flood warnings. This study solely examines severe thunderstorm and tornado warnings conveying threats for wind, hail, and tornadoes.

Since 1 October 2007, the NWS has issued storm-based warnings, which cover smaller areas than the previous county-based system. Situational awareness and appropriate staffing levels are necessary to make warning operations successful within a NWS Weather Forecast Office (WFO). If storm coverage and severity are great enough, warning outbreaks can occur in which an NWS WFO has an anomalously high number of warnings valid at the same time—covering large portions of their areas of responsibility. In the top cases, there have been ≥10 warnings in effect at the same time within a county warning area, and ≥30 across the country.

A dichotomy exists between the environments that are associated with local and national tornado warning and severe thunderstorm warning outbreaks. Tornado warning outbreaks occur with high-end supercellular storm modes in high convective available potential energy (CAPE) and shear profiles. These events are often identified by the Storm Prediction Center as moderate or high risk with particularly dangerous situation tornado watches issued. Meanwhile, severe thunderstorm warning outbreaks transpire in mostly slight or enhanced risk areas with modest CAPE and low shear, which produce mainly pulse and linear thunderstorms. Verification statistics of these warnings indicate poorer performance compared to national averages—whether on local or national scales—with lower critical success index scores and higher false alarm ratios, although most events are warned during these outbreaks.