An Investigation of NWS Warning Outbreaks for Severe Convective Events

The National Weather Service 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 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. If storm coverage and severity are great enough, warning outbreaks can occur, in which a NWS WFO has an anomalously-high number of warnings valid at the same time, covering large portions of their areas of responsibility. If the storm coverage extends beyond a single CWA national warning outbreaks can occur as well. In the top cases, there have been ten or more warnings in effect at the same time within a county warning area, and thirty or more across the country. A dichotomy exists between the environments that are associated with tornado warning and severe thunderstorm warning outbreaks. Tornado warning outbreaks occur during high-end supercellular storm modes with high 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 slight risk areas with modest CAPE and low shear, which produce mostly 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 rates, although most events are warned during these outbreaks.