A technique for spatial evaluation of severe thunderstorm warnings issued by the National Weather Service

In 2007, the National Weather Service (NWS) changed severe weather warnings from county-based warnings to storm-based warnings. The primary goal of this change was to reduce the area under warning in order to reduce the perception of a false alarm by people unaffected by the storm in the warned counties. While the NWS changed the way in which warnings were issued, the method in which they verify warnings has remained the same. Warnings are verified by reports of hail 25.4mm in diameter, winds 58 mph or greater, reports of wind damage, such as broken tree limbs, or a tornado. Only one report within the temporal and spatial bounds of a warning is needed for verification. This type of warning verification might assist in knowing the percentage of severe weather events which were warned, but it offers no information on how much of the warning area was justified or how much of the area outside of the warning experienced severe weather. The National Severe Storms Laboratory (NSSL) has developed a number of multi-radar, multi-sensor severe weather products which could serve as a proxy in determining the accuracy and efficiency of severe storm warnings. The efficiency of NWS warnings will be studied through the use of time-accumulated swaths of hail size fields by assessing the placement of warnings with respect to storm motion and intensity. These warnings will then be compared to the daily composite of severe thunderstorm warnings to determine the efficiency of warnings on an individual basis versus daily scale.