(Formerly P6.8) On the precision of threats in National Weather Service severe weather warnings

The National Weather Service (NWS) implemented storm-based warnings in the fall of 2007 to provide a greater level of precision in depicting the actual areal coverage of severe weather in contrast to the former county-based method. The national verification program provides statistical measures such as probability of detection (POD) and false alarm rate (FAR) for these “polygon” warnings. Most NWS severe thunderstorm warnings also indicate the type and/or magnitude of the expected threat by specifying a hail size and/or wind speed. However, there is no formal effort to verify the accuracy of these warning parameters.

The goal of this study is to quantify the current level of skill in threat forecasting to develop a baseline from which future training and research efforts can build and improvements be measured. Severe weather reports in the NWS Davenport, Iowa county warning area from 2005-2007 were paired with the associated warning. The maximum hail size and/or maximum measured wind gust, or estimated wind gust (based on the type and degree of damage reported), were compared to the values stated in the initial warning issuance, and verification statistics were generated from that comparison. Tornado warnings are included for comparison purposes, although these statistics are tracked in the national verification program.

Verification results will be presented, and potential efforts toward improving the level of skill in threat forecasting will be discussed.