Categorical thunderstorm and tornado warnings in the National Weather Service

The effectiveness of National Weather Service severe thunderstorm and tornado warnings is limited by long-standing operational procedures and policy. A tornado warning conveys the same level of threat whether the warned tornado is violent or weak. As a result, weak tornadoes tend to be overemphasized by the warning process. Since most tornadoes are weak, tornado warnings, in general, are overemphasized in the current operational system. The focus on tornado warnings can have unintended negative results. Very strong, non-tornadic thunderstorms may not receive adequate response from the public if covered by standard severe thunderstorm warnings. Forecasters have on occasion issued tornado warnings for very severe, non-tornadic thunderstorms in the hope that the public would better respond to the real threat if this higher visibility warning were issued. Since both severe and tornadic thunderstorms are defined by the phenomena they produce and damage they cause, forecasters are effectively required to warn for something they do not observe. This paper describes a new 4-tier categorical severe thunderstorm and tornado warning system that would mitigate the negative aspects of the current operational system. The proposed scheme allows forecasters to indicate the degree of threat posed by a particular severe and/or tornadic thunderstorm without complicating the warning interpretation. The new system provides objectively defined thunderstorm severity categories which can be directly monitored by NWS forecasters on AWIPS.