Developing an Impact-Based Strength Index for the Classification of Derechos

Derechos are large, and often disastrous, convective storms that have the potential to cause expansive swaths of wind damage. At the high end, straight-line winds associated with these storms can exceed 120 mph, well above the severe wind threshold. Using nine historic derecho events from 1991 to 2014 of various intensities, an impact-based scale is created. These events were selected using local news media reports and National Weather Service (NWS) overviews. In addition, these sources provide relevant information regarding societal impacts and estimates of surface-based winds at point locations. Next-Generation Radar (NEXRAD) reflectivity and velocity data from the Iowa State University Archive were utilized to further analyze the wind speeds and storm structure for each event. Based on all gathered information, these events were then grouped into low, medium, and high strength categories. The development of this index will allow a common nomenclature for scientists and the public to distinguish and analyze derecho events