A Detailed Look at Extreme Wind Damage in Derecho Events

The derecho, as defined by Johns and Hirt (1987), includes any family of downbursts produced by an extra-tropical mesoscale convective weather system (MCS). Johns and Hirt checked for a general "bowed shape" to the MCS convective system when identifying derecho events and generally assumed that the damaging winds were the result of a single bow echo or a series of bow echoes. Other studies (e.g. Przybylinski and Decaire 1985) have revealed that the convective elements associated with derecho events occasionally include supercells in addition to bow echo structure. When present, supercell storm structure has generally been associated with a potential tornado threat.

Over the past several years, detailed, high quality reflectivity and radial velocity data have been archived during many derecho events by the WSR-88D radar network. In this paper, several derecho events in which "extreme" non-tornadic damaging winds were reported are examined. For this study, "extreme" non-tornadic winds are defined as an 80 kt or greater measured wind gust and/or observed high-end F1 or greater intensity damage on the Fujita scale. These data indicate that while "extreme" non-tornadic wind damage events within the derecho damage path can be associated with intense bow echo storm structures, in some instances, these "extreme" non-tornadic wind damage events appear to be associated with supercell storms embedded within the derecho producing MCS. The development and evolution of supercells within a derecho producing MCS are examined and operational warning implications discussed.