Derecho-producing convective systems in the United States
Ashley, Walker Scott
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Convectively generated windstorms occur over broad temporal and spatial scales; however, the more widespread and longer-lived of these windstorms have been given the name “derecho.” The purpose of this study is to illustrate derecho-induced hazards, define derecho groupings and investigate their climatology, and examine the unique setting that produced the exceptional number of derechos and derecho groupings during the 1998 warm-season. Utilizing an integrated derecho database, this investigation first reveals the amount of insured property losses, fatalities, and injuries associated with these windstorms. Findings illustrate that fatalities occur more frequently in vehicles or while boating, while injuries are more likely to happen in vehicles or mobile homes. Both fatalities and injuries are most common outside the region with the highest derecho frequency. Casualty statistics and damage estimates from hurricanes and tornadoes are contrasted with those from derechos to emphasize that derechos can be as hazardous as many tornadoes and hurricanes. The second portion of the investigation illustrates the tendency for derechos to group together across the U.S. – forming “families.” The derecho family is recognized as any succession of derechos that develop within a similar synoptic environment with no more than 72 hours separating events. Over 62% of the derechos during 1994-2003 were members of a derecho family. On average, nearly six families affected the U.S. annually. Most derecho families consisted of two or three events; though, 14 families contained four or more events. Analyses reveal that families largely frequent regions of the Midwest, Ohio Valley, and the south-central Great Plains during May, June, and July. The final portion of this study examines the environmental conditions that led to the abnormally large number of derechos during May and June 1998. The investigation utilizes a multiscale perspective to illustrate the importance of the large-scale environment in promoting the convective ingredients required for derecho groupings. Analyses suggest three primary family types exist. A number of derechos during this period did not fit the conceptual models of derecho environments provided in the literature. This suggests that there is a need to continue to explore and document the range of environmental conditions conducive to derechos.