Tuesday, 5 October 2004
Convectively generated windstorms occur over broad temporal and spatial scales. However, the longer-lived, larger-scale, and most intense of these windstorms have been given the name "derecho" (Hinrichs 1888; Johns and Hirt 1987). This study outlines the tendency for derecho-producing MCSs to group together -- forming families. The derecho family is recognized as any succession of derechos with no more than 72 hours separating the individual events constituting the family [This approach is fairly consistent with the method that Johns (1982) utilized to identify northwest flow outbreaks]. Finally, all events that either were geographically (e.g., one event in Montana, another in Florida) or synoptically (e.g., associated with significantly differing flow patterns) disparate were discarded from the derecho family dataset.
Of the 290 events in the derecho dataset from 1994-2003, 180 (62% of the dataset) met the family criteria discussed in the previous section. The 180 events consisted of 59 families with a mean of 3.05 derechos per family. On average, there were 5.9 families and 18 family members annually. Most families involved only two or three derecho events; however, 14 families contained four or more events. Two separate families -- one in June 1998 and one in July 2003 -- involved nine derecho events within a period of nine days.
Data suggest that once a derecho occurs during the warm-season months, there is a 58% chance that this event is the first of a series of two or more, and about a 46% chance that this is the first of a derecho family consisting of three or more events. Analyses reveal that derecho families largely frequent regions of the Midwest, Ohio Valley, and the south-central Great Plains during May, June, and July.
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