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A revised, more physically based definition of "derecho" is proposed that is more specific to the type of intense convective windstorm that first inspired the term in the late nineteenth century.
On 4-5 April 2011, an expansive quasi-linear convective system (QLCS) crossed the southern and eastern United States accompanied by a broad swath of strong winds that produced a record number (1096) of nonduplicate damaging wind reports and nearly four-dozen tornadoes in a 24-h period (Figs. 1a and 3a). The windstorm extended more than 1000 km (620 mi) from the Ohio River southward to the Gulf Coast and was responsible for several deaths, at least 30 injuries, and millions of dollars in damage, mainly to trees and power lines.
Collectively, the spatial and temporal extent of the damage easily met the definition of a "derecho" as given by Johns and Hirt (1987, hereafter JH87): "Any family of downburst clusters associated with an extratropical mesoscale convective system," a definition that only has been slightly modified in more recent years (American Meteorological Society 2014):
A widespread, convectively induced straight-line windstorm, more specifically, any family of down-burst clusters produced by an extratropical mesoscale convective system.
The event also satisfied the following supplementary criteria established by JH87 based on Fujita and Wakimoto (1981) to identify derecho events in Storm Data and in the National Severe Storms Forecast Center (predecessor to the Storm Prediction Center) database:
1) There must be a concentrated area of reports consisting of convectively induced wind damage and/ or convective gusts > 25.7 m s-1 (50 kt). This area must have a major axis of at least 400 km (250 mi).
2) The reports within this area must also exhibit a nonrandom pattern of occurrence; that is, the reports must show a pattern of chronological progression, whether as a singular swath (progressive) or a series of swaths (serial).
3) Within the area there must be at least three reports, separated by 64 km (40 mi) or more, of either F1 or greater damage (Fujita 1971) and/or "significant" convective gusts of 33.4 m s-1 (65 kt) or greater.
4) No more than three hours can elapse between wind damage events (gusts).
The 4-5 April 2011 convective system traveled east-northeast nearly 1500 km (930 mi) in 24...