The derecho, a widespread convectively induced windstorm, is identified and defined in terms of current nomenclature. A comprehensive dataset consisting of 70 derecho cases has been developed from the warm season months of May through August for the 4-year period 1980–1983. Analyses of this dataset reveal that the warm season derecho typically emanates from a mesoscale convective system (MCS) moving along a quasistationary, low-level thermal boundary in an environment characterized by high potential instability and relatively strong midtropospheric winds. In the continental United States these windstorms are most frequent in a zone extending from eastern South Dakota to the Upper Ohio Valley, and typically commence during the afternoon and evening hours. Particular radar and satellite imagery characteristics are associated with the derecho-spawning MCS. Based upon the meteorological parameters and synoptic patterns associated with derecho events, a decision tree has been developed to assist the operational meteorologist in anticipating derecho development.

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