On the episodic nature of derecho-producing convective systems in the United States
Ashley, Walker S.
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Convectively generated windstorms occur over broad temporal and spatial scales; however, one of the larger-scale and most intense of these windstorms has been given the name ‘derecho’. This study illustrates the tendency for derechoproducing mesoscale convective systems to group together across the United States – forming a derecho series. The derecho series is recognized as any succession of derechos that develop within a similar synoptic environment with no more than 72 h separating individual events. A derecho dataset for the period 1994–2003 was assembled to investigate the groupings of these extremely damaging convective wind events. Results indicate that over 62% of the derechos in the dataset were members of a derecho series. On average, nearly six series affected the United States annually. Most derecho series consisted of two or three events; though, 14 series during the period of record contained four or more events. Two separate series involved nine derechos within a period of nine days. Analyses reveal that derecho series largely frequent regions of the Midwest, Ohio Valley, and the south–central Great Plains during May, June, and July. Results suggest that once a derecho occurred during May, June, or July, there was a 58% chance that this event was the first of a series of two or more, and about a 46% chance that this was the first of a derecho series consisting of three or more events. The derecho series climatology reveals that forecasters in regions frequented by derechos should be prepared for the probable regeneration of a derecho-producing convective system after an initial event occurs.