Analysis of a Progressive Derecho Climatology and Associated Formation Environments

A 1996-2013 May-August U.S. progressive derecho climatology existing entirely within the modern radar era is constructed identifying 256 derecho events over the 18-yr span. A corridor of enhanced derecho activity in agreement with previous derecho studies stretches from southern Minnesota to the border of Ohio and West Virginia with a marked decrease east of the Appalachian Mountains. A secondary maximum in progressive derecho activity exists in Kansas and Oklahoma. Analyses of derecho frequency by month of the warm season indicate a northward shift in frequency through July and an increase in derecho frequency through the first half of the warm season followed by a large decrease in August. The 256 identified derecho events are divided subjectively into seven distinct categories based on the synoptic environments in which they form. While the prevailing northwest flow conceptual model is upheld as the dominant progressive derecho synoptic category, the common occurrence of warm-season progressive derechos ahead of well-defined upper-level troughs is presented. This connection between upper-level troughs and progressive derecho formation expands on the relationship between upper-level troughs and serial derecho formation that has been the focus of past studies. In addition, a link between progressive derecho formation and easterly low-level flow to the north of a Rocky Mountain lee cyclone is bolstered. Consistent with previous derecho studies, all composite categories are characterized by large low-level moisture and the presence of an upper-level jet at derecho initiation.