A Comparison of Synoptically Influenced Mesoscale Features in High-End and Low-End Progressive Derechos

Progressive derechos are severe mesoscale convective systems that often form during the warm season (May–August) in benign synoptic environments east of the Rocky Mountains and cause, by definition, straight-line wind damage along paths upwards of 400 km long. This presentation will draw upon the results of an 18-year (1996-2013) climatology and composite analysis of warm season progressive derechos to illustrate synoptic-mesoscale interactions that govern the behavior of these types of derechos. The results from the composite analyses show that corridors of enhanced atmospheric moisture, midlevel lapse rates, and vertical wind shear characterize progressive derecho synoptic environments. Case studies will be presented of two progressive derechos on opposite ends of the progressive derecho formation spectrum, one forming on the evening of 12 June 2013 ahead of a well defined upper-level trough and surface cyclone, and the other along a weak, east-west oriented surface boundary on the morning of 29 June 2012 in the absence of substantial large-scale forcing for ascent. The life cycles of these two derechos will be linked to the distinct synoptic classes they represent.