SERIAL DERECHO

A serial derecho event (yellow hatched area in schematic below) typically is associated with an extensive squall line or quasi-linear convective system (red dashed / double-dotted line) with multiple embedded bow echoes. Joined together in the larger-scale squall line, the individual bows form a series of of scallop-shaped structures know as line echo wave patterns, or LEWPs. Within the squall line, the bow echoes may be of widely varying sizes. Serial derechos usually occur in conjunction with well-defined, migratory low pressure system. Overall convective system (and, therefore, derecho) motion is dominated by advection --- the downstream movement of individual thunderstorm cells by the environmental wind.

Schematic of a serial derecho-producing convective system, showing location of associated bow echoes (arced portions of dashed / double-dotted line) relative to the cold front along which the parent thunderstorms typically form. Line-echo wave patterns, or LEWPs, mark the inflection points between individual bows. (After Johns and Hirt 1987).

"Ping-pong" animation showing the development and evolution of a typical serial derecho-producing convective system over the lower Ohio and Tennessee Valleys as viewed by radar (composite reflectivity data; April 19-20, 2011). The squall line evolves from discrete supercell storms that form over Illinois, Missouri, and Arkansas during the late afternoon (beginning of loop). The convective system ultimately grows to extend from Michigan to Mississippi, and produces four, large-scale bow echoes. For a period of time, three of the bows are connected in a wave-like pattern. The crest of each "wave" (i.e., the point of connection between an adjacent pair of bows) is known as a line-echo wave pattern, or LEWP. The duration of the animation is twelve hours. Selected surface observations are shown using conventional station plot format.

PROGRESSIVE DERECHO

A progressive derecho event (yellow hatched area in schematic below) typically is associated with a squall line of restricted length that may involve a single bow echo or multiple bows (red dashed / double-dotted line). Progressive derechos usually are associated with a stationary front in a rather stagnant large scale weather pattern. Overall convective system (and, therefore, derecho) motion is dominated by thunderstorm propagation --- that is, the development of new storm cells downstream from existing ones.

Schematic of a progressive derecho-producing convective system, showing location of the large-scale bow echo relative to the stationary front near or along which the originating thunderstorms typically form. (After Johns and Hirt 1987).

"Ping-pong" animation showing the development of a typical progressive derecho-producing convective system over the Midwest and Ohio Valley as viewed by radar (composite reflectivity data; June 29, 2012). The large-scale bow squall line evolves from storms that form over northern Illinois during the late morning (beginning of loop). Selected surface observations are shown using conventional station plot format. Loop duration is seven hours.

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