Derechos are long-lived wind events that begin as a cluster of thunderstorms. When such clusters develop in a favorable wind environment (where surface and upper level winds are moving in the same direction), they often develop a "bow echo" formation, with its leading edge pointing in the direction of these steering winds.
Downbursts within the thunderstorms produce gust fronts of chilled air that move ahead of the bow echo; ominous shelf clouds commonly reflect the presence of these gust fronts. Lifting the warm, humid air into which they move, the gust fronts fuel another line of thunderstorms, propagating the bow-shaped cluster as the system moves forward. Rapid movement of the thunderstorms, combined with the high speed of the gust front winds can produce destructive straight line winds (often exceeding 75 mph) across the leading edge of the bow echo.
By definition, a derecho occurs when the above dynamics persist along a swath of 250 miles or more and produce wind gusts of at least 58 mph. As we have seen this summer, derechos often develop along the northern and northeast rim of a high pressure ridge, where an unstable atmosphere ignites the thunderstorms and strong winds both generate and steer the bow echo formation.