Sadly, I never took a course in meteorology so I would be a horrible blogger if I tried to describe the science of a derecho. However, NOAA has an excellent internet resource on the storms full of useful information including
A derecho (pronounced similar to "deh-REY-cho" in English, or pronounced phonetically as "") is a widespread, long-lived wind storm that is associated with a band of rapidly moving showers or thunderstorms. Although a derecho can produce destruction similar to that of tornadoes, the damage typically is directed in one direction along a relatively straight swath. As a result, the term "straight-line wind damage" sometimes is used to describe derecho damage. By definition, if the wind damage swath extends more than 240 miles (about 400 kilometers) and includes wind gusts of at least 58 mph (93 km/h) or greater along most of its length, then the event may be classified as a derecho.
Derecho winds are the product of what meteorologists call "downbursts." A downburst is a concentrated area of strong wind produced by a convective downdraft. Downbursts have horizontal dimensions of about 4 to 6 miles (8 to 10 kilometers), and may last for several minutes. The convective downdrafts that comprise downbursts form when air is cooled by the evaporation, melting, and/or sublimation of precipitation in thunderstorms or other convective clouds. Because the chilled air is denser than its surroundings, it becomes negatively buoyant and accelerates down toward the ground. Derechos occur when meteorological conditions support the repeated production of downbursts within the same general area. The "downburst clusters" that arise in such situations may attain overall lengths of up to 50 or 60 miles (80 to 100 kilometers), and persist for several tens of minutes. Within individual downbursts there sometimes exist smaller pockets of intense winds called "microbursts." Microbursts occur on scales (approximately 2 1/2 miles or 4 km) that are very hazardous to aircraft; several notable airline mishaps in recent decades resulted from unfortunate encounters with microbursts. Still smaller areas of extreme wind within microbursts are called "burst swaths." Burst swaths range from about 50 to 150 yards (45 to 140 meters) in length. The damage they produce may resemble that caused by a tornado.