NOAA on Derecho Storms

The news has been full of stories about the "Derecho" storm which slammed into the Eastern United States bringing death and destruction from Ohio to Delaware and Pennsylvania to South Carolina.  I was even caught in the initial blast of the storm due to poor analytic skills (note to self:  just because one measures lightning being three miles/five kilometers away does not mean the rain and wind are that far away as well).

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.

A derecho may be associated with a single bow echo or with multiple bows. Bow echoes themselves may consist of an individual storm, or may be comprised of a series of adjacent storms (i.e., a "squall line" or "quasi-linear convective system") that together take on a larger scale bow shape. Bow echoes may dissipate and subsequently redevelop during the course of given derecho. Derecho winds occasionally are enhanced when a rotating band of storms called a "bookend vortex" develops on the poleward side of the bow echo storm system. Derecho winds also may be augmented on a smaller scale by the presence of embedded supercells in the derecho-producing convective system.

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.

Be sure to read more about these storms on the website!   And remember, NOAA is a great resource for all weather related phenomenon.