Derecho

DERECHO-PRODUCING STORMS

What kind of storms cause derechos?
Derechos typically are associated with bands of showers or thunderstorms (collectively referred to as "convection") that assume acurved or bowed shape. The bow-shaped storms are called bow echoes. A derecho may be associated with a single bow echo or with multiple bows. Bow echoes themselves may consist of an individual storm, ormay 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 subsequentlyredevelop 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 enhanced on a smaller scale by the presence of embedded supercells in the derecho-producing convective system.

TYPES OF DERECHOS

Are there different kinds of derechos?Three main types of derechos may be distinguished. This classification largely is based on the overall organization and behavior of the associated derecho-producing convective system and reflects, inpart, the dominant physical processes responsible for the thunderstorms that produce the damaging winds.

The type of derecho most often encountered during the spring and fall is called a serialderecho. These are produced by multiple bow echoes embedded in an extensive squall line (typically many hundreds of miles long) that sweeps across a very large area, both wide and long. This type ofderecho typically is associated with a strong, migratory low pressure system. An example of a serial derecho with a very extensive squall line and embedded smaller scale bow echoes is the one thataffected Florida, Cuba, and adjacent parts of the Gulf of Mexico, the Caribbean Sea, and the Atlantic Ocean during the early stages of the "Storm of the Century" on March 12-13, 1993.

The second type...