Derechos

Straight-line winds associated with convective weather activity cause property and crop damage and loss of life each year, comparable with tornado losses. A derecho is such a strong linear wind that is not tornadic, with wind speeds in excess of 26 m/s (meters per second) (58 mph [miles per hour]). The term plow wind applies in the Canadian vernacular. The name, coined by University of Iowa physicist G. Hinrichs in 1888, derives from a Spanish word meaning “direct” or “straight ahead.”

Derecho winds generated convectively from a downburst cluster forms a progressive derecho (major axis of effects less than 400 km [kilometers]; 250 miles), whereas, on a larger scale a group of downburst clusters produces a serial derecho (damage axis more than 400 km). Derechos tend to blast in linear paths fanning out along singular or multiple curved-wind fronts, known as bow echos, over a wide swath of land. Their duration and spatial dimensions provide data for identification.

The danger stems from the potentially large areas affected by the highly variable derecho winds. And derechos are associated with unstable bow echo systems that migrate rapidly, sometimes exceeding 50 mph or more in speed, making prediction difficult and forecasts uncertain. People can be overtaken by such a system with little warning.

Regarding their climatology, derechos can occur in any month but peak in May through July and form principally east of the 100th meridian in North America. Derechos pose distinct hazards to summer outdoor activities by swamping boats, throwing flying objects, and breaking trees and limbs. Typically, there are warm-season and coolseason patterns. May to August finds them in the region stretching from Southern Wisconsin across Illinois, Indiana, and Western Ohio in the upper Midwest and in a region of Oklahoma, adjoining Kansas and Missouri. This accounts for nearly 70% of occurrences.

From September through April, areas of activity migrate southward to Eastern Texas through Alabama, extending along the lower Mississippi Valley to the Ohio River and Southern Indiana. The causative association for this cool season is with migratory low-pressure systems. Derechos have been reported elsewhere in the world; however, data are lacking for specific assessment.

As an example, the speed of a derecho in 1998 in Eastern Wisconsin exceeded 57 m/s (128 mph): Overall, some 139 counties in nine states were affected in related storms. In August 2007, as [Page 715]eries of derechos across Northern Illinois reached this same intensity. Researchers, such as Walter Ashley, identified 377 events in the United States between 1986 and 2003, with 153 deaths and more than 2,600 injuries—an average of about 21 derechos per year.