A derecho [deh-rey-cho] is a widespread and damaging group of severe thunderstorms which often represent with rapid forward speeds. They have a distinct appearance on radars, known as bow echos (after an archer’s bow). The common definition of the word is: a thunderstorm complex that produces a damaging wind swath of at least 240 miles, featuring a concentrated area of convectively induced wind gusts exceeding 58 mph. Some studies add further criteria, such as a requirement that no more than two or three hours separate any two successive wind reports.

Unlike other thunderstorms, which typically can be heard in the distance when approaching, a derecho seems to strike suddenly. Within minutes, extremely high winds can arise, strong enough to knock over highway signs and topple large trees. These winds are accompanied by spraying rain and frequent lightning from all directions. It is dangerous to drive under these conditions, especially at night, because of blowing debris and obstructed roadways. A derecho moves through quickly, but can do much damage in a short period of time.

Derecho comes from the Spanish word for ‘straight’ (‘direct’). The word was first used in the ‘American Meteorological Journal’ in 1888 by Gustavus Detlef Hinrichs in a paper describing the phenomenon and based on a significant derecho event that crossed Iowa in 1877.

There are three types of derechos: 1) Serial derechos appear as multiple bow echoes embedded in a large squall line typically around 250 miles long. This type of derecho is usually associated with a very deep low. One example of the serial type is a derecho that occurred during the 1993 ‘Storm of the Century’ in Florida. Because of embedded supercells, tornadoes can spin out of these types of derechos. 2) Progressive derechos are small lines of bow-shaped thunderstorms and may travel for hundreds of miles along stationary fronts. Tornado formation is less common in a progressive than serial type. 3) Hybrid derechos have characteristics of serial and progressive derechos.

Derechos come from a band of thunderstorms that are bow- or spearhead-shaped on radar and, thus, also are called a bow echo or spearhead radar echo. The size of the bow may vary, and the storms associated with the bow may die and redevelop. Winds in a derecho can be enhanced by downburst clusters embedded inside the storm. In these clusters wind gusts of up to 200 mph are possible in the most extreme cases. Tornadoes sometimes form within derecho events, although such events are often difficult to confirm due to the additional damage caused by straight-line winds in the immediate area.

Derechos in North America form predominantly from May to August, peaking in frequency during the latter part of June into July. During this time of year, derechos are mostly found in the Midwestern United States in order of frequency over the Upper Mississippi Valley, Great Lakes region including southern Canada, and the Ohio Valley. During mid-summer when a hot and muggy airmass covers the north-central U.S., they will often develop farther north into Manitoba or Northwestern Ontario, sometimes well north of the U.S.-Canadian border. North Dakota, Minnesota, and upper Michigan are also vulnerable to derecho storms when such conditions are in place. They often occur along stationary fronts on the northern periphery of where the most intense heat and humidity bubble is occurring. Late-year derechos are confined to Texas and the Deep South, although a late-summer derecho struck upper parts of New York State in September of 1998.

Derechos have been known to occur in other parts of the world. One such event occurred in July 2002 in Germany: a serial derecho killed eight people and injured 39 near Berlin. They have occurred in Argentina and South Africa as well, and on rarer occasions, close to or north of the 60th parallel in Northern Canada. Primarily a mid-latitudes phenomenon, derechos do occur in the Amazon Basin of Brazil.

Since derechos occur during warm months and often in places with cold winter climates, people who are most at risk are those involved in outdoor activities. Campers, hikers, and motorists are most at risk because of falling trees toppled over by straight line winds. Wide swaths of forest have been felled by such storms. People who live in mobile homes are also at risk; mobile homes that are not anchored to the ground may be overturned from the high winds. Derechos may also severely damage an urban area’s electrical distribution system, especially if these services are routed above ground. The derecho that struck Chicago in July 2011, left more than 860,000 people without electricity. The June 2012 North American derecho took out electrical power to more than 3.7 million customers starting in the Midwestern United States, across the central Appalachians, into the Mid-Atlantic States during a heat wave.

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