A derecho is a widespread and long-lived, violent convectively induced windstorm that is associated with a fast-moving band of severe thunderstorms usually taking the form of a bow echo. Derechos are usually not associated with a cold front, but a stationary front. A warm weather phenomenon, they occur mostly in late spring to summer, especially July, but can occur at any time of the year.

Word origin

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Derecho comes from a Spanish word for "direct" or "straight ahead". The word first came about in the American Meterological Journal in 1888 by Gustavus Hinrichs. In contrast, tornado comes from the Spanish word "tornar" meaning "to turn".

Derechos come from a band of thunderstorms that are bow- or spearhead-shaped, and hence are also called a bow echo or spearhead radar echo. The size of the bow may vary and the storms may die and redevelop. Winds in a derecho can be enhanced by downburst clusters embedded inside the storm. These straight-line winds can exceed 100 mph (160 km/h) in these clusters. Tornadoes sometimes form within derecho events, although such are often difficult to confirm due to the additional damage caused by straight-line winds.

Derechos form from May to August, mostly July. During this time of year, derechos are usually confined to the Ohio Valley, upper Mississippi Valley and the Great Lakes region including southern Canada, mostly in Southwestern Ontario. They often occur during a period of extreme heat along stationary fronts on the northern side of where the most intense heat is.

Late-year derechos are confined to Texas and the deep south, although a late-summer derecho struck upper New York State after midnight on September 7, 1998. Derechos have been known to occur in other parts of the world. One such event occurred on July 10, 2002 in Germany. A serial derecho killed eight people and injured 39 near Berlin.

Types of derechos

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There are three types of derechos:

• Serial derecho - Multiple bow echoes embedded in a massive squall line typically around 250 miles (400 km) 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 Great Blizzard of 1993 in Florida. Also because of embedded supercells, tornadoes can easily spin out of these types of derechos.

• Progressive derecho - A small line of thunderstorms take the bow-shape and can travel for hundreds of miles. An example of this is a derecho is one that moved through Minnesota, Wisconsin, Michigan, Ontario, Quebec and Maine on July 4 and July 5, 1999. This particular derecho is named the "Boundary Waters-Canadian Derecho".

• Hybrid derecho - Has characteristics of a serial and progressive derecho. These types of derechos are associated with a deep low like serial derechos, but are relatively small in size like progressive derechos. An example is the one that moved through the central Northern Plains and the Southern Great Lakes on May 30, 1998.

According to the National Weather Service criterion, a derecho is classified as a band of storms that have winds of at least 50 knots (58 mph or 93 km/h) along the entire derecho.

Who is at risk during derechos?

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Since derechos occur during warm months, people who are most at risk are those involved in outdoor activities. Campers, hikers, and motorists are most at risk because of falling trees. Most casualties in derechos come from trees falling on cars.

People who live in mobile homes are also at risk. Mobile homes are not bolted to the ground, so they can be overturned from the high winds.

When a strong derecho hits a city, power outages caused by damaged powerlines may occur. The largest city to have been hit by more than one derecho is Grand Rapids, which has been hit by two such events (once on July 7, 1991 and again on May 30, 1998).

A derecho hit the city of Indianapolis, Indiana on April 2, 2006 causing widespread damage, including blowing multiple windows out of three sides of a 36-story office building.

See also

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• List of notable derecho events
• Weather
• Severe weather
• Meteorology
• Extreme weather

External links and sources

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• Facts about derechos (Storm Prediction Center)
• What is a derecho? (University of Nebraska at Lincoln)
• What is a derecho? (Meteorologist Jeff Haby's education page)
• Extreme Convective Windstorms: Current Understanding and Research (Report of the Proceedings (1994) of the U.S.-Spain Workshop on Natural Hazards (Barcelona, Spain, 8-11 June 1993), J. Corominas and K.P. Georgakakos, Eds., pp. 44-55)
• Derecho Hazards in the United States (Walker Ashley)
• Examination of Derecho Environments Using Proximity Soundings (Weather and Forecasting, Jeffry S. Evans, Charles A. Doswell III)
• StormWiki

Derechos | Storms | Severe weather and convection | Weather hazards

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