A derecho (from Spanish: "derecho" meaning "straight") is a widespread and long-lived, violent convectively induced straight-line windstorm that is associated with a fast-moving band of severe thunderstorms in the form of a squall line usually taking the form of a bow echo. Derechos blow in the direction of movement of their associated storms, similar to a gust front, except that the wind is sustained and generally increases in strength behind the "gust" front. A warm weather phenomenon, derechos occur mostly in summer, especially June and July in the Northern Hemisphere. They can occur at any time of the year and occur as frequently at night as in the daylight hours.
The traditional criteria that distinguish a derecho from a severe thunderstorm are sustained winds of 58 mi/h (50 kt or 93 km/h) during the storm (as opposed to gusts), high or rapidly increasing forward speed, and geographic extent (typically 250 nautical miles [280 mi or 460 km] long). In addition, they have a distinctive appearance on radar (bow echo); several unique features, such as the rear inflow notch and bookend vortex, and usually manifest two or more downbursts.
Although these storms most commonly occur in North America, derechos can occur elsewhere in the world, although infrequently. Outside North America they are sometimes called by different names. For example, in Bangladesh and adjacent portions of India, a type of storm known as a "Nor'wester" can be a progressive derecho.
Derecho comes from the Spanish word for "straight". The word was first used in the American Meteorological Journal in 1888 by Gustavus Hinrichs in a paper describing the phenomenon and based on a significant derecho event that crossed Iowa on 31 July 1877.
Derechos come from a band of thunderstorms that are bow- or spearhead-shaped on radar, and hence are also 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. These straight-line winds can exceed 100 mi/h (160 km/h) (in some cases, sustained wind) in these clusters and straight-line wind gusts of up to 200 mi/h (320 km/h) 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.
On the other hand, with the average tornado in the United States and Canada rating in the low end of the F/EF1 classification at 85 to 100 mi/h peak winds and most or all of the rest of the world even lower, derechoes tend to deliver the vast majority of extreme wind over much of the territory in which they occur. Data compiled by the United States' National Weather Service and other organizations shows that a large swath of the north-central United States and presumably at least the adjacent sections of Canada and much of the surface of the Great Lakes can expect winds above 85 mi/h to as high as 120 mi/h (135 to 190 km/h) over a significant area at least once in any 50-year period, including both convective events and extra-tropical cyclones and other events deriving power from baroclinic sources. Only in 40 to 65 percent or so of the United States resting on the coast of the Atlantic basin and a fraction of the Everglades are derechoes surpassed in this respect—by landfalling hurricanes, which in the worst case can have winds as severe as EF3 tornadoes.
Certain derecho situations are the most common instances of severe weather outbreaks which can become less favourable to tornado production as they become more violent; the height of the 30-31 May 1998 upper Middle West-Canada-New York State derecho and the latter stages of significant tornado and severe weather outbreaks in 2003 and 2004 are but only three examples of this. Some upper-air measurements used for severe-weather forecasting can reflect this point of diminishing returns for tornado formation, and the mentioned three situations were instances during which the rare but quite possible particularly dangerous situation severe thunderstorm variety of severe weather watches were issued from the Storm Prediction Center of the US National Oceanic & Atmospheric Administration.
Downbursts in general are extremely common in derechoes and the rotor variety can also cause damage in a circular pattern.
Derechos in North America form predominantly from May to August, peaking in frequency during July. During this time of year, derechos are mostly found in the Ohio Valley, Upper Mississippi Valley and the Great Lakes region including southern Canada, mostly in Southwestern Ontario. During mid-summer if a hot and muggy airmass covers most of the north-central US they will often develop in Manitoba or Northwestern Ontario, near or just north of the US-Canadian border. North Dakota, Minnesota and upper Michigan are also vulnerable to derecho storms when such conditions are in place. They often occur during periods of extreme heat along stationary fronts on the northern side of where the most intense heat and humidity is occurring. Late-year derechos are confined to Texas and the Deep South, although a late-summer derecho struck upper parts of the New York State area after midnight on September 7, 1998.
Derechos have been known to occur in other parts of the world. One such event occurred on 10 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, in addition to Canada in rare occasions close to or north of the 60th parallel.
There are three types of derechos:
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 mi/h or 93 km/h) along the entire span of the derecho, which occurs over a time span of at least 6 hours.
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 can be overturned from the high winds.