Derecho

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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 within a highly buoyant, warm airmass. A warm weather phenomenon, derechos occur mostly in summer, especially July (in the northern hemisphere), but can occur at any time of the year and occur as frequently at night as in the daylight hours.

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[edit] Etymology

Derecho comes from a Spanish word for "direct" or "straight ahead". The word first came about in the American Meteorological 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 some cases sustained wind) in these clusters and straight-line wind gusts of up to 155 mp/h (250 km/h) are possible in the most extreme cases. Tornadoes sometimes form within derecho events, although such are often difficult to confirm due to the additional damage caused by straight-line winds in the immediate area.

Derechos in North America form 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 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.

[edit] Types of derechos

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.
  • 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/May 31, 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 span of the derecho.

[edit] Who is at risk during derechos?

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.

Recently, two campers were killed and others injured by fallen trees in separate locations during powerful storms that struck Northeastern and Central Ontario on July 17, 2006.

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.

[edit] Cities affected

The cities of Detroit and Windsor were hit by four major derecho events, on July 5, 1977, July 7, 1991, July 13, 1995 and May 31, 1998. London, Cleveland, Chicago, Milwaukee, Rochester, Pittsburgh and Saint Louis were hit three times.

Saint Louis was hit by two major events in a span of three days on July 19 and July 21, 2006 (with other events elsewhere on July 17 and 18) making it the shortest interval between derechos in a major city. However, parts of northern Minnesota were hit by successive derechos on July 12, 13 and 14, 1995.

Several other cities such as Toronto, Grand Rapids, Minneapolis, Saint-Paul, New York, Dallas, Fort Worth and Columbus were hit at least twice by derechos.

[edit] Narrow misses

Some other major cities were also more fortunate by avoiding the worst of these storms. The city of Ottawa narrowly escaped three major derecho events by less than 20 miles on July 15, 1995, July 5, 1999 and July 17, 2006 but still had considerable thunderstorm activity in some parts. Detroit in addition of being hit four times, narrowly escaped events on July 4, 1969, July 5, 1980 and September 7, 1998.

[edit] Effects of storms in cities

When a strong derecho hits a city, power outages caused by damaged powerlines may occur. The most crippling derecho to ever hit a major city was in Memphis, Tennessee on July 22, 2003. According to the National Weather Service, maximum wind gusts of 102mph occurred all across the metropolitan area. Memphis and Shelby County were declared a federal disaster area in the wake of the storm.

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.

Numerous windows were blown out of downtown Toronto, Ontario office towers when a derecho hit that city on May 31, 1998. Fortunately no one was killed on the ground, as it was a Sunday and the streets were not packed like they are during the week. Adequate storm warnings were also posted by Environment Canada after the same storm wreaked havoc in Ontario's western neighbor, Michigan, further discouraging people from venturing outdoors.

[edit] See also

[edit] For further reading

  • Ashley, Walker S., et al (2004). "Derecho Families". Proceedings of the 22nd Conference on Severe Local Storms [1], American Meteorological Society, Hyannis, MA.
  • Ashley, Walker S., et al (2005). "Derecho Hazards in the United States" [2]. Bulletin of the American Meteorological Society, 86 (11), pp. 1577–92.
  • Ashley, Walker S., et al (2005). "On the episodic nature of derecho-producing convective systems in the United States" [3]. International Journal of Climatology, 25 (14), pp 1915-32.
  • Bentley, Mace L., and Thomas L. Mote (1998). "A Climatology of Derecho-Producing Mesoscale Convective Systems in the Central and Eastern United States, 1986–95. Part I: Temporal and Spatial Distribution" [4]. Bulletin of the American Meteorological Society, 79 (11), pp. 2527–40.
  • Bentley, Mace L., et al (2000). "A synoptic climatology of derecho producing mesoscale convective systems in the North-Central Plains" [5]. International Journal of Climatology, 20 (11), pp 1329-49.
  • Burke, Patrick C., and David M. Schultz (2004). "A 4-Yr Climatology of Cold-Season Bow Echoes over the Continental United States" [6]. Weather and Forecasting, 19 (6), pp. 1061–74.
  • Coniglio, Michael C., et al (2004). "An Observational Study of Derecho-Producing Convective Systems" [7]. Weather and Forecasting, 19 (2), pp. 320–37.
  • Coniglio, Michael C. and David J. Stensrud (2004). "Interpreting the Climatology of Derechos" [8]. Weather and Forecasting, 19 (3), pp. 595–605.
  • 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)
  • Doswell, Charles A., and Jeffry S. Evans (2003). "Proximity sounding analysis for derechos and supercells: an assessment of similarities and differences" [9]. Atmospheric Research, 67–68, pp. 117–33.
  • Evans, Jeffry S., and Charles A Doswell III (2001). "Examination of Derecho Environments Using Proximity Soundings" [10]. Weather and Forecasting, 16 (3), pp. 329–42.
  • Johns, Robert H., and W. D. Hirt, W. (1987). "Derechos: Widespread Convectively Induced Windstorms" [11]. Weather and Forecasting, 2 (1), pp 32-49.
  • Przybylinski, Ron W. (1995). "The Bow Echo: Observations, Numerical Simulations, and Severe Weather Detection Methods" [12]. Weather and Forecasting, 10 (2), pp. 203–18.

[edit] External links

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