Weather 101: Derechos

Discussion started by Dean D Davison 2 years ago

Derechos are heard about now more then ever. What is a Derecho? What is the Definition of one and what can be done to better warn people of this threat?

A Derecho by definition is a widespread, long-lived, straight-line wind storm that is associated with a land-based, fast-moving group of severe thunderstorms.

Derechos can cause hurricane force winds, tornadoes, heavy rains, and flash floods. Convection-induced winds take on a bow echo (backward "C") form of squall line, forming in an area of wind divergence in upper levels of the troposphere, within a region of low-level warm air advection and rich low-level moisture. They travel quickly in the direction of movement of their associated storms, similar to an outflow boundary (gust front), except that the wind is sustained and increases in strength behind the front, generally exceeding hurricane-force. A warm-weather phenomenon, Derechos occur mostly in summer, especially during June, July, and August in the Northern Hemisphere, within areas of moderately strong instability and moderately strong vertical wind shear. They may occur at any time of the year and occur as frequently at night as during the daylight hours.

There are four types of Derechos:

  • Serial Derecho — this type of Derecho is usually associated with a very deep low.
    • Single-bow — a very large bow echo around or upwards of 250 miles (400 km) long. This type of serial derecho is less common than the multi-bow kind. An example of a single-bow serial derecho is the derecho that occurred in association with the October 2010 North American storm complex.
    • Multi-bow — Multiple bow derechos are embedded in a large squall line typically around 250 miles (400 km) long. One example of a multi-bow serial derecho 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. This is a much more common type of serial derecho than the single-bow kind. Multi-bow serial derechos can be associated with line echo wave patterns on weather radar.
  • Progressive derecho — A line of thunderstorms take the bow-shape and may travel for hundreds of miles along stationary fronts. An example of this is the Boundary Waters-Canadian Derecho of 4–5 July 1999. Tornado formation is less common in a progressive than serial type.[why?][citation needed]
  • Hybrid derecho — A derecho with characteristics of both a serial and progressive derecho. Similar to serial derechos and progressive derechos, these types of derechos are associated with a deep low, but are relatively small in size. An example is the Late-May 1998 tornado outbreak and derecho that moved through the central Northern Plains and the Southern Great Lakes on 30–31 May 1998.
  • Low dewpoint derecho — A derecho that occurs in an environment of comparatively limited low-level moisture, with appreciable moisture confined to the mid-levels of the atmosphere. Such derechos most often occur between late fall and early spring in association with strong low pressure systems. Low dewpoint derechos are essentially organized bands of successive, dry downbursts. The Utah-Wyoming derecho of 31 May 1994 was an event of this type. It produced a 105 mph wind gust at Provo, Utah, where sixteen people were injured, and removed part of the roof of the Saltair Pavilion on the Great Salt Lake. Surface dewpoints along the path of the derecho were in the mid-40s °F.
  • Winds in a derecho can be enhanced by downburst clusters embedded inside the storm. These straight-line winds may exceed 100 miles per hour (160 km/h), reaching 130 miles per hour (210 km/h) in past events. 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.

 


Source NWS

 

Derechos depending on your location is more or less frequent to occur. One of the most Devastating Derechos in the last 5 years occurred on June 29/30 2012. This Derecho was powerful with 28 deaths, 2.9 million in damage and very High Winds some of which occurred in Roanoke VA where at the airport an 80 mph plus wind occurred. My weather station Recorded 91 mph matching the highest official gust of 91mph in Fort Wayne, Indiana

Areas with the most Derechos

 

Research by Meteorologist Dean D Davison for the National Weather Association Conference will be presented in  Norfolk VA in September.

 

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