Derecho - Classification and Criteria

Classification and Criteria

Its common definition is a thunderstorm complex that produces a damaging wind swath of at least 250 miles (400 km), featuring a concentrated area of convectively-induced wind gusts exceeding 50 knots (58 mph; 93 km/h). According to the National Weather Service criterion, a derecho is classified as a band of storms that have winds of at least 50 knots (93 km/h; 58 mph) along the entire span of the storm front, maintained over a time span of at least six hours. Some studies add a requirement that no more than two or three hours separate any two successive wind reports. Derechos typically possess a high or rapidly increasing forward speed. They have a distinctive appearance on radar (known as a bow echo) with several unique features, such as the rear inflow notch and bookend vortices, and usually they manifest two or more downbursts.

There are three 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 echoes 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.
• 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.