Wind Damage >> Derechos, Tornadoes And Cyclones X

The rain produced by the newer storms reinforces the cold pool, strengthening the inflow of air from the back side of the developing storm complex and encouraging the downward transport of higher momentum air from aloft. These processes can enable the system Derechos, Tornadoes And Cyclones X to attain a nearly steady-state condition. 

At this point, the convective system typically exhibits a pronounced bow shape on radar (see figure below) and satellite, with an area of moderate to occasionally heavy rain located near the center of the cold pool, Derechos, Tornadoes And Cyclones X well behind the arc of intense rain immediately behind the gust front. 

As long as the thermodynamic and kinematic environments support the continued development of new thunderstorm cells in the downwind direction along the advancing gust front --- a process known as downwind or Derechos, Tornadoes And Cyclones X "forward" propagation --- the convective complex will persist, along with the potential for downbursts and microbursts. 

An animated, visible data satellite view of a derecho-producing convective system appears below. This particular event (May 31, 1994) occurred in a relatively dry environment over Utah and western Wyoming. Because of this, Derechos, Tornadoes And Cyclones X there was minimal supplementary cloud cover that tends to obscure the active storm towers of systems occurring in more moist regimes. 

As a result, the case well-illustrates the process of forward propagation along a convective system gust front. As the arc-shaped gust front advances from southwest to northeast across Utah, Derechos, Tornadoes And Cyclones X the rapidly-growing, new updrafts appear as sharply-outlined whitish clumps (cumulus and cumulonimbus clouds). 

These stand in marked contrast to the more fibrous debris clouds (cirrus "anvils") that are left behind as the gust front continues northeastward, Derechos, Tornadoes And Cyclones X undercutting the older updrafts and severing their link to warm, buoyant air. The derecho-producing convective system is therefore seen to be comprised of a series of discrete, downwind-developing, individual storms. 

For more on thunderstorm propagation and Derechos, Tornadoes And Cyclones X derechos, see Derechos and Flash Floods. TYPES OF DERECHOS Two main types of derechos may be distinguished. 

This classification largely is based on the overall organization and behavior of the associated derecho-producing convective system and reflects, Derechos, Tornadoes And Cyclones X in part, the dominant physical processes responsible for the thunderstorms that produce the damaging winds. 

Serial derechos. The type of derecho most often encountered during the spring and fall is called a serial derecho. These are produced by multiple bow echoes embedded in an extensive squall line (typically many hundreds of miles long) that sweeps across a very large area, Derechos, Tornadoes And Cyclones X both wide and long. 

This type of derecho typically is associated with a strong, migratory low pressure system. An example of a serial derecho with a very extensive squall line and embedded smaller scale bow echoes is the one that affected Florida, Cuba, and adjacent parts of the Gulf of Mexico, Derechos, Tornadoes And Cyclones X the Caribbean Sea, and the Atlantic Ocean during the early stages of the "Storm of the Century" on March 12-13, 1993. 

Progressive derechos The second type of derecho is called a progressive derecho. These are associated with a relatively short line of thunderstorms (typically from 40 miles to 250 miles in length) that may at times take the shape of a single bow echo, Derechos, Tornadoes And Cyclones X particularly in the early stages of development. 

In some cases, the width of a progressive derecho and its associated bow echo system remain relatively narrow even though they may travel for Derechos, Tornadoes And Cyclones X hundreds of miles. An example of this type is the "Boundary Waters-Canadian Derecho" that occurred on July 4-5, 1999. 

In other cases, the progressive derecho and associated bow echo system begin relatively small, with a narrow path, Derechos, Tornadoes And Cyclones X but over time grow to exceed 250 miles in width. The line of thunderstorms of a progressive derecho often begins as a single bow echo that evolves into a short squall line, typically with more than one embedded bowing segment. 

Such development occurred with the "I-94 Derecho" over the north central United States on July 19, 1983 (see Fig. 2 in that event's discussion page). Progressive derechos may travel for many hundreds of miles along a path that is relatively narrow compared to those Derechos, Tornadoes And Cyclones X of serial derechos. Often they are associated with an area of weak low pressure at the surface. 

Hybrid systems Occasionally, derechos having characteristics of both serial and progressive events are observed. These events are known as hybrid derechos. For example, Derechos, Tornadoes And Cyclones X the "Southern Great Lakes Derecho of May 30-31, 1998 was attendant to a strong, migrating low pressure system. 

However, the derecho path and the associated bow echo convective system had many characteristics of a progressive derecho. Low dewpoint derechos Most derecho-producing thunderstorm systems originate in and/or Derechos, Tornadoes And Cyclones X move through areas of very moist air. 

For example, nearly all of the cases included in Noteworthy Events occurred with surface dewpoint temperatures at or above 70° Fahrenheit. In contrast, Derechos, Tornadoes And Cyclones X bands of widespread wind-producing storms sometimes occur in environments of very limited moisture, for example, with dewpoints in the 40s or low 50s (°F). 

Such systems are known as low dewpoint derechos. Low dewpoint derechos most often occur between late fall and early spring in association with strong low pressure systems, and Derechos, Tornadoes And Cyclones X are a form of serial derecho. The May 31, 1994 Utah-Wyoming derecho shown in the animated satellite loop in Derecho Development is an example of a low dewpoint event. 

This system produced a 105 mph wind gust at Provo, Utah, where sixteen people were injured, and Derechos, Tornadoes And Cyclones X tore off part of the roof of the Saltair Pavilion on the Great Salt Lake. A wind gust of 140 mph was measured on Camel Back Mountain in nearby Dugway Proving Ground. The derecho occurred in an environment where average surface dewpoints were around 45 °F. 

More information about low dewpoint derechos is available in this study. Derechos are widespread, Derechos, Tornadoes And Cyclones X long-lived windstorms associated with a band of rapidly moving showers or thunderstorms. Coined by Dr. Gustavus Hinrichs in 1888, "derechos", a Spanish word which means "direct" or "straight ahead". 

Although a derecho's strength can produce destruction similar to tornadoes, the damage pattern produced by these events will occur along relatively straight lines. Thus the term, Derechos, Tornadoes And Cyclones X straight-line wind damage. Derechos are produced by a family of downbursts clusters. Downburst clusters have overall lengths of 50 to 60 miles (80 to 100 kilometers).

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