Wind Damage >> F5 Tornado Facts

As the cold pool elongates, repetitive storm growth on the downwind-moving, progressive part of the gust front comprises the derecho-producing convective system (red arc in bottom cross section of figure below, and F5 Tornado Facts on right side of plan view inset). 

Meanwhile, because of the unidirectional nature of the flow, the trailing, upwind part of the gust front often becomes stationary. This is especially true when the atmosphere is very moist through a deep layer, and F5 Tornado Facts the potential for very cool storm downdrafts is minimal. 

The stationary source of low-level uplift provided by the gust front can then serve as the seat of repetitive thunderstorm development in the upstream direction (green arc in top cross section, and on left side of plan view). As individual storms grow and F5 Tornado Facts mature, they move parallel to the boundary, causing multiple episodes of heavy rain at locations along the line. 

Such convective evolution is known as echo training. Prolonged echo-training in a moisture-rich environment nearly always results in flash flooding. Smaller scale or more intermittent episodes of echo training frequently occur on the rear flanks of progressive derechos, and F5 Tornado Facts may cause localized flooding in the wake of a derecho's high winds. 

More information on the relationship between derecho and flash-flood producing convective systems is available here. DERECHOS AND HEAT WAVES Some of the most intense summer derechos, F5 Tornado Facts especially those of the progressive type, occur on the fringes of major heat waves. 

Examples include the July 1983 "I-94" derecho in the upper Mississippi Valley, the Mid-July 1995 derechos in New York and Canada, and F5 Tornado Facts the more recent Ohio Valley / Mid-Atlantic derecho of June 2012. The relationship is more than statistical. It turns out that the meteorological conditions favorable for large-scale heat waves often also are conducive to derechos. 

In the United States, this is especially true from the Upper Missisippi Valley and Upper Great Lakes into the Ohio Valley and Northeast. The primary link between heat waves and F5 Tornado Facts derechos is the presence of an elevated mixed layer, or EML. An EML is a layer of mid-tropospheric air that originates over the arid, elevated terrain. 

Because of their origin, EMLs exhibit sharp decreases in temperature with height. The large vertical temperature differentials (or "steep" lapse rates) in EMLs are analogous to those observed over black-topped roofs and F5 Tornado Facts parking lots on sunny days. Such thermal stratification encourages the formation of strong updrafts that can lead to the development of thunderstorms. 

In fact, the frequent presence of an EML on days othwerwise favorable for thunderstorm formation to a large extent accounts for the intensity of the storms commonly encountered over the Great Plains. During a typical heat wave over the central and eastern United States, a large, F5 Tornado Facts stationary upper-level high pressure area usually is present over the south-central states. 

Persistent westerly winds on the poleward side of the high allow EMLs generated over the Rocky Mountains to extend eastward into the Ohio Valley and Northeast, F5 Tornado Facts well beyond their usual range over the Plains. 

Warm air aloft associated with the base of the EML acts as a "cap" or "lid" that prohibits thunderstorm development along much of the extent of the EML, F5 Tornado Facts southward into the heat wave-associated "high." 

But on the northern fringe of the EML, where low-level uplift frequently is focused along a stationary front marking the northern edge of the heat wave, F5 Tornado Facts updrafts that form in the strongly heated air near the ground may breach the cap, resulting in an explosive release of instability. 

If other conditions are favorable (e.g., low-level moisture is abundant along the front, winds are largely unidirectional, parallel to it, and increase with height), F5 Tornado Facts additional storms may erupt in concentrated fashion along the boundary, yielding a band of downstream-developing storms and, on occasion, a full-blown derecho. 

The schematic below illustrates the idealized, F5 Tornado Facts large-scale meteorological environment favorable for progressive derechos on the northern fringe of a quasi-stationary high pressure cell associated with a major heat wave over central and eastern United States. 

The location of the EML in such situations is approximated by the brown-tinted band behind the word "Progressive," with the mean mid-tropospheric flow depicted by the arrow marked "Airflow," and F5 Tornado Facts the high pressure center by an "H." The region most susceptible to serial-type derechos in such a pattern also is shown. 

TORNADOES AND OTHER CIRCULATIONS IN DERECHO ENVIRONMENTS Tornadoes Derechos and tornadoes can occur with the same convective system. This is particularly F5 Tornado Facts so with serial derechos associated with strong, migratory low pressure systems. 

The tornadoes may occur with isolated supercells (rotating thunderstorms) ahead of the derecho producing squall line, or F5 Tornado Facts they may develop from storms within the squall line itself. 

An example of a serial derecho that produced both extremely damaging straight-line winds and F5 Tornado Facts significant tornadoes from supercells embedded in the derecho-producing squall line is that which affected Florida during the early stages of the so-called "Storm of the Century" ofMarch 12-13, 1993. 

Although not as common, tornadoes sometimes occur with progressive derechos. When they do, F5 Tornado Facts the tornadoes typically form within the bow echo storm system itself, and only rarely are associated with isolated supercells ahead of the bow. 

The occurrence of tornadoes with derecho-producing convective systems reflects the fact that both tornadoes and F5 Tornado Facts strong convective wind gusts share, to some extent, common origins in the background atmospheric environment. 

In short, the great degree of thermodynamic instability; i.e., buoyancy, that gives rise to strong updrafts and, ultimately, F5 Tornado Facts the thunderstorms that spawn tornadoes also promotes the formation of storm downdrafts. 

In addition, both tornado and derecho environments are characterized by the presence of substantial vertical wind shear; i.e., F5 Tornado Facts large changes in wind speed and/or direction with height. While derecho-producing convective systems tend to be most favored when the vertical wind profile is unidirectional, a unidirectional wind profile may still contain appreciable shear. 

At the same time, in even a modestly sheared environment, small-scale stretching and tilting motions often present along storm gust fronts in a squall line may yield low-level circulations that, F5 Tornado Facts on occasion, can "tighten up" into a tornado.

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