Wind Damage >> Preventive Measures For Tornadoes

DERECHO FORECASTING Formation Reduced to simplest terms, a derecho-producing convective system is comprised of a line of downstream-replicating, Preventive Measures For Tornadoes ordinary thunderstorm cells. 

Forecasting derechos begins, therefore, Preventive Measures For Tornadoes with examination of the atmospheric environment to assess the likelihood for the sequential development of individual thunderstorms, and, in particular, thunderstorms that might produce strong surface winds.

Thunderstorms are reasonably well-forecast; the processes governing their formation have been fairly well-understood for the better part of a century. In contrast, Preventive Measures For Tornadoes derecho forecasting remains one of the most challenging tasks facing operational meteorologists today. 

This difficulty reflects two primary limitations: (1) the imprecise nature of the observational network (relative that necessary) to produce an accurate and detailed picture of the propensity for thunderstorms with strong surface winds, Preventive Measures For Tornadoes and (2) our incomplete understanding of the processes that lead to repetitive, linearly-organized storm development.

Many meteorological factors, some acting synergistically and Preventive Measures For Tornadoes some seemingly at odds, may come together to yield an environment conducive to derechos. The variation of these factors over space and time also is important in fostering or hindering development. 

Two nearly identical meteorological settings at a given location might yield vastly different outcomes --- that is, a derecho or no derecho --- depending upon how the atmosphere evolved to that point in time, Preventive Measures For Tornadoes and how conditions vary spatially around that location. 

Derecho formation also is dependent upon very small-scale processes involving condensation, Preventive Measures For Tornadoes melting, evaporation, and sublimation of water deep within rapidly changing clouds --- processes that are neither well-understood, nor readily observable.

In some ways, serial derechos are easier to forecast than progressive events because the primary forcing mechanisms responsible for their development (e.g., Preventive Measures For Tornadoes strong cold fronts or jet stream disturbances) are fairly well sampled by today's observing network. 

On the other hand, unlike many other significant weather events of any type, Preventive Measures For Tornadoes progressive derechos may arise in relatively benign large-scale environments, with little or no identifiable atmospheric forcing. 

Many progressive derechos, in fact, Preventive Measures For Tornadoes arise in a manner that might best be described as a form of "atmospheric boot-strapping," wherein seemingly random interactions between individual thunderstorm cells --- or a loosely organized group of cells --- sometimes lead to a much larger, more strongly-organized system. 

One factor that appears important in realizing such development is the release of very strong latent heating in a relatively confined region. This Preventive Measures For Tornadoes may occur through the simultaneous development of many vigorous individual thunderstorm cells near a weak front or other local lifting mechanism. 

Such latent heating involves the lofting of abundant rain, snow, and Preventive Measures For Tornadoes hail particles that can lead to the rapid formation of strong storm downdrafts and an expanding cold pool. The "Super Derecho" of May 8, 2009 is an example of a storm that exhibited this type of development.

Recent research has to some extent furthered our understanding of mechanisms behind repetitive, Preventive Measures For Tornadoes downstream thunderstorm development, and studies have shed light on the origins of strong winds in convective storms (e.g., the BAMEX program of 2003; see References). 

In addition, numerical forecast models exist that have, on occasion, Preventive Measures For Tornadoes correctly depicted derecho development. But significant advances in forecast skill likely will require deployment of a nationwide network of atmospheric sensing devices with greatly enhanced spatial and temporal resolution relative to that in place today --- in addition to further storm-scale research.

Movement Once a derecho has formed, the forecast task reduces to determining where the parent convective system will move, and how long will it last. As noted in Derecho Development, Preventive Measures For Tornadoes a derecho will persist as long as the environment on the downwind side of its elongating cold pool is favorable for the formation of new storms. 

But both the location and rate of new storm development in the downwind direction typically vary over space and time, with factors such as the distribution of low-level moisture, Preventive Measures For Tornadoes atmospheric instability, and "capping" (warm layers aloft that hinder storm formation) --- amongst others --- typically involved. 

Such variables complicate the forecast process. At the same time, Preventive Measures For Tornadoes the background wind field around a derecho rarely remains static. 

Because the overall motion of a derecho-producing convective system is a combination of (1)  Preventive Measures For Tornadoes advection (the movement of individual storm cells by the environmental wind outside the storms).

Propagation (the development of new storm cells relative to older ones), spatial and/or Preventive Measures For Tornadoes temporal changes in the environmental wind add further complexity to the forecasting of derecho tracks.Various parameters and techniques have been developed in recent years to help forecast derecho formation, movement, and longevity (see References). 

These tools will require modification as observational and Preventive Measures For Tornadoes modelling studies further our understanding of derecho mechanics. 

DERECHOS AND FLASH FLOODS On occasion, on the rear flank of a convective system responsible for a progressive derecho, a nearly stationary band of thunderstorms may form. If such a band persists for an extended period, Preventive Measures For Tornadoes and/or if the line forms in a region of rocky or steeply-sloped terrain, flash flooding may follow. 

Situations of this type are not common, Preventive Measures For Tornadoes but when they do occur, the results may be devastating as the convective system's high winds are followed by the ravages of flooding. The "Ohio Fireworks" derecho-flash flood of July 4, 1969 is an example of what might be termed a concurrent derecho-flash flood convective system. 

Eighteen people died, many others were injured, and widespread destruction occurred as derecho winds swept across Michigan, Ohio, Pennsylvania, and adjacent Lake Erie. A few hours later, Preventive Measures For Tornadoes more than two dozen people perished from subsequent flash flooding in Ohio. 

The devasting Johnstown, Pennsylvania flood of July 20, 1977 also was associated with a convective system that produced both flash flooding and Preventive Measures For Tornadoes high winds.

The same unidirectional wind profile that is conducive to downwind (or forward) propagation of a convective system --- that is, Preventive Measures For Tornadoes the repetitive, downwind development of new thunderstorm cells such as that which occurs in a derecho --- is, paradoxically, also favorable for repetitive storm development in the upwind direction. 

The figures below illustrates why this is so. When the environmental flow around a convective system is nearly unidirectional (as in the wind profile at left),Preventive Measures For Tornadoes the system's collective outflow of cool air at the surface --- the cold pool --- necessarily elongates downstream in the direction of the mean flow. This elongation is shown schematically in the plan view in the right part of the figure, where "T" refers to "time." 

As noted in Derecho Development, the leading edge of the cold pool, Preventive Measures For Tornadoes or gust front (depicted here with weather map frontal symbols), is the main site of new thunderstorm formation. Such development may occur repetitively along the boundary if thermodynamic conditions are favorable.

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