Wind Damage >> Storm Damage Roof Insurance Claim

Special hazards posed by derechos in urban areas Whether in an urban or rural area, those out-of-doors are at greatest risk of being killed or injured in a derecho. But of particular significance in urban areas is the vulnerability of electrical lines to high winds and Storm Damage Roof Insurance Claim falling trees. 

In addition to posing a direct hazard to anyone caught below the falling lines, derecho damage to overhead electric lines sometimes results in massive, long-lasting power outages. Hundreds of thousands of people may be affected; in the worst events, Storm Damage Roof Insurance Claim power may not be restored for many days. 

It is the complex and dense concentration of overhead distribution feeders in urban areas --- and Storm Damage Roof Insurance Claim their frequent proximity to large trees --- that make cities especially vulnerable to electrical outages following wind storms. The density and mileage of overhead electric distribution lines in urban areas far exceeds that of any rural or exurban area. 

Pole lines often carry multiple circuits and voltages, as well as lines for street lighting and customer service connections that further add to the vulnerability. Because of this, and because urban electrical feeders typically serve smaller territories relative to their rural counterparts, Storm Damage Roof Insurance Claim significantly greater manpower is necessary to restore service after major storms. 

In addition, unlike the localized damage produced by a tornado, derecho damage may be widespread. As a result, repairs often require greater effort, Storm Damage Roof Insurance Claim with additional delays related to shortages in supplies. Cities in which derechos have resulted in prolonged power outages include Baltimore (June 29, 1980), Kansas City (June 7, 1982), and Memphis (July 22, 2003). 

More recently, the Ohio Valley / Mid-Atlantic derecho of June 29, 2012 caused protracted, Storm Damage Roof Insurance Claim widespread power outages in cities from Cincinnati, Columbus, and Dayton to Atlantic City, Baltimore, and Washington, D.C. 

There also is evidence to suggest that the impact posed by derechos has increased in recent years due, in part, to the maturation of shade trees planted in suburban areas in the 1950s and 1960s. The vast tracks of post-war suburbs, with their overhead utility lines and Storm Damage Roof Insurance Claim older trees, are especially vulnerable to damage from high winds of any source. 

DERECHO-PRODUCING STORMS Derechos are associated with bands of showers or thunderstorms (collectively referred to as "convection") that assume a curved or Storm Damage Roof Insurance Claim bowed shape. The bow-shaped storms are called bow echoes. Bow echoes typically arise when a storm's rain-cooled outflow winds are strong, and move preferentially in one direction. 

A derecho may be associated with a single bow echo or with multiple bows. Bow echoes, in turn, may consist of an individual storm, or Storm Damage Roof Insurance Claim may be comprised of a series of adjacent storms (i.e., a squall line or, more formally, a quasi-linear convective system) that together take on a larger scale bow shape. 

Bow echoes may dissipate and Storm Damage Roof Insurance Claim subsequently redevelop during the course of given derecho. Derecho winds occasionally are enhanced when a rotating band of storms called a bookend vortex develops on the poleward side of the bow echo storm system. 

Derecho winds also may be augmented by the presence of embedded supercells (rotating thunderstorms) and other smaller-scale circulations in the derecho-producing convective system (see Tornadoes and Other Circulations). As noted previously, Storm Damage Roof Insurance Claim derecho winds are the product of what meteorologists call downbursts. 

A downburst is a concentrated area of strong wind produced by a convective downdraft. Downbursts have horizontal dimensions of about 4 to 6 miles (8 to 10 kilometers), and Storm Damage Roof Insurance Claim may last for several minutes. 

The convective downdrafts that comprise downbursts form when air is cooled by the evaporation, melting, and/or sublimation (the direct change to vapor phase) of precipitation in thunderstorms or Storm Damage Roof Insurance Claim other convective clouds. Because the chilled air is denser than its surroundings, it becomes negatively buoyant and accelerates down toward the ground. 

Derechos occur when meteorological conditions support the repeated production of downbursts within the same general area. The "downburst clusters" that arise in such situations Storm Damage Roof Insurance Claim may attain overall lengths of up to 50 or 60 miles (80 to 100 kilometers), and persist for several tens of minutes. 

Within individual downbursts there sometimes exist smaller pockets of intense winds called microbursts. Microbursts occur on scales (approximately 2 1/2 miles or 4 km) that are very hazardous to aircraft; Storm Damage Roof Insurance Claim several notable airline mishaps in recent decades resulted from unfortunate encounters with microbursts. 

Still smaller areas of extreme wind within microbursts are called burst swaths. Burst swaths range Storm Damage Roof Insurance Claim from about 50 to 150 yards (45 to 140 meters) in length. The damage they produce may resemble that caused by a tornado. 

A typical derecho consists of numerous downburst clusters ("families of downburst clusters") that are, in turn, comprised of many smaller downbursts, microbursts, and Storm Damage Roof Insurance Claim burst swaths. The schematic below illustrates the scalar relationships between these features. 

DERECHO DEVELOPMENT Derecho development necessarily is tied to the formation of bow echoes. A bow echo usually arises from a cluster of thunderstorms, Storm Damage Roof Insurance Claim but also may evolve from a single strong storm. Bow echoes most frequently occur when tropospheric winds are relatively strong and unidirectional (i.e., they vary little in direction with height). 

As the rain-cooled downdraft of a thunderstorm reaches the earth's surface, it spreads horizontally, most rapidly in the direction of the mean tropospheric flow. As the cool, dense air spreads outward, it forces the lighter, Storm Damage Roof Insurance Claim warm and moist air surrounding the storm up along the leading edge of the outflow, or gust front (see figure below, with mean flow assumed to be from left to right). 

The upward motion along the gust front typically is greatest along that part of the front that is moving most rapidly, Storm Damage Roof Insurance Claim that is, in the downwind direction (to the right in the figure). Gust fronts often are marked by a band of ominous, low clouds known as arcus. A photograph of an arcus (or shelf) cloud on the leading edge of a derecho appears at the top of this page. 

The development of a thunderstorm's downdraft ordinarily marks the Storm Damage Roof Insurance Claim dissipation stage of that particular storm. But air forced up along a gust front can give birth to new thunderstorms. As new storms mature, the rain they produce reinforces the existing "pool" of rain-cooled air produced by earlier storms, allowing the gust front to maintain its strength. 

As this cold pool increases in size and elongates in the direction of the mean wind, it may induce an inflow of air known as the rear-inflow jet (dashed brown arrow in figure below) on the trailing side of the thunderstorm complex. This causes the updraft to tilt toward the rear side of the storm (i.e., to the left in the figure). Tilting of the updraft allows the thunderstorm to further expand, Storm Damage Roof Insurance Claim increasing the aerial coverage of the rain. 

This, in turn, adds to the pool of cold air accumulating beneath the storm and strengthens the gust front, Storm Damage Roof Insurance Claim causing it to bow outward in the downwind direction. The resulting acceleration in forward motion of the gust front subsequently forces more warm, moist air upward, creating still more storms, and the process repeats.

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