Authorities confirm two dozen people lost their lives

Oklahoma tornado May 20 2013

The massive tornado that struck Moore, Oklahoma, a suburb of Oklahoma City, on Monday destroyed many homes, businesses and public buildings, including two elementary schools and a hospital.

According to reports and aerial images, property damage in the affected region is extensive, with whole neighbourhoods heavily damaged or destroyed.

AIR Worldwide estimates that the replacement value of properties within a 0.4-mile buffer zone around the track of Monday’s tornado (for a total width of 0.8 miles) is about $2bn. Within a one-mile buffer zone of the storm track (for a total width of two miles), AIR estimates a total replacement value of about $6bn.

These two buffer zones, and the differing estimated replacement value associated with them, reflect the uncertainty in the exact size of the tornado. 

“A slow-moving upper level low pressure system triggered a series of severe thunderstorm outbreaks across the Central Plains starting on 19 May,” said AIR senior principal scientist Tim Doggett. “The system coupled with very warm, humid air moving northward out of the Gulf of Mexico ahead of a surface cold front and wind shear associated with an approaching increase in jet-stream energy.

“Together, these factors produced the highly unstable atmospheric conditions that caused the Monday afternoon storms to rapidly evolve into tornadic supercell thunderstorms. The tornadoes spawned by these supercell events were long-lived due to the orientation of the track and the severity of the conditions, leaving behind long and wide swaths of damage.”

Doggett continued: “In all, this system produced 22 tornadoes on 20 May, largely in Oklahoma, although tornadoes were also reported in Missouri, Texas, Kansas, Arkansas, and Colorado. However, of these 22 tornadoes, the Moore event was by far the most damaging.”

Doggett noted: “An official estimate of the number of destroyed homes has not yet been released, however, the 1999 EF-5 tornado that followed a similar path to Monday’s EF-5 Moore tornado destroyed more than 8,000 homes.”

Although basements and underground shelters are the safest refuge from a tornado, many homes in Oklahoma are built on concrete slabs—and thus lack basements—because the soil in the region renders basement construction too costly.

According to AIR, residential structures in the affected area are typically of wood-frame construction, which are more vulnerable to high winds and windborne debris than masonry structures. Commercial buildings are, on average, less vulnerable than residential structures or automobiles, but exhibit a broader damage distribution because of wide variations in construction practices and design.

“The associated upper-level system and the surface cold front will be slowly moving east and north-east over the next couple of days. Therefore, a wide area of the central United States — including parts of Texas (including the Dallas region), Missouri, Illinois, Indiana, and Pennsylvania—may be at risk of strong winds, hail, and tornadoes as the week progresses,” said Doggett.

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