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North Texas hardest hit by weekend tornadoes, AIR reports

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Catastrophe modeling firm AIR Worldwide reports that the huge storm system that cut a wide swath across the U.S. over the Christmas weekend hit North Texas hardest, with at least nine tornadoes damaging as many as 1,450 structures.

“A cold front passed through North Texas on Dec. 26, where it entered a region of very warm and moist air, triggering the development of severe thunderstorms, including isolated tornadoes,” Scott Stransky, manager and principal scientist at AIR Worldwide in Boston, said in a release. “After the tornadoes had developed, the temperature in Dallas fell from 72 to 56 degrees.”

The biggest storm, an EF-3 with winds gusting between 136 and 165 mph, hit Rowlett in Dallas and Rockwall counties before strengthening to an EF-4 storm, with 166-200 mph winds, and leaving a 13-mile path of destruction through the Dallas suburb of Garland. Texas Gov. Greg Abbott has declared Collin, Dallas, Ellis and Rockwall counties disaster areas.

It's too soon for damage estimates, AIR says, but besides the 1,400 structures, mostly residences, vehicles, power lines and trees were hit hard.

“A direct hit by a tornado usually causes catastrophic damage to residential buildings and major damage to engineered structures,” Mr. Stransky said in the release. “Roofs, wall covers and windows are often the first part of a building to be damaged by tornado winds.”

According to AIR, commercial buildings are, on average, less vulnerable to the rotational and suction forces of a tornado than residential structures or automobiles. This is particularly true of older commercial buildings with reinforced concrete or brick exteriors.

Though tornado activity is somewhat unusual in December — the peak season on the Gulf Coast is during the spring — 2015 has seen a below-average number of the storms. “According to the (National Weather Service), an average of about 1,200-1300 tornadoes hit the U.S. yearly. The Storm Prediction Center notes that 2015 has seen only 1,056 through … Dec. 27,” Mr. Stransky noted in the release.