Dallas Area - The convergence of warm, moist gulf air, an approaching cold front and an upper-level low pressure system with plenty of wind to set things spinning triggered Saturday’s deadly tornado outbreak across the Dallas area.

It was the worst outbreak in the region almost 90 years, when 34 people died in an outbreak of tornadoes in Collin and Dallas counties on May 9, 1927.

A preliminary estimate by the National Weather Service in Fort Worth counted 11 tornadoes in the area, although meteorologist Eric Martello said weather service teams had time Sunday to investigate only three — in Garland, Rowlett and Copeville, east of Lavon Lake.

“We can only cover so much ground,” he said. “The teams focused on the highly urban areas and Copeville, and they’ll investigate more” on Monday.

Many in Texas associate tornadoes with spring weather, and almost 63 percent of tornadoes hit the state in April, May and June, according to the Texas Almanac.

But tornadoes are possible every month of the year and at any time, Martello said.

“This isn’t our most active time of year for this kind of weather,” he said. “But it does happen because of our proximity to the Gulf of Mexico and the Rocky Mountains. With tornadoes, it doesn’t matter if it’s December, April or July.”

On average, Texas sees about five tornadoes in December, although the last outbreak in North Texas came nine years ago, with 15 tornadoes reported in and near Johnson County on Dec. 29, 2006.

And as Saturday’s storms showed, December tornadoes can be deadly.

In Garland, an EF4 tornado with winds of 166 to 200 mph damaged or destroyed hundreds of buildings, many of them homes, and killed eight people in storm-related accidents near Interstate 30 and the Bush Turnpike, according to police.

In Rowlett, a tornado measured as an EF3, with winds of 136 to 165 mph, caused significant and widespread property damage, destroying dozens of homes and injuring more than 20 people, officials said.

The Copeville tornado brought winds of 111 to 135 mph and rated an EF2, according to the National Weather Service.

Collin County Sheriff’s Capt. Jim Moody said the tornado destroyed four to six homes and a store in Copeville and others in Blue Ridge, with a trail of significant damage that basically followed the route of State Highway 78 in rural eastern Collin. Two died in Copeville, and a baby was killed in Blue Ridge.

The last Dallas-area F4 tornado ripped through Lancaster on April 24, 2004, before the current Enhanced Fujita scale was put in place. Saturday’s death toll eclipsed the Dallas tornado of April 2, 1957, which killed 10.

Source: Dallas Morning News