Tornado season is at its height in ‘Dixie Alley’

April 28, 2011

Many Americans still think of twisters as almost always occurring in “Tornado Alley” — an area that roughly includes Nebraska, Kansas, Oklahoma and Texas. Mother Nature has heartily cleared up that misconception this month with two huge, tornado-producing storm systems in the South, one on Wednesday and one two weeks ago.

Tornadoes in the South are actually so common that it has its own meteorological nickname: “Dixie Alley.”

“The South is pretty close to being in its prime time of the year for tornadoes,” says John Snow, a meteorologist at the University of Oklahoma.

Though tornadoes can occur any day of the year and in any state, every region has an annual season that most often produces conditions for severe thunderstorms and tornadoes, Snow says. It starts in March in the South, then moves north to the Great Plains in May and June, following the same progression as spring.

Two or three tornado-producing thunderstorms will often occur over a small region during the season. Every four or five years, tornado conditions cover a bigger area and create 20 or 30 severe storms. But the spate of storms April 14-16 was “a once-in-10-or-20-years type of event,” Snow said.

Those tornadoes earlier in the month caused about 45 deaths. For Wednesday’s storm, the Associated Press reports there have been at least 131 deaths in Alabama, 32 in Mississippi, 16 in Tennessee, 13 in Georgia, four in Virginia and one in Kentucky.

Tornadoes happen all over the world, but most often in North America. Between 900 and 1,400 tornadoes occur each year in the United States, followed by China with 100.

It’s difficult for the atmosphere to put together the right combination of extreme conditions to produce even one tornado, according to Greg Carbin, a meteorologist with National Oceanic and Atmospheric Administration’s Storm Prediction Center in Norman, Okla.

“Any tornado is extreme,” Carbin says. “People want to talk about ‘normal tornadoes,’ but a tornado is not normal in any sense of the word.”

Tornado-producing thunderstorms require cold air, warm air, wind and moisture. The April outbreaks were created when dry, cold air from the north, powered by a strong jet stream, collided with warm, humid air originating from the Gulf of Mexico and, as it traveled east, from the Atlantic Ocean.

Almost 300 reports of tornadoes were received during this month’s rampage through the South. But multiple witnesses reported the same one, and the National Weather Service is still figuring out exactly how many twisters occurred in the first wave in April, Cabin says. He expects the final count to be about 135 or 145 — “and that’s still a big three-day number.” The Storm Prediction Center says it received 137 tornado reports throughout the South on Wednesday.

The worst recorded tornado outbreak in U.S. history was in 1974, when 148 twisters touched down in 13 states, stretching from Alabama to Michigan. The 16-hour event caused 330 deaths, according to NOAA. The statistics from Wednesday are not expected to eclipse those of 1974.

One reason that people usually picture tornadoes occurring in the center of the country is that Plains twisters tend to be more photogenic. (And, of course, there’s the lasting impression from “The Wizard of Oz.”) The Southeast often gets “rain-wrapped tornadoes,” Carbin says, which are harder to see than dramatic funnel clouds roaring across the flatlands. Trees and a diverse terrain can also obscure the view. However, the mile-wide tornado that ripped through Tuscaloosa, Ala., on Wednesday, produced a very dramatic funnel cloud.

“The one in Raleigh was pretty ominous-looking, but it was hard to say you were looking at a tornado with the rain around the vortex,” Carbin said of the April 16 storm that caused five deaths in North Carolina’s capital.

Jason Samenow of The Washington Post’s Capital Weather Gang contributed to this report.

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