National Geographic Daily News
An aerial view of damaged homes in Alabama.
Buildings in Tuscaloosa, Alabama, lie in ruins on April 28, a day after a tornado demolished the city.

Photograph by Marvin Gentry, Reuters

Willie Drye

for National Geographic News

Published April 28, 2011

ON TV: Witness: Tornado Swarm 2011 airs Sunday, May 29, 9 p.m. ET/PT >>

The monster tornado that devastated Tuscaloosa, Alabama, (see map) on April 27 was spawned by unusual "perfect storm" conditions, experts say. (See pictures of the Alabama tornado.)

Those conditions—which stretched across Mississippi, Alabama, and Georgia—included warm, moist air rising and mixing with colder, dry air at higher altitudes.

Unfortunately for those living in the tornadoes' path, "weather conditions came together perfectly," said Tim Samaras, a Denver, Colorado-based tornado expert and producer of the tornado-research website TWISTEX.

"Mississippi, Alabama, and Georgia had that down to a T. It was a very, very rare day for everything to come together for this type of event," said Samaras, also a National Geographic Society Emerging Explorer. (The Society owns National Geographic News.)

Upper-level winds known as the jet stream also caused the storm system to rotate, according to meteorologist Jeff Masters, director of the website Weather Underground.

Rotating thunderstorms—known as super cells—spawn tornadoes. In the South on Wednesday, such storms spawned an outburst of a hundred or more twisters, which barreled through six states and killed at least 283 people.

"This is a history-making tornado outbreak," Masters said. "You don't see many like this."

Tuscaloosa Tornado Shattered Record?

The mile-wide (1.6-kilometer-wide) Tuscaloosa tornado may have had winds exceeding 260 miles an hour (418 kilometers an hour), which would make it an F5 storm on the Fujita scale. The scale ranks tornadoes from F1 to F5 based on wind speeds and destructive potential.

(Learn what happens inside a twister.)

Investigators are trying to determine how long the tornado, which originated just southwest of Tuscaloosa, stayed on the ground.

Tornadoes usually touch the ground for only a few miles before they dissipate. But favorable meteorological conditions may have sustained the Tuscaloosa twister for a record-breaking trek of 300 miles (482 kilometers) across Alabama and Georgia. (See more tornado pictures.)

"There were no limitations," said tornado chaser Samaras. "It went absolutely crazy. It had nothing but hundreds of miles to grow and develop."

The current record for a tornado's ground time is three and a half hours, set in 1925 by a twister that killed 747 people as it moved 219 miles (352 kilometers) across Missouri and Illinois before falling apart in Indiana.

Wednesday night's tornado outburst was the worst since April 3, 1974, when 330 people were killed from Alabama to Indiana (see map), experts say. (Also see pictures: "Tornadoes Ravage U.S. South" [2008].)

The outbreak was also the second to strike the South in fewer than two weeks. On April 16, a similar system of violent thunderstorms spawned about 140 tornadoes, killing 22 people in North Carolina.

Stormy Spring Still a Mystery

Some meteorologists think La Niña conditions—which have existed since summer 2010—could be a factor in the South's stormy spring. For instance, Russell Schneider, director of the U.S. Storm Prediction Center, told msnbc.com that La Niña can create more thunderstorms.

Such cyclical conditions are caused by cooler sea-surface temperatures in the equatorial region of the eastern Pacific Ocean, which can affect weather patterns in the U.S.

But neither Masters nor Samaras is willing to attribute this year's tornado outburst to La Niña.

"We don't have enough statistics to discuss this and decide what's causing it," Samaras said. "I don't know why [there's been a tornado outburst]. It's hard to tell why this year is so violent."

While tornadoes form in other parts of the world, the U.S. by far has the most, experiencing about 1,200 each year. (Watch: "Tornadoes, Lightning in Rare Video.")

And some unique weather conditions in an area known as Tornado Alley can spark powerful, tornado-producing storms.

This alley, which extends from the Dakotas south to the Gulf of Mexico, is bordered by the Rocky Mountains to the west and the Appalachian Mountains to the east.

Here, dry air comes off the Rockies and collides with warm, moist air from the Gulf and cold Arctic air from the north—producing energy for powerful storms.

(Read about tornado chasers.)

Tornado Season Just Starting

This year's stormy April marks only the beginning of the tornado season, which continues through June, experts say.

Samaras noted that twisters usually move northward as the season progresses. Oklahoma, Kansas, and Nebraska typically see tornadoes in May, while in June the funnels form in Minnesota and parts of the Dakotas.

"We still have two very busy months left," Weather Underground's Masters cautioned.

"Hang on to your hats."

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