Weekend tornadoes that carved a path of injury and destruction from Alabama to the hills of Northwest Georgia followed a well-worn path.
Strong twisters have pummeled Northeast Alabama hard over the last two decades. Since 1990, nearly 100 funnels have touched down in the six Alabama counties closest to Chattanooga, according to data from the National Climatic Data Center.
The storms have caused nearly $40 million in damage, injured more than 150 people and claimed four lives. In the same period, the eight counties of Northwest Georgia saw 22 funnels and Eastern Tennessee saw 50 tornadoes over a much larger 11-county area, records show.
"It seems like we certainly get our fair share and then some," said Michael Leath, director of the DeKalb County, Ala., Emergency Management Agency. Over the years, the storms have injured nearly 70 people in DeKalb County alone and caused $7.5 million in property damage there.
"I don't know what causes us to have so many," Mr. Leath said. "It could be the air currents, the terrain or just our geography, but we get a lot of tornadoes."
Last weekend, at least seven tornadoes were confirmed in Alabama -- including two in DeKalb County, one in Geraldine and one in Mentone -- according to the Associated Press.
Elsewhere across the Deep South, the weekend storms killed 12 people -- 10 in Mississippi and two in Alabama, though not in the northeast region of the state -- and damaged hundreds of homes.
In Chattooga County, Ga., right across the stateline from DeKalb, eight homes had tornado damage, but no one was injured.
"Georgia has a lot of tornadoes, too," said David Ashburn, director of Walker County's Emergency Management Agency. "We may not have as many as Alabama, but people should be prepared for anything ... that includes tornadoes."
Why tornadoes strike the region is tricky. Most meteorologists blame warm, moist air from the Gulf of Mexico mixing with cooler, drier air from the Appalachian Mountains. What keeps the tornadoes mostly isolated in Northeast Alabama is a little trickier, but there are some prevailing theories.
"We think the main energy source is in the Gulf of Mexico, so by the time it travels all that terrain and goes over the hills and trees, the storm gets torn up," said Dr. Tim Coleman, a University of Alabama in Huntsville researcher.
That doesn't mean mountains or valleys are safe zones. In fact, some research suggests mountains give fuel to tornadoes.
"Once a tornado starts, they will go up and down mountains, and trees are just something for them to toss around," said Mike Coyne, meteorologist in charge at the National Weather Service in Huntsville.
The Sunday twister in Chattooga County started on Lookout Mountain, came down into Chattooga Valley and cut a swath of destruction a half-mile wide and eight miles long.
"We think valleys may intensify tornadoes," Dr. Coleman said. "When a tornado goes over a mountain and into a valley, it gets stretched out and it spins faster."
Some tornadoes studied by Dr. Coleman started as EF-1 funnels on mountaintops, then strengthen to EF-4 storms by the time they reach homes in the valley below.
In 1997, Chattanooga was battered by a tornado that started in Lookout Valley. The twister first touched down in Tiftonia, hopped over Lookout Mountain to land in St. Elmo, then carved a path to East Brainerd, destroying 50 homes and damaging another 600. Overall, the twister caused nearly $45 million in damage.
A strong 2002 tornado decimated Mossy Grove, a community in Morgan County, Tenn., killing 17 people, records show.
The Enhanced Fujita Tornado ScaleEF-0: 65-85 mphEF-1: 86-110 mphEF-2: 111-135 mphEF-3: 136-165 mphEF-4: 166-200 mphEF-5: More than 200 mphSource: National Weather Service
April and May are classified as tornado season by forecasters, so twisters are expected -- especially in this part of the country.
The area is so prone for tornadoes, University of Akron researchers have branded the region from Texas to metro Atlanta "Dixie Alley." Those researchers say the region is second in tornado touchdowns only to the "Wizard of Oz" plains of Kansas, Oklahoma and Nebraska.
But Alabama researchers think this part of the country might have just as many, if not more, tornadoes than the Great Plains.
"We think we have as many tornadoes as out there, but the difference is that in Kansas you can see for 50 miles and you can spot every tornado," said Dr. Tim Coleman, a University of Alabama in Huntsville researcher. "Here, our tornadoes are obscured by mountains and trees, and they mostly happen at night."
Interactive graphic: Tornado probability
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Storm carves path through Alabama, Georgia