Why Did April Spawn so Many Deadly Tornadoes in the South?

BY Jenny Marder  May 12, 2011 at 11:20 AM EDT


Photo courtesy Katrina Floyd

A scary thing happened to Katrina Floyd on the morning of April 27. A thunderstorm swept through her northeast Alabama town of Ider in the early morning hours, uprooting trees and knocking out power and cell signals. She had no access to television, radio, the Internet, or her smartphone. On a day of extreme weather, she had no way to access weather alerts.

“We were going into a severe weather day, thinking, “How in the world are we going to keep up with everything?’” Floyd recalled. “We were going into all these storms knowing there was supposed to be bad weather, knowing it could be dangerous, but we didn’t have a way to keep track of what was going on.”

Her family got lucky. Their house was spared by tornadoes that passed on both sides, chewing up homes, ripping out trees and flattening businesses, leaving a trail of destruction and debris. Her neighbors and parents were not spared.

“Both bathrooms in the house were standing, but they were so filled with debris that I don’t believe they would have survived,” she said of her parents’ longtime home. (Her parents, luckily, were out of state when the storm struck.) “We went through the rubble, and it’s almost all gone. They pretty much lost everything.”

Mike Smith, meteorologist and CEO of AccuWeather’s Weather Data, thinks the loss of power to more than 200,000 Alabama homes and businesses — that’s as many as a million residents without power — explains the high rate of fatalities on April 27th. More than 300 people were killed in six southern states. For many, power was out during the entire duration of the storm.

An outbreak two weeks earlier in the South struck a similarly populated area with roughly the same number of tornadoes, but the April 27 tornadoes caused ten times as many deaths, Smith wrote in a recent report, titled, “What If They Never Got the Warnings?”

“If you don’t have the TV on, if you don’t have the radio on, if you live in an area without a siren, you’re at risk of not getting a warning,” said Smith, also author of the book, “Warnings: The True Story of How Science Tamed the Weather.” “The consequence of not getting a warning could be terrible.”

Others, like Harold Brooks, research meteorologist with NOAA’s National Severe Storms Laboratory, thinks power outages factored only minimally into the death toll. Brooks, who has been studying firsthand accounts of people affected by the storm, said that of the 30 specific accounts he’s heard, only two people blamed the loss of power.

“Most were clearly aware there was a threat,” he said. “If that sample is anywhere close to being accurate, I don’t think the power had a big impact.”

More dangerous, he said, was the number of mobile homes in the southeastern United States. Mobile home residents are 15 times more likely to be killed than those in permanent homes, he said. “You put a bunch of violent tornadoes on the ground for a long time in the path of mobile homes, and that’s a bad combination.”

April was a devastating month of tornadoes, with hundreds touching down in America’s Midwest and South. And April is not even the region’s prime tornado month. That prize goes to May. This frightening fact has residents in low-lying areas bracing for the possibility of even more storms in coming weeks.

How exactly are these twisters formed? A tornado is a violently rotating column of air, and needs several ingredients to take shape: warm, moist air coming in from the Gulf of Mexico at low levels, dry air at about 10,000 feet, and colder, higher air moving in at roughly 18,000 feet.

Changing wind direction and windspeed during a thunderstorm create a horizontal spinning effect; this increases the likelihood that a tornado will form. Then, as the low humid air rises, creating what’s known as an “updraft,” it tilts the horizontal spin upward into a vertical wind funnel. The hot gulf air fuels the thunderstorm like wood fuels a fire.

“There is an aspect to rising humid air that once it starts rising, it rises at an accelerated speed,” Smith explained. “In thunderstorms that produce tornadoes, the air in the center rises upward at about 120 miles an hour. It would be like shooting a bullet straight up. Without that updraft, thunderstorms would collapse after a short period of time.”

It was the frequency of these ingredients converging during April in the South that made that month so volatile, says Dan Kottlowski, expert senior meteorologist at accuweather.com. But two other factors contributed too: warmer-than-normal water temperatures in the Gulf of Mexico, combined with a colder upper air pattern from Montana, the Dakotas, and the Northern Plains states.

These patterns have now passed, he said. “That pattern which helped bring us the unsettled weather through much of April was a pattern that helped spawn multiple tornado outbreaks. And the odds of us getting the same pattern in the same place would be very, very, very unusual. It would be very rare.”