April is the cruelest month

Dana Coulter in Terra Daily via NASA Science News: You might agree if you live in the southeastern United States. Last April, a historic outburst of 202 tornadoes turned broad swaths of that part of the country into a disaster zone. "The event of April 27th and 28th 2011 was the costliest convective storm in U.S. history," said Kevin Knupp, professor of atmospheric sciences at the University of Alabama-Huntsville. And he doesn't just mean costly in terms of property damage - 316 people lost their lives.

Of the 202 twisters that day, 62 tore through Alabama, where Knupp works. Ten of them were ranked EF 4 and 5 on the Enhanced Fujita Scale. Three tornadoes churned paths more than 120 miles long, and a large number of the twisters cut swaths more than a half mile wide.

Knupp saw the results first-hand, and he's been studying them ever since. Aided by a team of graduate students and colleagues, he's sifted through gigabytes of data1 collected by NASA and NOAA satellites and local ground sensors. A year later, they have drawn some interesting conclusions.

One discovery was how rapidly an EF-5 spun up near the small town of Hackleburg, Alabama. "The Hackleburg storm got its act together really quickly," says Knupp. This particular twister formed only 50 minutes after the underlying thunderstorm appeared. For comparison, the average time for tornado formation is 2 hours. The twister blasted through north Alabama with winds over 200 mph, killing 72 people.

Knupp's team believes that something called a "thermal boundary" set the stage for the birth of the killer. Cool, moist air on one side of the boundary formed a low cloud base -"kind of like a wall," he explains. Warm air from the storm ran into the wall and swept upward. Updrafts are a key ingredient of tornadoes. In this case, updrafts as swift as 75 feet per second were recorded.

...Could local topography have attracted the twisters? There does appear to be a link between the shape of the landscape and the path of these tornadoes....

Damage from an EF-5 tornado which struck the town of Phil Campbell, Alabama during the April 27, 2011 tornado outbreak. From the National Weather Service