By Anthony R. Wood
April 29, 2011
Well before the first tornado touched down, John DeBlock sensed that something horrific was brewing in the steamy Alabama air Wednesday.
“The air was palpable with the potential,” said DeBlock, the warning-coordinator meteorologist at the National Weather Service office in Birmingham.
By the time it all ended Thursday morning, the havoc and destruction awed even veteran storm observers – 148 reported tornadoes, winds perhaps over 300 m.p.h., and 290 or more deaths from Alabama to Virginia. At least one twister even touched down in York County, Pa., on Thursday, but no injuries were reported.
Overall, it was a dramatic climax to one of the wildest months in U.S. weather history; The atmosphere has been in a state of riot.
So far this month, 900 tornado sightings have been reported, according to the government’s Storm Prediction Center. Those figures include some double-counting, and the final total should be around 600. Until now, no April on record has surpassed the 267 twisters recorded in 1974. The all-time record for any month is 543, in May 2003.
What’s going on?
Every spring, the atmosphere over the United States becomes a battleground between the winter cold retreating into Canada and the summer heat invading from the Gulf of Mexico.
That’s a big reason why this nation is far and away the world capital for tornadoes, the most violent storms on Earth.
Thunderstorms form at the collisions of cold and warm air, as moisture-laden warm air is forced to rise and water vapor condenses into pounding rains. Under the right circumstances, the storm can mutate into a “supercell” that spawns tornadoes.
This year, those storms have been especially potent, thanks in part to the Gulf, where surface temperatures have been significantly warmer than normal, said Henry Margusity, a meteorologist with AccuWeather Inc. in State College, Pa.
“There’s a bathtub there just pumping warm air into the Mississippi Valley,” he said. The warmer the air, the more moisture it can hold.
“That warm air is what feeds that super-cell beast,” said DeBlock.
The moisture has been getting an extra lift from ultra-energized jet-stream winds, said Russell Schneider, chief of the government’s Storm Prediction Center. Those are the upper-air westerlies that circle the planet moving weather systems and detonating storms.
As those winds blow over a storm, they force the air underneath to rise violently – think of smoke lifted from a chimney on a windy evening.
Margusity said the strength of the jet-stream winds can be traced to the tropical Pacific. During the winter, surface temperatures over tens of millions of miles of the Pacific were about as chilly as they get, a phenomenon known as La Niña.
Jet streams form over the boundaries of warm and cold air, and unusually cool or warm waters in the Pacific have a powerful impact on those contrasts, and thus the speed of the winds.
On Wednesday, jet-stream winds over Alabama were estimated at 150 m.p.h. Meanwhile, near the surface in the tornado zone, the winds were blowing strongly from the south.
The conflicting winds at different levels of the atmosphere helped give the storms the spins that turned them deadly.
“Yesterday was remarkable for the environmental conditions,” said Schneider.
Recent research shows a possible link between La Niña and exceptionally strong tornado outbreaks, said Harold Brooks, a researcher at the National Severe Storms Laboratory in Norman, Okla.
La Niña coincided with the aforementioned April 1974 severe-storm season. That year, an April outbreak was blamed for 310 deaths, the most in the modern reporting era dating to 1950.
It may be days before a final death toll or a final tornado count are available for the Wednesday-Thursday storms.
DeBlock said Thursday that investigators were tracking down the preliminary reports. It is tough to compare recent tornado data with those of past decades, since radar detection today is superior, and more people are living in places few lived in before.
His investigators are trying to infer the wind strength through the damage evidence, but Schneider said that it’s likely some of the twisters reached levels 4 and 5 – that would be winds of 208 to 318 m.p.h. – on the Enhanced Fujita scale.
Margusity said that given the warnings, it was puzzling that so many people were killed. “You have more communication devices than ever before, and people are dying,” he said.
DeBlock said that the fatalities evidently included some people who had taken precautions and sought shelter. But a 300 m.p.h. wind is going to overmatch almost anything in its path.
“There aren’t many structures that are going to withstand the impact of that beast,” he said. “A safe place reduces risk; it doesn’t eliminate it.”