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 the storms ended Thursday, the destruction awed even veteran storm observers — 148 reported tornadoes, winds perhaps over 300 mph, and 280 or more deaths from Alabama to Virginia.
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. The figures include some double-counting, and the final total should be about 600. Until now, no April on record had surpassed the 267 tornadoes recorded in 1974.
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 the United States 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 rain. Under the right circumstances, such a storm can mutate into a "supercell" that spawns tornadoes.
This year, those storms have been especially potent, in part because of the gulf, where surface temperatures have been significantly higher than normal, said Henry Margusity, a meteorologist with AccuWeather 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 supercell beast," DeBlock said.
The moisture has had an extra lift from ultra-energized jet-stream winds, said Russell Schneider, chief of the 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.
Margusity said the strength of the jet-stream winds can be traced to the tropical Pacific. During the winter, surface temperatures over the Pacific were about as low 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 mph. 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," Schneider said.
"There's a pretty good chance some of these were a mile wide, on the ground for tens of miles and had wind speeds over 200 mph," said Harold Brooks of the National Severe Storms Laboratory in Norman, Okla.
It may have been a single long-ranging twister that battered Tuscaloosa, Ala., and then covered the 60 miles to Birmingham, Brooks said.
Only 1 percent of twisters reach the most powerful readings, but Brooks thinks several of those that left death and destruction in Alabama and five other states fall into that category.
Brooks also said recent research shows a possible link between La Niña and exceptionally strong tornado outbreaks.
La Niña coincided with the April 1974 severe storm season. That year, an April outbreak was blamed for 330 deaths, the most in the modern reporting era dating to 1950.
It may be days before a final death toll or tornado count are available for the storms on Wednesday and Thursday.
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 the fatalities evidently included some people who had taken precautions and sought shelter. But a 300-mph 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.
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