Veteran tornado watchers saw Wednesday's mega-twisters coming.
But they were still staggered by the destruction that a massive storm system unleashed across the South and Southeast that leveled broad swaths of Tuscaloosa, Ala., and killed more than 280 people.
"I've been at this for a while, and I've never seen anything like this," said Chris Weiss, an associate professor of atmospheric science at Texas Tech University in Lubbock, Texas.
Some of Wednesday's tornadoes could have traveled dozens, or perhaps even hundreds, of miles before eventually dissipating. Typical twisters survive only minutes. Based on early observations, a few of the twisters probably were more than a mile wide, Weiss said.
As of Thursday evening, there had been 173 reports of twisters touching down in the U.S. on Wednesday, said Greg Carbin, warning coordination meteorologist for the National Weather Service's Storm Prediction Center in Norman, Okla. The National Oceanic and Atmospheric Administration estimated that there were 164 tornadoes Wednesday.
That number of reports might be typical for an entire April, he said. This year's national preliminary tornado estimate for April has already hit 600, with several days left to go.
"We will finish out with more in this April than in any month we've seen in the last 60 years," Carbin said. "It's really hard, even for me, to get my mind around that number."
It's typical for tornadoes to develop in the spring as warm, wet air from the south bumps up against cool, dry air from the north. The warmer air makes the atmosphere unstable, said Stu Ostro, a senior meteorologist at the Weather Channel.
Throw in wind shear — when winds at different levels in the atmosphere blow in different directions — and you might get enough twisting convection currents to produce super-cell thunderstorms. These, in turn, can spawn tornadoes.
This year, the winds from the Gulf of Mexico have been "exceptionally warm and humid for this early in the spring," Ostro said. Those warm winds from the south blew close to the ground this week while a very strong jet stream came in from the west higher up.
"The ingredients were classic, and extreme," Ostro said.
Weiss said conditions favorable for tornadoes also extended over a very wide portion of the eastern third of the U.S., from Mississippi, where the storms originated, east to the mid-Atlantic coast and into the Ohio River Valley, reaching as far north as far as Indianapolis. That allowed the storm system to thrive, and it may account for the widespread devastation on the ground, he said.
The role of global warming in the phenomenon is unclear, he added, noting that it's hard to relate individual weather events to the long-term sweep of climate change, and that even if one could, there's "significant debate" in the scientific literature about whether warming will increase or decrease the number of tornadoes.
By Thursday afternoon, the storm system had edged into the Atlantic Ocean, and the danger of additional tornadoes had subsided.
Over the coming months, Carbin said, the National Weather Service will study the destruction to plot the twisters' paths and intensity, assigning them ratings from 0 to 5 on the Enhanced Fujita Scale. Three-second gusts at 65 to 85 mph are rated EF-0; twisters with gusts over 200 mph are rated EF-5.
Measuring tornado winds directly isn't feasible, so experts who assign ratings instead assess damage on the ground, considering the types of structures that are damaged, and extrapolate a likely wind speed. NASA has directed data from one of its monitoring satellites to agencies working on the ground to help identify hard-hit areas.
Weiss said he was "fairly certain" one or more of these storms would get an EF-4 or EF-5 rating.
Carbin remained cautious. "It's going to be a very tedious process to come up with what we think represents a good figure," he said.
But he agreed that the storms will be historic, perhaps rivaling the "super outbreak" of April 1974, when 148 tornadoes in 13 states killed 335 people.