Tornadoes and violent storms ripped through seven southern states, killing at least 284 people in the country's deadliest series of twisters in nearly four decades.
The clusters of powerful tornadoes -- more than 160 reported in total -- combined with storms to cut a swath of destruction heading west to east over several days. In some areas, whole neighborhoods were flattened, cars flipped over and trees and power lines felled, leaving tangled wreckage.
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Given the apparent scale of the destruction, insurance experts were wary on Thursday of estimating damage costs, but believed they would run into the billions of dollars, with the worst impacts concentrated in the Alabama cities of Tuscaloosa and Birmingham.
"In terms of the ground-up damage and quite possibly the insured damage, this event will be of historic proportions," Jose Miranda, an executive with the catastrophe risk modeling firm EQECAT, told Reuters.
At least 184 people died in Alabama, the worst-hit state which suffered "massive destruction of property," Governor Robert Bentley said.
The mile-wide monster twister that on Wednesday tore through the town of Tuscaloosa, home to the University of Alabama, may have been the biggest ever to hit the state, AccuWeather.com meteorologist Josh Nagelberg.
Many people told tales of narrow misses. "I made it. I got in a closet, put a pillow over my face and held on for dear life because it started sucking me up," said Angela Smith of Tuscaloosa, whose neighbor was killed.
President Barack Obama said he will visit Alabama on Friday to view damage and meet the governor. Obama declared a state of emergency for Alabama and ordered federal aid.
In preliminary estimates, other state officials reported 32 killed in Mississippi, 33 in Tennessee, 11 in Arkansas, 14 in Georgia, eight in Virginia and two in Louisiana.
Miranda said the estimated costs would be "in the same ballpark" as an Oklahoma City tornado outbreak in 1999 that caused $1.58 billion of damage and a 2003 tornado outbreak in Kansas, Missouri, Arkansas and Oklahoma that caused $4.5 billion of damage.
"I would not be surprised to see it in the mid-level billions, singular billions, of dollars," Miranda said.
The Browns Ferry nuclear power plant in Alabama was expected to be shut for days, possibly weeks, as workers repaired damaged transmission lines.
But the backup systems worked as intended to prevent a partial meltdown like the nuclear disaster in Japan.
"The reactors will remain shut until we have restored the reliability of the transmission system," said Ray Golden, spokesman for the Tennessee Valley Authority, which owns the 3,274-megawatt plant.
Up to 1 million people in Alabama were left without power.
Daimler said it had shut down its Mercedes-Benz vehicle assembly plant in Tuscaloosa until Monday because of the tornadoes, but the plant itself sustained only minor damage.
'SOUNDED LIKE CHAIN-SAW'
Some of the worst devastation occurred in Tuscaloosa, where at least 37 people were killed, including some students.
"It sounded like a chain-saw. You could hear the debris hitting things. All I have left is a few clothes and tools that were too heavy for the storm to pick up. It doesn't seem real," said student Steve Niven, 24.
"I can buy new things but you cannot replace the people. I feel sorry for those who lost loved ones," Niven told Reuters.
Shops, shopping malls, drug stores, gas stations and dry cleaners were all flattened in one section of Tuscaloosa, a town of around 95,000 in the west-central part of Alabama.
Alabama's governor declared a state of emergency and said he was deploying 2,000 National Guardsman. Governors in Arkansas, Mississippi, Tennessee and Virginia also declared states of emergency.
Tornadoes are a regular feature of life in the U.S. South and Midwest, but they are rarely so devastating.
Wednesday was the deadliest day of tornadoes in the United States since 310 people lost their lives on April 3, 1974, weather forecasters said.
"We have never experienced such a major weather event in our history," said the Tennessee Valley Authority, which operates the Browns Ferry nuclear plant and provides electricity to 9 million people in seven states.
Among the Alabama counties affected was Jefferson, which is struggling to avert what would be the largest bankruptcy in municipal history over a $3.2 billion bond debt.
The county suffered "widespread damage," a local emergency spokesman said, and at least 17 people were killed.
"Everybody says it (a tornado) sounds like a train and I started to hear the train," Tuscaloosa resident Anthony Foote, whose home was badly damaged, said.
"I ran and jumped into the tub and the house started shaking. Then glass started shattering."
The campus of the University of Alabama, home of the famous Crimson Tide football team, was not badly damaged but some students were killed off campus, Bentley said.