In Virginia, four people died — three in the small town of Glade Spring — in Washington and Halifax counties, when twisters roared through overnight, officials said.

A truck stop on Interstate 81 and a new factory were destroyed, according to Christy Parker, assistant administrator in Washington County, in southwestern Virginia.

Tractor trailers “were flipped and thrown about the interstate like toys,” she said Thursday.

“The outbreak is the biggest in terms of tornadoes and in terms of impact since ’74 and it’s possible that its actually bigger than ’74,” said Harold Brooks, research meteorologist at the National Severe Storms Laboratory in Norman, Okla.

The April 3, 1974, outbreak sparked twisters across the eastern United States, claiming 310 lives, Brooks said. Wednesday’s outbreak may be most similar to the tornado outbreak of March 21, 1932, when 332 people were killed, including 268 in Alabama, he said. Nothing, however, comes close to the destruction of March 18, 1925, when 747 people died, most of them along the path of a single twister, after the so-called Tri-State Tornado tore up Missouri, Indiana and Illinois.

Brooks said the conditions have been ripe in recent weeks for just such a catastrophe. Cold air aloft blows in from the Rocky Mountains, meeting the low-level, warm, moist air moving northward from the Gulf of Mexico. If the cold fronts are strong enough they’ll suppress tornado formation. But if they’re a bit weaker than that, the lethal combination results: Air at ground level is moving in a different direction from the air higher up, and the rotational energy spawns twisters.

Alabama is suffering most in the aftermath of Wednesday’s twisters. The city of Tuscaloosa took a direct hit. The city is home to the flagship of the state’s university system, the University of Alabama, which has canceled or postponed final exams scheduled for next week.

“This place looks like a war zone,” Jackie Wuska Hurt, director of development for the University of Alabama honors college, said in an e-mail. “Folks looked like refugees walking single file with suitcases or grocery carts of their belongings down the sidewalks of University Blvd.”

“It’s almost total disbelief,” said Phyllis Little, director of emergency management for Cullman County, Ala., a largely rural area of 82,000 peppered with small towns. “The county court house lost its roof. The Baptist church has a skeleton for a steeple. Old buildings that have been there for hundreds of years have just collapsed.”

The entire county is without power, and the emergency responders are operating on natural gas generators. Little has been turning away volunteers who have begun calling her office, offering to come to Cullman to help. She is turning them away.