Severe weather is threatening the eastern states, after two days of powerful storms killed up to 34 people.
Alabama had flash flooding and tornadoes were reported in North Carolina on Tuesday.
Thunderstorms and severe weather are expected along the east coast on Wednesday as the storm shifts.
Tornadoes cut a broad track of death and destruction through the southern states on Sunday and Monday.
The storms flattened buildings, overturned cars, and left thousands of residents without power.
“Widespread thunderstorms are forecast across the central Gulf Coast region, with several severe [or] supercell storms possible,” the National Weather Service wrote in a morning forecast.
More than two million people are said to be at risk of tornadoes and high winds in parts of Alabama and Mississippi on Tuesday.
As the storm moves further east, parts of Florida, Georgia, North Carolina and South Carolina are expected to be under threat of severe weather, forecasters say.
Meanwhile, the search for survivors continues in the southern US states that were smashed by huge tornadoes on Monday and Tuesday.
In Mississippi, officials reported 12 dead, including nine near Louisville and three in separate traffic accidents.
Early on Tuesday, residents of Louisville crept from their shelters to find their homes demolished by a tornado that punched holes in the roof of a local hospital.
Giles Ward huddled in a bathroom with his wife and four other relatives as a tornado destroyed his brick house and overturned his son-in-law’s four-wheel-drive parked outside his home in Louisville.
“For about 30 seconds, it was unbelievable,” said Giles Ward, a Republican state senator.
“It’s about as awful as anything we’ve gone through.”
Three people were reported dead in Alabama, including a University of Alabama student who perished when he took shelter in a basement and a retaining wall collapsed on him.
As many as 16 people were killed on Sunday, including 14 in the towns surrounding Little Rock, Arkansas, with Mayflower and Vilonia bearing the brunt of the damage.
Many homes and businesses, including a new secondary school worth $14 million, were left in ruins in Vilonia after the storm.
President Barack Obama promised assistance to those affected by the storms, saying federal emergency officials would be on the ground to help.
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