Hurricane Dorian is expected to bring a "life-threatening storm surge" up the US east coast after causing destruction and at least 20 deaths in the Bahamas.
Residents from Georgia all the way up to Virginia are warned to listen to emergency advice as the category three hurricane slowly moves north.
Police say parts of Charleston, South Carolina, have flooded and conditions are "beginning to deteriorate".
Dorian weakened after hitting the Bahamas but has now strengthened again.
It currently has maximum sustained winds of 115mph (185km/h).
On 1 September it hit the Bahamas with winds of up to 185mph (298km/h) - equalling the highest ever recorded at landfall. It battered the Abaco Islands and Grand Bahama, in the north of the archipelago, for two days.
Bahamas PM Hubert Minnis called the storm "one of the greatest national crises in our country's history". He expected the number of fatalities to increase.
What's the latest on the storm?
The US National Hurricane Center (NHC) said that at 06:00GMT Dorian was 170km (105 miles) south-east of Charleston and moving north at about 11km/h (7mph).
It is expected to turn north-east on Thursday and speed up.
The NHC warns that a "life-threatening storm surge with significant coastal flooding is expected along a large portion of the south-east and mid-Atlantic coasts of the United States in the next couple of days".
South Carolina is preparing for a record storm surge and the effects are already being felt. Some 20,000 people are without power in Charleston County.
Dorian is forecast to move "near or over" the coast of South Carolina on Thursday, then the coast of North Carolina overnight and on Friday.
Graphics showed waters could rise up to 8ft (2.4m) above ground level on the South Carolina coast, and up to 15in of rain could fall in the coming days.
"Time to get out is running out," the state's governor Henry McMaster told reporters.
"Water levels could rise well in advance of the arrival of strong winds," the NHC said, adding these storm surges are expected "regardless of the exact track of Dorian's centre".
More than 2.2 million people have been ordered to evacuate along the eastern seaboard.
US President Donald Trump earlier held up a map incorrectly suggesting Dorian could hit Alabama. Some observers said it looked as though a line around the state was drawn with a felt pen.
The president has since said he did not know why the chart showed this.
Presidential candidate Marianne Williamson meanwhile earlier suggested on Twitter that prayer could drive the storm away. She defended her now-deleted tweet, claiming that those praying should not be treated with "mockery or condescension".