Hurricane Dorian is expected to bring “life-threatening storm surges” up the US east coast after causing destruction and at least 20 deaths in the Bahamas.
The category three storm is currently lashing the coast of the Carolinas as it moves slowly north. Tens of thousands of people are without power.
Residents from Georgia up to Virginia are being warned to listen to emergency advice.
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.
Flooding in central Charleston
The US National Hurricane Center (NHC) said that at 09:00 GMT Dorian was 130km (80 miles) south-east of Charleston and moving north at about 13km/h (8mph).
The NHC said Dorian “continues to lash the coast of the Carolinas. Hurricane conditions are likely over portions of the area later on Thursday.”
It is expected to turn north-east later on Thursday and speed up.
South Carolina has prepared for a record storm surge and the effects are already being felt. More than 100,000 people were without power in coastal areas on Thursday morning, energy officials said.
Social media footage showed flooding in central Charleston.
Dorian is forecast to move close to the coast of South Carolina on Thursday, then “near or over” 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.
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.