Although it's been downgraded to a category two storm with 105mph winds, officials say there could be feet - rather than inches - of rain.
A buoy off the North Carolina coast recorded waves almost 30 feet (9 metres) high as Florence churned toward shore.
Hurricanes can push masses of water from the sea over land when they come ashore, raising local sea levels by as much as 20 feet (6 meters), according to the National Hurricane Center.
The biggest danger to life could be from storm surges, which are still predicted to be as high as 13ft (4m) between Cape Fear and Cape Lookout in North Carolina. Florence is expected to arrive in southeast North Carolina on Friday morning before drifting south into SC.
Energy companies have also warned that the storm could knock out power for the foreseeable future in some areas.
Wilmington resident Julie Terrell said she was concerned after walking to breakfast past a row of shops fortified with boards, sandbags and hurricane shutters.
"The first bands are upon us", Gov. Roy Cooper said Thursday evening. "If we lose the house, oh well, we can get housing".
Schools and businesses closed as far south as Georgia, airlines canceled more than 1,500 flights, and coastal towns in the Carolinas were largely emptied out.
Another video from NOAA's GOES-East weather satellite caught a different view of Hurricane Florence.
"But we can't replace us so we made a decision to come here".
North Carolina has had this problem before.
About 5.25 million people live in areas under hurricane warnings or watches, and 4.9 million live in places covered by tropical storm warnings or watches, the National Weather Service said.
More than a million people along the coastlines of North Carolina, South Carolina and Virginia have been ordered to evacuate.
A state of emergency has been declared in five coastal states - North Carolina, South Carolina, Georgia, Maryland and Virginia - as well as the U.S. capital Washington.
In Wilmington, residents who had decided not to evacuate were lining up to get ice from a vending machine - US$2 for a 7.2kg bag.
It said the additional declaration would bring more federal help with debris removal, search and rescue teams, meals and generators, among other items.
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