"Dangerous" Florence has maximum sustained winds of 130mph and will be a Category 4 storm when it makes landfall on Friday morning.
The NHC said the first tropical storm-force winds of at least 39 miles per hour (63 kph) would hit the region early on Thursday with the storm's center reaching the coast Friday.
"This rainfall would produce catastrophic flash flooding and significant river flooding", the hurricane center says.
A search-and-rescue team is being deployed by FEMA from Montgomery County in Maryland to help in SC as the area braces for Hurricane Florence.
The landfall location is expected to be between Wilmington and Morehead City, North Carolina.
About 1.7 million people across the three states affected - North Carolina, South Carolina and Virginia - have been told to leave their homes ahead of the storm's arrival.
"So right now, we're thinking it's going to be tropical-storm-force conditions for the Beaufort area, but I would not rule out the potential for hurricane force winds or hurricane force conditions for Beaufort County".
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Florence is still about 530 miles south-east of Cape Fear, North Carolina approaching the coast at 17mph (28kph). Gradual weakening is forecast over the next couple of days, and Helene is expected to become a tropical storm on Thursday.
North Carolina Department of Transportation officials said maintenance crews are prepping their vehicles and will be holding swift water rescue training Wednesday.
Florence could strengthen some over open water and then weaken as it nears land, but the difference won't make it any less risky, forecaster Stacy Stewart wrote in a National Hurricane Center discussion.
Life-threatening storm surges of up to 13 feet (3.9 meters) were also forecast in some areas of North Carolina along with the possibility of tornadoes. Residents in some parts of the Carolinas have been ordered to evacuate.
The National Weather Service is forecasting heavy rainfall Sunday through Tuesday across central SC with flooding potential in the Upstate, northeast Georgia and the mountains of western North Carolina.
The front that has been giving us showers and storms the past few days will finally dissipate and lift northwestward.
It will also pass directly over two nuclear power plants - the Brunswick Nuclear Plant, which is located 30 miles south of Wilmington, as well as the Shearon Harris Nuclear Plant in New Hill, about 23 miles from Raleigh.
To back up that point, Graham cited a sobering statistic: "50 percent of the fatalities in these tropical systems is the storm surge - and that's not just along the coast". But the change was probably temporary and didn't do anything to lessen the danger, said Jeff Byard, an administrator with the Federal Emergency Management Agency.