The storm was heading for the coast of the two states but heavy rain was also expected in Virginia to the north and Georgia to the south.
More than 1 million people have been ordered to evacuate the coastlines of the Carolinas and Virginia.
At 2 p.m. on September 12, the NHC had storm surge and hurricane warnings in place for most of the North Carolina and SC coast. "I encourage Georgians to be prepared for the inland effects of the storm as well as the ensuing storm surge in coastal areas". "Heed those warnings and evacuate if you're in one of the zones".
The southeast coast of North Carolina will experience a prolonged storm surge reaching up to 9 to 13 feet from near the mouth of the Cape Fear to Cape Lookout.
"This will likely be the storm of a lifetime for portions of the Carolina coast, and that's saying a lot given the impacts we've seen from Hurricanes Diana, Hugo, Fran, Bonnie, Floyd and Matthew", it said.
"They haven't seen anything like what's coming at us in 25-30 years, maybe ever", President Trump said while appearing with FEMA officials in the Oval Office on Tuesday.
"In collaboration with state emergency agencies and the Georgia Hospital Association, we are monitoring bed counts daily so that we can quickly and accurately assist with any necessary evacuations", said Sydney Walker, spokesperson for Piedmont Athens Regional Medical Center.
Duke Energy, a power company in the Carolinas, estimated that one million to 3 million customers could lose electricity because of the storm and that it could take weeks to restore.
The governors of North Carolina, South Carolina, and Virginia ordered mandatory evacuation in areas of their states earlier this week.
"The state is mobilizing all available resources to ensure public safety", Deal said.
The eastbound lanes of several major highways have been shut to allow traffic to flow inland, but the exodus was slow along roads jammed with outward-bound vehicles.
"It's surreal to think we may have nothing to go home to", she said.
Mayor Joe Benson of Carolina Beach, a small town near Wilmington with a permanent population of about 6,300, said he believes half of the residents have stayed on the island. "You can't stop Mother Nature".
Another commenter said: 'That hurricane better be taking me to dinner first'.
Her home in Hillsborough, North Carolina, is at risk for flooding, and her family and nearby relatives will ride out the storm in Chattanooga, Tennessee.
Forecasters said parts of North Carolina could get 20in of rain, if not more, with as much as 10in elsewhere in the state and in Virginia, parts of Maryland and Washington DC.
"Hurricane Florence is looking even bigger than anticipated".