The hurricane is generating swells that are already affecting Bermuda and the U.S. East Coast.
Meanwhile, a storm surge watch was sent out for Edisto Beach in SC to the North Carolina-Virginia border as well as Albemarle and Pamlico Sounds, including the Neuse and Pamlico Rivers. "With the size and the intensity and the current forecast track, it's a risky time for people in North Carolina with regard to this storm".
It could make landfall on Thursday, near Wilmington, North Carolina.
"It is an extremely risky, life-threatening, historic hurricane".
Those in states where the Category 4 hurricane is expected to make direct landfall may have to wait weeks before power is restored, FEMA Associate Administrator Jeff Byard said in a call Tuesday with reporters. "Isaac appeared to have been in relatively favorable conditions during the past day or so, but it did not strengthen".
Florence was expected to remain an extremely risky, major hurricane and was forecast to approach the southeastern coast of the United States on Thursday, the NHC added.
Mandatory evacuation orders have been issued for the coasts of South Carolina, North Carolina and Virginia, and states of emergency have been declared there as well as Maryland and the District of Columbia.
South Carolina's Governor Henry McMaster gave the entire coastline of his state until noon on Tuesday to leave.More news: EA under Criminal Investigation by Belgium Government over Illegal FIFA Lootboxes
"On the forecast track, the center of Florence will move over the southwestern Atlantic Ocean between Bermuda and the Bahamas Tuesday and Wednesday and approach the coast of SC or North Carolina on Thursday".
A map of the eight watersheds in SC.
The office of Virginia Governor Ralph Northam predicted "catastrophic inland flooding, high winds and possible widespread power outages".
Residents of affected areas have been flocking to stores to stock up on essentials.
Some petrol stations were running dry as customers filled up.
"This storm is a monster", North Carolina Governor Roy Cooper told a Tuesday news conference about the most powerful storm to approach the Carolinas in almost three decades.
Miller said residents should prepare for the storm by stocking up on bottled water and batteries for flashlights, ensure storm drains are free of debris and clean gutters and downspouts to prevent basement flooding.
He said the agency has implemented changes since 2017, improving their partnerships with the private sector, in particular fuel, transportation and electrical companies, to better meet needs on the ground after a disaster. "With the storm of this magnitude, the planning factors, and everything, we will meet the needs". Schools in 26 of the state's 46 counties were ordered closed from Tuesday. Ten to 15in (25 to 38 cm) of rain is forecast for the Big Island.