By the time it reaches the USA mainland, the NHC says Florence will strengthen into "an extremely risky major hurricane". That means sustained winds of at least 130 miles per hour and expectations of catastrophic damage, the hurricane center says.
North Carolina State University canceled classes beginning Sept. 12 at 5 p.m. until the evening of Sunday, Sept. 16, although dining and residence halls will remain open.
The National Hurricane Center says a new report from an Air Force Reserve Unit hurricane hunter aircraft indicates that Hurricane Florence's top sustained winds have decreased slightly to 130 miles per hour (215 kph), with higher gusts.
Tropical storm winds are forecast to hit the region sometime Thursday morning, with landfall occurring late Thursday night. It was centred east-south-east of Cape Fear, North Carolina, and moving west.
Before that, Hurricane Hugo made landfall just north of Charleston, W. Va., at Category 4 strength in 1989.
A hurricane watch has been issued for the East Coast from Edisto Beach, S.C., to the North Carolina-Virginia border, including the Pamlico and Albemarle Sounds. Hurricane force winds now extend 40 miles from the eye.
At noon on Tuesday, CBS News correspondent David Begnaud says lanes will be reversed on four of the largest roads leading to the SC coast, so cars will only be able to drive inland.
As astronauts aboard the International Space Station (ISS) saw first-hand on Monday morning, hurricanes Isaac and Helene are trailing behind Florence, making for a trio of powerful storms. But the storm is expected to restrengthen over the next day or so. That Category 4 storm destroyed 15,000 buildings and 19 people in North Carolina.
"Additional strengthening is forecast during this time, but some fluctuations in intensity are likely due to eyewall replacement cycles".
Further to the east, forecasters are also watching hurricanes Isaac and Helene.
For many people, the challenge could be finding a safe refuge: If Florence slows to a crawl just off the coast, it could bring torrential rains to the Appalachian mountains and as far away as West Virginia, causing flash floods, mudslides and other unsafe conditions. Large swells could be generated and cause damage in some parts of the state, which survived brushes with hurricanes Hector and Lane in recent weeks.
"We are in the bull's-eye", North Carolina Governor Roy Cooper told a news conference.