Up to 10 million people lie in the path of the storm, which still poses a risk to life and property, although it has been downgraded to a Category 2 hurricane.
Florence's large size - tropical storm force winds extend 170 miles from the center in all directions - means its fury will arrive long before the center of the storm comes ashore, Graham said. The storm will bring dangerously high amounts of rain, forecasters say.
Jeff Byard of the Federal Emergency Management Agency invoked a former boxing champion to warn residents that it would bring "a Mike Tyson punch to the Carolina coast".
South Carolina Governor Henry McMaster has ordered the mandatory evacuation of one million coastal residents while neighboring North Carolina ordered an evacuation of the Outer Banks, barrier islands that are a popular tourist destination, and parts of coastal Dare County. Governors of those states have already declared states of emergency, as have the governors of Virginia and Maryland.
Florence weakened slightly to a Category 3 storm on a five-step scale but had maximum sustained winds of 125 miles per hour (201 km per hour) as of 2 p.m. EDT (1800 GMT), down from 130 mph earlier in the day.
Areas along the coast from Cape Fear to Cape Lookout, NC, including the Neuse, Pamlico, Pungo and Bay rivers may experience storm surges from 9 to 13 feet.
In Columbia, South Carolina, Barry Sparks, a 66-year-old retiree, was thinking of getting out after the path of the storm shifted somewhat to the south.
On Wednesday, McMaster said that emergency lane reversals - using all lanes of I-26 out of Charleston and Highway 501 near Myrtle Beach for westbound traffic - will cease on Thursday, two days after the measure was instituted.
Warm weather returns to Montreal - hurricane Florence expected to strengthen
The storm was getting bigger and better organized and is expected to continue to strengthen for the next day or so, the NHC said. Freight haulers need to be prepared for many washed out roadways and damage to rail infrastructure across this multi-state area.
People in areas vulnerable to a unsafe hurricane have left or are fleeing ahead of the storm's expected Friday or Saturday landfall.
Emergency preparations in SC now include more than 2,100 of the state's National Guard soldiers. Three college football games with Top 25 teams have been called off: East Carolina at No. 11 Virginia Tech, No. 15 West Virginia at NC State and No. 18 Central Florida at North Carolina.
Satellites orbiting Earth have also been keeping tabs on Florence from above.
"Rather than a very narrow and intense band of winds, the winds are slightly weaker - but [they] cover a much larger area", NHC senior hurricane specialist Stacy Stewart said on Wednesday.
Predictions now have the storm stalling and making a slight southward turn after it makes landfall.
Florence is one of the strongest hurricanes to hit the eastern coast in decades.
The intensity of the wind and rain we'll see here is dependent on the exact track of the storm.
"Even if you've ridden out storms before, this one is different. You can't argue with mother nature".