President Trump says the Federal Emergency Management Agency and first responders are poised to help - but he also warned people to get out of the storm's path.
Tropical storm-force winds extended 195 miles (315 kilometres) from Florence's centre, and hurricane-force winds reached out 70 miles (110 kilometres).
"Don't play games with it".
"This one really scares me", Graham said.
"Later in the period, the dynamical model spread increases but most of the guidance has continued its southward shift, and now take Florence southwestward near the coast of SC by day 4", the National Hurricane Center said Wednesday.
The threat has sparked a rush of evacuation efforts in SC and North Carolina, with more than 1 million people urged to get out of Florence's way. Airlines had cancelled almost 1,000 flights and counting.
A wind pattern that allows a storm to get strong and stay strong. The two hardware chains said they sent in a total of around 1,100 trucks.
"The concern is about people who are very young or very old, who don't have much money or access to transportation, who do not speak the predominant language or have access to information", Thomas Tighe, of Direct Relief, said.
"It certainly is a challenge forecasting precise impacts when its exact track won't be known until a day in advance", Landsea said.
Computer models of exactly what the storm might do varied, adding to the uncertainty.
South Carolina, western and northern North Carolina are expected to receive five to 10 inches and in isolated areas up to 20 inches.
For many of those under evacuation orders, getting out of harm's way has proved difficult, as airlines canceled flights and motorists had a hard time finding gas. "Hurricane Florence is looking even bigger than anticipated", Trump said.
"We've had our lessons".
With the change in the forecast, Georgia Gov. Nathan Deal issued an emergency declaration for the entire state to ease regulations on trucks hauling gasoline and relief supplies, and asked people to pray for those in Florence's path.
The coastal surge from Florence could leave the eastern tip of North Carolina under more than 9ft of water in spots, projections showed. "Get food, water. Medications they may need". "We've just never seen anything like this".
"We're expecting heavy rainfalls across the ACP footprint, so we're taking all necessary precautions to prepare for the storm", he said. The worst of the storm's effects will be felt over the weekend as Florence moves on through.
This contrasts with other storms, such as Harvey, which sat for days over Houston and dumped more than 51 inches of rain, killing 82 people past year.