Packing winds of up to 220 km/h (140 mph) late Monday, Florence was expected to further strengthen and become "an extremely risky major hurricane" by the time in makes landfall in the Carolinas on Thursday, the National Hurricane Center (NHC) said in a 21:00 UTC update.
The National Hurricane Centre said it was a potentially catastrophic Category-4 storm, but was expected to keep drawing energy from the warm water and intensify to near Category 5, which means winds of 157 miles per hour or higher.
"This storm is a monster".
US President Donald Trump said the "storm looks very bad" and urged people to take precautions.
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.
In neighboring Maryland, Governor Larry Hogan said his state was readying for potentially "historic and catastrophic rainfall, life-threatening flooding, and high winds". "It's a big one", he said. "The waves and wind this storm may bring is nothing like you've ever seen".
Some 38cm (15in) to 64cm (25in) of rain is forecast in some areas - with up to one metre (40in) at the centre of the storm.
The longer the storm stalls near the coast, the more precipitation it will be able to pull from the Atlantic and dump inland.
More than 5.4 million people live in areas under hurricane warnings or watches on the US East Coast, according to the NWS, and another four million people were under a tropical storm watch.
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