Florence crossed the 74mph threshold from tropical storm to hurricane on Sunday morning, and by evening its winds were up to 85mph.
Drawing energy from the warm water, it could be a fearsome Category 4 with winds of 130mph or more by Tuesday, the National Hurricane Center said.
People up and down the south east coast have been warned to get their emergency kits ready, map out escape routes and fill sandbags as the governors of North and South Carolina and Virginia declared states of emergency.
Red flags flying on beaches warned swimmers to stay out of waters already roiled by the distant storm, and cruise ships and Navy vessels were being steered out of harm's way.
The Miami-based hurricane agency said that it is too early to know what path the storm will take but that it could roll ashore in the Carolinas by Friday.
Forecasters urged residents from South Carolina to the mid-Atlantic to get ready and not just for a direct blow against the coast.
They warned that Florence could slow or stall after coming ashore, with some forecasting models showing it could unload a foot or two of rain in places, causing devastating inland flooding.
South Carolina Gov. Henry McMaster said: "Pretend, assume, presume that a major hurricane is going to hit right smack dab in the middle of South Carolina and is going to go way inshore."
The state's emergency management agency said it is "preparing for the possibility of a large-scale disaster."
In Charleston, South Carolina, along the coast, city officials offered sandbags to residents.
Myrtle Beach Mayor Brenda Bethune urged people to secure their homes but said it's too early to know if evacuations will be ordered.
Hardware stores and supermarkets were busy ringing up sales of bottled water, plywood and generators.
Ryan Deeck, grocery department manager at a Walmart, told The Sun News: "Literally, they are filling buggies full of water, shopping carts full of water.
"They're coming in and buying water and plates, and that's about all they're buying."
North Carolina officials started getting bulldozers and chain saws ready.
Across the south east, people were urged to put together emergency supply kits, prepare their homes and research evacuation routes.
The Navy planned to send ships from the Hampton Roads area of Virginia out to sea.
Florida-based Carnival Cruise Line re-routed its cruise ships.
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