A rapidly strengthening Hurricane Florence chugged across the Atlantic on Sunday toward a possible direct hit on the U.S. Southeast late this week, triggering warnings to people up and down the coast to get their emergency kits ready, map out escape routes and fill sandbags.

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. People rushed to buy bottled water, plywood and other supplies.

Florence crossed the 74 mph threshold from tropical storm to hurricane Sunday morning, and by evening its winds were up to 85 mph (140 kmh). As of 5 p.m., it was centered about 720 miles (1,160 kilometers) southeast of Bermuda, moving west at 7 mph (11 kmh).

Drawing energy from the warm water, it could be a fearsome Category 4 with winds of 130 mph (209 kph) or more by Tuesday, the National Hurricane Center said.

The Miami-based hurricane center 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.

"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," South Carolina Gov. Henry McMaster said. 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.

Myrtle Beach hardware stores and supermarkets were busy ringing up sales of bottled water, plywood and generators.

"Literally, they are filling buggies full of water, shopping carts full of water," Ryan Deeck, grocery department manager at a Walmart, told The Sun News. "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 Southeast, people were urged to put together emergency supply kits, prepare their homes and research evacuation routes.

Florence's effects were already being felt along the coast, with dangerous swells and rip currents in some spots. On North Carolina's Outer Banks, the town of Nags Head posted no-swimming flags on beaches.

In Jacksonville, North Carolina, about 20 miles (32 kilometers) from the coast, some residents picked up hurricane supplies during their normal weekend shopping, The Daily News reported. Ilija Cesal told the newspaper he wouldn't worry about buying extra water or other supplies for a few more days.

"I'll see by Wednesday how that goes — we got over 48 hours before that happens," Cesal said.

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.

The governors of North and South Carolina and Virginia declared states of emergency far ahead of the storm to get ready.

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