Navy ships off Virginia's coast are getting set to sail out of the path of the powerful hurricane, one North Carolina university has canceled classes and people have begun stocking up on plywood, bottled water and other supplies even with Florence still far out at sea southeast of Bermuda.

Florence crossed the 74 mph threshold from tropical storm to a hurricane Sunday morning, and by evening its winds were up to 85 mph (140 kph) as the National Hurricane Center warned a hurricane hunter plane found the storm strengthening quickly. Forecasters said Florence was expected to become an extremely dangerous major hurricane sometime Monday and remain that way for days.

As of 11 p.m. EDT, Florence was centered about 685 miles (1,100 kilometers) southeast of Bermuda, moving west at 7 mph (11 kmh). Its maximum sustained winds are at 90 mph (150 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 Miami-based center said.

Forecasters said it is too early to know the exact path the storm will take but warned that it could roll ashore in the Carolinas by Thursday. But authorities were taking no chances as the governors of North and South Carolina and Virginia declared states of emergency far ahead of the approaching storm.

People up and down the densely populated coast were told to be ready for the worst — and not just for a possible direct blow against the coast. They warned that Florence could slow or stall on or near shore, with some forecasting models showing it could unload a foot or two of rain in places, causing devastating inland flooding. Forecasters also warned of a rising threat of life-threatening storm surge, along with the damage of a hurricane's high winds.

Red flags flying on beaches warned swimmers to stay out of waters as seas began kicking up. People rushed to get emergency kits ready, map out escape routes and fill sandbags and secure their homes.

"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 was 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.

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.

As Hurricane Florence approaches the East Coast, the University of North Carolina at Wilmington canceled classes and encouraged its students to leave campus for a safer location.

In southeast Virginia, Naval Station Norfolk told its employees they should not leave their vehicles parked at the sprawling base in coming days because of the flood threat. 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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Bynum reported from Savannah, Georgia. AP writers Sarah Rankin in Richmond, Virginia, and Gary Fineout in Tallahassee, Florida, contributed to this story.