“The waves and the wind this storm may bring is nothing like you’ve ever seen. Even if you’ve ridden out storms before, this one is different. Don’t bet your life on riding out a monster,” he said.

Forecasters said Florence was expected to blow ashore late Thursday or early Friday, then slow down and dump up to 60cm (23.6 inches) of rain. Flooding inland could wreak environmental havoc by washing over industrial waste sites and hog farms.

President Donald Trump declared states of emergency for North and South Carolina and Virginia, opening the way for federal aid. He said the federal government is “absolutely, totally prepared” for Florence.

“We got A Pluses for our recent hurricane work in Texas and Florida (and did an unappreciated great job in Puerto Rico, even though an inaccessible island with very poor electricity and a totally incompetent Mayor of San Juan). We are ready for the big one that is coming!”

The Carolinas and Virginia ordered mass evacuations along the coast. But getting out of harm’s way has proven difficult.

American and Southwest Airlines were among the carriers cancelling flights to and from the hurricane zone starting from Wednesday. Charleston International Airport in South Carolina tweeted that it expected to close runways by the end of the day.

Michelle Stober loaded up valuables at her home on Wrightsville Beach to drive back to her home in Cary, North Carolina. Finding fuel for the journey was tough.

“This morning I drove around for an hour looking for gas in Cary. Everyone was sold out,” she said.

Florence is so wide that a life-threatening storm surge was being pushed 485km (300 miles) before its eye, and so wet that a large area from South Carolina to Ohio and Pennsylvania could get deluged.

People across the region rushed to buy bottled water and other supplies, board up their homes, pull their boats out of the water and get out of town.

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Long queues formed at service stations, and some started running out of petrol as far west as Raleigh, with bright yellow bags, signs or rags placed over the pumps to show they were out of order. Some store shelves were picked clean.

“There’s no water. There’s no juices. There’s no canned goods,” Kristin Harrington said as she shopped at a Walmart in Wilmington.

People were not the only ones evacuating. Eight dogs and 18 cats from a shelter in Norfolk, Virginia, were sent to two shelters in Washington to make room for pets expected to be displaced by the hurricane.

Florence is the most dangerous of three tropical systems in the Atlantic. Tropical Storm Isaac was east of the Lesser Antilles and expected to pass south of Puerto Rico, Hispaniola and Cuba, while Hurricane Helene was moving northward away from land. Forecasters also were tracking two other disturbances.

The coastal surge from Florence could leave the eastern tip of North Carolina under more than 2.75 metres (nine feet) of water in spots, projections showed.

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“This one really scares me,” National Hurricane Centre Director Ken Graham said.

Federal officials begged residents to put together emergency kits and have a plan on where to go.

“This storm is going to knock out power days into weeks. It’s going to destroy infrastructure. It’s going to destroy homes,” said Jeff Byard, an official at the Federal Emergency Management Agency.

Florence’s projected path includes half a dozen nuclear power plants, pits holding coal-ash and other industrial waste, and numerous hog farms that store animal waste in huge lagoons.

Duke Energy spokesman Ryan Mosier said operators would begin shutting down nuclear plants at least two hours before hurricane-force winds arrive.

North Carolina’s governor issued what he called a first-of-its-kind mandatory evacuation order for all of North Carolina’s fragile barrier islands. Typically, local governments in the state make the call on evacuations.

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“We’ve seen hurricanes before,” Cooper said, “but this one is different.”

But 65-year-old Liz Browning Fox plans to ride the storm out in the Outer Banks village of Buxton, North Carolina, despite a mandatory evacuation order. Her 88-year-old mother refused to leave and will stay with her.

“Everyone who is staying here is either a real old-timer, someone who doesn’t know where would be better, or someone involved in emergency operations one way or another,” said Fox.