HOLDEN BEACH, N.C. (Reuters) - More than 1 million people along the Carolina coast fled toward higher ground on Monday in a mass evacuation ordered three days before the expected arrival of Hurricane Florence, a Category 4 storm and the most powerful to menace the region in nearly three decades.

With maximum sustained winds of 140 miles per hour (220 kph), Florence was due to grow even stronger before making landfall on Thursday, mostly likely in southeastern North Carolina near the South Carolina border, the National Hurricane Center in Miami said.

Authorities also warned of life-threatening coastal storm surges and the potential for Florence to unleash prolonged torrential rains and widespread flooding across several states, especially if it lingers inland for several days.

“Florence is expected to be an extremely dangerous major hurricane through Thursday,” the NHC said in its latest bulletin. North Carolina Governor Roy Cooper told a news conference his state was in “the bull’s eye.”

Cooper and his counterparts in South Carolina, Virginia and Maryland all declared states of emergency.

Mindful of devastation wrought by a string of deadly U.S. hurricanes last year, jittery residents in the Carolinas began the rituals of disaster preparation - boarding up windows and stocking up on groceries, water and gasoline.

Classified as a Category 4 on the five-step Saffir-Simpson scale of hurricane strength, Florence stood as the most severe storm to threaten the U.S. mainland this year and the first of its magnitude to take aim at the Carolinas since 1989, when Hurricane Hugo barreled over Charleston, South Carolina.

As a precaution against Florence, South Carolina Governor Henry McMaster ordered an estimated 1 million coastal residents in his state to move inland. Mandatory evacuation orders also were issued for more than 50,000 people from Hatteras and Ocracoke, the southernmost of North Carolina’s Outer Banks barrier islands.

At least 250,000 more were due to be evacuated from the northern Outer Banks on Tuesday, along with some coastal Virginia residents.

Emergency management officials reported waves already starting to crash over Hatteras Island’s main highway where dunes were breached by heavy surf, slowing traffic to a crawl.

“Traffic was backed up for about 9 miles (14 km) in places,” said Paul Jones, a retiree who owns a home on the island.

U.S. President Donald Trump, whose administration was criticized as being slow to respond to Hurricane Maria in Puerto Rico last year, canceled a political rally planned for Friday in Jackson, Mississippi, over safety concerns related to Florence, his campaign said.

Trump said in a Twitter message he has spoken to the governors of North and South Carolina and Virginia, adding: “Federal government stands by, ready to assist.”

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‘GETTING FRANTIC’

In Holden Beach, North Carolina, in the storm’s path, longtime residents were busy securing their homes and possessions.

“It’s scary to all of us. We know we can’t play around with this,” said Jennifer Oosterwyk, who owns the Sugar Britches boutique on Holden Beach and lives in nearby Wilmington.

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A BP service station and mini-mart in Wilmington ran out of gasoline for about two hours on Monday as motorists rushed to fill up their tanks. “People are getting frantic,” assistant manager Nadine Schrock said, adding her shop also sold out of cases of bottled water.

Classes were canceled starting on Monday at the Wilmington campus of the University of North Carolina.

The U.S. military said it was sending an advance team to Raleigh, North Carolina, to coordinate with state and local authorities. It said about 750 military personnel would be designated to provide support.

The U.S. Navy said it was sending nearly 30 ships from coastal Virginia out of port to ride out the storm.

Other military personnel, however, could find themselves directly in harm’s way.

The largest U.S. Marine Corps base on the East Coast, Camp Lejeune and its extensive beachfront northeast of Wilmington, North Carolina, lie within the NHC’s forecast “cone” for possible landfall.

Coastal properties were especially vulnerable to possible flooding from the hurricane’s storm surge, which according to the California-based risk assessment service CoreLogic, will put more than 758,000 homes in the Carolinas and Virginia at risk.

NHC Director Ken Graham also warned of “staggering” amounts of rainfall that may extend hundreds of miles inland and cause flash flooding across the mid-Atlantic region.

Forecasts call for 10 to 15 inches (25-38 cm) of rain in the hardest-hit areas, possibly more if the storm stalls over land, as expected, Graham said.

Virginia’s emergency operations chief, Jeffrey Stern, told reporters that residents should brace for “something that no one in Virginia has experienced in their lifetimes.”