More than a million people were ordered Monday to evacuate the path of Hurricane Florence as the powerful Category 4 storm packing winds of 140 miles (220 kilometers) per hour bore down on the East Coast of the United States.

"This is one of the worst storms to hit the East Coast in many years," President Donald Trump warned on Twitter. "Please be prepared, be careful and be SAFE!"

South Carolina Governor Henry McMaster ordered as many as one million residents of the state's eastern coast to leave their homes ahead of the storm's possible arrival on Thursday. Schools in 26 of the state's 46 counties were to close from Tuesday.

The governor of neighboring North Carolina ordered an evacuation of the Outer Banks, barrier islands that are a popular tourist destination, and parts of coastal Dare County, while a state of emergency was declared in Virginia.

"This is a very dangerous hurricane," McMaster said, adding that the evacuation order for coastal counties was "mandatory, not voluntary."

"We do not want to risk one South Carolina life in this hurricane," the governor told reporters. "We're liable to have a whole lot of flooding."

Hurricane Florence has the potential to bring catastrophic flooding to areas of the eastern United States already soaked by heavy rain and may be the strongest storm to hit the region in decades.

A Category 4 on the five-level Saffir-Simpson hurricane wind scale, Florence was 525 miles south-southeast of Bermuda and the center of the hurricane was forecast to pass between Bermuda and the Bahamas on Tuesday and Wednesday, the National Hurricane Center (NHC) said in its 5:00 pm (2100 GMT) advisory.

Florence is currently moving west at around 13 miles per hour.

"Florence has continued to rapidly strengthen," the NHC said. "Florence is expected to be an extremely dangerous major hurricane through Thursday."

Storm surge and hurricane watches 

At a hardware store in downtown Charleston, South Carolina, store manager John Johnson said the rush on batteries, flashlights, plastic tarps and sandbags began Friday.

"From eight o'clock 'til two we were slammed," said Johnson, who sold scores of bags of sand over the weekend, saving just a few to barricade the store's own doors.

"We were nonstop."

Nurse Barbara Mack was using a small shovel to fill sandbags at a public works facility in Charleston -- but she saw a silver lining in the hurricane preparations.

"This is good exercise," she quipped. "This is probably the only exercise I get this week."

Also out for sandbags was Deborah LaRoche. Half her supply was going to barricade a basement soup kitchen she managed, she said, and the other half was going to protect her own home on nearby Johns Island.

She and her husband would decide Tuesday morning whether to evacuate their family of two kids and a dog, said LaRoche. Having grown up in storm-prone Florida, LaRoche said she's careful not to underestimate any hurricane.

"It doesn't matter what happened in (previous) storms," said LaRoche, a social services director. "This one is different."

Storm surge and hurricane watches may be issued early Tuesday for portions of southeastern US states, the NHC said.

On its current track, Florence is expected to slam the Carolinas and Virginia the hardest.

"Don't concentrate on the exact forecast track of Hurricane #Florence. Significant effects will extend outside the cone, and will arrive at the coast sooner than the eye," the National Weather Service warned.

Virginia Governor Ralph Northam's office predicted "catastrophic inland flooding, high winds and possible widespread power outages," cautioning that the deadliest risk would come from flooding.

The US Navy said it was preparing to send about 30 ships stationed in Virginia out to sea.

The vessels will get underway from Naval Station Norfolk and Joint Expeditionary Base Little Creek to avoid potential damage from winds and tidal surges, said Colonel Rob Manning, a Pentagon spokesman.

Heavy rain in the Washington area over the weekend has already led to flooding in historic Alexandria, Virginia, and the National Weather Service issued a flood watch for part of the Potomac River.

Two more hurricanes

North Carolina Governor Roy Cooper's office said Florence is already being felt along the state's coast, with large sea swells resulting in life-threatening rip currents and surf.

"This is a huge storm," said Robert Woodward, chairman of the Dare County Board of Commissioners, predicting 15 to 20 inches (38 to 50 centimeters) of rain.

"Never have we seen quite this type of a storm approach us."

At this height of the Atlantic hurricane season, Florence was being trailed on east-to-west paths by two other hurricanes, Helene and Isaac.

Helene -- 475 miles west of the Cape Verde islands off the African coast -- had winds up to 105 miles per hour, and was expected to continue moving west-northwest for another couple of days, the NHC said.

Hurricane Isaac -- which late Sunday became the fifth hurricane of the season -- is heading west toward the Caribbean.

Isaac, which the NHC called a small hurricane, was about 1,090 miles east of the Lesser Antilles -- a region still recovering from last year's powerful Hurricane Maria -- with maximum sustained winds of 75 miles per hour.