Florence strengthens 'rapidly,' now powerful Category 4 hurricane

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

Florence was 575 miles (925 kilometers) south-southeast of Bermuda and the center of the hurricane was forecast to pass between Bermuda and the Bahamas on Tuesday and Wednesday, the NHC said.

It said Florence, a Category 4 storm on the five-level Saffir-Simpson hurricane scale, could approach the coast of South Carolina or North Carolina on Thursday.

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

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.

On its current track, Florence is expected to slam the Carolinas and Virginia the hardest -- and the three states issued emergency declarations to speed preparations.

Virginia Governor Ralph Northam's office described Florence as possibly the state's "most significant hurricane event in decades," and warned of "catastrophic inland flooding, high winds and possible widespread power outages."

"The largest threat to life from hurricanes is not the high winds," it said. "Flooding is the deadliest result of these storms."

The US Navy ordered ships at its major base in Hampton Roads, Virginia, to put to sea, saying "the forecasted destructive winds and tidal surge are too great to keep the ships in port."

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

Florence is currently moving west at around 13 mph (20 kph), and is forecast to drench a large swath of the US East Coast running from northern Florida to New Jersey.

- 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.

"Everyone in North Carolina needs to keep a close eye on Florence and take steps now to get ready for impacts later this week," Cooper said.

A mandatory evacuation was ordered for residents of Hatteras Island and some other areas of Dare County in North Carolina.

The storm "is too powerful and its path is too uncertain to take any chances," South Carolina Governor Henry McMaster said, issuing his state's emergency declaration.

Florence was producing large swells expected to reach from the northern Caribbean to the southern coasts of Canada's Maritime provinces.

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 -- 375 miles (600 kms) west of the Cape Verde islands off the African coast -- had winds up to 105 mph (165 kph), and was expected to continue moving west-northwest for another couple of days, the NHC said in its 1500 GMT bulletin.

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

At 1500 GMT, Isaac, which the NHC called a small hurricane, was about 1,150 miles (1,855 kms) east of the Windward Islands -- a region still recovering from last year's powerful Hurricane Maria -- with maximum sustained winds near 75 mph (120 kph).

Isaac is expected to gain strength in the next day or two before beginning to weaken mid-week when it approaches the Lesser Antilles.

Maria -- which killed at least 3,057 people, most in Puerto Rico -- is believed to be the third-costliest tropical cyclone on record.