Tropical Storm Florence is expected to grow into an "extremely dangerous" hurricane by Monday as it approaches the US East Coast, the National Hurricane Center said, urging residents to make preparations.
"Florence is forecast to rapidly strengthen to a major hurricane by Monday, and is expected to remain an extremely dangerous major hurricane through Thursday," the NHC said in an advisory issued at 11:00am (1500 GMT) Sunday.
The states of North and South Carolina and Virginia have already issued emergency declarations to help speed preparations.
"This storm is too powerful and its path is too uncertain to take any chances," South Carolina Governor Henry McMaster said in issuing his state's emergency declaration.
The US navy has ordered ships at its Hampton Roads, Virginia base to put to sea, saying "the forecasted destructive winds and tidal surge are too great to keep the ships in port."
The NHC said the vast storm had maximum sustained winds of nearly 75 miles per hour (120 kph), reaching up to 125 miles from its center, as it tracked west-northwestward across the western Atlantic.
The storm was producing large swells expected to reach from the northern Caribbean to the southern coasts of Canada's Maritime provinces.
These swells, already affecting Bermuda, "are likely to cause life-threatening surf and rip current conditions," the NHC said.
Florence had been the first Category 4 hurricane of the Atlantic season, but weakened as it passed through cooler waters. But it is now entering a warmer zone, meteorologists said.
"Interests along the US East Coast, particularly Florida through North Carolina, should closely monitor the progress of Florence, ensure they have their hurricane plan in place and follow any advice given by local officials," the NHC said.