North Carolina Governor Roy Cooper declared a state of emergency on Friday and urged residents to prepare for the storm's arrival.
The National Hurricane Center said Florence was at Category 4 strength and will produce extremely unsafe conditions for land in its path.
The storm's center was 720 miles (1,160 km) southeast of Bermuda at 5 p.m. EDT on Sunday, and on a track to pass between Bermuda and the Bahamas on Wednesday, the NHC said.
The National Hurricane Center said despite Florence eventual path, large swells will begin to affect the U.S. East Coast this weekend, resulting in life-threatening surfs and rip currents. In the Atlantic there are three named storms, with Florence likely posing a threat to the east coast later this week.
The storm's maximum sustained winds are 65 miles per hour (100 kph) and it is moving west at 14 miles per hour (22 kph). Florence is a 140-mph, major Category 4 storm, moving to the west-northwest at 13 mph, according to the National Hurricane Center's latest advisory.
"A big question on this track is how intense the storm remains over the Research Triangle area of North Carolina - Raleigh-Durham-Chapel Hill", Watson said. Isaac, which was moving toward the lesser Antilles in the Caribbean Sea, could bring heavy rain and become a hurricane overnight, CNN meteorologist Tom Sater said.
"Now is the time for your family also to prepare and stay tuned for more updates", South Carolina Gov. Henry McMaster said. It was moving west at 6 miles per hour.
1M Ordered to Evacuate as Florence Approaches
Behind Isaac, Helene has strengthened with maximum sustained winds estimated to be at 110 miles per hour . "It's right on target". Many newcomers have moved to the coast in the almost 19 years since the last strong hurricane - Floyd - threatened the area.
Officials in North Carolina were already preparing for impact.
The National Hurricane Center said Florence's maximum sustained winds on Friday night were estimated to be 60mph.
In addition, a Storm Surge Watch is in effect for Edisto Beach, South Carolina to South Santee River, South Carolina; and North of Duck, North Carolina to the North Carolina/Virginia border.
Two other storms were spinning in the Atlantic.
Hurricanes impacting the Carolinas isn't rare, but it has been some time since a major hurricane has made landfall in North Carolina.
This will offer a tracking map, interactive radar, United States radar, advisories, and other helpful tools as we head into the peak of hurricane season. "The last Atlantic hurricane to intensify as rapidly as far north as Florence's current location was Hurricane Humberto in 2007", he said.
The low pressure area that has formed into Depression Nine has moved very little during the past several hours.