The hurricane was gaining strength as it traveled over warm Atlantic waters, about 1,240 miles (2,000 km) east-southwest of Cape Fear, North Carolina, the National Hurricane Center said in an advisory.
Forecasters say Florence will pass between Bermuda and the Bahamas on Tuesday and Wednesday and start its approach to the USA on Thursday morning. "We are taking Hurricane Florence seriously".
The first effects were already being seen on barrier islands Monday as risky rip currents and seawater flowed over the state highway.
"Florence is expected to be an extremely risky major hurricane through Thursday (Sept. 13)", said officials. "Damaging hurricane-force winds are likely along portions of the coasts of SC and North Carolina, and a Hurricane Watch will likely be issued by Tuesday morning".
The hurricane is projected to make a direct hit on the East Coast, somewhere along the coast of North Carolina, South Carolina or Virginia.
The Miami-based National Hurricane Centre (NHC) said that Isaac, packing winds of 75 miles per hour (mph) located near latitude 14.7 north, longitude 43.9 West and moving towards the west near 14 mph.
The other storms Tropical Storm Isaac and Tropical Storm Helene are both expected to become hurricanes Sunday or Monday.More news: Tropical storm Florence likely to become a hurricane, forecasters say
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"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.
Mandatory evacuations have begun along North Carolina's coast.
It warned the storm would be "an extremely risky major hurricane" by the time it made landfall, forecast in the Carolinas on Thursday.
"We are just going to do everything we can here to get everything secure as possible and then give the rest to God and see what happens", one resident said. It strengthened rapidly last week before it was weakened by wind shear. Any Category 3, 4, or 5 hurricane is classified as a major hurricane.
Further to the east, forecasters are also watching hurricanes Isaac and Helene.
"The Storms in the Atlantic are very risky", Trump tweeted.
Southeastern North Carolina remains in the middle of the storm's "cone of uncertainty", with the cone extending from roughly Hilton Head, S.C., in the south to the Chesapeake Bay in the north.