The hurricane center said it was still too early to predict its exact path.
Tropical Storm Helene developed off the western coast of Africa late Friday, and it is slowly moving south and west of the Cabo Verde Islands.
Currently, Florence is moving to the west near 7 miles per hour with maximum sustained winds of 65 miles per hour. a general west to northwestward motion is expected to continue for the next several days with an increase in forward motion expected by mid-week.
Meteorologist Eric Holthaus warned on Twitter that it could become one of the worst storms in U.S. history.
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The South Carolina Emergency Management Division tweeted Sunday that officials there are "preparing for the possibility of a large-scale disaster".
The tropical storm could begin to batter the southeastern United States by late on Wednesday, with Florence possibly making landfall around the Carolinas on Thursday or Friday.
McMaster said he had asked President Donald Trump to declare a federal emergency in SC in anticipation of the storm's arrival.
Dave Loewenthal, a meteorologist with the National Weather Service's Wilmington office, said the storm's track is still uncertain.
Florence regained Hurricane strength this morning, and is considered a potential threat as it is expected to pass within 460 miles of the island in the next 72 hours. As Florence's path moves closer to the US coastline in the next week Florence is expected to regain category three hurricane status tonight.
And another storm, Tropical Storm Isaac spinning farther out in the Atlantic, could also become a hurricane by Monday as it gains strength, forecasters with the National Hurricane Center (NHC) in Miami said in an advisory early Sunday.
Isaac is the ninth named storm of the Atlantic hurricane season.
Officials in North Carolina were already bracing for impact.
Strong high pressure will develop north of Florence later this week, which should cause the storm to move northwest toward the Southeast coastline. As of Saturday morning the hurricane was 1280 miles east of Honolulu.
Florence could begin to batter parts of the US East Coast this weekend as the storm swirls across the Atlantic, according to forecasters at the National Hurricane Center.