Florence intensified into a hurricane on Sunday and was expected to strengthen rapidly into a major threat later this week as it churned across the mid-Atlantic with winds of 75 miles (120 kilometers) per hour with possible landfall on the U.S. East Coast at the end of the week.
Another storm, Tropical Storm Isaac spinning farther out in the Atlantic, could also become a hurricane later on Sunday as it gains strength, forecasters with the National Hurricane Center (NHC) in Miami said in an advisory on Sunday.
With an eye on Hurricane Florence, governors in Virginia, North Carolina and South Carolina have declared states of emergency, warning residents to prepare for a dangerous storm.
Forecasters at the NHC said Florence would continue to strengthen and "is expected to remain an extremely dangerous major hurricane through Thursday."
On its current track, the storm could make landfall around the Carolinas on Thursday or Friday.
The storm's center was 750 miles southeast of Bermuda at 11 a.m. EDT on Sunday. It will move between Bermuda and the Bahamas on Wednesday and Thursday, before approaching the southeastern U.S. coast, the NHC said in an advisory.
Swells generated by Florence were affecting Bermuda and starting to reach parts of the U.S. East Coast. They were likely to cause life-threatening surf and rip current conditions, according to the NHC.
Tropical Storm Isaac, with maximum sustained winds of 65 mph (100 km/hr) on Sunday, was about 1,500 miles east of the Windward Islands, forecasters said. It was expected to strengthen into a hurricane then weaken by the middle of the week.
A third storm, Helene, off the Cabo Verde Islands was expected to become a hurricane later on Sunday. It did not immediately pose a threat to land.