Florence intensified into a hurricane on Sunday and was expected to strengthen rapidly as it churned across the Atlantic Ocean and threatened to make landfall on the US East Coast by the end of the week.
With winds at 75 miles per hour (120km per hour), Florence became a Category 1 hurricane and was expected to develop into a major storm, defined as Category 3 or higher, according to the National Hurricane Center (NHC) in Miami.
"All indications are that Florence will be an extremely dangerous Category 4 hurricane while it moves over the western Atlantic toward the southeastern United States," the NHC said.
Forecasters were also tracking two more storms further out in the Atlantic, with Tropical Storm Isaac expected to become a hurricane later on Sunday as it barrelled toward the Caribbean.
US residents from South Carolina to Virginia were warned that Florence posed an increasing risk of life-threatening coastal storm surge, as well as flooding from heavy rainfall inland.
On its current trajectory, winds from the hurricane could reach the southeastern United States late on Wednesday, with Florence possibly making landfall around the Carolinas on Thursday or Friday.
The governors of Virginia, North Carolina and South Carolina have declared states of emergency, warning residents to prepare for a dangerous storm.
"Review your emergency plans and gather your supplies now," North Carolina Governor Roy Cooper said in a statement on Sunday.
The storm's centre was 750 miles (1,200km) southeast of Bermuda at 11am EDT on Sunday. It was on track to move between Bermuda and the Bahamas on Wednesday, the NHC said.
Swells generated by Florence already were impacting Bermuda and starting to reach the US coast, with life-threatening surf and rip current conditions likely, according to the NHC.
Meanwhile, Tropical Storm Isaac, with maximum sustained winds of 65 mph (100kph) on Sunday, was about 1,500 miles east of the Windward Islands, forecasters said. The forecast showed Isaac tracking south of Puerto Rico as it strengthened into a hurricane.
A third storm, Helene, off the Cabo Verde Islands was also expected to become a hurricane later on Sunday but did not appear to pose an immediate threat to land.