Tropical storm Florence strengthened to a hurricane on Sunday.
Forecasters predict it will make landfall on the US east coast later this week.
Governors in Virginia, North Carolina, and South Carolina have declared states of emergency, warning residents to prepare for a dangerous storm.
The US Navy on Saturday told all its ships in the Hampton Roads, Virginia — home to more than a fifth of the fleet — to prepare for an order to take to sea to avoid storm damage.
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, 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 barreled 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.