Forecasters have warned that a “major hurricane” is expected to hit the US coast next week.
Tropical Storm Florence is becoming a “better organised” weather system that is likely to strengthen into a hurricane, weather forecasters said.
The storm is expected to approach the southeast US coast on Thursday.
Officials in North Carolina were already preparing for impact.
Governor Roy Cooper declared a state of emergency on Friday evening, urging residents to use the weekend to prepare for the possibility of a natural disaster.
"We are entering the peak of hurricane season and we know well the unpredictability and power of these storms," Mr Cooper said.
South Carolina's Emergency Management Division also advised coastal residents to start making contingency plans.
Swells generated by Florence could cause dangerous rip currents and coastal flooding Saturday in areas including coastal Delaware and New Jersey, the National Weather Service said.
Additional reporting by Associated Press
