In addition to inundating the coast with wind-driven storm surges of seawater as high as 13 feet (4 meters) along the Carolina coast, Florence could dump 20 to 30 inches (51-76 cm) of rain, with up to 40 inches in parts of North Carolina, the NHC said.

Downpours and flooding would be especially severe, lasting for days, if the storm stalls over land. Heavy rains were forecast to extend into the Appalachians, affecting parts of Alabama, Tennessee, Kentucky and West Virginia.

Tens of thousands of homes and businesses could be flooded in North Carolina alone, Governor Cooper warned.

Georgia Governor Nathan Deal, concerned the storm would bring devastation south, issued an emergency declaration for all 159 counties in his state. Similar declarations were made earlier in North and South Carolina, Virginia, Maryland and the District of Columbia.

More than 1 million have been ordered to evacuate the coastlines of the Carolinas and Virginia. Authorities in Chatham County, Georgia, which borders South Carolina and includes the historic port city of Savannah, urged residents who feel unsafe "to evacuate as they see fit."