While Florence was moving slowly toward the coasts of North Carolina and South Carolina as a Category 2 hurricane, federal and state officials issued final appeals to residents to get out of the way of the storm "once in a lifetime" .
"This storm will bring destruction," said North Carolina Governor Roy Cooper. "The catastrophic effects will be felt."
"It's getting late to evacuate," said South Carolina Governor Henry McMaster. "You have to go on and go now."
Federal emergency management officials warned that Florence, although weakening a bit, remains a "very dangerous storm" capable of wreaking havoc across a wide swathe of the east coast.
"Just because the wind speed went down, the intensity of this storm was reduced to a Cat 2, please do not let your guard down," said Brock Long, the administrator of the Federal Emergency Management Agency (FEMA).
Florence was downgraded to a Category 2 storm overnight on the five-tiered Saffir-Simpson wind scale, but is still packing hurricane force winds of 165 kilometers per hour, the National Hurricane Center (NHC) said.
"Small changes in the strength are expected before the eye of Florence reaches the coast, with a weakening expected after the center moves inland," said the Miami-based NHC.
Winds were already collecting along the coast on Thursday morning and some minor flooding was reported in the Outer Banks of North Carolina and in some coastal cities.
Myrtle Beach was practically deserted, with empty streets, boarded-up shops and very little traffic.
"I felt good until I got up this morning and this is a ghost town," said Kristin Beard, a 40-year-old Myrtle Beach saleswoman. "I'm going to Charlotte."
US television networks UU They said curfews had been established since 7:00 p.m. until 7:00 a.m. in several towns around Myrtle Beach.
Monster storm is expected
At 2:00 pm (1800 GMT), Florence was over the Atlantic Ocean about 145,200 kilometers east-southeast of Wilmington, North Carolina, and was moving northwest at 10 miles per hour, the NHC said.
Steve Goldstein, of the National Oceanic and Atmospheric Administration, said the Florence advance had slowed overnight and was not expected to land in the Carolinas until "some Friday afternoon, Friday or Saturday morning. "
He said hurricane force winds extend 80 miles from the center of the storm and tropical storm force winds extend nearly 200 miles.
A storm tide of between nine and 12 feet (2.7-3.6 meters) was expected along the North Carolina coast, Goldstein said, and some areas could receive up to 40 inches (one meter) of rain.
"This rain will produce catastrophic flash floods and prolonged significant river floods," the NHC said.
A tornado watch was also in effect for parts of North Carolina.
"This is a very dangerous storm," Long said of FEMA, urging people still in evacuation zones to obey orders to flee to safer terrain.
"Time is running out," he said.
Long said the danger was not only along the coast. "Inland floods could kill many people, unfortunately, and that's what we're about to see," he said.