The center was located about 325 miles (520 km) east-southeast of Myrtle Beach, South Carolina.
Forecasters said conditions will continue to deteriorate as the storm pushes ashore early Friday near the North Carolina-South Carolina line and makes its way slowly inland.
Storm surge flooding does not include floods caused by the heavy rain from the hurricane, such as what happened past year in Houston with Hurricane Harvey and in North Carolina in 2016 during Hurricane Matthew.
The National Hurricane Center said Florence was "forecast to be an extremely risky major hurricane when it nears the USA coast late Thursday and Friday".
"This is going to be a Mike Tyson punch to the Carolina coast", said Jeff Byard, associate administrator for response and recovery at the Federal Emergency Management Agency.
SC ordered the mandatory evacuation of one million coastal residents while North Carolina ordered an evacuation of the Outer Banks, barrier islands that are a popular tourist destination.
The mayor said at least 4,200 homes and 300 businesses sustained damage from flooding. "So, again, the time to act is now".
Apple unveils iPhone XS Max, largest iPhone ever made
In terms of battery , Apple reckons that it can last up to 1.5 hours longer than the iPhone 8 Plus . This model will be available in 64GB, 128GB, and 256GB options starting at $749 Dollars .
Overall, risky storm surge is possible all the way from Edisto Beach, South Carolina, to the North Carolina/Virginia border.
Helping with rescues there were members of the so-called Cajun Navy, a group of Louisiana-based volunteers who became famous during last year's Hurricane Harvey, locals said on Twitter.
By 1:50 pm (1750 GMT) the winds had dropped to 75 miles per hour (120 kph) and the center was moving west at a 6 miles per hour (10 kph), the NHC said, and parts of North and SC would get as much as 40 inches of rain (1 meter).
Weak steering current will allow the hurricane to drift westward along the SC coast and inland, and center over the state's interior Sunday morning, the NHC projected.
President Donald Trump said his administration was "as ready as anybody has ever been" to respond to the storm. "We'll handle it. We're ready, we're able".
"We have a situation that has developed here, at the station, and that is that the water is getting close to the building, that the building has been evacuated", Cox said. "We're fully prepared. Food, medical, everything you can imagine, we are ready". Don't play games with it.
About 10 million people could be affected by the storm, and more than one million were ordered to evacuate the coasts of the Carolinas and Virginia. They have faced gas shortages at some stations, while those who have chosen to stay put are clearing store shelves of emergency supplies.
The likelihood that Florence will pound the coast for days not hours underscored flooding risks from both storm surge and rainfall measured in feet. In Craven County, authorities say they received more than 150 telephone calls to rescue people in the historic town of New Bern because water had entered their homes. She says it had not run out at any of its 27 locations in Georgia, the Carolinas and Virginia.