On Friday, the storm's center will hug the coast of southeast North Carolina, where the most severe effects are expected.
The NHC said the threat of tornadoes was increasing as Florence neared shore and South Carolina Governor Henry McMaster said the heavy rain could trigger landslides in the west of his state. Hurricane-force winds extended 130 kilometres from its centre, and tropical-storm-force winds up to 315 kilometres.
About150 people were awaiting rescue in New Bern, N.C. early Friday morning, and multiple news outlets report some 60 people had to be evacuated from a motel in Jacksonville, N.C. overnight after it sustained structural damage as the Category 1 hurricane made landfall.
Officials in several states have declared states of emergency, including in the Carolinas, Georgia, Virginia and Maryland, where coastal areas are still recovering from summer storms.
The storm remained a Category 2 hurricane with maximum sustained winds of 100 miles per hour.More news: Paul Manafort Agrees to Plead Guilty in Mueller Probe
But one innovative weather report has been praised for showing exactlyhow high water levels could get thanks to the storm. He said parts of North Carolina had seen storm surges - the bulge of seawater pushed ashore by the hurricane - as high as 10 feet.
National Weather Service forecaster Brandon Locklear predicted Florence would drop up to eight months of rain in two or three days. The storm is threatening catastrophic flooding while leaving scores in need of rescue from rising waters and hundreds of thousands without power.
For people living inland in the Carolinas, the moment of maximum peril from flash flooding could arrive days later, because it takes time for rainwater to drain into rivers and for those streams to crest.
Authorities warned, too, of the threat of mudslides and the risk of environmental havoc from floodwaters washing over industrial waste sites and hog farms.
According to poweroutage.us, more than 500,000 customers in North Carolina and 50,000 in SC were without power on Friday morning, though the Weather Channel estimates up to 3 million customers could lose power in the Carolinas due to Florence. The few cars out on a main street in Wilmington had to swerve to avoid fallen trees, metal debris and power lines.
Wilmington Police reported on Twitter that a tree fell on a house on Mercer Avenue, killing the pair around 9:30 a.m.More news: Florence isn’t alone. More tropical storms are brewing
The Wilmington airport had a wind gust clocked at 105 miles per hour (169 kph), the highest since Hurricane Helene in 1958, the weather service said.
And about 46 miles farther up the waterfront, in New Bern, about 150 people were waiting to be rescued from floods on the Neuse River, WXII-TV reported.
Sheets of rain splattered against windows of a hotel before daybreak in Wilmington, where Sandie Orsa of Wilmington sat in a lobby lit by emergency lights after the electricity went out. "Trees are blowing down in the wind".
Florence, which is now a slow-moving Category 2 storm, is expected to linger over the Carolinas before turning north and sliding into western Virginia.
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. "Persons located within these areas should take all necessary actions to protect life and property from rising water and the potential for other unsafe conditions", the hurricane center briefing said.More news: Wind approaches outer banks of North Carolina