The storm was expected to move across parts of southeastern North Carolina and eastern SC on Friday and Saturday, then head north over the western Carolinas and central Appalachian Mountains early next week, the NHC said.
On Friday afternoon, the US National Hurricane Centre warned that the weather system would bring life-threatening storm surges and catastrophic freshwater flooding.
On the other side of the world, meanwhile, more than a dozen people have died as Typhoon Mangkhut rips through the Philippines.
Serious flooding is expected throughout the two states - and some rivers may not crest for another three to five days.
"This is not the end of it", said Jeff Byard, associate administrator for response and recovery at FEMA.
The National Hurricane Center's forecast for Florence as of 11am Saturday.
Social media users mocked the reporter and claimed he was exaggerating the storm.
Parts of North and SC can expect an additional 10 to 15 inches. That should spell some relief for the inundated areas of Onslow County, but unfortunately puts the city of Wilmington, southeast coastal communities of North Carolina and northeast areas of SC in the line of fire instead.
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Florence will continue to move very slowly to the southwest on Saturday, which in turn will drag the axis of heavy rain bands to the southwest as well.
Wildlife also found itself directly affected by Florence's floodwaters, including these deer seen swimming through neck-deep water.
As it moves near OH and West Virginia, it will become a remnant low.
In North Carolina, fears of what could be the worst flooding in the state's history led officials to order tens of thousands to evacuate, though it wasn't clear how many had fled or even could.
Florence peaked at a terrifying Category 4 with top winds of 140 mph (225 kph) over warm ocean water before making landfall as a Category 1 hurricane at 7:15 a.m.at Wrightsville Beach, a few miles (kilometres) east of Wilmington and not far from the SC line. By the time it made landfall at 7:15 a.m., the storm was whipping winds at 90 miles per hour.
Authorities fear the death toll will go higher as the tropical storm crawls westwards across SC.
In North Carolina, a mother and infant died when a tree fell on their house, a 78-year-old man was electrocuted while plugging in a generator in the rain, and a 77-year-old man died after being blown to the ground while checking on his hunting dogs. Later, Trump tweeted his condolences, writing, "Deepest sympathies and warmth go out to the families and friends of the victims".
North Carolina Governor Roy Cooper said the hurricane was likely to "continue its violent grind for days" and described the severity of the downfalls as a "1,000 year event". "Be extremely careful and stay alert".
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Eduardo Munoz / Reuters Water from the Neuse river floods houses during the passing of Hurricane Florence in the town of New Bern, North Carolina.
New Bern has been one of the hardest hit areas by Florence, experiencing a 10-and-a-half-foot storm surge.
More than 100 people were rescued and up to 75 more were awaiting rescue on Saturday morning, according to a town official.
Water could reach as high as 5ft in some areas.
Roberts, the city spokeswoman, said Friday preliminary estimates show about 4,300 residences and 300 commercial buildings had been damaged.
Two people in Carteret County died due to the storm, officials said.
Victor Merlos was overjoyed to find a store open for business in Wilmington since he had about 20 relatives staying at his apartment, which still had power.
The White House says President Donald Trump has issued a disaster declaration for North Carolina and that will make federal money available to people in the counties of Beaufort, Brunswick, Carteret, Craven, New Hanover, Onslow, Pamlico and Pender.
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Supreme Court nominee Judge Brett Kavanaugh testifies before the Senate Judiciary Committee on September 6, 2018. Kavanaugh, from the moment Trump announced his nomination, has stressed he has supported and promoted women.
This post has been updated throughout.