Landfall is expected late Thursday or early Friday, and the National Hurricane Center fears the storm "will slow considerably or stall, leading to a prolonged and exceptionally heavy and risky rainfall event Friday-Sunday".
Adding to concerns, forecasters warned the larger and slow-moving storm could linger for days around the coast, leaving many without power and supplies.
A view of Hurricane Florence is shown churning in the Atlantic Ocean in a west, north-westerly direction heading for the eastern coastline of the United States.
The National Hurricane Center warned the threat of tornadoes was increasing as Florence neared shore and South Carolina Governor Henry McMaster said the heavy rains could trigger landslides in the western part of his state.
Because of its enormous size and power, the storm has been a recent - if not frightening - muse for astronaut photography. "Today the threat becomes a reality".
The National Hurricane Center said the storm's strength should continue through Thursday. As the storm moves inland it will find a relatively flat area for hundreds of miles.
The station reached 70 million viewers past year and averaged more than a million viewers during the week Hurricane Irma hit Puerto Rico, the Bahamas and Florida.
About 10 million people live in areas that have been under either hurricane or tropical storm warnings and watches. Flights from Fort Lauderdale to Greer, South Carolina, Richmond and Washington also are on schedule.
Around midday, Spanish moss blew sideways in the trees as the winds increased in Wilmington, and floating docks bounced atop swells at Morehead City.
Maryland's largest natural gas and electric company, BGE, said it was also preparing for the storm. "Because it's Mother Nature". Her team manages 32 wild horses living in the Rachel Carson Reserve near Beaufort, North Carolina. That's enough water to fill the Empire State Building almost 40,000 times.
Just as Hurricane Florence closes in on the Southeast, the area covered by hurricane-force winds has doubled - meaning far more people will get blasted with winds topping 73 miles per hour.
Body surfer Andrew Vanotteren, of Savannah, Ga., crashes into waves from Hurricane Florence, Wednesday, Sept., 12, 2018, on the south beach of Tybee Island, Ga. Along the coast, fewer homes have flood insurance than five years ago. She said a hurricane has a way of bringing everyone to the same level.
"It's going to coming roaring up to the coast Thursday night and say 'I'm not sure I really want to do this and I'll just take a tour of the coast and decide where I want to go inland, '" said Jeff Masters, meteorology director of the private Weather Underground. "Whether you have a house or not, when the storm comes it will bring everyone together".
Duke Energy Co. said Florence could knock out electricity to three-quarters of its 4 million customers in the Carolinas, and outages could last for weeks. Workers are being brought in from the Midwest and Florida to help in the storm's aftermath, it said.