The storm's maximum sustained winds were clocked at 175kmph late on Wednesday, down from a peak of 225kmph a day earlier, before it was downgraded to a Category 3 and then a Category 2.
The storm was about 235 miles east-southeast of Wilmington, North Carolina and was moving at about 17 mph on Thursday morning.
Florence's nighttime winds were down to 115 miles per hour (185 kph) from a high of 140 miles per hour (225 kph), and the Category 4 storm fell to a Category 3, with a further slow weakening expected as the storm nears the coast. But forecasters say there's an increased chance for tropical storm winds to reach Savannah.
An estimated 10 million people live in areas expected to be placed under a hurricane or storm advisory, said Marc Chenard of the US Weather Prediction Center.
"You put your life at risk by staying", North Carolina Gov. Roy Cooper said. Executives in North and South Carolina, Virginia, Maryland and the District of Columbia declared emergencies earlier in the week.
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When the National Hurricane Updates shared a map of the category four hurricane, dubbed "the storm of a lifetime", on Facebook, people were quick to compare the pink round-headed shape displaying the devastation to something a lot less serious.
About 1.7 million people have already been told to leave their homes in the latter three states, but some who thought they were safely out of the path of the storm are now desperately working to board up their homes and businesses before its arrival.
He said electric power could be out for weeks. "It's going to destroy homes", said Jeff Byard, an official at the Federal Emergency Management Agency. You're going to be displaced from your home in the coastal areas.
If the European model is true or the overall trend persists, University of Miami hurricane researcher Brian McNoldy said it "is exceptionally bad news, as it smears a landfall out over hundreds of miles of coastline, most notably the storm surge".
North Carolina's governor issued what he called a first-of-its-kind mandatory evacuation order for North Carolina's fragile barrier islands from one end of the coast to the other. And if local officials recommend evacuation, evacuate immediately. The shopping list for people who have made a decision to ride out the storm at home: plywood to board up their windows, sandbags, bilge pumps, generators, trash bags, potato chips, bottled water and wine.
"Watch out, America! #HurricaneFlorence is so enormous, we could only capture her with a super wide-angle lens from the @Space_Station, 400 km directly above the eye", German astronaut Alexander Gerst tweeted.