The storm's centre was about 100 miles (160 km) southeast of Wilmington, North Carolina, at 3 p.m. EDT (1900 GMT) but already some 19,000 homes and businesses in the Carolinas and Virginia were without power by mid-afternoon.
Even with some weakening that is predicted just before it makes landfall, the storm "is expected to remain a risky major hurricane as it approaches the coastline", the hurricane center said. In a series of tweets Wednesday morning, Trump said the government "got A Pluses" for storm recovery in Texas and Florida and "did an unappreciated great job in Puerto Rico".
"These enormous waves are produced by being trapped along with very strong winds moving in the same direction [of] the storm's motion", the agency said.
The US Coast Guard said it closed the ports of Wilmington and Moorehead City in North Carolina and restricted port operations in Charleston, South Carolina.
"More significant weakening is forecast on Saturday as Florence moves farther inland over central SC", the NHC said on Thursday.
Federal emergency management officials warned that Florence - while weakening a bit - remains a "very risky storm" capable of wreaking havoc along a wide swathe of the East Coast.
LATER NEXT WEEK: The remnants of Hurricane Florence could bring some heavy rain to the D.C. region by next week.
"Think of it like a marathon", said Mark Robinson, from The Weather Network, also in Wilmington with Kourounis. "We're about to be in the thick of it".
Though Florence won't be making landfall until Friday, tropical-storm-force winds - 39 to 73 miles per hour (63 to 117 km/h) - will begin whipping through coastal regions as early as tomorrow, making it exceptionally risky to be outside, Graham said. Florence, the most ominous for United States residents, is expected to make landfall today. "If they're telling you to leave, you have to leave", Graham said.
In North Carolina, Florence is expected to dump up to 40 inches of rain. There is a chance some of that tropical rainfall could pass through portions of the Northeast next Tuesday or Wednesday before we can finally say good riddance to Florence for good.
"If you have not left now, you need to leave", McMaster said. Flash floods will be a major problem for some areas, and the overall flooding could be catastrophic.
The center of Florence is expected to hit North Carolina's southern coast Friday, then drift southwest before moving inland on Saturday.
Forecasters say the storm is likely to remain constant in strength, weakening only after the center of the storm moves inland.
The National Weather Service said 5.25 million people live in areas under hurricane warnings or watches, and 4.9 million live in places covered by tropical storm warnings or watches. "Significant river flooding is expected as a result of excessive rainfall across area hydrologic basins".
The impact of Florence will be widespread, with destructive winds, life-threatening storm surge, risky surf, torrential rainfall, flooding and the potential for tornadoes.
"This is a life-threatening situation".
Media captionWhy do people ignore hurricane warnings? Storm surge of 13 feet on top of a high tide at 7 feet could overwhelm Carolina Beach. On Thursday morning, South 17th Street, usually teeming with commuter traffic by 6:30 a.m., was almost devoid of cars.
"All the way from Arizona to ME, if you just trace a diagonal line on a map and take it right through Chicago, most areas under that line have clear skies right now", he said.
"We're on the wrong side of this thing", he said.