Even though the storm has began to weaken, Florence is still forecast to be an “extremely dangerous major hurricane” when it nears the US south-east coast late on Thursday and Friday.
The hurricane is about 470 miles (755km) east-southeast of Myrtle beach, South Carolina with maximum sustained winds of 125mph (205km/h).
In an update issued at 2pm EDT (7pm BST) the NHC said Florence’s peak winds have decreased slightly but the wind field has increased.
Federal Emergency Management Agency associate administrator Jeff Byard said: “This is not going to be a glancing blow. This is going to be a Mike Tyson punch to the Carolina coast.
“Today is the last good day to evacuate.”
A storm surge warning is in effect for South Santee River, South Carolina to Duck, North Carolina, Albemarle and Pamlico Sounds, including the Neuse and Pamlico Rivers.
The Carolinas and Virginia will be smashed with the life-threatening storm surge, catastrophic winds and inundating rain.
Latest forecasts show the storm lingering near the coast, bringing days of heavy that could bring intense inland flooding from South Carolina to Virginia – parts of North Carolina could get 40 inches (1 metre).
More than 1.7 million people have been ordered to evacuate the coastline of the three states, while schools and factories were being shuttered.
However some residents have decided to board up their homes and ride out the storm.
The NHC said the first tropical storm-force winds of at least 39 mph (63kph) would hit the region early on Thursday with the storm’s centre reaching the coast on Friday.
Forecast models show the storm slowing to a crawl off the North Carolina coast late on Thursday or Friday and make a southern turn.
This means some coastal areas will get damaging hurricane-force winds for more than 24 hours.
A NHS forecaster said: “This will likely be the storm of a lifetime for portions of the Carolina coast.
“And that’s saying a lot given the impacts we’ve seen from Hurricanes Diana, Hugo, Fran, Bonnie, Floyd and Matthew.
“I can’t emphasise enough the potential for unbelievable damage from wind, storm surge and inland flooding with this storm.”
President Donald Trump has urged residents to heed the official evacuation warnings.
Mr Trump wrote on Twitter: “Get out of its way, don’t play games with it. It’s a big one, maybe as big was they’ve seen, and tremendous amounts of water.
“Bad things can happen when you are talking about a storm this size. It’s called Mother Nature. You never know, but we know. We love you all, we want you safe.”
Emergency preparations in the area included activating 2,800 National Guard troops in North Carolina, stockpiling food, setting up shelters, switching traffic patterns so that major roads led away from shore and securing 16 nuclear reactors in the three-state region.