At 2300 EDT (0300 GMT), the NHC said Florence had maximum sustained winds of 40 miles per hour (65 km), and was slowly drifting westward over SC.
Windows and doors are boarded up in anticipation of the arrival of Hurricane Florence September 12, 2018 in Morehead City, North Carolina.
Speaking at a news conference on Wednesday, he said: Disaster is at the doorstep and it's coming in.
Now, while the US East Coast gets prepared for the Category 3 storm (earlier downgraded from four), Hurricane Florence satellite images revealed that two more storms are swirling above the Atlantic Ocean, forming a rare phenomenon.
Some of the weather models predict that it may stall and sit and spin for a couple of days, just as Hurricane Harvey did over Houston previous year. Heavy rains were forecast to extend into the Appalachians, affecting parts of Alabama, Tennessee, Kentucky and West Virginia.
"Don't bet your life on riding out a monster", stated Roy Cooper, North Carolina governor.
Emergency declarations were in force in Georgia, South and North Carolina, Virginia, Maryland and the District of Columbia.
With South Carolina's beach towns more in the bull's-eye because of the shifting forecast, OH vacationers Chris and Nicole Roland put off their departure from North Myrtle Beach to get the maximum amount of time on the sand.
The country's No. 2 power company said Wednesday that it's anticipating 1 million to 3 million homes and businesses could lose power for lengthy periods, depending on the storm's track. Spokesman Howard Fowler said restoration could take "weeks instead of days", even though 20,000 workers, including crews from outside the area, were standing by.
Storm surge watches and warnings are separated from hurricane alerts because hurricane-force winds and storm surges don't always occur at the same place or the same time, said Rick Knabb, the former director of the hurricane center, and now the hurricane expert at the Weather Channel.
Emergency preparations in the region included activating over 2,700 National Guard troops, stockpiling food, setting up shelters, switching traffic patterns so major roads led away from shore, and securing 16 nuclear power reactors in the Carolinas and Virginia.
For 62 years, Robert Faircloth has lived in Wilmington, North Carolina. The hurricane centre said Florence will approach the coast on Friday and linger for a while before rolling ashore.
He said: We are ready but this is going to be one of the biggest ones to ever hit our country.
"I've never seen the president and all these other people talk the way they're talking", he said while waiting for an evacuation bus. "But no matter how bad it's going to be, it will pass and our job will be to rebuild this community together, and that's what we're going to do".