Forecasters say Florence, the first major hurricane to take direct aim at the USA mainland this year, could batter coastal areas with whipping winds, torrential rain and storm surges as high as 9 feet.
In addition to inundating the coast with wind-driven storm surges of seawater as high as 9 feet (2.7 meters) along the Carolina coast, Florence could dump 20 to 30 inches (51-76 cm) of rain, with up to 40 inches (1 metre) in parts of North Carolina, the NHC said.
"This is a very unsafe hurricane", Mr McMaster said, adding that the evacuation order for coastal counties was "mandatory, not voluntary".
But its slow march over the two states, crawling west at only 3 miles per hour (6 km per hour), is expected to leave large parts of the region deluged in the coming days, the National Hurricane Center in Miami said late Saturday.
"It's possible that the west coast of Wales and Cornwall could get some winds as well, but we will get a clearer picture of what to expect very soon".
Windows and doors are boarded up in anticipation of the arrival of Hurricane Florence September 12, 2018 in Morehead City, North Carolina.
Night-time winds were down to 110 miles per hour from a high of 140 miles per hour, and the Category 3 storm fell to a Category 2, with a further slow weakening expected as the storm nears the coast.
More than 1 million have been ordered to evacuate the coastlines of the Carolinas and Virginia.
According to the National Hurricane Center, the eye of the hurricane will likely not blow ashore until Friday afternoon local time, after which it will then push westward and bring with it the potential for catastrophic inland flooding.
"It's a very unsafe storm", Federal Emergency Management Authority administrator Jeff Byard said.
As of 11 p.m. EDT Wednesday, the storm was centered 260 miles (455 kilometers) east southeast of Wilmington, North Carolina, moving northwest at 17 mph (28 kph).
Duke Energy, the nation's No. 2 power company, said Florence could knock out electricity to three-quarters of its 4 million customers in the Carolinas, and outages could last for weeks.
A state of emergency has also been declared in Maryland, North Carolina, South Carolina, Virginia and Washington.
"It was pitch black and I was just scared out of my mind", said Tracy Singleton, who with her family later drove through torrential rain and high winds from her home near New Bern.
Nurse Barbara Mack was using a small shovel to fill sandbags at a public works facility in Charleston, but she saw a silver lining in the hurricane preparations. Despite its unpredictable path, it was forecast to make landfall near Cape Fear, North Carolina, at midday on Friday.