With maximum sustained winds of 140 miles per hour (220 kph), Florence was due to grow even stronger before making landfall on Thursday, mostly likely in southeastern North Carolina near the SC border, the National Hurricane Center in Miami said.
Residents in eight counties along the coast - Jasper, Beaufort, Colleton, Charleston, Dorchester, Berkeley, Georgetown and Horry - must begin evacuating by noon Tuesday, McMaster said at a news briefing.
James City County has declared a state of emergency in anticipation of effects from Hurricane Florence. The National Weather Service reported Sunday that more than 20 people had to be pulled from the ocean off Wrightsville Beach and Carolina Beach, The Wilmington Star reported.
The storm's potential path also includes half a dozen nuclear power plants, pits holding coal-ash and other industrial waste, and numerous pig farms that store animal waste in massive open-air lagoons.
"Pretend, presume, assume that a major hurricane is going to hit smack dab in the middle of SC and may go way inshore", McMaster warned.
Virginia Governor Ralph Northam's office predicted "catastrophic inland flooding, high winds and possible widespread power outages", cautioning that the deadliest risk would come from flooding.
McMaster said the state will reverse lanes on four main roads - so that all lanes are leading away from the coast - to facilitate the evacuation.
At this height of the Atlantic hurricane season, Florence is being trailed on east-to-west paths by two other hurricanes, Helene and Isaac.
Maximum sustained winds were clocked at 140 miles per hour (220 kph) as it moved west-northwest at 13 miles per hour (20 kph). The storm is so huge that SC won't be spared even if it escapes the eye of the hurricane. Its centre will move between Bermuda and the Bahamas on Tuesday and Wednesday and approach the coast of South or North Carolina on Thursday, the National Hurricane Centre said.More news: Hurricane watch issued for Lowcountry coast; Florence maintains Category 4 strength
More news: Pres. Trump: North Korea Parade Is 'A Big And Very Positive Statement'
More news: Petrol Touches Rs.90.11 In This City
Storm over the Atlantic ocean "looks very bad", wrote in his Twitter the President of the United States Donald trump.
Like Hurricane Harvey, which stalled over Texas in 2017, Florence could linger over the Southeast for several days after landfall.
Preparations for Florence were intensifying up and down the densely populated coast.
Crystal Kirwan didn't see she had much choice as she got her family ready to leave their home Monday on the coast of North Carolina, with Category 4 Hurricane Florence threatening the state.
"We know the evacuation order I'm issuing will be inconvenient", McMaster said.
Even if Florence hits as "only" a Category 3 hurricane, it will still cause "devastating damage".
"We want to make sure that we get the best data going to the National Hurricane Center. and to emergency managers and to the media so we can get people out of the way", Flaherty said. As the forecast becomes clearer, Florence poses three challenges: coastal storm surge, strong winds and inland flooding that inundates rivers and low-lying ground.
"I highly encourage people to move out of the county", Brunswick Emergency Management Director Brian Watts said.