It could be the most devastating to hit the region in three decades.
"We've basically got everything we need. and we're just going to wait it out for a few days", he said.
Heavy rain in the Washington area over the weekend had already led to flooding in historic Alexandria, Virginia, local media said, and the National Weather Service issued a flood watch for part of the Potomac River.
Hurricane Florence is expected to make landfall either late Thursday night or early Friday morning, first touching the United States somewhere between northern SC and the Outer Banks in North Carolina.
"If the hurricane miraculously, overnight, decides to go somewhere else, we can take corrective action", he said, saying it was easier to "stand down" state resources than have them "stand up" at a moment's notice.
Schools, state government offices and medical facilities, including nursing homes in those counties, will be evacuated as well, McMaster said. SC ordered an estimated 1 million people to evacuate its coast.
On Monday, residents emptied shelves of storm essentials - such as bread, water and batteries - at area grocery stores as soon as they were stocked. "We live paycheck to paycheck, being a military family, and it's four days until payday".
For many, the challenge could be finding a safe refuge.
Similar scenes were repeated up and down the US East Coast, from Virginia to Savannah, Georgia.
This happened previous year, too, when Hurricane Jose and Tropical Storms Maria and Lee were churning at the same time.
A "major" hurricane is one with sustained winds of more than 110 miles per hour.
Helene is expected to become a major hurricane by Monday night, but forecasters say it will weaken by late Tuesday. "If the track brings higher rainfall amounts that axis of rain farther south and into our area". SC emergency management officials said they were "preparing for the possibility of a large-scale disaster".
It remains an extremely risky major hurricane through Thursday night. Maximum sustained winds have increased to 105 miles per hour. Well-built homes can lose walls and roofs.
The 1st effects were already being seen on barrier islands Monday as unsafe rip currents and seawater flowed over the state highway. North Carolina State, University of North Carolina and UNC-Wilmington have all suspended classes, with UNC-Wilmington evacuating its coastline campus by noon Tuesday.
In Virginia, mandatory evacuations will begin on Tuesday morning for about 245,000 residents in a portion of Hampton Roads and the Eastern Shore area, Governor Ralph Northam said.
The National Hurricane Center predicted Monday that Florence would be a category 4 storm with winds of 135 miles per hour at landfall. "So, that high pressure is now driving Florence toward the East Coast", he said.
"Action today can avoid losses due to Florence", he said. And the timing couldn't be more apt: Monday was the peak of the Atlantic hurricane season, when storm activity has historically been at its highest.
Many people in the Southeast and Mid-Atlantic probably have not experienced a storm of the potential magnitude of Florence. Isaac's forward movement was accelerating on a path to cross into the lower Caribbean on Thursday.
During his press conference Monday, McMaster appeared open to the possibility of modifying his evacuation plan.