Tropical-storm-force winds are due to reach the coasts of North and SC on Thursday morning, and hurricane-force winds may be felt around Thursday night, ahead of an anticipated Friday morning landfall.
Sustained winds Monday afternoon were up to 140 miles per hour.
"This is a very unsafe hurricane", McMaster said, adding that the evacuation order for coastal counties was "mandatory, not voluntary".
We are in communication with the National Weather Service, state and local authorities, the Southeastern Conference, Marshall University and schools that other programs that we compete against this week regarding potential weather issues.
Maximum wind speeds were 140mph with hurricane-force winds extending up to 40 miles (65km) from the centre.
A Category 4 hurricane can cause catastrophic damage, the hurricane center warned: "Well-built framed homes can sustain severe damage with loss of most of the roof structure and/or some exterior walls", the hurricane center said.
The center of the eye of the hurricane made landfall in Wrightsville, North Carolina early Friday morning. "It's expected to be a large storm so it will have impacts well away from the center". "This will likely be a big problem not just Thursday but potentially through the entire weekend".
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The governors of South Carolina, North Carolina and Virginia have declared states of emergency and evacuation orders have been issued in Dare County, N.C., beginning at noon Monday.
Maue calculates that 34 million people will get at least 3 inches, with more than 5.7 million getting at least a foot and about 1.5 million getting 20 inches or more.
A list of evacuation shelter locations will be available at scemd.org and in the SC Emergency Manager mobile app as soon as they open, the governor's office said.
My people just informed me that this is one of the worst storms to hit the East Coast in many years.
"More significant weakening is forecast on Saturday as Florence moves farther inland over central SC".
Cape Hatteras National Seashore, Fort Raleigh National Historic Site, and Wright Brothers National Memorial all shut down Monday, the national seashore announced.
On Monday Florence had strengthened into a category 4 hurricane. "The Federal Government is closely monitoring and ready to assist". Florence was gaining strength in the warm waters of the Atlantic and expected to make landfall sometime Friday morning.
The NHC is already warning for the possibility of "life-threatening inundation" from storm surge from Edisto Beach, South Carolina to the North Carolina-Virginia border in the next 48 hours. Please take all necessary precautions.
"We have already began mobilizing our assets to respond accordingly, and we are here for you!" he wrote on Twitter.