"On the forecast track, the center of Florence will approach the coasts of North and SC later [Thursday night], then move near or over the coast of southern North Carolina and northeastern SC in the hurricane warning area on Friday", the NHC said.
Governors in the Carolinas and Virginia already had declared emergencies, and some sought federal aid as well. "This helps everyone find vital information more easily".
If Florence slows to a crawl just off the coast, it could bring torrential rains to the Appalachian mountains and as far away as West Virginia, causing flash floods, mudslides and other risky conditions.
National Hurricane Center Director Ken Graham is warning that the "staggering" size of Florence means its impacts will be felt far and wide. People living well inland should prepare to lose power and anticipate flooding and other hazards, he warned.
"The centre of Florence will move over the southwestern Atlantic Ocean between Bermuda and the Bahamas Tuesday and Wednesday, and approach the southeastern coast of the USA on Thursday", the NHC said.
Hurricane Florence, Hurricane Isaac, Hurricane Helene track in Atlantic Ocean.
"We do not want to risk one SC life in this hurricane", McMaster said.
As for a hit on the US mainland, the most recent Cat 4 at landfall was Hurricane Harvey a year ago, which hit Texas with winds of 130 miles per hour. This includes cities in the path of Florence such as Wilmington and Cape Hatteras in North Carolina and Norfolk, Virginia.
Florence's centre was expected to move over the Atlantic Ocean between the Bahamas and Bermuda on Tuesday and Wednesday before approaching North and SC on Thursday. Two other storms also are churning in the Atlantic.
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Once the storm makes landfall, the threats are far from over.
As Florence approaches the coast, it is forecast to strengthen.
The hurricane will weaken slightly before it makes landfall on Thursday as it encounters more wind shear, but Florence should still be a major hurricane at landfall along the central North Carolina coast.
"We do not want to risk one SC life in this hurricane", the governor told reporters.
President Donald Trump is canceling a campaign rally in advance of Hurricane Florence's landfall.
The storm is expected to turn towards the north on Wednesday. People rushed to get emergency kits ready, map out escape routes, fill sandbags and secure their homes.
Many newcomers have moved to the coast in the almost 19 years since the last strong hurricane - Floyd - threatened the area.
SHAPIRO: What is your message to the people who say, I've been through storms in the past; I can ride out this one?
"This one is hard", she said, having been through hurricane damage before.
The storm "is too powerful and its path is too uncertain to take any chances", SC governor Henry McMaster said, issuing his state's emergency declaration.