Florence "is expected to remain an extremely risky major hurricane through Thursday", the NHC said in a statement. The counter-clockwise rotation of hurricanes means more damage to the storm's upper right quadrant, viewing it from above, but less impact to the lower left side.
Hurricane Florence's top winds exploded to 130 miles (209 km) per hour, making it a Category 4 storm, just one-step short of the most severe level as it pushed westward toward the U.S. East Coast.
At least 340 dead people have been killed by Hurricane Matthew in Haiti and the Dominican Republic as the United States braces itself for the worst storm in a decade. The storm is expected to bring 15 to 30 inches of rain.
North Carolina State University canceled classes beginning Sept. 12 at 5 p.m. until the evening of Sunday, Sept. 16, although dining and residence halls will remain open. And with little wind shear to pull the storm apart, Florence's hurricane wind field was expected to expand over the coming days, increasing its storm surge and inland wind threats. That information will help public officials assess storm damage, discern between wind and flood damage, and improve computer models used to forecast future floods. "Flooding is the deadliest result of these storms".
The Navy's U.S. Fleet Forces Command has ordered all Navy Ships in the Hampton Roads, Virginia area to "set Sortie Condition Alpha".
"This is going to be big", said Victoria Oliva, a meteorologist with the National Weather Service in Wilmington.
NHC spokesman Dennis Feltgen said historically, 90 percent of fatalities from hurricanes, tropical storms and tropical depressions have been caused by water.
US landfall could come Friday between Charleston, South Carolina and Norfolk, Virginia, the Hurricane Center said.
"This is a very risky hurricane", McMaster said, adding that the evacuation order for coastal counties was "mandatory, not voluntary".
The storm "is too powerful and its path is too uncertain to take any chances", South Carolina Governor Henry McMaster said, issuing his state's emergency declaration.
At the moment, Florence, Hurricane Isaac, and Hurricane Helene are all swirling in different parts of the ocean at the peak of hurricane season.
The hurricane was gaining strength as it traveled over warm Atlantic waters, about 1,240 miles (2,000 km) east-southwest of Cape Fear, North Carolina, the National Hurricane Center said in an advisory. "These tropical storms and hurricanes are very unsafe". Its maximum sustained winds are at 105 miles per hour.
Isaac is expected to reach landfall in the Caribbean later this week.