It said that the storm, which was located at 775 miles east of the Lesser Antilles, had maximum sustained winds of 70 miles per hour.
The hurricane is rapidly strengthening, according to the NHC.
Hurricane Florence is rapidly intensifying and could strike a direct and risky blow anywhere from the Carolinas to the Mid-Atlantic region later this week, possibly as a fearsome category 4, forecasters said.
Forecasters say Florence will pass between Bermuda and the Bahamas on Tuesday and Wednesday and start its approach to the USA on Thursday morning.
Hurricane Florence appears to be taking aim at the largest U.S. Marine Corps base on the East Coast.
Forecasters also warned of a rising threat of life-threatening storm surge, along with the damaging winds.
A "major" hurricane is one with sustained winds of more than 110 miles per hour.
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Helene is a Category 2 hurricane that's blowing winds between 96-110 miles per hour.
Helene is moving toward the west-northwest near 12 miles per hour.
This enhanced satellite image shows Hurricane Florence, third from right, in the Atlantic Ocean on Sunday, with Isaac and Helene behind. SC emergency management officials said they were "preparing for the possibility of a large-scale disaster".
Steve Pfaff, a warning coordination meteorologist at the National Weather Service's Wilmington office, warned impacts could include storm surge and wind damage, although the storm's path remains uncertain at this time.
A very active scene in the Tropics is what we're waking up to on this Monday morning with THREE hurricanes and two more disturbances with the potential to become tropical depressions. Forecasters say that because of the size the storm is expected to attain, it will wreak havoc regardless of how strong its winds are.
A warm ocean gives hurricanes their fuel, and Florence is moving over an area with water temperatures nearing 85 degrees (30 Celsius), hurricane specialist Eric Blake wrote.
Virginia Gov. Ralph Northam's evacuation order applies to about 245,000 people, including parts of the Hampton Roads area and Eastern Shore.
The storm "is too powerful and its path is too uncertain to take any chances", South Carolina Governor Henry McMaster said in issuing his state's emergency declaration. "I don't know where to go from here", she said.