The NHC warned they're likely to cause "life-threatening surf and rip current conditions".
The exact track, overall size and forward speed of the storm will determine which locations along the coast receive the worst of Florence's damaging winds, heavy rain and storm surge flooding.
"The water could overtake some of these barrier islands and keep on going".
"With time, the wind pushes the water into every nook and cranny you can think of. That, folks, is extremely life-threatening".
Virginia Governor Ralph Northam ordered evacuations in flood-prone areas in the Hampton Roads area, which is home to one of the busiest ports in the United States as well as the largest naval base in the world.
Several meteorologists said Florence could do what Hurricane Harvey did a year ago over Texas, dumping days of rain, although not quite as bad. That Category 4 storm destroyed 15,000 buildings and killed 19 people in North Carolina.
Since then, the coastal population has grown by thousands of people.
The storm's first effects were already apparent on barrier islands as unsafe rip currents hit beaches and seawater flowed over a state highway.
The watch covers the area from Edisto Beach, South Carolina, north to the North Carolina-Virginia border, according to a statement issued Tuesday morning.
Rainfall will be very heavy north of the storm.
"With those high winds you are going to have soggy ground and soggy ground means you are going to have fallen tree limbs into power wires", he said.
Those in Florence's path weren't wasting time on Monday.
A fleet of federal hurricane hunting craft - including planes and personnel from NASA, the Air Force, and NOAA - take flights directly through hurricanes to give storm forecasters an improved idea of what's churning inside the storm, and what might happen next. With little wind shear to pull the storm apart, hurricane-strength winds have been expanding to 40 miles (64 kilometres) from the eye of the storm, and tropical-storm-force winds 150 miles from the centre.
"Unfortunately, the models were right". And of course there would be 130 to 140+ miles per hour sustained winds, driving rains and even the potential of tornadoes to boot. While national reconnaissance aircraft noted this morning the hurricane weakened slightly, it's expected to strengthen again later today.
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. It was located farther north in the Atlantic than any other storm to ever hit the Carolinas, so what we're forecasting is unprecedented.
North Carolina should brace for three dangers from Florence, Cooper said: coastal ocean surges, strong winds and flooding.
South Carolina Walker Townsend, at right, from the Isle of Palms, S.C., fills a sand bag while Dalton Trout, in center, holds the bag at the Isle of Palms municipal lot where the city was giving away free sand in preparation for Hurricane Florence at the Isle of Palms S.C., September 10, 2018. If you have family or friends in that area, please urge them to heed all warnings and evacuation orders and continue to stay tuned to hourly update from reputable sources.