In addition to wind-driven storm surges of seawater, Florence could dump up to 35 inches (89 cm) in some spots as it moves inland, forecasters said.
It is then forecast to close in on North or SC on Thursday, hitting a stretch of coastline that is vulnerable to rising sea levels due to climate change.
The center reported late Tuesday night that Florence was roughly 670 miles east-southeast of Bald Head Island with maximum sustained winds of 140 mph.
NHC forecasts showed the effects of Florence would be widely felt, with tropical storm-force winds extending almost 300 miles across three states.
President Donald Trump has declared of State of Emergency in both North and SC, in anticipation of damage done by Hurricane Florence.
White House officials released a statement Monday saying Trump had declared emergency in SC and ordered Federal assistance for the state in connection with Hurricane Florence.
But while wind speed offers an easily quantifiable way to rate unsafe storms, forecasters are warning people not to fixate on that, saying that saltwater from the storm surge and freshwater from heavy rains pose a serious threat, no matter what the top winds are when the hurricane makes landfall.
A Category 4 storm has sustained winds from 130-156 miles per hour.
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Boeing Co suspended work on Tuesday at the SC plant where it assembles 787 widebody jetliners, and a Volvo automobile plant in South Carolina's evacuation zone was also closed, company officials said.
The hurricane watch notice is significant because it signals the opening of a 48-hour window for the expected first arrival of tropical storm-force winds.
North Carolina Governor Roy Cooper issued a mandatory state evacuation for the barrier islands of North Carolina.
Sailors cast off mooring lines to the Command hospital ship USNS Comfort as the ship evacuates Naval Station Norfolk in preparation for Hurricane Florence in Norfolk, Virginia, U.S., September 11, 2018. "Any amounts of money, whatever it takes, we're going to do it". "Florence is expected to produce total rainfall accumulations of 15 to 20 inches with isolated maxima to 30 inches near Florence's track over portions of North Carolina, Virginia and northern SC through Saturday". Some isolated areas could get 30 inches, forecasters said.
"This is going to produce heavy rainfall, and it may not move very fast".
"That's good news and bad news", he said. By Monday, it was evident that the worst-case-scenario was becoming reality, as Florence exploded in strength from a Category 2 hurricane to a Category 4 storm with winds in excess of 130 miles per hour. Fallen trees and power poles will isolate residential areas. That means that once rainfall begins late this week, it could continue into early next week, with flooding possible anywhere from the Appalachian Mountains to the Atlantic Coast.
Florence's growth from an ordinary tropical storm to a major hurricane had been well-forecasted since the cyclone originated from a strong tropical wave that emerged off the coast of Africa on August 30.
Farther to the east, Hurricane Helene is now a Category 2 storm, but it's expected to curve harmlessly northward and weaken in the central Atlantic.
With the remnants of one tropical storm already dumping rain across Atlantic Canada on Tuesday, residents were keeping a close watch on three other storms churning their way across the Atlantic.