The storm will most likely make landfall in North Carolina or SC on Thursday. Hurricane watches and storm surge watches have been issued from Edisto Beach, S.C.
Officials say that the storm will likely strengthen.
A state of emergency has been declared in both North and SC.
Hurricane Florence will create a "life-threatening storm surge" along parts of the Carolina and Virginia coasts, according to the National Hurricane Center. It is expected to intensify to near Category 5 status as it slows over very warm ocean water near North and SC.
In addition to flooding the coast with wind-driven storm surges of seawater as high as 13 feet (4 m), Florence could drop 15 to 25 inches (38 to 64 cm) of rain, with up to 35 inches (89 cm) in some spots, forecasters said.
Florence is expected to be an extremely unsafe major hurricane through landfall. "So, that high pressure is now driving Florence toward the East Coast", he said.
Orange traffic cones with the words "Hurricane" and "Evacuation" are seen ahead of the arrival of Hurricane Florence near Orangeburg, South Carolina, U.S., September 11, 2018.
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There, the vehicle rammed through two glass doors at the airport's Terminal 1 to drive onto the runways, she said. The airport has been closed and all flights have been successfully grounded, according to reports.
Florence is expected to produce total rainfall accumulations of 15 to 20 inches and possibly 30 inches in some spots along its track over portions of North Carolina, Virginia and northern SC through Saturday. A year ago, people would have laughed off such a forecast, but the European model was accurate in predicting 60 inches for Hurricane Harvey in the Houston area, so "you start to wonder what these models know that we don't", University of Miami hurricane expert Brian McNoldy said. "That means the more trees that could fall, the more power outages", National Hurricane Center Director Ken Graham said. It took Matheson and her husband almost the whole day Monday to drive the 100km off the barrier island.
"If you are now under an evacuation order, we strongly urge you to be cognizant, heed the warning and evacuate", Byard said. "All you have to do is look up at your ceiling and think about 12ft of water - that, folks, is extremely life-threatening".
"Get to a safe place now or by Thursday morning", he said.
In addition to thousands of homes and businesses, five military bases and half a dozen nuclear power plants are also in the path of the storm.
Nasa said the storm, now brewing above the Atlantic Ocean, had weakened to Category 1 strength, but is expected to strengthen into a Category 3 hurricane by the time it makes landfall on the United States mainland.
"The largest threat to life from hurricanes is not the high winds".
Meanwhile, there are two other storms in the vicinity - Isaac and Olivia have become tropical storms, with the former heading for the Caribbean and the latter set to hit Hawaii early on Wednesday.