Hurricane Helene was gradually weakening and may become a tropical storm by Thursday, the U.S. National Hurricane Center (NHC) said on Tuesday.
The National Hurricane Centre estimated Florence would blow ashore as early as Friday afternoon around the North Carolina-South Carolina line, then push its rainy way westward with a potential for catastrophic inland flooding.
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"This is a very risky hurricane", Mr McMaster said, adding that the evacuation order for coastal counties was "mandatory, not voluntary".
Helene, which has been downgraded from a hurricane, is among a glut of tropical storms brewing in the region, with mass evacuations under way in southern U.S. states as Hurricane Florence barrels across Virginia and the Carolinas.
Officials in North Carolina were already preparing for impact.
Life-threatening storm surges of up to 13 feet were also forecast in some areas along with the possibility of tornadoes in North Carolina. (AAP) Millions of residents on the United States east coast are on high alert as Hurricane Florence approaches.
USA media say more than one million people have been ordered to evacuate.
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"Strength, track, and forward speed of Florence will be the major players in determining the scope and amount of rainfall and correspondingly the severity of inland flooding", according to AccuWeather forecaster Dan Kottlowski on September 12.
At least 88,000 people were without power in North Carolina with the brunt of the storm yet to come, according to the state's emergency management agency.
The governor of neighbouring North Carolina ordered an evacuation of the Outer Banks, barrier islands that are a popular tourist destination, and parts of coastal Dare County, while a state of emergency was declared in Virginia.
The storm was centred about 835 miles (1340 kilometres) southeast of Bermuda and moving west at 7 mph (11 kph).
SC ordered the mandatory evacuation of one million coastal residents while North Carolina ordered an evacuation of the Outer Banks, barrier islands that are a popular tourist destination.
Bridges, airports and ferry services in the affected areas face closure if winds grow too violent, while roads and rail routes might also be cut off.
Tens of thousands of homes and businesses could be flooded in North Carolina alone, Governor Cooper warned.
"We hope to have something left when we get home", she said.
Interestingly, the Atlantic Ocean housed three hurricanes at the same time, that before Tropical Storm Isaac being downgraded from its former hurricane status.