Even worse: Florence is expected to hover over the Carolinas, whipping hurricane-force winds and dumping relentless rain at least through Saturday.
The storm is expected to slowly move inland, battering much of the U.S. coast for days.
Warm sea surface temperatures and low wind shear (which, when high, can zap the energy from a storm) had set up ideal conditions for Florence to intensify. It was located 85 miles from Wilmington at 8 p.m. Thursday and its eye was expected to make landfall Friday, the advisory said.
Undeniably enormous, and frightfully powerful, the storm has captured the imagination of astronauts watching over it from orbit.
The surf and storm surge are already cutting through North Carolina's Outer Banks and bands of rain are lashing the coast.
"The deepest water will occur along the immediate coast in areas of onshore winds, where the surge will be accompanied by large and destructive waves".
General O'Shaughnessy said there were about 7,000 United States military personnel now in place and ready to respond to the storm, along with ships, helicopters, high-wheeled vehicles and other equipment.
Hurricane Florence is growing in both strength and size as it prepares to make landfall.
The storm was about 100 miles southeast of Wilmington and had maximum sustained winds of 100 mph, which is slightly weaker than earlier Thursday, the briefing said.More news: Elizabeth Smart calls decision to free captor 'incomprehensible'
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Forecasters say Florence will likely turn to the west-northwest and slow down its forward motion - a situation that will bring even more rainfall to the area.
Forecasters anxious the storm's damage will be all the worse if it lingers on the coast.
"Even the rescuers can not stay there", he said. Rainfalls up to 40 inches (102 centimeters) in some places are expected in a region ranging from coastal North Carolina to northeastern SC, with flash flooding likely, according to the NHC.
As the storm - with those winds - nears shallow water along the coast, its forces water inland.
Measurements by an Air Force Hurricane Hunter plane indicate that maximum sustained winds have decreased to near 110 miles per hour, with higher gusts.
One climate model is predicting that as much as 11 trillion gallons of rain will fall on North Carolina in the coming week - an amount that's enough to fill the Empire State Building 40 times over. Roads from the Outer Banks have been closed and ferries have stopped running as storm surge cuts across the islands, carving new inlets, stirring sea foam, and clawing at the foundations and stilts lifting homes above the normally tranquil beach.
Frustrated after evacuating his beach home for a storm that was later downgraded, retired nurse Frederick Fisher grumbled in the lobby of a Wilmington hotel several miles inland.
Preceded first by the storm surge and the winds, heavy rains were picking up as of late Thursday afternoon, the beginning of an onslaught that for some areas may not relent for days.
Will Epperson, a 36-year-old golf course assistant superintendent, said he and his wife had planned to ride out the storm at their home in Hampstead, North Carolina, but then reconsidered.
He said the magnitude of the storm may exceed the ability of the state-activated National Guard troops.