To whip up a monstrous storm like the one chugging for the Carolinas you need a handful of ingredients - and Florence has them all.
Nuclear power plants in the area were preparing for the storm, with Duke Energy Corp's Brunswick and Harris plants in North Carolina's were most likely to be affected, said U.S. Nuclear Regulatory Commission spokesman Roger Hannah. Get out of the storm's way.
Once the storm makes landfall, the threats are far from over.
The American Red Cross said more than 700 workers were headed to the target area while shelters were set up to take in those who could not evacuate. That prediction shows Florence stalling out and looping over or near the North Carolina coastline, which would bring tremendous amounts of rain and potential flooding.
"The longer it stays, the more wind, the more rain".
"I would say everybody should get out, he said".
The slow-moving storm, the most severe hurricane to threaten the USA mainland this year, was located about 785 miles (1,260 km) southeast of Cape Fear, North Carolina, at 4:30 p.m. EDT (2030 GMT), according to the NHC. But a computer simulation known as the European model predicts some places could get 45 inches. The storm led to disastrous flooding in Honduras.
"We've never seen anything quite like this on the east coast". "This is not a pretty sight".
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The chief meteorologist for WCBD-TV in South Carolina, Rob Fowler, told the BBC that Florence was getting bigger, and those even 100 miles away would feel the impact.
First comes the storm surge - a rise in sea levels caused by storm winds.
North Carolina, South Carolina, Maryland, and Virginia have declared states of emergency ahead of the storm.
The University of North Carolina canceled its scheduled home game against the 18th-ranked University of Central Florida in Chapel Hill due to the storm's expected landfall.
Florence's path remains uncertain. And with the Appalachian Mountains to the west, there could be flooding and mudslides, experts worry. The storm is now about 950 miles from the Carolina coast line, but moving in that direction at about 15 mph.
Florence is the most risky of three tropical systems in the Atlantic. Hurricane Harvey, they said, is a recent example of what could happen.
The ocean waters that Florence is travelling over are about 2.7 degrees (1.5 degrees Celsius) warmer than normal, McNoldy said.
Long cautioned coastal residents via Twitter of storm surges up to 12ft - "rapid rise of water that is overwhelming and deadly". The air is holding 10 percent more water that can be dumped as rain. "Most of the area will be uninhabitable for weeks or months", according to the agency. The parking lot has been full for three days at the Ace Hardware store in coastal Calabash, North Carolina, where manager Tom Roberts said he sold 150 gas cans in two hours Monday, along with generators, plywood, rope, manual can openers, sand bags and a plethora of other items.