A Category 4 storm could devastate the East Coast, simulation shows.
Its maximum sustained winds have dropped slightly to 110 miles per hour.
Massive waves of up to 83 feet were measured inside the storm on Wednesday, the National Hurricane Center's Tropical Analysis and Forecast Branch said, citing satellite altimeter data.
Almost 1000 prisoners in SC will not be moved from their cells, despite a mandatory evacuation order in the area.
Their entire neighborhood evacuated in Wilmington, North Carolina, David and Janelle Garrigus planned to ride out Florence at their daughter's one-bedroom apartment in Charlotte. "Don't bet your life on riding out a monster".
The threat has sparked a rush of evacuation efforts in SC and North Carolina, with more than 1 million people urged to get out of Florence's way.
As of 2 p.m. Wednesday, the Category 4 storm was about 435 miles southeast of Wilmington, N.C., and moving northwest at about 16 mph.
U.S. President Donald Trump on Twitter warned of the storm's dangers and praised his administration's handling of past hurricanes, rejecting criticism for its response to Hurricane Maria previous year in Puerto Rico. Some 3,000 people died in the aftermath of that storm.
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And it led to mixed signals from officials in SC, whose governor had canceled mandatory evacuation for several coastal counties.
The NHC said the storm was on a trajectory that showed its centre most likely to strike the southern coast of North Carolina by Friday.
Communities could lose electricity for weeks, Federal Emergency Management Agency Administrator Brock Long said. The only exception was Edisto Beach - that and other areas to the north remain under evacuation orders.
The American Red Cross said more than 700 workers were headed to the region while shelters were set up to house those unable to flee.
Axios: "The ties between Hurricane Florence and climate change" - "Hurricane Florence is a unique Atlantic hurricane, projected to stall out after hitting land and forecast to dump upwards of 2 feet of rain on several states, much like Hurricane Harvey did in Texas a year ago". "What is flooding going to do to our home, our city?"
The storm was expected to become a tropical depression on Saturday and significant weakening was expected over the weekend, the NHC said in a bulletin. She said a hurricane has a way of bringing everyone to the same level.
"We're just trying to plan for the future here, not having a house for an extended period of time", David Garrigus said.
Florence is now heading for ocean water that has surface temperatures of about 85 degrees, meaning it will most likely strengthen on its way to the East Coast.
Meteorologists suggest that the hurricane will reach the U.S. East coast on September 13.