The category 4 storm carrying maximum sustained winds of 215 km/h would probably strike the southern coast of North Carolina by late Thursday or early Friday, the National Hurricane Center (NHC) said.
The NHC warned on Tuesday that the storm may strengthen to Category 5, with wind speeds in excess of 157 miles per hour (253 km/h), as it moves over a band of warmer water off of North Carolina's coast.
Meteorologists are warning of potentially catastrophic flooding in the area, with isolated amounts of up to 40 inches of rain expected in parts of SC.
The monster storm showed signs of slowing down as it inched closer to the USA, according to the National Hurricane Center.
Hurricane-force winds will reach the Carolina coasts late Thursday or early Friday.
"We are already experiencing heavily impacted traffic on some of the evacuation routes", said Jeff Byard, the associate administrator for response and recovery at the Federal Emergency Management Agency (FEMA). You're going to be displaced from your home in the coastal areas.
The North and SC coasts are on high alert as residents brace for Hurricane Florence's brutal lashing.
More than 1 million residents have been ordered to evacuate the coastline of the three states, while university campuses, schools and factories were being shuttered. I believe that was a Category 1 storm when it hit Charlotte.
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While a man who gave his name as Joe, a contractor from Carolina, admitted he was "scared" but said he had survived storms before and was determined to make it through this one. Even if you've ridden out storms before, this one is different.
Florence is now churning through the Atlantic, only a few hundred miles off the coast of Wilmington, N.C.
WCNC-TV meteorologist Brad Panovich told WFAE Morning Edition host Marshall Terry that the shift of Florence means more rain for the Charlotte area.
As residents of the region prepared by stocking up on essentials, officials called on them to evacuate to higher ground. "Everyone was sold out", she said.
In a morning news conference, Gov. Roy Cooper warned that residents in every part of North Carolina will feel Florence's impact this weekend and should prepare - especially for flooding.
In neighboring Maryland, Governor Larry Hogan said his state was readying for potentially "historic and catastrophic rainfall, life-threatening flooding, and high winds".
Hurricane Florence is now a massive Category 4 hurricane.
Abigail Darlington, covers the city of Charleston for the South Carolina Post and Courier.
The coastal surge from Florence could leave the eastern tip of North Carolina under more than 9 feet of water in spots, projections showed.