Forecasters with the National Hurricane Center said that while Florence is expected to slow down considerably by late Thursday and into Friday, it will still be an "extremely risky major hurricane" when it makes landfall.
As of 8 p.m., the storm was centered 335 miles (540 kilometers) southeast of Wilmington, North Carolina, moving northwest at 16 mph (26 kph).
It diminished from hurricane force as it came ashore, but forecasters said the 350-mile-wide storm's slow progress across North and SC could leave much of the region under water in the coming days.
The hurricane center said Florence will approach the coast Friday and linger for a while before rolling ashore.
Officials in at least one area that ordered an evacuation will be going to homes to ask people planning to ride it out for information on their next of kin, CNN reported Wednesday.
"The big picture: There are several characteristics of the changing climate that are helping to increase the risks of damage from Hurricane Florence, even though global warming is not directly causing such a storm to spin up".
In addition to the dangers the expected deluge of water poses, the hurricane center cautioned, "A few tornadoes are possible in eastern North Carolina beginning late Thursday morning".
Florence is forecast to dump up to one metre of rain in some areas after it makes landfall in North and SC.
Officials said some 1.7 million people in the Carolinas and Virginia were warned to evacuate, but it is unclear how many did.
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The hurricane could also produce heavy and excessive rainfall - up to 40 inches in isolated areas in the Carolinas and anywhere between 6 to 12 inches elsewhere in the Appalachians and Mid-Atlantic region.
A police vehicle patrols the beach ahead of the arrival of Hurricane Florence in Myrtle Beach, South Carolina, U.S. September 11, 2018.
Henry McMaster lifted the mandatory evacuation order for Beaufort, Jasper and Colleton counties Tuesday morning.
The storm is expected to hit the Carolinas hard later this week. President Trump has declared an emergency in the three states and says the government is "as ready as anybody has ever been".
"I'm scared we'll get 30 inches or more of rain", said Carol Trojniar, 69, a longtime Wilmington resident and retired real estate agent who has never experienced a Category 4 hurricane.
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.
A storm surge watch is in effect for Edisto Beach, SC, to South Santee River, SC, and for north of Duck, NC, to the North Carolina/Virginia border.
The storm's centre is expected to slow when it reaches inland, meaning it could rain for days in places and some areas could see three feet of rain. Hurricane-force winds extended 80 miles from its centre, and tropical storm force winds up to 195 miles.
To back up that point, Graham cited a sobering statistic: "50 percent of the fatalities in these tropical systems is the storm surge - and that's not just along the coast". She said it will become a "major flooding event".