Florence is expected to make landfall late Thursday or early Friday (Sept. 13 or 14), likely as a Category 3 hurricane, meaning it will have winds blowing at a minimum of 111 miles per hour (178 km/h), Samson said.
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).
Florence's top sustained wind speeds dropped from a high of 140mph to 110mph which reduced the hurricane's power from Category 4 to Category 2.
Like Hurricane Harvey in 2017, Florence is expected to slow significantly when it reaches the coast, allowing the storm to dump a catastrophic amount of rain in the Carolinas.
Since Tuesday, forecasts have shifted the storm track towards the south and southwest after it reaches the coast, which could increase the storm's severity in coastal SC through Myrtle Beach and Charleston and even into parts of Georgia.
Florence is forecast to dump up to one metre of rain in some areas after it makes landfall in North and SC.
More than one million people have been ordered to evacuate the coastline of the three states, while university campuses, schools and factories were being shuttered.
"This will likely be the storm of a lifetime for portions of the Carolina coast", the National Weather Service in Wilmington, N.C., said, "and that's saying a lot given the impacts we've seen from hurricanes Diana, Hugo, Fran, Bonnie, Floyd, and Matthew".
A crew of about 80 rescuers from Montgomery County is headed to SC to help deal with Hurricane Florence and its aftermath. The only exception was Edisto Beach - that and other areas to the north remain under evacuation orders.
Then, as the high-pressure system steering Florence weakens, the hurricane is expected to hang out inland for days, showering the region with rain much like Hurricane Harvey did to Houston previous year.
Amanda Kinseth, reporter for WPDE ABC 15 News in Myrtle Beach, South Carolina. North Carolina corrections officials said more than 3,000 people were relocated from adult prisons and juvenile centres in the path of Florence, and more than 300 county prisoners were transferred to state facilities.
Florence is the most risky of three tropical systems in the Atlantic.
The storm is expected to hit the Carolinas hard later this week. "I encourage Georgians to be prepared for the inland effects of the storm as well as the ensuing storm surge in coastal areas".
Atlantic Beach on North Carolina's Outer Banks islands had already received 30 inches of rain, the U.S. Geological Survey said, while more than 25 inches have fallen in the Newport, Morehead City area since Thursday.
On Wednesday morning, Florence was extending hurricane-force winds outward up to 70 miles from its center, with tropical-storm-force winds up to 175 miles outward. "I'm a little bit scared right now", she said, "but I have my iPad and I'm watching Netflix".
As Florence closed in, President Donald Trump and state and local officials urged residents in the path of the storm to evacuate before it was too late.
Forecasters anxious the storm's damage will be all the worse if it lingers on the coast.