Florence is now expected to pass over much of SC, with storm force winds beginning Thursday and hurricane force winds on Friday, said John Quagliariello with the National Weather Service in Columbia.
"This will likely be the storm of a lifetime for portions of the Carolina coast", the National Weather Service in Wilmington, North Carolina, wrote in its Tuesday evening area forecast discussion, adding in a Wednesday report that flooding in southeast North Carolina and northeast SC could be "unprecedented".
A state of emergency has been declared in Georgia, making it the fifth state to do so, as authorities warned of storm surges and catastrophic flooding.
The message from North Carolina Governor Roy Cooper to residents was bleak.
The NHC said the first tropical storm-force winds of at least 39 miles per hour (63 kph) would hit the region early on Thursday with the storm's center reaching the coast Friday.
"I've been through hurricanes before but never with kids", she said.
Gov. Henry McMaster is not yet issuing mandatory evacuation orders for Jasper and Beaufort counties, but officials have not ruled out the possibility of another evacuation order.
"As we have been predicting, this hurricane is unpredictable", McMaster said at a news briefing updating the state's response to Florence on Wednesday. Although wind speeds have dropped - Florence was a Category 4 on Monday with 140 miles per hour winds - the National Hurricane Center warned wind fields are still expanding, and the hurricane has more total energy, which will "create a significant storm-surge event".
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.
Melody Rawson evacuated her first-floor apartment in Myrtle Beach and arrived at Atlanta Motor Speedway in Hampton, Georgia, to camp for free with three other adults, her disabled son, two dogs, and a pet bird.
"Get out of its way, don't play games with it, it's a big one, maybe as big as they've seen", Trump said.
Forecasters anxious the storm's damage will be all the worse if it lingers on the coast.
With South Carolina's beach towns more in the bull's-eye because of the shifting forecast, OH vacationers Chris and Nicole Roland put off their departure from North Myrtle Beach to get the maximum amount of time on the sand. "You feel like you should have already left".