With the storm expected to make landfall somewhere between SC and Virginia, AAA says the storm will "likely" have an impact on East Coast gas prices.
"Bad things can happen when you're talking about a storm this size", Trump said Wednesday morning.
It's unclear exactly how many people fled, but more than 1.7 million people in the Carolinas and Virginia were warned to clear out.
"This rainfall would produce catastrophic flash flooding and significant river flooding", the Miami-based National Hurricane Center (NHC) said.
"To those in the storm's path, if you can hear me, please stay sheltered in place", North Carolina Governor Roy Cooper said at a news conference in Raleigh, adding that Florence would "continue its violent grind across the state for days".
In the 11 p.m. ET advisory, Florence was 280 miles southeast of Wilmington, NC, packing maximum-sustained winds of 110 mph and moving to the northwest at 17 mph.
Jeff Byard of the US Federal Emergency Management Agency said it was imperative locals heed the evacuation warnings.
To hasten evacuations from coastal SC, officials switched traffic patterns so that all major roads led away from shore.
Manure lagoons are about as pleasant as they sound-vast open-air ponds of manure located next to the many hog farms near the North Carolina coast.
Another FEMA spokesman said: "I need to re-emphasise this is a unsafe storm".
In North Carolina, the state Department of Information Technology is bracing for inevitable power outages by supplying the Transportation and Environmental Quality departments with generators to keep traffic signals, road-alert signs and other critical equipment functioning, the IT agency told StateScoop.
"Against my better judgment, due to emotionalism, I evacuated", he said.
"I rode out Hugo", Miller said of a 1989 hurricane that caused widespread damage. "I was in the same house and it stood fine". "The water level will get a little high but we'll be fine. We have everything. We're ready".
North Carolina's governor issued what he called a first-of-its-kind mandatory evacuation order for North Carolina's fragile barrier islands from one end of the coast to the other.
Lt. Col. Lindy White, an Oklahoma National Guard spokeswoman, said officials from East Coast states are expected to request additional assistance in the days to come.
A hurricane warning - meaning hurricane conditions are expected within 36 hours - is in effect for a long stretch of the coast, from the Santee River in SC to Duck, N.C., which is part of the Outer Banks. "Disaster is at the doorstep and it's coming in". After its initial impact, Byard said the most serious concern will be massive rainfalls that would produce unsafe inland flooding.
In Virginia, where about 245,000 residents were ordered to evacuate low-lying areas, officials urged people to remain away from home despite forecast changes showing Florence's path largely missing the state. Isaac could bring heavy rainfall to Martinique, Dominica and Guadeloupe.