A line of heavy traffic moved away from the coast on Interstate 40, the main route between the port city of Wilmington, North Carolina, and inland Raleigh.
Hurricane Florence is predicted to bring "catastrophic flash flooding" to parts of North and SC, along with strong winds and a massive storm surge.
The storm's winds may have weakened in recent days - they are now at 110mph (175km/h) - but there are fears Florence's slow-moving nature could bring different problems.
Florence is now moving to the northwest at 17 kilometers per hour.
"If you're going to leave. you should leave now because time is running out", McMaster said on Thursday. The storm was moving northwest at 12 miles per hour.
Vickie Grate, left, waits in a shelter with her son Chris, center, and his girlfriend Sarah, who only gave their first names, for Hurricane Florence to pass after evacuating from their nearby homes, in Conway, South Carolina, Wednesday, September 12, 2018. People in the Carolinas had the daytime hours Thursday to complete their final preparations to ride out or escape the storm.
After criticism for its response in Puerto Rico to last year's Hurricane Maria, which officials there said was responsible for 3,000 deaths, Trump has vowed a vigorous response to Florence and defended his handling of Maria. That forecast area also includes part of southwest Virginia.
The 400-mile wide storm is predicted to start slamming the N.C. coast Thursday night to early Friday, then weaken to a tropical storm as it drifts into SC, said John Quagliariello, meteorologist with the National Weather Service in Columbia.
"The combination of a risky storm surge and the tide will cause normally dry areas near the coast to be flooded by rising waters moving inland from the shoreline", according to the National Hurricane Center.
Air Force General Terrence J O'Shaughnessy, head of US Command, said search and rescue is a top priority but that the magnitude of the storm may exceed the ability of rescuers.
Brock, meanwhile, said the surges "are going to be a major problem way up into the streams and tributaries that come out of sound areas". Storm surge is why many of you have been placed under evacuation and we are asking citizens to please heed a warning.
Hurricane Florence is shaping up to be a terrible storm, and it does not look any better when you look at it from space. Hundreds of thousands of people have already evacuated. Additionally, tornadoes could arise in southeast North Carolina on Thursday and Friday. "But even worse than that is coming back in because you don't know what you're coming back to".
"Sometimes these systems, they expand", she said. "Remember that floods will occur in places that do not receive a hurricane or even tropical storm level winds". Here's why this storm threatens not only the coast but millions of people inland. Emergency declarations were in force in Georgia, South and North Carolina, Virginia, Maryland and the District of Columbia.
US President Donald Trump touted the government's readiness and urged people to get out of the way. On Wednesday, officials said some of those shelters were at capacity, but in a statement early Thursday, county officials said they were "able to mobilize additional resources to accommodate the need for shelter space". "We're fully prepared. Food, medical, everything you can imagine, we are ready".