Hurricane warnings continue in parts of eastern North Carolina and northern coastal SC.
This hurricane downgraded slightly from a Category 4 to Category 3 hurricane with winds sustained at 115mph.
Duke Energy has around 4 million customers in North and SC, and says it could take several weeks to restore the electricity.
Four South Carolina motorways have been diverted one-way to speed the exodus.
The states of Georgia, South Carolina, North Carolina, Virginia, and Maryland have all declared states of emergency ahead of Florence.
Weather Underground meteorology director Jeff Masters said Florence eventually could strike as merely a Category 1 hurricane with winds less than 100 miles per hour, but that's still enough to cause at least $1 billion in damage.
"And that's saying a lot given the impacts we've seen from Hurricanes Diana, Hugo, Fran, Bonnie, Floyd and Matthew".
"It's a very risky storm", Federal Emergency Management Authority administrator Jeff Byard said. It is an extremely, dangerous, life-threatening, historic hurricane, ' said North Carolina Gov. Roy Cooper.
Waves 83ft (25m) tall were recorded at sea on Wednesday morning.
Though Florence has weakened slightly, it's still a very risky storm, and a life-threatening storm surge and rainfall are expected.
As Florence drew near, President Donald Trump tweeted that FEMA and first responders are "supplied and ready", and he disputed the official conclusion that almost 3,000 people died in Puerto Rico, claiming the figure was a Democratic plot to make him look bad. "Don't play games with it".
One officer is seen restraining a young boy as another shopper drops several bottles of water.
"Bad things can happen when you are talking about a storm this size". "You can't stop Mother Nature". You never know, but we know. "We love you all, we want you safe".
Hurricane and storm surge warnings are in place from South Santee River, South Carolina, to Duck, North Carolina.
It is forecast to bring 20-40in (50-100cm) of rain and life-threatening storm surges of up to 13ft.
The storms path is promising to bring even more devastation than first predicted to the Carolinas and parts of Georgia.
National Hurricane Center Director Ken Graham warned that waterways up to 40 miles inland may flood.
Three feet of storm surge moving onto the coastline will result in higher tides and higher water moving onshore. When Hurricane Floyd made landfall near Cape Fear in 1999 as a Category 2 storm, bloated carcasses of hundreds of thousands of hogs, chickens and other drowned livestock bobbed in a nose-stinging soup of fecal matter, pesticides, fertilizer and gasoline so toxic that fish flopped helplessly to escape it.
Is global warming to blame? But previous research has shown that the strongest hurricanes are getting wetter, more intense and intensifying faster because of human-caused climate change.