"Do you want to get hit with a train or do you want to get hit with a cement truck?" said Jeff Byard, an administrator with the US Federal Emergency Management Agency.
Florence's maximum sustained winds were clocked on Thursday at 100 miles per hour (170 km per hour) as it spun in the Atlantic Ocean, down from a peak of 140 mph (224 kph) earlier this week when it was classified as a Category 4.
Forecasters say it could slow dramatically when it nears land and then linger until Saturday, moving unpredictably along the coast and bringing torrential rain.
The coastal area of SC features hundreds of inlets while North Carolina has barrier islands miles away from the mainland.
Winds were already picking up along the coastline on Thursday morning and Myrtle Beach was virtually deserted with empty streets, boarded up storefronts and very little traffic.
"I don't care if this goes down to a Category 1".
As of 11 p.m., the storm was centered 280 miles east southeast of Wilmington, N.C., and was moving northwest at 17 mph, and this general motion, accompanied by a gradual decrease in forward speed, was expected to continue through Thursday.
The US Coast Guard said it closed the ports of Wilmington and Moorehead City in North Carolina and restricted port operations in Charleston, South Carolina. Hurricane-force winds extended 80 miles (130 kilometers) from its center, and tropical-storm-force winds up to 195 miles (315 kilometers).
"This rainfall will produce catastrophic flash flooding and prolonged significant river flooding", the briefing said.
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A tornado watch was also in effect for parts of North Carolina.
It was unclear exactly how many people fled, but more than 1.7 million people in the Carolinas and Virginia were warned to clear out.
"In 12 or 18 hours, they may be saying different things all over again", he said. "Inland flooding kills a lot of people, unfortunately, and that's what we're about to see", he said.
Now, there are signs that Hurricane Florence could deliver a similar blow to the East Coast beginning on Thursday, making it even more important to evacuate before the storm arrives.
The Carolinas and Virginia are preparing for the onslaught of Hurricane Florence later this week and one of the most life-threatening elements of the storm is the amount of rainfall predicted to hit the area, reports Vox. Only one storm scared them enough to leave the island.
Emergency declarations were in force in Georgia, South and North Carolina, Virginia, Maryland and the District of Columbia. "The combination of a unsafe storm surge and the tide will cause normally dry areas near the coast to be flooded by rising waters moving inland from the shoreline".
Evacuation orders for low-lying areas were issued Monday and continued Tuesday.
Antonio Ramirez, a construction worker from El Salvador living in Leland, North Carolina, said he planned to ride out the storm with his dog Canelo.
Florence could do more than $170bn (£130bn) of damage and affect almost 759,000 homes and businesses, says analytics firm CoreLogic. "I'm not leaving him here".
"When that last ferry pulls out ... it's unnerving to see it pull away and know, 'That's the last chance I have of getting off this island, '" she said Wednesday.