The center of Florence is expected to approach the coasts of North and SC later on Thursday, the US National Hurricane Center (NHC) said.
It is predicted to deliver tropical-storm-force winds by noon Thursday to North Carolina's coast, and hurricane-force winds and risky storm surges by late Thursday or early Friday.
"Just because we have a landfall to the south doesn't mean your out of the woods because the wind around this storm is huge", National Hurricane Center Director Ken Graham said Wednesday morning.
Florence weakened slightly to a Category 3 storm on a five-step scale but had maximum sustained winds of 125 miles per hour (201 km per hour) as of 2 p.m. EDT (1800 GMT), down from 130 mph earlier in the day.
This same zone will be hammered by winds gusting up to hurricane force for almost a day while tropical-storm conditions could linger twice that long.
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.
A storm surge watch is in effect for Edisto Beach, SC, to South Santee River, SC, and for north of Duck, NC, to the North Carolina/Virginia border.
"This storm will bring destruction to North Carolina", Gov. Roy Cooper said Thursday morning.
Rising tides moved closer to the dunes in Kill Devil Hills, N.C., on Thursday as Hurricane Florence approached the east coast.More news: Stewart, Storm sweep Mystics for WNBA title
North and SC and Virginia ordered mass evacuations along the coast. However, the surge is expected to be accompanied by large and destructive waves, regardless of when the storm arrives. Washington, DC, Mayor Muriel Bowser and the governors of North Carolina, South Carolina, Virginia, Georgia and Maryland have declared states of emergency.
An estimated 10 million people live in the storm's path, according to the US Weather Prediction Center, and coastal businesses and homes were boarded up in anticipation. North Carolina alone could get from 20 to 30 inches, with isolated spots possibly receiving 40 inches. We'll discuss the latest on the storm.
"I'm not approaching Florence from fear or panic", he said. The actual landfall - when the center of the eye reaches land - will be Friday afternoon at the earliest, said Neil Jacobs of the National Oceanic and Atmospheric Administration.
"If you're going to leave. you should leave now because time is running out", McMaster said on Thursday.
Elsewhere in the Atlantic basin, Tropical Storm Isaac will strike the islands of the Lesser Antilles, which are expected to experience tropical storm conditions and rainfall amounts of up to 6 inches.
"While that rain is still coming down for a couple of days, one to two days later you're going to have stuff coming from North Carolina and things coming in from the Upstate".
A power outage model run at the University of MI projects that 3.2 million customers will be without electricity because of the storm, mostly in the eastern half of North Carolina.
A spokeswoman said: "Basically Helene is in the Atlantic at the moment and is crossing the Atlantic and is expected to track towards the UK".