Meanwhile, the National Hurricane Center (NHC) said that the maximum sustained winds from Florence had eased slightly and that it had been demoted to a Category 3 category 4 on the five-level Saffir-Simpson scale.
The NHC emphasized, however, that although a slow weakening is expected in the next 24 hours, "Florence is expected to be a very dangerous hurricane when it approaches the US coast on Thursday and Friday."
While Florence was advancing across the Atlantic with 205km / h winds, President Donald Trump and state officials intensified appeals to residents on the path of the monstrous storm to evacuate before it is too late.
"Get out of the way, do not play with him, he's big, maybe as big as they've seen him," Trump said. "We will solve it, we are ready, we can".
"But despite that, bad things can happen when you talk about a storm of this size," he added.
Up to 1.7 million people are under voluntary or mandatory evacuation orders, and residents of the coast frantically embarked on homes and businesses and hit the road on Wednesday as the storm approached.
Florence is forecast to shed up to three feet (almost one meter) of rain in some areas after touching land in North and South Carolina.
"This rain would cause catastrophic flash floods and significant river flooding," the NHC said.
Storm surges of up to 3.9 meters were also predicted that threaten life in some areas of North Carolina along with the possibility of tornadoes.
"This is going to be a blow from Mike Tyson on the Carolina coast," said Jeff Byard, the associate response and recovery administrator for the Federal Emergency Management Agency (FEMA).
"This is not going to be an oblique blow," Byard said, warning of blackouts, road closures, damage to infrastructure and possible loss of life.
Ride the storm
At 2:00 p.m. (1800 GMT), the eye of the storm was 435 miles (700 km) southeast of Wilmington, North Carolina, moving northwest at 16 mph (26 kph).
The storm was heading to the coast of North and South Carolina, but heavy rains were also expected in Virginia to the north and Georgia to the south.
Georgia Governor Nathan Deal declared a state of emergency "in light of the forecast trajectory south of the storm after touching land."
"The state is mobilizing all available resources to guarantee public safety," Deal said. "I encourage the Georgians to be prepared for the terrestrial effects of the storm and the storm surge that occurs in the coastal areas."
A state of emergency has also been declared in Maryland, North Carolina, South Carolina, Virginia and Washington.
People fleeing the coasts of North Carolina and South Carolina blocked the roads Wednesday when Florence pounced on the coast to receive a direct impact on a low region with holiday homes facing the sea.
The eastbound lanes of several major freeways have been closed to allow traffic to flow inland, but the exodus was slow along roads full of vehicles heading outward.
In Columbia, South Carolina, Barry Sparks, a 66-year-old retiree, was thinking of leaving after the path of the storm moved south.
"If I need to evacuate I can go to my son's house" in North Carolina, Sparks said as he loaded a load of water bottles in his car.
"I was thinking about coming here until this morning, and now he asked me if I wanted to go there," he said.
Kevin Miller, a 50-year-old electrician, said he planned to pass the storm at his home near Charleston.
"I rode Hugo," Miller said of a 1989 hurricane that caused widespread damage. "The water level will go up a bit, but we'll be fine.
"Hugo was a direct hit," he said. "I was in the same house and I was fine."
& # 39; Disaster is at the doors & # 39;
In a trailer park on the outskirts of Wilmington, Alondra Espinoza was preparing to leave with her two small children.
"Everything is full," said Espinoza. "I want to take them as far as possible."
"I've been through hurricanes before, but never with children," he said. "If it were not for them, I would not have minded staying here."
The governor of North Carolina, Roy Cooper, warned residents that time was running out to seek security for what he called a "once-in-a-lifetime" storm.
"Disaster is just around the corner and it's coming in," Cooper said. "If you're on the coast, there's still time to get out safely."
South Carolina ordered the mandatory evacuation of a million residents of the coast, while North Carolina ordered the evacuation of the Outer Banks, barrier islands that are a popular tourist destination.
In Virginia, 245,000 coastal residents were ordered to evacuate.
Florence is being dragged into the Atlantic by two other storms: Hurricane Helene and Tropical Storm Isaac.
However, Helene was weakening and presented no danger to the earth, the NHC said, while Isaac could bring heavy rain to Martinique, Dominica and Guadeloupe.