South Carolina Governor Henry McMaster said the heavy rain could trigger landslides in the west of his state. "Get prepared on the East Coast, this is a no-kidding nightmare coming for you".
Cooper spoke at a news conference Thursday afternoon with state emergency management officials.
Florence's winds weakened as it drew closer to land, dropping from a peak of 140 miles per hour (225 kph) earlier in the week, and the hurricane was downgraded from a terrifying Category 4 to a 2.
Roads and intersections on North Carolina's Outer Banks barrier islands were already inundated with water.
Myrtle Beach was virtually deserted with empty streets, boarded up storefronts and very little traffic. "He is the only caregiver to me and my son", Browning said.
"Afterwards, I'm going to drink a bottle of whiskey and take a two-day nap, but right now I'm walking the neighbourhood making sure my neighbours are fine because nobody can get in here". "We're able", he said.
On Thursday morning, Florence was spotted 378 km east southeast of Wilmington, North Carolina, and about 450 km east southeast of Myrtle Beach, South Carolina, moving northwest at 28 kph.
The National Hurricane Center noted the following at 11 a.m.: "Florence is moving toward the northwest near 10 miles per hour (17 km/h)". Hurricane-force winds extended 90 miles (150 kilometers) from its center, and tropical-storm-force winds up to 195 miles (315 kilometers).
Heavy rain, storm surge, damaging winds, rough surf and isolated tornadoes will be possible, especially in eastern North Carolina.
The advisory read: "This rainfall will produce catastrophic flash flooding and prolonged significant river flooding".
After the storm, crews will give priority to critical infrastructures first, then work on fixing lines that will restore power to the most people, but downed trees and flooding could cause delays.
"Your time is running out", he said.More news: EPL: Arsenal provides latest injury updates ahead of Newcastle clash
She said a hurricane has a way of bringing everyone to the same level.
Forecasters said that given the storm's size and sluggish track, it could cause epic damage akin to what the Houston area saw during Hurricane Harvey just over a year ago, with floodwaters swamping homes and businesses and washing over industrial waste sites and hog-manure ponds.
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.
Its surge could cover all but a sliver of the Carolina coast under as much as 11 feet (3.4 meters) of ocean water, and days of downpours could unload more than 3 feet (0.9 meters) of rain, touching off severe flooding.
In Virginia, 245,000 coastal residents were ordered to evacuate.
So far, a state of emergency has been declared in five states - South Carolina, North Carolina, Virginia, Georgia and Maryland and Washington DC.
Duke Energy Co. said Florence could knock out electricity to three-quarters of its 4 million customers in the Carolinas, and outages could last for weeks.
Not everybody was heeding orders to evacuate, however.
Will Epperson, a 36-year-old golf course assistant superintendent, said he and his wife had planned to ride out the storm at their home in Hampstead, North Carolina, but reconsidered due to its ferocity. "I'm not leaving him here".
US President Donald Trump touted the government's readiness and urged people to get out of the way.
"I'm not going to lie: I'm scared", she said.