Officials in North Carolina's Harnett County, about 90 miles inland, urged residents of about 1,100 homes to clear out because the Lower Little River was rising toward record levels.
"Rivers are rising to risky levels, and the relentless rains will continue through the weekend", North Carolina Gov. Roy Cooper warned on Friday.
Parts of West-central Virginia and far eastern West Virginia will see "life-threatening flash flooding and river flooding, along with an elevated risk for landslides", according to the agency.
"This is a hurricane event followed by a flood event", said South Carolina Governor Henry McMaster.
"I feel like the dumbest human being who ever walked the face of the earth", he said.
NC resident dies plugging in a generator, marking 4th storm-related death
Numerous calls are from residents who are trapped in their homes, some of whom reported floodwater rising up to the second floor. September 14, 2018. "If you've ever doubted the destructiveness of a hurricane, what's happening here will make you a believer".
The Miami-based center had said earlier Friday Florence's arrival would come with "catastrophic" fresh water flooding over portions of the Carolinas.
View of Florence in Wilmington, N.C., on Friday. As of Saturday afternoon, about 752,000 homes and businesses were without power in North Carolina, along with 119,000 in SC.
As 400-mile-wide (645-kilometer-wide) Florence pounded away at the coast with torrential downpours and surging seas, rescue crews used boats to reach more than 360 people besieged by rising waters in New Bern, North Carolina, while many of their neighbors awaited help.
"Sooner or later we need to get out of there", Orrin H. Pilkey, a professor emeritus of geology at Duke University, told The Associated Press before the storm hit.
Elsewhere in North Carolina, a man was knocked to the ground while outside and died.
Two people also died in Harkers Island, North Carolina, on Friday, Carteret County Director of Emergency Services Stephen Rea said Saturday.
Nationwide warning system test scheduled for next week, officials want Alaska feedback
The system is already used to warn the public about "dangerous weather, missing children, and other critical situations". Everyone will then receive this message in English: "THIS IS A TEST of the National Wireless Emergency Alert System".
More than 80,000 people were already without power as the storm began buffeting the coast, and more than 12,000 were in shelters.
The tree was about 6 feet above the road surface.
It already had, in Beaufort County and its capital city, Washington, where 300 people were rescued from flooded homes. A 61-year-old woman died Friday night when her auto struck a tree that fell during the storm, state emergency officials said. That could affect the infrastructure, bridges, residences, farms and other buildings, Dahler said.
Coastal Albemarle and Pamlico Sounds will likely become bottlenecks, filling with runoff faster than they can drain into the ocean, he said.
"Hurricane Florence is going to continue its violent grind across our state for days". Duke Energy said late Friday that it anticipates 1 million to 3 million outages in the Carolinas, and that full power restoration could take weeks.
In Lumberton, North Carolina, volunteers are working to try to prevent a repeat of what happened with 2016's Hurricane Matthew, which left the city underwater for days. On Friday afternoon, it was traveling at just 3 to 5 miles per hour, still near the coast. The evacuation zone included part of the city of Fayetteville, population 200,000.
At least 12 dead as Typhoon Mangkhut lashes Philippines
On Guam, where Mangkhut passed, residents dealt with flooded streets, downed trees and widespread power outages. At least four million people are in the direct path of Mangkhut.