"I can not overstate it: Flood waters are rising".
More than 30 inches (75cm) of rain has fallen across North and SC since Friday, with tens of thousands of people evacuated from their homes.
But one meteorologist said, "This is still a catastrophic, life-threatening storm". "Those rivers are going to start to crest later today and Tuesday and maybe longer".
Millions of Americans are facing "historic and unprecedented flooding" from record levels of rainfall as Florence continues to pummel the Carolinas, with the worst yet to come.
Major river flooding is expected on some rivers from southern Virginia to northern SC, including most of North Carolina.
Florence made landfall Friday as a Category 1 hurricane but has since been downgraded to a tropical storm, even as it continued to wreak havoc along the East Coast, downing trees and power lines and forcing 20,000 people to flee to shelters.
It has dumped up to 40 inches (100 cm) of rain on North Carolina since Thursday and continued to produce heavy rain over much of North Carolina and eastern SC, the NWS said. "This smashes the previous yearly rainfall record of 83.65 inches set in 1877.with over 3 months to go", the Weather Service tweeted. Radar showed parts of the sprawling storm over six states, with North and SC at the centre.
James Berthold checks on his van stuck in floodwaters in Newport, N.C.
Several victims died on flooded roads.
County spokesman James McConnell confirmed to The Associated Press that officials believe the tree fell because of the rain and wind from the storm's remnants.
Woody White, the New Hanover county commission chair, said officials were planning for food and water to be flown into the coastal city. "We want you home, but you can't come yet".
More than 641,000 homes and businesses were without electricity in North and SC and surrounding states, down from a peak of almost 1 million. Hundreds of water rescues have been carried out by local authorities, the Cajun Navy volunteers and the U.S. Coast Guard since Friday morning.
-63-year-old Mark Carter King and 61-year-old Debra Collins Rion of Loris, South Carolina, died of carbon monoxide poisoning from running a generator indoors, authorities said.
Water on the Cape Fear River, which passes through Fayetteville, kept rising Sunday, reaching more than 41 feet (12.5 meters), according to the National Weather Service.
The rain from Florence in Wilmington pushed its 2018 rainfall total to 220.4cm.
A least seven people have died so far in the storm in North Carolina, including a mother and child killed by a falling tree and three people who drowned, state officials said.
In the Lehigh Valley, Davis said the rainfall could result in some localized flash flooding and wind gusts of about 20 miles per hour, much less than the over 105 miles per hour gusts Thursday night that sent seawater surging ashore along the Outer Banks in North Carolina.