Its storm surge and the prospect of 1-3ft of rain were considered a bigger threat than its winds, which had dropped off from an alarming 140mph earlier in the week.
More than 2 feet of rain already had fallen in places, and the drenching went on and on as Florence, a hurricane-turned-tropical storm, practically parked itself over the two states.
"This storm is relentless and excruciating", North Carolina Governor Roy Cooper told CNN late on Friday.
"And we're not done yet", Graham said, adding that some hard-hit areas could get an additional 15 to 20 inches because the storm was moving so slowly.
Heavy rainbands along with tropical storm-force winds are spreading across the outer banks and coastal southeastern North Carolina, the NHC said on Thursday. Rescue crews used boats to carry more than 360 people from rising water in the river town of New Bern, N.C., while many of their neighbours awaited help.
Hurricane Florence is continuing on its deadly path, causing widespread flooding and damage despite its downgrading to a tropical storm.
New Bern, home to about 300,000 people, has seen more than 10 feet of storm surge and almost 10 inches of rain since Friday evening.
Storm surges - the bulge of ocean water pushed ashore by the hurricane - were as high as 10 feet.
The center of the storm made landfall around 7:15 a.m. near Wrightsville Beach, N.C., close to the SC border.
The town of Oriental, North Carolina, had gotten more than 18 inches of rain just a few hours into the deluge, while Surf City had 14 inches and it was still coming down.
Forecasters said catastrophic freshwater flooding is expected well inland over the next few days as Florence crawls westward across the Carolinas all weekend.
About 9,700 National Guard troops and civilians were deployed with high-water vehicles, helicopters and boats.
Authorities warned, too, of risks of mudslides and environmental disasters from floodwaters washing over industrial waste sites and hog farms. After criticism for its response in Puerto Rico to last year's Hurricane Maria, which officials there said was responsible for 3,000 deaths, Trump has vowed a vigorous response to Florence and defended his handling of Maria.
Flood waters from Tropical Storm Florence cover a golf club near Wallace, N.C., on September 15, 2018.
The National Hurricane Center said Florence will eventually break up over the southern Appalachians and make a right hook to the northeast, its rainy remnants moving into the mid-Atlantic states and New England by the middle of next week.
Charlotte and Asheville in North Carolina, and Roanoke, Virginia, could be in for heavy rains as Florence plods inland. That's enough to fill the Chesapeake Bay or cover the entire state of Texas with almost 10 centimetres of water, he calculated.
The rest of SC and North Carolina into southwestern Virginia will experience 5 to 10 inches, with isolated totals up to 15 inches. The Neuse River rose rapidly, overcoming the streets of New Bern. "You may need to move up to the second story, or to your attic, but WE ARE COMING TO GET YOU".
"The wind was so hard, the waters were so hard ..." "We got thrown into mailboxes". She retreated and was eventually rescued by a boat crew.
"Honestly, I grew up in Wilmington".
"Hurricane Florence was uninvited but she's just about here anyway", he said at a news conference.