At least four people were killed.
Immigration officers have been dispatched to help with response and recovery as Florence lashes North and SC with life-threatening winds, rain and floods.
Some of the worst flooding from the monster storm was in the town of New Bern, North Carolina, where the Neuse River overflowed its banks, flooding streets and trapping many people in their homes. More than 60 others had to be rescued as a cinderblock motel collapsed.
North Carolina Governor Roy Cooper told a news conference that whole communities "could be wiped away". Forecasters said catastrophic freshwater flooding is expected well inland over the next few days as Florence crawls westward across the Carolinas all weekend.
A mother and baby were killed when a tree fell on their home in Wilmington, North Carolina.
"Hurricane Florence is powerful, slow and relentless", he said.
Flooding caused by Hurricane Florence covers blocks of Front Street in downtown New Bern on Friday.
"I feel like the dumbest human being who ever walked the face of the earth", he said.
The center of Florence made technical landfall at about 7:15 a.m. on Friday near Wrightsville Beach, N.C. As of 12 p.m., the storm was about 24 miles west-southwest of Wilmington, N.C., and about 45 miles east-northeast of Myrtle Beach, S.C. It came ashore along a mostly boarded-up, emptied-out stretch of coastline. "I'm not sure if it's a shift in the wind or low tide - but the water has miraculously disappeared".
Keith Beavers, a farmer in Duplin County, North Carolina, was rushing on Thursday to harvest as much corn as he could ahead of the storm. Other communities got well over a foot. CoBank ACB, an agricultural lender, estimates damage to North Carolina farming could hit $1 billion before the storm slows.
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For people living inland in the Carolinas, the moment of maximum peril from flash flooding could arrive days later, because it takes time for rainwater to drain into rivers.
Preparing for the worst, about 9,700 National Guard troops and civilians were deployed with high-water vehicles, helicopters and boats.
Authorities warned too of the threat of mudslides and the risk of environmental havoc from floodwaters washing over industrial waste sites and pig farms.
Florence was seen as a major test for the Federal Emergency Management Agency, which was heavily criticized as slow and unprepared a year ago for Hurricane Maria in Puerto Rico, where the death toll was put at almost 3,000.
It is expected to move across parts of southeastern North Carolina and eastern SC on Friday and Saturday, then head north over the western Carolinas and central Appalachian Mountains early next week, the NHC said.
The hurricane has darkened the skies and homes of hundreds of thousands of residents of both North & South Carolina.
Farther up the coast, in New Bern, about 150 people waited to be rescued from flooding on the Neuse River, WXII-TV reported.
All of the occupants, who included children and pets, were safely rescued. He says that "over the years the state government and the federal government have become very coordinated in their ability to manage the pre-deployment of assets (and) the response to the citizens of those states, and we will soon be into the recovery".
"We got thrown into mailboxes, houses, trees", said Holt, who had stayed at home because of a doctor's appointment that was later canceled.