The storm's intensity diminished as it neared land, with winds dropping to around 90 miles per hour (144 kph) by nightfall.
The hurricane's storm surge - the wall of water it pushed in from the Atlantic - "overwhelmed" the town of New Bern at the confluence of the Neuse and Trent rivers, North Carolina Governor Roy Cooper said.
The outer bands of the cyclone drenched the Carolinas on Thursday, flooding roads and knocking out power in an ominous glimpse of the damage it could inflict when it makes landfall on Friday.
Florence's impacts: Florence's over-all wind intensity is down, but the impact to the Carolinas will not be greatly diminished: extreme, prolonged storm surge reaching 15 feet or more, high wind gusts over 100 miles per hour, and tremendous inland flooding where more than 30 inches of rain could fall through the weekend.
The center said the threat of freshwater flooding will increase over the next several days.
Rain bands are already affecting the eastern part of North Carolina and SC, with rain increasing and waves growing.
It is the storm's movement and not its strength that has forecasters and officials anxious.
Florence's forward movement during the day slowed to a near-standstill - sometimes it was going no faster than a human can walk - and that enabled it to pile on the rain.
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National Guard soldiers from the state, North Carolina and Virginia are mobilizing to prepare for hurricane recovery efforts.
By late Thursday afternoon, Florence's fierce headwinds were already uprooting trees and tearing down power lines and had ripped the roof off of at least one building in coastal North Carolina, according to news station WGHP. Millions were expected to lose power from the storm and restoration could take weeks.
Of those who left, more than 4,000 had sought housing at emergency shelters across the state.
People are seen inside a shelter run by Red Cross before Hurricane Florence comes ashore in Grantsboro, N.C., September 13, 2018.
Even though Florence is moving to the northwest at this time, AccuWeather meteorologists believe that the hurricane will stall and meander near the Carolina coast from Thursday night to Saturday. Heavy rains were forecast to extend into the Appalachian Mountains, affecting parts of Alabama, Tennessee, Kentucky and West Virginia. "And I can not stress enough the importance of adhering to the governor's orders for mandatory evacuation".
Hurricanes and typhoons not only bring severe coastal flooding via storm surge-they create awful inland flooding as well.
Wilmington is the state's fifth-largest city, with about 121,00 residents.
"We might leave", she said. Some of those who stayed went to shelters while others stuck it out in their homes.
It said the additional declaration would bring more federal help with debris removal, search and rescue teams, meals and generators, among other items. "We're fully prepared. Food, medical, everything you can imagine, we are ready".
Mayor Beach Mayor Brenda Bethune told CNN traffic out of the SC city has more than quadrupled as residents and visitors evacuate. The city said early Friday that two out-of-state FEMA teams were working on swift-water rescues and more teams were on the way.