In New Bern, North Carolina, the storm surge overwhelmed the town of 30,000, located at the confluence of the Neuse and Trent rivers.
Florence had been a Category 3 hurricane on the five-step Saffir-Simpson scale with 120-mph winds as of Thursday, but dropped to a Category 1 hurricane before coming ashore near Wrightsville Beach close to Wilmington.
The storm looked likely to make landfall on the coast of North and SC but heavy rain was also expected in Virginia to the north and Georgia to the south.
The National Hurricane Center downgraded it to a tropical storm on Friday afternoon, but warned it would dump as much as 30 to 40 inches of rain on the southeastern coast of North Carolina and into the northeastern coast of SC in spots.
Boarding up his home in Myrtle Beach, South Carolina, Chris Pennington watched the forecasts and tried to decide when to leave.More news: I Believe In Maurizio Sarri's Philosophy, Chelsea Defender Insists
As of 11 p.m., the storm was centered 280 miles (455 kilometers) southeast of Wilmington, North Carolina, moving northwest at 17 mph (28 kph). The only exception was Edisto Beach - that and other areas to the north remain under evacuation orders.
"Historical rainfall amounts from #Florence are possible with UNPRECEDENTED flooding", according to the National Weather Service office in Newport and Morehead City, on North Carolina's coast.
In Wilmington, the fire department is urging people to prepare for at least seven days of hardship, collecting water and nonperishable foods. By late Wednesday, authorities in North Carolina reported almost 7,000 evacuees staying in 71 emergency shelters throughout the state.
The ocean waters that Florence is travelling over are about 2.7 degrees (1.5 degrees Celsius) warmer than normal, McNoldy said.
US President Donald Trump promised Wednesday that the federal government was "ready" and "able" to handle Hurricane Florence, warning residents to get out of the way as the monster storm bears down on the East Coast.
The NHC said Helene was weakening, however, and posed no danger to land. As the hurricane center says, "Florence is expected to slow down considerably by late Thursday into Friday, and move slowly through early Saturday".More news: Manafort Wants "No Cooperation" Plea Deal
A warning is typically issued 36 hours before the anticipated first occurrence of tropical-storm-force winds, conditions that make outside preparations hard or unsafe.
Some areas are expected to see between 20 and 30 inches of rain through early next week in the area shaded in purple in the second image above.
"This rainfall will produce catastrophic flash flooding and prolonged significant river flooding", the hurricane center said.
South Carolina Governor Henry McMaster ordered the mandatory evacuation of one million coastal residents.
The shift in the projected track had areas that once thought they were out of range anxious. She said it will become a "major flooding event".
Higher sea levels to make a storm surge worse.More news: Amazon chief Jeff Bezos gives $2bn to help the homeless
Jeff Byard of the US Federal Emergency Management Agency said it was imperative locals heed the evacuation warnings.