"It can not be emphasized enough that the most serious hazard associated with slow-moving Florence is extremely heavy rainfall, which will cause disastrous flooding that will be spreading inland through the weekend", the National Hurricane Center said in its 5 a.m. update.
Brock Long, the administrator of the Federal Emergency Management Agency (FEMA), warned the danger was not only along the coast: "Inland flooding kills a lot of people, unfortunately, and that's what we're about to see", he said.
More than 12,000 were in shelters in North Carolina.
According to the latest figures by the National Flood Insurance Program, fewer homeowners in North and SC own flood insurance today than five years ago.
About 10 million people could be affected by the storm and more than 1 million were ordered to evacuate the coasts of the Carolinas and Virginia, jamming westbound roads and highways for miles.
The southern portion of the Virginia coast is under a tropical storm warning - from north of Duck, North Carolina, to Cape Charles Light, Virginia, and the Chesapeake Bay south of New Point Comfort.
The storm is moving inland at a speed of 6 miles per hour and conditions are likely to deteriorate as it pushes its way southwest along the coast Friday through Saturday.
The worst of the storm's fury had yet to reach coastal SC, where emergency managers said people could still leave flood-prone areas. Coastal towns in the Carolinas were largely empty, and schools and businesses closed as far south as Georgia.
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Forecasters predict as much as 101 centimeters (40 inches) of rain for some parts of North Carolina and storm surges as high as 4 meters - taller than many houses. Categories only represent the speed of sustained winds, and these are still destructive.
Florence was seen as a major test for Fema, which was heavily criticised as slow and unprepared previous year for Hurricane Maria in Puerto Rico.
Others were at home hoping for the best.
Emergency officials on Wrightsville Beach were able to assess the beginning impacts of Hurricane Florence before day turned to night Thursday. But James said she and her family are prepared and stocked up on several items. "We have a safe basement and generator that comes on automatically". We probably will be getting heavy winds and rain for the next few days.
The Wilmington airport had a wind gust clocked at 105 miles per hour (169 kph), the highest since Hurricane Helene in 1958, the weather service said. The storm surge could rise up to 13 feet - that's water inundating homes up to the first-floor ceiling, the National Hurricane Center said.
One electricity company fears that three-quarters of its four million customers will lose power as a result of the storm, and may not be reconnected for weeks.
Populated coastline: 11 million Americans live in areas under storm watches and warnings.
Grounded: almost 1,200 flights cancelled. "So will the power lines, as the trees fall down".