Hurricane warnings are issued 36 hours before tropical-storm-force winds hit the areas.
But North Carolina Gov. Roy Cooper warned: "Don't relax, don't get complacent". "Catastrophic effects will be felt".
The NHC predicts the storm will continue in that direction and approach the coast of the Carolinas, entering the hurricane warning area by Thursday and moving slowly near the coastline through Saturday. 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.
The National looks at the science behind Hurricane Florence and why it's being called a once-in-a-generation storm.
The storm's center was about 170 miles southeast of Wilmington at 8 a.m. Thursday, according to a National Hurricane Center briefing.
"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.
It is the storm's movement and not its strength that has forecasters and officials anxious. Large, battering waves will ride atop this surge. The Wilmington area can now expect 9-to-13-foot storm surges if Florence's peak impact coincides with high tide, Brunswick County coastal areas west of Southport can expect 6-to-9-foot surges.
"This rainfall will produce catastrophic flash flooding and prolonged significant river flooding", the hurricane center said.
NWS confirms that "major to extreme flooding" is likely across central and eastern North Carolina, as Hurricane Florence threatens to pound the area for a full 24 hours.
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Anyone remaining in coastal areas on the East Coast is warned that the surges can cause life-threatening surf and rip current conditions. Utility companies said millions were expected to lose power and that restoring it could take weeks.
North Carolina corrections officials said more than 3,000 people were relocated from adult prisons and juvenile centres in the path of Florence, and more than 300 county prisoners were transferred to state facilities.
Florence was seen as a major test for FEMA, which was heavily criticized as slow and unprepared a year ago for Hurricane Maria in Puerto Rico, which was blamed for almost 3,000 deaths. Hurricane-force winds extended 80 miles (130 kilometers) from its center, and tropical-storm-force winds up to 195 miles (315 kilometers). Heavy rains were forecast to extend into the Appalachian mountains, affecting parts of Alabama, Tennessee, Kentucky and West Virginia.
The Pentagon has dispatched two ships and a Marine unit offshore to provide help if needed. As the storm moves inland, Georgia, Virginia and Maryland will also be in peril.
Rescue helicopters and trucks that can navigate floodwaters are also standing by.
"This is a very risky hurricane", Mr McMaster said, adding that the evacuation order for coastal counties was "mandatory, not voluntary".
If you're anxious about flooding away from the coast, then the Southeast River Forecast Center is your go-to stop.
"Our sand dunes are healthy, but they're not going to be able to keep back a wall of water like that", he said.
The police chief of Wrightsville Beach suggested that those who made a decision to stay give him their next-of-kin contact information. "Life-threatening storm surge and rainfall expected". "We are ready. We're as ready as anybody's ever been".
He said there are about 7,000 USA military forces now in place and ready to respond to the storm - along with ships, helicopters and high-wheeled vehicles. "You never know where there are active downed powerlines that could cause an electric situation", Parker says.