And an unusual combination of other weather systems that are likely to stall Florence when it hits the Carolinas, allowing it to sit for days and dump huge amounts of rain.
Speaking at a news conference, the official said the hurricane was expected to spread out into the Central Appalachian mountains, the Tennessee Valley, and southeastern parts of the United States after Saturday.
The storm, which fell from category four to category three on the five-step Saffir-Simpson scale, is expected to strengthen despite it weakening on Wednesday.
"Even if you've ridden out storms before, this one is different".
People in areas vulnerable to a risky hurricane have left or are fleeing ahead of the storm's expected Friday or Saturday landfall.
The threat of storm surges loom for areas in the path of the storm, meaning life-threatening inundation from rising water moving inland is possible in the next 36 hours. North and SC and Virginia declared emergencies earlier in the week.
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New projections indicate the now-Category 3 hurricane will move slightly southeast, with landfall expected between Wilmington and Morehead City, North Carolina.
As of Tuesday, about 1.7 million people in North and SC and Virginia were under warnings to evacuate the coast, and hurricane watches and warnings extended across an area with about 5.4 million residents.
"This rainfall would produce catastrophic flash flooding and significant river flooding", the Miami-based National Hurricane Center (NHC) said.
The storm is expected to hit the Carolinas hard later this week.
Michelle Stober loaded up valuables at her home on Wrightsville Beach to drive back to her primary residence in Cary, North Carolina.
"These enormous waves are produced by being trapped along with very strong winds moving in the same direction [of] the storm's motion", the agency said.
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Wednesday, the Category 4 storm was about 435 miles southeast of Wilmington, N.C., and moving northwest at about 16 mph. The category three storm is ploughing towards the east coast and is due to make landfall in SC early on Friday morning.
The percentage of customers who may be affected by power outages following Hurricane Florence.
In the first report of storm-related fatalities, a mother and infant died after a tree fell on a house in Wilmington, North Carolina, according to local police.
Washington, DC, Mayor Muriel Bowser and the governors of North Carolina, South Carolina, Virginia, Georgia and Maryland have declared states of emergency. The port in Charleston, South Carolina, though, was suspending operations from Thursday through Saturday and possibly on Sunday.
If some of the computer projections hold, "it's going to come roaring up to the coast Thursday night and say, 'I'm not sure I really want to do this, and I'll just take a tour of the coast and decide where I want to go inland, '" said Jeff Masters, meteorology director of the private Weather Underground forecasting service. Workers at Florence Toyota in Florence, South Carolina, plan to pull high-value luxury vehicles into the service bays on Thursday to protect them.
On Wednesday morning, Florence was extending hurricane-force winds outward up to 70 miles from its center, with tropical-storm-force winds up to 175 miles outward.
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Authorities have advised people, however, to move to higher levels in their homes while they wait to be rescued. At least 150 people were waiting to be rescued in New Bern, N.C., officials tweeted Thursday evening.