The National Hurricane Center or NHC noted that hurricane-force winds extend outward up to 80 miles (130 km) from the center and tropical-storm-force winds extend outward up to 195 miles (315 km). "Don't plan to leave once the winds and rains start".
"While the peak winds have fallen, they are still strong enough to be destructive to houses and trees, and the water-related threats (storm surge and rain) remain a very big concern", he wrote in a blog post Thursday.
At 2:20 a.m. EDT (0230 UTC) on September 13, Moderate Resolution Imagine Spectroradiometer or MODIS instrument aboard NASA's Aqua satellite analyzed Hurricane Florence in infrared light. This is a powerful storm that can kill.
"This is a horrific nightmare storm from a meteorological perspective", University of Georgia meteorology professor Marshall Shepherd said.
"I don't care if this goes down to a Category 1", said CNN meteorologist Chad Myers. "Flooding is nearly guaranteed".
Florence's nighttime winds were down to 110 miles per hour from a high of 140 miles per hour, and the Category 3 storm fell to a Category 2, with a further slow weakening expected as the storm nears the coast.
The National Hurricane Center's best guess was that Florence would blow ashore as early as Friday afternoon around the North Carolina-South Carolina line, then push its rainy way westward with a potential for catastrophic inland flooding.
On Thursday morning, a tweet from South Carolina's Emergency Management Division brought this perspective on Florence's sheer size: It's "larger than SC and North Carolina combined".
A satellite altimeter under the northeast quadrant, the strongest area of the storm, recorded the wave.
The storm is not expected to change much in strength before making landfall.
Florence could threaten the largest rainfall event in US history, Hurricane Harvey, the 2017 whirlwind that settled over Texas and dropped more than 60 inches of rain, turning roadways into rivers, destroying homes and buildings and killing almost 90 people. Both North Carolina and SC are updating their lists of emergency shelters for people caught out by the storm.
Around 5.25million people live in areas under hurricane warnings or watches and 4.9 million in places covered by tropical storm warnings or watches, the National Weather Service said.
As Florence closed in, some residents weighed whether to ride it out. In Myrtle Beach, S.C., and nearby areas, the surge could hit 6-9 feet.
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In Wilmington, Richard King, 64, said Wednesday that he, his wife and perhaps 60 of their neighbors planned to stay.
Florence's winds dropped from a peak of 140 miles per hour to 105 miles per hour early Thursday.
"We're a good community up there".
President Trump says the Federal Emergency Management Agency and first responders are poised to help - but he also warned people to get out of the storm's path.
People on barrier islands or peninsulas were running out of time Wednesday to retreat by bridge or ferry.
"Hurricane Florence was uninvited but she's just about here anyway", he said at a news conference. She packed up what she could and took a ferry Tuesday night. "Everyone was sold out", she said.
"Just because we have a landfall to your south, that doesn't mean you're out of the woods, because the winds are huge around this system", NHC Director Ken Graham said, pointing out the wide risk of flooding.
Officials said traffic was moving well Wednesday though there have been two minor accidents in lanes where cars were traveling in the opposite direction from normal.
About 300,000 people have been evacuated from South Carolina, McMaster said. "This is going to be a Mike Tyson punch to the Carolina coast". It urged residents to heed evacuation orders.
Georgia Governor Nathan Deal, concerned the storm would bring its devastation south, issued an emergency declaration for all 159 counties in his state.
Florence's expanse has even captured the attention of the astronauts and cosmonauts aboard the International Space Station, who have been tweeting pictures of the storm back to Earth. "If God's coming for you, you can't run from him".
By early Saturday morning, Florence is expected to have weakened to a tropical storm, and by early Sunday, it will be a tropical depression.