Meteorologists are calling the Category 4 storm "extremely dangerous" and of a "staggering size", with flooding expected in thousands of buildings along the Carolina coast. At 11am (1500 GMT) on Wednesday, the storm was located about 485 miles (785 km) southeast of Wilmington, North Carolina.
As of Wednesday afternoon, Florence is a powerful Category 3 storm with maximum sustained winds of about 125mph as it continued on its path toward the Carolinas Wednesday. The destructive storm surge will occur as the hurricane's eye comes ashore from Thursday night into Friday, the Weather Channel said, and coastal flooding may persist through multiple high tide cycles into this weekend east of the center of Florence.
For a swath of the North Carolina shore from Cape Fear to Cape Lookout, the storm surge could range from 9-13 feet, the NHC said.
McMaster ordered mandatory evacuation of coastal SC counties on Tuesday.
The slow movement, combined with the massive amount of moisture this storm holds, will bring risky rains - from 20 to 30 inches in coastal North Carolina, with 40 inches possible in isolated areas, the weather service says. US President Donald Trump on Twitter warned of the storm's dangers and praised his administration's handling of past hurricanes, rejecting criticism for its response to Hurricane Maria previous year in Puerto Rico.
The number of people told to evacuate in the Carolinas and Virginia has been estimated at more than 1 million, according to an NPR report.
Forecasters have adjusted Hurricane Florence's predicted path, saying that after it makes landfall, it will likely take a more southerly route than had previously been expected.
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By Saturday, total rainfall could accumulate to 30 inches (76 centimetres) - or even 40 inches in places - in parts of the Carolinas and Virginia, the NHC said.
For anyone needing a place to stay to escape the hurricane, both North Carolina and SC are updating their lists of emergency shelters.
"The concern is about people who are very young or very old, who don't have much money or access to transportation, who do not speak the predominant language or have access to information", Thomas Tighe, of Direct Relief, said. From there, people will be transported to open shelters, he said. "Florence is expected to slow down considerably by late Thursday into Friday, and move slowly through early Saturday". "Get food, water. Medications they may need".
If the forecast holds, Hurricane Florence will become the highest north on the East Coast that a Category 4 hurricane has ever made landfall. In South Carolina and inland parts of North Carolina, 5 to 10 inches of rain could fall, with 20 inches possible in some areas.
The tropical storm-force winds extend 280 miles across the Atlantic Ocean, which is far enough to reach from Boston to Philadelphia.
Tecklenburg said his flood-prone city is preparing for "copious rain" by clearing out the city's drainage system and getting boats and portable pumps ready. "It goes well inland".
Communities could lose electricity for weeks, Federal Emergency Management Agency Administrator Brock Long said. "As guests stock up, we know some of these items are selling out, but we're working around the clock to restock them as long as we safely can", the company said in a statement.