The category 4 hurricane is expected to hit as an "extremely unsafe hurricane" NHC says, despite predictions of some weakening after the next 48 hours.
The governors of North and South Carolina, Maryland and Virginia declared states of emergency ahead of the approaching storm.
The storm's first effects were already apparent on barrier islands as unsafe rip currents hit beaches and seawater flowed over a state highway.
With Hurricane Florence spinning through the Atlantic Ocean and threatening to slam into SC later this week, Gov. Henry McMaster ordered mandatory evacuations for the state's entire coastline beginning at noon Tuesday.
Hurricane Florence has slightly increased in speed as it heads toward the U.S. East Coast.
Describing Hurricane Florence as "a no-kidding nightmare" is certainly appropriate, with Gerst warning Americans on the East Coast to prepare for the storm's arrival.
In North Carolina, all Hatteras Island residents and visitors were ordered to evacuate starting at noon Monday, according to Dare County Emergency Management.
The evacuation order extends to eight counties along the state's coast, with McMaster estimating that around one million people - residents and visitors - will be affected by the order.
Meanwhile, two other storms were also spinning in the Atlantic.
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But the landfall intensity of that storm was 250 km/h when it hit Puerto Rico, it added. Cooper already issued a state of emergency late last week.
Florence is expected to bring heavy rain along the coast of the Carolinas, but several towns located inland are expected to see a lot of rain. SC is one-waying several interstates including I-26 out of Charleston.
Virginia Governor Ralph Northam's office predicted "catastrophic inland flooding, high winds and possible widespread power outages", cautioning that the deadliest risk would come from flooding.
"We know the evacuation order I'm issuing will be inconvenient", McMaster said.
In the eastern Pacific Ocean, forecasters are keeping a close watch on Tropical Storm Paul and Hurricane Olivia.
Sustained winds Monday afternoon were up to 140 miles per hour.
Life-threatening storm surges - as high as 10 feet or more - also are likely along portions of the coast, in addition to high surf and "deadly" rip currents, which Quagliariello said will continue through the week.
"Florence is expected to be an extremely risky major hurricane through Thursday (Sept. 13)", said officials.
South Carolina National Guard soldiers transfer bulk diesel fuel into fuel tanker trucks for distribution in advance of Hurricane Florence, in North Charleston, South Carolina, U.S. September 10, 2018.
"We've basically got everything we need. and we're just going to wait it out for a few days", he said.