"This is one of the worst storms to hit the East Coast in many years", President Donald Trump warned on Twitter.
"Please take all necessary precautions".
At a hardware store in downtown Charleston, South Carolina, store manager John Johnson said the rush on batteries, flashlights, plastic tarps and sandbags began Friday.
The US president said he had spoken with governors of the threatened states, adding that the "federal government stands by, ready to assist 24/7".
HurricaneIsaac, stirring in the mid-Atlantic Ocean with twist rates of around 75 miles per hour, is following firmly behind Florence. She grew up on the island chain, and says this will be only the second time she's evacuated. "This is probably the only exercise I get this week".
A tracker map from the center on Monday forecast the hurricane making landfall in the Carolinas Thursday morning, and expected the storm to continue through Saturday morning, when it will have weakened to a tropical depression.
"The track that Florence is taking is very atypical", Klotzbach said in an email. Emergency declarations were issued by the governors in both states, as well as in Virginia and Maryland. "Everyone in Virginia needs to prepare", he said.
No coastal watches or warnings were in effect as of Monday morning.
Now rated as a category 4 storm - the second most powerful on the national weather service's (NWS) classification system - Florence would be the first storm of that magnitude in recorded history to strike the eastern coastline so far north if it remains on its most likely track.
RAINFALL: Olivia is expected to produce total rainfall accumulations of 10 to 15 inches.
"Never have we seen quite this type of a storm approach us".
"A few moments later, #Isaac & the outer bands of #Helene were also visible, "Arnold tweeted, referring to two other hurricanes swirling to the east of Hurricane Florence".
"This could be the first Category 4 hurricane to make landfall in the Carolinas since Hugo", Quagliariello said, referring to the Category 5 hurricane that caused widespread damage in the Caribbean and Southeast U.S.in 1989.
Hurricane conditions may arrive by late Thursday or Thursday night.
The airlines affected by the waivers vary from city to city, but most are now in the path of Florence as meteorologists are tracking the category 4 storm. There is the chance that Isaac will strike Puerto Rico - still recovering from the deadly Hurricane Maria last September - as a tropical storm.
In announcing his evacuation order, South Carolina Gov. Henry McMaster said an estimated 1 million people would be fleeing the coast.
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