At 11pm Atlantic Time on Monday night (4am Tuesday BST), Florence was churning at 465 miles south-southeast of Bermuda, and 1,085 miles east-southeast of Cape Fear North Carolina, according to the National Hurricane Centre (NHC).
The formidable storm is packing maximum sustained winds of 140 mph, making it a Category 4 Hurricane on the Saffir-Simpson scale, and it is expected to strengthen further.
Florence is moving in a west-northwest motion at 13 mph.
The NHC warned Florence will approach the coast of North or South Carolina on Thursday and is “expected to be an extremely dangerous major hurricane through Thursday.”
The storm, which forecasters were previously hoping could yet be diverted, now looks certain to be a direct and life-threatening hit.
US President Donald Trump tweeted earlier, saying: “My people just informed me that this is one of the worst storms to hit the East Coast in many years. Also, looking like a direct hit on North Carolina, South Carolina and Virginia. Please be prepared, be careful and be SAFE!”
He added the “Federal Government stands by, ready to assist 24/7.”
Mr Trump has signed approvals for the declarations of emergency in the Carolinas.
Mandatory evacuations have been ordered in South Carolina, North Carolina and Virginia, affecting more than a million people.
South Carolina’s Governor Henry McMaster has given the entire coastline until noon on Tuesday to leave.
The University of North Carolina at Wilmington has also ordered a mandatory evacuation of students.
The storm is expected to pass between Bermuda and the Bahamas on Tuesday and Wednesday, making landfall on the North Carolina coast on Thursday around 10pm local time (3am Friday BST).
Schools in affected areas will begin to close on Tuesday and lanes on some highways will be reversed to aid evacuations.
The office of Virginia Governor Ralph Northam predicted «catastrophic inland flooding, high winds and possible widespread power outages».
Rainfall could be up to 20in (50cm) in some area.
This could worsen if, as some meteorologists predict, Florence stalls after it makes landfall.
The NHC is also warning of swell generated by Florence which is already affecting Bermuda and parts of the US East Coast.
The advisory said: “These swells are likely to cause life-threatening surf and rip current conditions.”
The US Navy is sending 30 ships stationed in Virginia out to sea to wait at anchor while the storm passes.
The NHC expects to issue its first official hurricane and storm surge warnings at some point on Tuesday.