Information gathered Tuesday by a hurricane-hunting aircraft suggests it will intensify again as it nears the coast, approaching the 157 miles per hour (253 kph) threshold for a worst-case Category 5 scenario.
Meanwhile, a storm surge watch was sent out for Edisto Beach in SC to the North Carolina-Virginia border as well as Albemarle and Pamlico Sounds, including the Neuse and Pamlico Rivers.
North Carolina governor Roy Cooper described the storm as "a monster". "It's big and it's vicious".
"It's an extremely unsafe, life-threatening, historic hurricane".
Trump faced severe criticism for his administration's response to Hurricane Maria previous year in Puerto Rico.
"It's too early at this point to be precise with how these impacts are going to stack up", he said, "but there is a huge potential someone is going to sustain devastating impacts as a result of this storm".
North Carolina, South Carolina, Maryland, and Virginia have declared states of emergency ahead of the storm.
HOLDEN BEACH, N.C.: The powerful Hurricane Florence threatened to bring "days and days" of rain and potentially deadly flooding to the USA southeast coast, North Carolina's governor warned on Tuesday, as some 1 million people were ordered to evacuate their homes.
A shop owner boards up his Wrightsville Beach, North Carolina, building.
The Brunswick Nuclear Plant in Southport, North Carolina, is just a few miles from where Florence is expected to make landfall.
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The current upper air pattern indicates Florence will probably move west into NY and Pennsylvania as it weakens.
"Don't concentrate on the exact forecast track of Hurricane Florence".
Virginia Governor Ralph Northam also issued mandatory evacuations to begin Tuesday morning for the most low-lying, flood prone areas of the state - the Eastern Shore and a section of coast identified by officials as "Zone A".
Rainfall could be up to 20in (50cm) in some areas.
"I know they've been constantly meeting about the hurricane", North Carolina coach Larry Fedora told reporters.
The US Navy is sending 30 ships stationed in Virginia out to sea.
Power companies warned residents may be without power for days or weeks.
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, Sept. 10, 2018.
"Emergency management resources are expected to be fully utilized and needed throughout the region, and today's decision will help give ample time to deploy those resources across the state", NC State said.
Two tropical storms are following behind it: Isaac and Helene. The most powerful winds will arrive overnight Thursday and throughout the day on Friday.
The Beaufort County School District is "re-evaluating its capacity to reopen schools later this week based on the availability of staff, many of whom have already evacuated", according to district spokesperson Jim Foster.