More than one million people in the United States have been asked to evacuate ahead of what could potentially be one of the most catastrophic storms to hit the country's East Coast region in decades.
Carrying winds of up to 220 km/h as a Category 4 storm, Hurricane Florence is expected to strengthen and possibly become a Category 5 storm on Tuesday.
It is forecast to make landfall in the southeastern states of North or South Carolina on Thursday.
"This is one of the worst storms to hit the East Coast in many years," US President Donald Trump warned. "Please be prepared, be careful and be SAFE!"
South Carolina Governor Henry McMaster ordered as many as one million residents of the state's entire eastern coastline to leave their homes starting at noon on Tuesday.
"This is a very dangerous hurricane," McMaster said, adding that the evacuation order for coastal counties was "mandatory, not voluntary".
"We do not want to risk one South Carolina life in this hurricane," the governor told reporters. "We're liable to have a whole lot of flooding."
Schools in 26 of the state's 46 counties were to close from Tuesday.
The governor of neighbouring North Carolina also ordered an evacuation of the Outer Banks, barrier islands that are a popular tourist destination, and parts of coastal Dare County, while a state of emergency was declared in Virginia.
Trump has approved emergency declarations for both coastal states, a standard move allowing the release of federal funds and equipment to aid in protection and recovery efforts.
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".
Hurricane Florence has the potential to bring catastrophic flooding to areas of the eastern US already soaked by heavy rain.
On its current track, Florence is expected to slam South Carolina, North Carolina and Virginia the hardest.
"Don't concentrate on the exact forecast track of Hurricane Florence. Significant effects will extend outside the cone, and will arrive at the coast sooner than the eye," the National Hurricane Centre warned.
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.
The US Navy said it was preparing to send about 30 ships stationed in Virginia out to sea.