Florence, packing winds of 140 miles per hour (220 kph), was expected to grow even stronger before making landfall on Thursday, mostly likely in southeastern North Carolina near the SC border, the National Hurricane Center in Miami said.
The 1st effects were already being seen on barrier islands Monday as unsafe rip currents and seawater flowed over the state highway.
For many people, the challenge could be finding a safe refuge.
If Florence slows to a crawl just off the coast, it could carry torrential rains up into the Appalachian mountains, causing flash floods, mudslides and other risky weather across a wide area.
"Don't concentrate on the exact forecast track of Hurricane #Florence".
A warm ocean is the fuel that powers hurricanes, and Florence will be moving over waters where temperatures are peaking near 85 degrees, hurricane specialist Eric Blake wrote.
With little wind shear to pull the storm apart, Florence's hurricane wind field is expected to expand over the coming days, increasing its storm surge and inland wind threats.
Florence, a Category 4 hurricane, is growing in size and strength and is expected to strengthen into an extremely risky storm through Thursday, the U.S. National Hurricane Center (NHC) said on Monday.
The storm was edging closer to the USA and was 975 miles (1570km) east-south-east of Cape Fear, North Carolina. Should Florence make landfall as a Category 4, it would be the first Category 4 hurricane since Hazel in 1954 to hit North Carolina.
Some strengthening is expected during the next 36 hours, and Florence is expected to be an extremely unsafe major hurricane through Thursday.
Preparations intensified up and down the densely populated coast.
Governors in North and SC declared states of emergency in advance of the storm, which already was creating risky conditions in waters off the coast.
Officials at the Savannah-Hilton Head International Airport are monitoring conditions and advise travelers to check with the airlines before heading to North Carolina, South Carolina or Virginia, Jones said. People rushed to get emergency kits ready, map out escape routes, fill sandbags and secure their homes.
The storm "is too powerful and its path is too uncertain to take any chances", SC governor Henry McMaster said, issuing his state's emergency declaration.
White House press secretary Sarah Sanders said in a statement Monday that President Donald Trump had a briefing call at the White House about Hurricane Florence with the Department of Homeland Security and the Federal Emergency Management Agency.
Myrtle Beach hardware stores and supermarkets were busy ringing up sales of bottled water, plywood and generators.
Forecasts expect 10 to 15 inches (25-38 cm) of rain in the hardest-hit areas, possibly more if the storm stalls over land, as expected, Graham said.
South Carolina Gov. Henry McMaster ordered a mandatory evacuation of all eight counties on the coast on Monday, The State newspaper in Columbia reported. The Navy planned to send ships from the Hampton Roads area of Virginia out to sea.
Larger vessels, such as cruise or cargo ships, may have to reroute their courses to avoid Florence's unsafe seas.
Maximum sustained winds were clocked at 140 miles per hour (220 kph) as it moved west-northwest at 13 miles per hour (20 kph).