AccuWeather Hurricane Expert Dan Kottlowski said that a Florence landfall along the U.S. East coast is becoming more likely, with the Carolinas at greatest risk on Thursday. Mandatory evacuation orders also were issued for more than 50,000 people from Hatteras and Ocracoke, the southernmost of North Carolina's Outer Banks barrier islands.
More than a million people in the southeast United States faced mandatory evacuation orders.
The storm's potential path also includes half a dozen nuclear power plants, pits holding coal-ash and other industrial waste, and numerous eastern pig farms that store animal waste in massive open-air lagoons.
A warm ocean is the fuel that powers hurricanes, and this area of the ocean is seeing temperatures peak near 85 degrees (30 Celsius), hurricane specialist Eric Blake wrote.
"We want to make sure that we get the best data going to the National Hurricane Center. and to emergency managers and to the media so we can get people out of the way", Flaherty said.
Horry and Georgetown counties shifted their operating conditions at noon Monday as Hurricane Florence continued to strengthen and head toward the Carolina coast.
As of 12pm EST, the storm was centered approximately 1,230 miles from Cape Fear, North Carolina.
People fleeing coastal North and SC clogged highways Wednesday as Florence bore down on the coast for a direct hit in a low-lying region dense with beachfront vacation homes.
Helene, meanwhile, was still in the Atlantic's spawning ground for hurricanes off the coast of Africa, swirling with 105 mph (165 kmh) winds and forecast to become a major hurricane, about 375 miles (600 kilometers) west of the Cape Verde islands.
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The National Hurricane Center upgraded the storm twice Monday, to a Category 4 hurricane.
North Carolina, South Carolina and Georgia coasts could experience prolonged hurricane-force winds and rain. The governors of both states declared states of emergency.
Major US military installments may be in the path of the Florence, including the largest Marine base at Camp Lejeune in North Carolina and the world's biggest Navy base in Norfolk, Virginia.
"Let me be clear: North Carolina is taking Hurricane Florence seriously and you should too", Mr Cooper said at a news conference.
Hurricane Isaac and Helene can be seen to the east of Florence.
With "monumental" rainfall totals of up to 40 inches possible, "the rain from Florence may break all-time state records for rainfall from a hurricane or tropical storm", Weather Underground meteorologist Jeff Masters said.
An evacuation order from Virginia governor Ralph Northam applies to about 245,000 people, including parts of the Hampton Roads area and Eastern Shore.
"People are getting frantic, I know some people were upset when we were out of gas", Schrock said in a phone interview.
The storm was edging closer to the USA and was 975 miles (1570km) east-south-east of Cape Fear, North Carolina.
SHAPIRO: Wilmington, N.C., is now in Florence's path.