Category 4 storms, with winds of at least 130 miles per hour, can cause "catastrophic" damage, the hurricane center said, snapping trees, and toppling power polls.
The storm had maximum sustained winds of 115 miles per hour and was moving west at about 13 miles per hour, the hurricane center said.
Hurricane Isaac was expected to lose strength as it reaches the Caribbean, and Helene, much farther out to sea, may veer northward into the open ocean as the 2018 hurricane season reaches its peak.
"The bottom line is that there is high confidence that Florence will be a large and extremely unsafe hurricane, regardless of its exact intensity", the hurricane center said. Gov. Ralph Northam ordered a mandatory evacuation to begin at 8 a.m. Tuesday for 245,000 people in parts of the Hampton Roads area and the Eastern shore. Even before the evacuation order, Dare County emergency officials warned that rough seas and strong rip currents already were creating unsafe conditions.
McMaster said he had asked President Donald Trump to declare a federal emergency in SC in anticipation of the storm's arrival.
In Charleston, South Carolina, along the coast, city officials offered sandbags to residents.
Editor's note: How are you preparing for Hurricane Florence? It was picking up speed over the Atlantic's warm waters, causing concerns about landfall and flooding from heavy rains afterward, possibly late Thursday or Friday.
"Now is the time for your family also to prepare and stay tuned for more updates", South Carolina Gov. Henry McMaster said.
Florence is likely to bring a deadly coastal storm surge to the Carolinas and Virginia, with "staggering" rainfall that may extend hundreds of miles inland and cause flash flooding across parts of the mid-Atlantic region, NHC Director Ken Graham said in an evening webcast. National Hurricane Center update.
The storm's winds grew by 35 miles-per-hour in the last 24 hours, and Florence is forecast to reach Category 4 status, one step short of the the most severe level, with 150 mile-per-hour winds by Wednesday.
"We're not going to gamble with the lives of the people of SC", the state's governor, Henry McMaster, said in an afternoon press briefing Monday as he announced that eight counties along the state's 187-mile coastline would begin to evacuate on Tuesday.
Hurricane Florence is approaching the East Coast and is expected to make landfall as a major hurricane on Thursday.
Isaac is moving toward the west near 14 miles per hour.
A tropical storm warning and hurricane watch are in effect for the Cabo Verde islands of Santiago, Fogo, and Brava.
The NHC predicts it could become a major hurricane by Monday and remain that way through Thursday.
Helene was also upgraded to a hurricane on Sunday, the center said, but neither Helene nor Tropical Storm Isaac are expected to hit the mainland United States.
Helene - now just southeast of the Cabo Verde islands off the African coast - had winds of 85 miles per hour, and was expected to turn northwest and then north into the open Atlantic by midweek, the NHC said.
But authorities were taking no chances as the governors of North and SC and Virginia declared states of emergency far ahead of the approaching storm.
August 28 - Florence formed as a tropical wave off of the west coast of Africa.
Two other storms were spinning in the Atlantic.
"I dont think many of us have ever been through a Category 4".
At a Home Depot in Wilmington, North Carolina, Billy Nivens told CNN they ran out of water, plywood, generators, flashlights, propane, batteries and five-gallon gas cans by Monday afternoon.
North Carolina officials started getting bulldozers and chain saws ready.
"Im telling them to go inland, but Im anxious about the rain and tornadoes too", Roberts said.
Crowds of shoppers formed at supermarkets on Sunday, as people tried to stock up on supplies before the storm.
Florida-based Carnival Cruise Line was re-routing its cruise ships, but there were other hurricanes to contend with.