It is expected to pickup speed, moving between Bermuda and the Bahamas on Tuesday and Wednesday and could make landfall as a Category 3 or higher on the five-step Saffir-Simpson scale, the NHC said. Not long after, a Hurricane Hunter flight measured sustained winds near 130 miles per hour, prompting the National Hurricane Center to issue a rare special advisory, upgrading the storm to Category 4.
As of early Monday, Isaac was about 1,230 miles east 1,305 miles (1,985 km) east of the Windward Islands with maximum sustained winds of 75 mph (120 kph), the NHC said.
In South Carolina alone, more than 1 million residents and tourists are expected to flee from coastal areas, Gov. Henry McMaster said Monday, vowing that state officials "are not going to gamble" with people's lives. "This storm gets stronger and stronger" and is on its way to a head-on impact on the Carolinas, CNN meteorologist Chad Myers said.
Gov. Roy Cooper declared a state of emergency Friday night to ease load restrictions on farmers moving harvests from fields to market or storage. Hurricane Floyd was a Category 4 storm before it hit Florida, traveled up the coast and struck Cape Fear as a strong Category 2 storm.
Florence is forecasted to become a major hurricane on Monday and will become a Category 4 Tuesday, maintaining that strength through Wednesday and into Thursday.
It said Florence could become a major hurricane "very soon".
"North Carolina is no stranger to hurricanes", Cooper said at a press briefing, noting that previous hurricanes - Matthew in 2016, Floyd in 1999 and Fran in 1996 - brought heavy wind and rain to the entire state.More news: Russian Federation calls on United Kingdom to help identify poisoning suspects
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Maps of the storm's trajectory showed it most likely to hit the North Carolina shore near the SC border, making it the most powerful storm to threaten the US mainland this year and the first of its magnitude in 29 years to strike the Carolinas.
A warm ocean is the fuel that powers hurricanes, and Florence will be moving over waters where temperatures are peaking near 85 degrees (30 Celsius), hurricane specialist Eric Blake wrote.
"I don't think many of us have ever been through a Category 4".
By comparison, Hurricane Matthew was a weak Category 1 storm when it hit Columbus County in 2016.
Red flags flying on beaches warned swimmers to stay out of waters as seas began kicking up.
As a result Florence is now being closely watched by insurance and reinsurance firms, as well as insurance-linked securities (ILS) funds and investors, as some of the forecasts show a path that could cause significant impacts.
Residents along the Southeast and Mid-Atlantic coasts should continue to prepare for a major hurricane landfall and have an evacuation plan. Myrtle Beach Mayor Brenda Bethune urged people to secure their homes but said it was too early to know if evacuations will be ordered.
In southeast Virginia, Naval Station Norfolk told its employees they should not leave their vehicles parked at the sprawling base in coming days because of the flood threat. The University of North Carolina Wilmington announced it has canceled classes and issued a voluntary evacuation for students starting at noon today.