Classified as a Category 4 on the five-step Saffir-Simpson scale of hurricane strength, Florence was the most severe storm to threaten the US mainland this year and the first of its magnitude to target the Carolinas since 1989, when Hurricane Hugo barreled over Charleston, South Carolina.
"This idea, first of all, of a very strong hurricane making landfall is always going to be very, very bad news", says Kossin, "but then adding the potential for it to stall out once it hits land - that's a very, very frightening prospect to think about".
Immediately to the north of Virginia, Maryland Governor Larry Hogan said his state was also "preparing for the potential of historic, catastrophic and life-threatening flooding".
The evacuation order becomes mandatory at noon Tuesday, but cameras show traffic already backing up along the main interstate connecting Charleston and Columbia.
ROY COOPER: The forecast places North Carolina in the bull's eye of Hurricane Florence, and the storm is rapidly getting stronger.
The National Hurricane Center said Monday the monster storm will be close to Category 5 strength by Tuesday.
A Category 4 hurricane has winds of 130 to 156 miles per hour on the Saffir-Simpson Scale of Hurricane Intensity.
Mandatory coastal evacuations were in effect for civilians in South Carolina, North Carolina and Virginia, but the military base posted on Facebook that different chains-of-command would decide whether to release non-essential personnel, and some relatives vented fears that they wouldn't be able to evacuate in time. Forecasters say Florence will arrive on land as an extremely risky major hurricane by the end of the week.
Two other storms are spinning in the Atlantic.
"We know the evacuation order I'm issuing will be inconvenient", McMaster said. "Everyone in Virginia need to be prepared", said Northam.
The storm was heading for the coast of North and SC but heavy rain was also expected in Virginia to the north and Georgia to the south.
South Carolina Gov. Henry McMaster said an estimated 1 million people would be fleeing his state's coast, with eastbound lanes of Interstate 26 heading into Charleston and USA 501 heading into Myrtle Beach reversed to ease the exodus.
Isaac, which the NHC called a small hurricane, was about 1,090 miles east of the Lesser Antilles - a region still recovering from last year's powerful Hurricane Maria - with maximum sustained winds of 75 miles per hour.
"Well-built framed homes can sustain severe damage with loss of most of the roof structure and/or some exterior walls". Power outages will last weeks to possibly months.
"This is a huge storm", said Robert Woodward, chairman of the Dare County Board of Commissioners, predicting 15 to 20 inches (38 to 50 centimetres) of rain.
Hurricane-force winds are extending up to 65 kilometres from the storm's centre, while tropical-storm-force winds are blowing out at up to 240 kilometres. If Florence weakens enough to arrive as a Category 3 storm, it would likely be near the upper limit of that classification.
Hurricanes Florence, Helene, and Isaac are all now brewing in the Atlantic Ocean.
The Atlantic hurricane season officially began June 1, but cooler water and higher wind shear - winds moving at different speeds and directions - early in the season are less than ideal for tropical systems to gain and maintain strength.
"We expect it to be at least a major hurricane at landfall", he said.