Tropical-Storm-force winds extend outward up to 195 miles from the center. The Hurricane Center forecasts that Florence will approach the coasts of North and SC later on Thursday, then move near or over the coast of southern North Carolina and eastern SC on Thursday night and Friday.
"Other sources are predicting a financial toll for Florence of up to $170 billion, and we think that is extreme when looking at Florence's track and impacts to people and their lives", Myers said.
But astonishing winds aren't the biggest danger.
The centre issued a public advisory with a summary of watches and warnings in effect for several areas.
Some parts of North Carolina have already seen surges as high as 10ft (3m) in places. But previous research has shown that the strongest hurricanes are getting wetter, more intense and intensifying faster because of human-caused climate change.
"This will have a storm surge in the 20-foot range", Myers said.
If it stalls over land, downpours and flooding would be especially severe.
On the forecast track, the center of Florence will approach the coasts of North and SC later today, then move near or over the coast of southern North Carolina and northeastern SC in the hurricane warning area tonight and Friday.
Heavy flooding is expected, with a storm surge up to 13ft (4 metres) in the area stretching from Cape Fear to Cape Lookout, North Carolina.
The main concerns for us from Florence will be moderately heavy rainfall, which will cause flooded roadways, strong gusty winds, coastal flooding, moderate potential for downed trees and widespread power outages, and unsafe travel conditions.
Waves crash around the Oceana Pier as the outer edges of Hurricane Florence being to affect the coast in Atlantic Beach on Thursday.
What is the latest on the storm?"So this is not just going to be a coastal threat".
With their entire neighborhood evacuated in Wilmington, North Carolina, David and Janelle Garrigus planned to ride out Florence at their daughter's one-bedroom apartment in Charlotte. By comparison, Washington, D.C., gets an average of 40 inches of rain per year. Currently, the water and sewer systems have been shut off in Wrightsville Beach, though the power remains on. Up to 13 feet of ocean water could push inland about 2 miles, which depends on how long the storm stays put.
SC authorities reported one death. Slower storms mean more rainfall over a smaller area.
And in the 29 years since Hurricane Hugo struck, the population of the coastal Carolinas has skyrocketed.
Gradually, Friday through the weekend (local time), the massive storm - containing a zone of tropical-storm-force winds almost 643km wide - will drift inland, engulfing much of SC and southern North Carolina.
This vulnerable area has been under mandatory-evacuation orders since Wednesday night at 8 p.m., and those who are choosing to ride out the storm have been under a mandatory curfew since that time, to keep people inside.
Residents along the south-eastern coast of the United States have been boarding up their properties, stockpiling supplies and some have left their homes as the Category 2 storm heads toward land.
He warned residents to take precautions "because once this storm is upon us, we're not going to be able to send emergency personnel out to rescue you".