Weather officials have warned not to underestimate the threat the storm poses.
The size of the hurricane-force winds doubled over 12 hours from 30 to 60 mph, the National Hurricane Centre (NHC) said Monday afternoon.
The wind field is expected to keep growing, which will increase the storm surge and inland wind threats, the service said in a statement.
"None of the guidance suggest that Florence has peaked in intensity, and this is supported by a continuation of a low-shear environment, and even warmer waters over he next 36 hours. Thus, the intensity forecast is raised from the previous one, bringing Florence close to Category 5 strength tomorrow," the NHC said.
In North Carolina, six more counties were placed under mandatory evacuation orders on Monday night, hours after the first order in Hatteras Island.
Florence became a hurricane on Sunday with sustained winds of at least 74 mph, CNN reported.
By Monday morning, the NHC classified Florence as a "major" Category 3 hurricane, with sustained winds of at least 111 mph.
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.
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