The hurricane is moving towards land with maximum sustained wind speeds of 90mph (150 km/h).
It lost power as it approached North and South Carolina, but officials warn it could still kill "a lot of people" with risks of "catastrophic" flooding.
Evacuation orders are in place for more than a million people.
The governor of North Carolina, where Florence is expected to make landfall at noon local time on Friday, said surviving the storm would be a test of "endurance, teamwork, common sense, and patience".
"The first bands of the storm are upon us but we have days more to go," Roy Cooper said.
National Weather Service forecaster Brandon Locklear said North Carolina is likely to see eight months' worth of rain in two to three days.