Portions of the North and South Carolina coast, along with Virginia inland, are expected to feel the direct hit of the storm as it heads for landfall in the coming days.
At 5am local time (10am BST), the storm was 975 miles east-southeast of Cape Fear, North Carolina and 400 miles from Bermuda.
Hurricane Florence is packing winds of 140mph with higher gusts, and the storm is heading in a west-northwestward direction at 15mph.
The NOAA warned these are just the first warnings to be implemented, and more may be issued as the day goes on.
Watches and warnings issued
A Storm Surge Watch is in effect for:
- Edisto Beach South Carolina to the North Carolina-Virginia border
- Albemarle and Pamlico Sounds, including the Neuse and Pamlico Rivers
A Storm Surge Watch means there is a possibility of life-threatening inundation, from rising water moving inland from the coastline, in the indicated locations during the next 48 hours.
A Hurricane Watch is in effect for:
- Edisto Beach South Carolina to the North Carolina-Virginia border
- Albemarle and Pamlico Sounds
A Hurricane Watch means that hurricane conditions are possible within the watch area. A watch is typically issued 48 hours before the anticipated first occurrence of tropical-storm-force winds, conditions that make outside preparations difficult or dangerous.
Potential hazards
Storm surge, rainfall, wind and surf will prose the greatest risk to life and infrastructure.
NOAA warns that the inundation of water in normally dry areas could reach the following heights:
- Edisto Beach to Murrells Inlet: 2-4 ft
- Murrells Inlet to Cape Fear: 4-6 ft
- Cape Fear to Cape Lookout including The Neuse and Pamlico River: 6-12 ft
- Cape Lookout to Ocracoke Inlet: 5-8 ft
- Ocracoke Inlet to North Carolina/Virginia Border: 3-5 ft
The surge will be accompanied by large and destructive waves along the coast.
Florence is also expected to bring up to 20 inches of rain with 30 inches in isolated areas.
This influx of rainfall could create life-threatening flash flooding.
Officials are urging residents to take the storm seriously and make all necessary preparations now.
Mandatory evacuations are in place for more than 1 million people in coastal areas of North Carolina, South Carolina and Virginia.
Florence is forecast to strengthen further, with the storm expected to hit Category 5 – the highest on the Saffir-Simpson scale, in the next 24 to 36 hours, the NOAA said.
The storm is expected to pass between Bermuda and the Bahamas on Tuesday and Wednesday, making landfall on the North Carolina coast on Thursday around 10pm local time (3am Friday BST).
Schools in affected areas will begin to close on Tuesday and lanes on some highways will be reversed to aid evacuations.
The US Navy is sending 30 ships stationed in Virginia out to sea to wait at anchor while the storm passes.