The hurricane though weakening slightly remains a “very dangerous storm” and can potentially lead to massive devastation.
In a prelude to the damaging storm, the outer bands of the hurricane began lashing the Carolinas with heavy rain and flooding.
The storm that has made its advent in the US East Coast can become Category 5 storm as it makes landfall as per reports.
More than 1.5 million people have been asked to evacuate their homes along the Virginia, North and South Carolina coasts ahead of the storm that is capable of wreaking havoc in a wide swathe of the coast with gusty winds and massive inland flooding.
"Just because the wind speed came down, the intensity of this storm came down to a Cat 2, please do not let your guard down," warned Brock Long, administrator of the Federal Emergency Management Agency (FEMA).
Florence was downgraded to a Category 2 storm overnight on the five-level Saffir-Simpson scale but it is still packing hurricane-force winds of 105 miles per hour (165 kilometers per hour), the National Hurricane Center (NHC) said.
Winds were already picking up along the coastline on Thursday morning and Myrtle Beach was virtually deserted with empty streets, boarded up storefronts and very little traffic.
Hurricane-force winds extend outward 80 miles (130 kms) from the center of the storm and tropical storm-force winds extend nearly 200 miles (322 kms) out.
A storm surge of nine to 12 feet (2.7-3.6 meters) was expected along the North Carolina coast and some areas could receive as much as 40 inches (one meter) of rain. A tornado watch was also in effect for parts of North Carolina.
South Carolina ordered the mandatory evacuation of one million coastal residents while North Carolina ordered an evacuation of the Outer Banks, barrier islands that are a popular tourist destination.
In Virginia, 245,000 coastal residents were ordered to evacuate. A state of emergency has been declared in five coastal states: North Carolina, South Carolina, Georgia, Maryland and Virginia and the US capital Washington.
Duke Energy, a power company in the Carolinas, estimated that one million to three million customers could lose electricity because of the storm and that it could take weeks to restore.
"We call them disasters because they break things," said FEMA's Long. "The infrastructure is going to break, the power is going to go out."
North Carolina Governor Roy Cooper warned residents not to "underestimate" the storm.
(With inputs from agencies)