WASHINGTON, Sept. 12 (Xinhua) -- Hurricane Florence to hammer U.S. East Coast on Wednesday weakened to a Category 3 storm over the Atlantic but its wind field size has increased, the U.S. National Hurricane Center (NHC) said.

It remains "an extremely dangerous major hurricane" when the storm reaches the North Carolina coast late Thursday and Friday and then make a southward turn, according to the center, warning a 'life-threatening storm surge' is highly likely on the low-lying coasts of U.S. states of North and South Carolina. Governor of southeastern U.S. state Georgia, Nathan Deal, declared on Wednesday a state of emergency for the state's all 159 counties after the newest models show the storm possibly impacting the state as it moves inland into next week.

"In light of the storm's forecasted southward track after making landfall, I encourage Georgians to be prepared for the inland effects of the storm as well as the ensuing storm surge in coastal areas." said Deal.

Governors of South Carolina, North Carolina, Virginia and Maryland have already declared a state of emergency ahead of the storm, joined by Washington D.C. mayor on Tuesday. West Virginia has declared a state of preparedness.

"This will likely be the storm of a lifetime for portions of the Carolina coast," a National Weather Service spokesperson said Tuesday night. "I can't emphasize enough the potential for unbelievable damage from wind, storm surge, and inland flooding with this storm."

The major power supplier for North and South Carolina, Duke Energy, told the New York Times on Wednesday that the storm could knock out power for up to three million customers across the two states and that it could take several weeks to restore electricity.

The life-threatening rain may last for days, flooding tens of thousands of structures, North Carolina Governor Roy Cooper said Wednesday. Florence may dump as much as 40 inches of rain in North Carolina alone.

Florence has the potential to become the most intense and damaging storm to strike the U.S. East Coast in at least 25 years since Hurricane Hugo, U.S. meteorologists say.