Krystal Day, of Homosassa, Fla., left, leads a sandbag assembly line at the Old Port Cove restaurant Tuesday, Oct. 9, 2018, in Ozello, Fla. Employees were hoping to protect the restaurant from floodwaters as Hurricane Michael continues to churn in the Gulf of Mexico heading for the Florida panhandle. (AP Photo/Chris O'Meara)

Washington - A storm threatening the US coast was upgraded to an "extremely dangerous" Category 4 hurricane, the US National Hurricane Center (NHC) announced early Wednesday.

"This is a life-threatening event for portions of the north-eastern Gulf Coast," the NHC wrote on Twitter.

Hurricane Michael was expected to make landfall over the Florida panhandle on Wednesday, while bringing winds and heavy rain across portions of southern Georgia and south-east Alabama, it said.

"Its outer rainbands are beginning to reach the coast," it added.

Heavy rainfall could also produce "life-threatening" flash flooding in Florida, Georgia, North Carolina, South Carolina and Virginia, the weather service said.

Florida Governor Rick Scott urged people to evacuate coastal areas as quickly as possible, writing on social media late Tuesday: "The decisions you & your family make over the coming hours could be the difference between life & death."

Scott had already called a state of emergency for 26 counties along Florida's Gulf coast.

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