Georgia Governor Nathan Deal declared a state of emergency on Tuesday for 92 counties in his state.
Hurricane Michael became a Category 3 storm with 120 miles per hour maximum sustained winds on October 9.
"Do not risk your life, leave NOW if you were told to do so", it said.
The storm appeared to be so powerful that it is expected to remain a hurricane as it moves over Georgia early Thursday. "The forecast keeps getting more unsafe", said Republican Gov. Rick Scott on Tuesday as he urged all residents in the storm's path to evacuate or seek safe shelter.
At 5 a.m., the center of the hurricane was bearing down on a stretch of the Florida Panhandle, still about 140 miles (225 kilometers) from Panama City and 130 miles (209 kilometers) from Apalachicola, but moving relatively fast at 13 mph (21 kph).
Northwest Florida as a region attracted almost 17 million visitors in 2016, or 15 percent of total tourists in the state, according to Visit Florida, the state's tourism marketing arm.
A year ago saw a string of catastrophic storms batter the western Atlantic - including Irma, Maria and Hurricane Harvey - causing a record-equaling $125 billion in damage when it flooded the Houston metropolitan area.
More than 300 miles of coastline was under threat, the National Weather Service has said. And isolated tornadoes were also possible.
President Donald Trump said federal emergency officials are standing by to help with recovery efforts.
He added that it is predicted to be "the most destructive storm to hit the Florida panhandle in decades".
Mr Scott warned that he may order more evacuations due to the size of the potential storm surge.
Scott said that he's called up 2,500 members of the National Guard to assist, and has more than 1,000 rescuers at the ready.
Michael was forecast to have the power to uproot trees, block roads and knock out power for days by the time it hits Florida on Wednesday.
At least 13 storm-related deaths have been reported in El Salvador, Honduras and Nicaragua.
Late Monday, residents of far western Cuba were forecast to get up to eight inches of rain.
By late Tuesday, Michael was causing major disruptions to US oil and gas production as it churned north, gathering greater strength over the warm waters of the Gulf of Mexico.President Donald Trump declared a state of emergency for the entire state of Florida, freeing up federal assistance to supplement state and local disaster response. US producers in the Gulf cut oil production by about 40 percent and natural gas output by 28 percent on Tuesday, the Bureau of Safety and Environmental Enforcement said.