"Residents in these areas should follow all advice given by their local officials", the hurricane center said.
"We are running out of time", the Republican governor said on Twitter. "We can rebuild your house, but we cannot rebuild your life".
Forecasters anticipate Michael will reach land either in the Panhandle or somewhere along the state's Big Bend as soon as Wednesday morning. "There's nothing between us and this storm but warm water and I think that's what terrifies us about the potential impacts".
Most of the rain was blamed on a low-pressure system off the Pacific coast, but Hurricane Michael in the Caribbean could have also contributed.
Alabama Gov. Kay Ivey signed an emergency declaration for her entire state, expecting widespread power outages and other problems from Michael. That's just 4 miles below a Category 4 storm. High surf warnings were in effect already at the Alabama coast.
A tropical storm watch is in effect along the southeastern coast of the US from Fernandina Beach, FL, to South Santee River, SC, the Chassahowitzka River to Anna Maria Island in Florida, including Tampa Bay, and the Mississippi-Alabama border to the mouth of the Pearl River.
Hurricane Michael is gaining strength as it lashes Cuba's western tip and lumbers northward toward the U.S.
The storm's rapid intensification over the past two days, despite shifting winds, "defies traditional logic", according to hurricane forecasters. Hurricane-force winds extend outward up to 35 miles (56 kilometres) from the core and tropical-storm-force winds out up to 175 miles (282 kilometres). Its center is located 200 miles south southwest of Apalachicola and 225 miles south of Panama City. This is in contrast to recent major storms, such as Harvey in 2017 and Florence in 2018, which made landfall in the United States and then stalled out.
A hurricane warning has been issued for the Alabama-Florida border to the Suwannee River, and a hurricane watch is in effect from the Alabama-Florida border to the Mississippi-Alabama border.
"You can not hide from storm surge", Florida Gov. Rick Scott warned at a news conference Tuesday. The agency said it is possible the storm could strengthen further before it makes landfall later Wednesday. More than 100,000 people were ordered out of a long swath of low-lying communities, including many in Bay County, home of Panama City.
If it makes landfall as a Category 3 it will be the first major hurricane to hit the Panhandle since Hurricane Dennis came ashore near Pensacola on July 10, 2005 with 130 miles per hour winds.
There will be no shelters open in Wakulla County, the sheriff's office warned on Facebook, because they are rated safe only for hurricanes with top sustained winds below 178kmh.
Mr Scott urged residents to evacuate, saying: "It could be the difference between life and death".
Tallahassee Mayor Andrew Gillum, who is the Democratic nominee for governor, had planned to campaign in South Florida on Monday and Tuesday, but he said he would return to Tallahassee to help with storm preparations.