The storm has already killed 13 people in Central America as it battered Mexico’s Yucatan Peninsula and western Cuba with rain and winds of up to 85 mph.
By the time it reaches Florida on Wednesday local time, it is expected to be a category three hurricane with winds of more than 100mph and storm surges of up to 12ft (3.7 metres).
This would make it the strongest storm to hit the Panhandle of the state since Hurricane Dennis in 2005.
Florida governor Rick Scott has declared a state of emergency in 35 counties, warning that the region must be “prepared for a direct hit with sustained hurricane force winds”.
He described Michael as a “monstrous hurricane”.
Some 1,250 National Guard soldiers are helping residents in coastal areas to find safety, and another 4,000 are on standby, Mr Scott said.
On the Panhandle, Escambia County Sheriff David Morgan warned people against staying in their homes, saying emergency services would not be able to help them.
He said: “If you decide to stay in your home and a tree falls on your house or the storm surge catches you and you’re now calling for help, there’s no one that can respond to help you.”