Florida Gov. Rick Scott called the storm "monstrous."
It is the worst hurricane to hit the Panhandle since the mid-1800s, the director of FEMA said.
As the storm barrels ahead with 150 mph winds, thousands have fled the coastline to shelters. But by Wednesday morning, officials said the time to evacuate is now over.
"This was a shock waking up knowing it was a [Category] 4," said Panama City Beach resident Julie Gordon. "Thinking it was a [Category] 2 was a very different story."
All bridges from Panama City Beach to further inland have closed, so Gordon said she is riding out Michael at home, "hoping and praying that the storm will continue to drift to the northeast ... [an area] where it's not quite as populated."
Life-threatening storm surge
Hurricane Michael, set to crash into the coast midday Wednesday, is forecast to bring heavy rain of up to 12 inches.
Water levels were already quickly rising Wednesday and as Michael approached the coast its pressure dropped to about 923 millibars (mb). The lower the pressure, the more intense the storm. Hurricane Katrina in 2005 made landfall with a pressure of 920 millibars.