The National Hurricane Center says Michael is now a hurricane and will continue to strengthen as it approaches US. A Tropical Storm Watch covered the area from the Chassahowitzka River to Anna Maria Island, including Tampa Bay.
Maximum sustained winds have increased to near 75 miles per hour with higher gusts, the National Hurricane Center said.
Right now, the National Hurricane Center's forecast path shows Michael may make landfall between Pensacola and Destin, Florida, on Wednesday.
Although few forecast models were predicting it would reach major hurricane status at the time, it seemed possible given that the storm was now moving away from the Yucatan Peninsula landmass and toward warmer waters in the southern Gulf of Mexico, where it will encounter lower overall wind shear.
According to the hurricane center, one potential impact from a Category 3 hurricane is that "electricity and water will be unavailable for several days to weeks after the storm passes". Storm surge and hurricane watches are in effect for some areas along the Gulf Coast. Officials also alerted residents living on the coast of the possibility of flooding caused by the storm.
Meanwhile, broad swaths of the Carolinas could find themselves exposed to tropical storm-force winds as early as Wednesday evening.
Current advisory shows winds are sustained at 75 miles per hour.
Mandatory evacuation orders were issued on Monday for residents in various areas in Gulf County, Florida, according to a statement from county emergency officials.
The tropical storm is expected to swell into a Category 1 hurricane as soon as Monday night as it rolls into the US Gulf of Mexico, with winds of 70 miles per hour, forecasters said.
The National Weather Service in Tallahassee said: "This is a storm to definitely take very seriously". These chances could go up as we get greater confidence in the forecast.
Leon County Schools, Florida State University and its Panama City campus will be closed Tuesday through the end of the week.
Masters compared Hurricane Michaels course to Hurricane Dennis, a Category 3 storm that hit near the Florida-Alabama border in 2005.
After hitting Florida, the storm is forecast to move northeast along the Atlantic Coast and batter the Carolinas, which are still recovering from Hurricane Florence last month.
Michael is set to become a hurricane sometime today.
The storm was centered about 80 kilometers off the western tip of Cuba, and about 220 kilometers east-northeast of Cozumel, Mexico.