The storm is expected to become a major Category 3 hurricane by the time it reaches the northern Gulf Coast on Wednesday.
Forecasters say Michael could move inland over the Florida Panhandle or Big Bend area Wednesday.
The National Hurricane Center says Michael is now a hurricane and will continue to strengthen as it approaches the U.S this week. "Families should take the opportunity TODAY to make sure they have three days of food and water, as well as all needed medications", he tweeted.
It is expected that Michael, now centered about 90 miles east of Cozumel, Mexico, will dump four to eight inches of rain, with totals reaching more than 12 inches in isolated areas, potentially causing "life-threatening" flash floods and mudslides, according to the National Hurricane Center. Tropical storm and storm surge watches have been issued from the Mississippi-Alabama border to Tampa Bay.
A hurricane watch is in effect for the Alabama-Florida border to the Suwannee River in Florida.
For us here in the Tennessee Valley, the system could help enhance showers and storms in North Alabama mid-week, but significant impacts still look to be confined to the south and southeast.
Tampa Bay has about a 40 percent chance of tropical storm-force winds within the next five days.
Hurricane Michael is strengthening as it enters the Gulf of Mexico, and Florida is its target.
Michael is 140 miles east-northeast of Cozumel, Mexico, moving toward the north near 7 mph as it moves into the southern Gulf of Mexico with 75 mph winds. A northward motion at a slightly faster forward speed is expected through Tuesday night, followed by a northeastward motion on Wednesday and Thursday.
The governor warned that storm surge could affect areas of Florida not in the storm's direct path. This rainfall could lead to life-threatening flash floods and mudslides.Florida Panhandle and Big Bend into the Carolinas.4 to 8 inches, with isolated maximum amounts of 12 inches.
Tallahassee Mayor Andrew Gillum, who is also the Democratic gubernatorial candidate in the November 6 elections, said on Sunday he was cancelling his campaign events and returning to Tallahassee, the state capital, to focus on storm preparations.
The storm has maximum sustained winds of 50 miles per hour, and slow strengthening is forecast in the next few days.