Manatee County and Anna Maria Island also were under a storm surge watch as of 6 a.m., and the storm surge watch stretches to Navarre.
Michael has undergone a period of "rapid Intensification" - defined as an increase of sustained winds of 35 mph in a 24-hour period.
NHC tweeted: "Michael could produce three-life threatening hazards along portions of the northeastern Gulf Coast: storm surge, heavy rainfall and hurricane-force winds, with storm surge and hurricane watches in effect". Hurricane watches are issued 48 hours before the arrival of tropical-storm-force winds, which is when outside preparations become risky. Another Watch is in effect for the Alabama-Florida border to the Mississippi-Alabama border.
An Air Force hurricane hunter airplane was sent into the storm to investigate, the hurricane center said. A major hurricane is defined as Category 3 or greater on the Saffir-Simpson scale, with sustained winds of 111mph or greater.
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Florida State University announced it was closing for the week on Tuesday, along with schools in Leon county, home to the state capital Tallahassee. Scott also activated 500 members of the Florida National Guard ahead of the storm. The storm is expected to gain hurricane status by Monday night or Tuesday as its core slowly crawls into the Gulf of Mexico, nearing the Florida Panhandle coast around midweek.
"If this storm hits Panama City, Tampa could still have storm surge", Scott said.
Masters compared Hurricane Michaels course to Hurricane Dennis, a Category 3 storm that hit near the Florida-Alabama border in 2005. "Don't take a chance", he said.
Southwest Airlines is the first airline to waive change fees as Hurricane Michael approaches the U.S. The storm was moving north around 7 miles per hour (11 kph).
The storm is about 120 miles east of Cozumel, Mexico and about 70 miles west of Cuba.
Democratic gubernatorial nominee Andrew Gillum canceled campaign events in South Florida on Monday and Tuesday to prepare for the storm, his campaign said.