UPDATE 6-Hurricane Michael expected to strengthen en route to Florida

(Adds school closures, evacuations, quotes from Florida governor)

Oct 8 (Reuters) - Mandatory evacuation orders and school closures were issued on Monday in the Florida Panhandle as Hurricane Michael was expected to strengthen rapidly before slamming into the state on Wednesday, the U.S. National Hurricane Center (NHC) said.

Michael was currently a Category 1 hurricane but could become a Category 3 storm by Tuesday on the five-step Saffir-Simpson scale, with winds of at least 100 miles per hour (160 km per hour). The hurricane was predicted to dump 4 to 12 inches (10 to 30 cm) of rain with life-threatening flash flooding, according to forecasters. Storm surges of 2 to 12 feet (0.6 to 3.7 meters) were also expected.

Florida Governor Rick Scott declared a state of emergency in more than 20 counties along the Florida Panhandle and Big Bend. Scott advised Gulf Coast residents to prepare for possible evacuation orders, and he put more than 5,000 National Guard soldiers on alert.

"We are running out of time. TODAY is the day to get a plan, because tomorrow could be too late. It is critical that you take care of yourself, your family, and your business as Hurricane Michael approaches FL," Scott wrote on Twitter on Monday.

Michael battered parts of Mexico and Cuba with powerful winds and drenching rains on Sunday and into early Monday. The storm was about 145 miles (230 km) northeast of Cozumel, Mexico, with maximum sustained winds of 75 mph (120 kph), forecasters said.

Tallahassee Mayor Andrew Gillum, who is also the Democratic gubernatorial candidate in the Nov. 6 election, said on Sunday he was cancelling his campaign events and returning to Tallahassee, the state capital, to focus on storm preparations.

Florida State University said its campuses in Tallahassee and Panama City will be closed from Tuesday through Friday. All schools in Panama City and surrounding Bay County will be closed on Tuesday and Wednesday. Leon County officials in Tallahassee were closing all schools from Tuesday through Friday.

Mandatory evacuation orders were issued on Monday for residents in various areas in Gulf County, Florida, according to a statement from county emergency officials. Residents were given 24 hours to relocate to shelters.

Jeff Hanson, the owner of Paisley Cafe in Tallahassee, said he was waiting until Wednesday morning to determine whether he would close the restaurant.

"There's a possibility we're going to be in business if we still have power," Hanson said by telephone. "It depends on what the city says in terms of the people getting around. That's our biggest concern, if our staff is safe."

After hitting Florida, the storm is forecast to move northeast on Wednesday and Thursday along the Atlantic Coast and batter the Carolinas, which are still recovering from Hurricane Florence last month.

Oil companies, including BP Plc and Exxon Mobil Corp , on Monday began evacuating personnel from Gulf of Mexico production platforms.

The Commodity Weather Group said Michael was not likely to cause much interruption to oil and gas production.

The Gulf of Mexico is home to 17 percent of daily U.S. crude oil output and 5 percent of daily natural gas output, according to the U.S. Energy Information Administration.

More than 45 percent of the nation's refining capacity is located along the U.S. Gulf Coast, which also is home to 51 percent of total U.S. natural gas processing capability.

(Reporting by Gina Cherelus in New York; Additional reporting by Rich McKay in Atlanta and Jon Herskovitz in Austin, Texas; Editing by Paul Simao and Dan Grebler)