(Adds upgrade to hurricane, details from NHC advisory)
Oct 8 (Reuters) - Tropical Storm Michael became a hurricane on Monday and was expected to strengthen rapidly before hitting the Florida Panhandle midweek, packing winds of at least 100 miles per hour (160 km per hour), the U.S. National Hurricane Center (NHC) said.
Michael was currently a Category 1 hurricane but could make landfall as a Category 3 on the five-step Saffir-Simpson scale and bring 4 to 12 inches (10 to 30 cm) of rain with life-threatening flash flooding, according to forecasters.
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 has put more than 5,000 National Guard soldiers on alert.
"Families should take the opportunity TODAY to make sure they have three days of food and water, as well as all needed medications," Scott wrote on Twitter on Monday. "EVERY FAMILY must be prepared. We can rebuild your home, but we cannot rebuild your life."
Michael battered parts of Mexico and Cuba with powerful winds and drenching rains on Sunday and into early Monday. The storm was about 140 miles (220 km) east-northwest of Cozumel, Mexico with the maximum sustained winds of winds of 75 mph (120 kph), forecasters said.
Tallahassee Mayor Andrew Gillum, who is also the Democratic gubernatorial candidate in the Nov. 6 elections, said on Sunday he was cancelling his campaign events and returning to Tallahassee, the state capital, to focus on storm preparations.
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.
BP and Exxon Mobil on Monday began evacuating personnel from Gulf of Mexico oil and gas 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 Rich McKay in Atlanta and Jon Herskovitz in Austin, Texas; Additional reporting by Gina Cherelus in New York; Editing by Lisa Shumaker and Paul Simao)