UPDATE 3-Tropical Storm Michael expected to slam into Florida as hurricane

(Adds comments from Florida governor, Tallahassee mayor)

Oct 8 (Reuters) - Tropical Storm Michael is expected to become a hurricane on Monday and was on track to hit the Florida Panhandle midweek, with the potential to pack winds of at least 100 miles per hour (160 km per hour), the U.S. National Hurricane Center (NHC) said.

The tropical storm is expected to swell into a Category 1 hurricane as soon as Monday night as it rolls into the U.S. Gulf of Mexico, with winds of 70 mph, forecasters said. Michael could reach or be close to a Category 3 by Tuesday night or Wednesday.

A storm is designated a Category 1 hurricane if it reaches speeds of 74 mph (119 kph) or more, and a Category 3 hurricane at 111 mph (178 kph) or more on the five-step Saffir-Simpson hurricane scale.

After hitting Florida, Michael will move northeastward across the southeastern part of the country Wednesday night and Thursday, according to the NHC.

Florida Governor Rick Scott declared a state of emergency in more than 20 counties along the Florida Panhandle and Big Bend on Sunday. He advised Gulf Coast residents to prepare for possible evacuation orders and 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 as it churned in the Caribbean. The storm moved north on a path between Cozumel in southeastern Mexico and the western tip of Cuba, the Miami-based hurricane center said.

The Florida Panhandle, Big Bend and southern Georgia were expected to receive as much as 8 inches (20 cm) of rainfall, with isolated maximum amounts of 12 inches that may lead to severe flash flooding.

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 along the Atlantic Coast and batter the Carolinas, which are still recovering from Hurricane Florence last month.

The Commodity Weather Group said on Sunday some oil rigs in the Gulf area may be evacuated, which may slow operations but was not likely to cause much interruption.

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)