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 100mph (160kph), according to the US National Hurricane Center (NHC). The tropical storm is expected to swell into a Category 1 hurricane as soon as Monday night as it rolls into the US Gulf of Mexico, with winds of 70mph, forecasters said. Michael could reach or be close to a Category 3 by Tuesday night or Wednesday, Reuters reports. A storm is designated a Category 1 hurricane if it reaches speeds of 74mph (119kph) or more, and a Category 3 hurricane at 111mph (178kph) 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, the NHC said. Florida Governor Rick Scott has declared a state of emergency in more than 20 counties along the Florida Panhandle and Big Bend.