Thousands of Florida residents evacuated homes on Sunday as Subtropical Storm Alberto picked up strength as it headed north through the Gulf of Mexico, with forecasters saying it could bring life-threatening inundation to Southern coastal states.

The storm was about 105 miles (165 km) south of Apalachicola, Florida, on the Gulf of Mexico coast as of 8 pm EDT (0000 GMT) and was expected to make landfall along the Florida Panhandle on Monday, the US National Hurricane Center said.

Alberto, the first named Atlantic storm of 2018, which spun up days before the formal June 1 start of the hurricane season, was packing maximum sustained winds near 65 miles per hour (105 kph) and was expected to drop as much as 12 inches (30 cm) of rain, slamming an area from Mississippi to western Georgia, the Miami-based hurricane center said.

After that, it will bring powerful winds and heavy rains as it moves into the Tennessee Valley on Tuesday and Wednesday, the hurricane center said. The storm comes during the Memorial Day weekend and was expected to scramble transportation on Monday as many people return from holiday travel.

Isolated tornadoes were possible across the central and northern Florida peninsula on Sunday night, according to the hurricane center, while Alberto was also forecast to bring storm swells of about 2 to 4 feet (60-120 cm) to low-lying areas that were "likely to cause life-threatening surf and rip current conditions."

There were a number of deadly hurricanes in the United States and Caribbean last year that walloped places including Texas, Florida and Puerto Rico, causing hundreds of billions of dollars in damage, massive power outages and devastation to hundreds of thousands of structures.

Franklin County, in the Florida Panhandle, has issued a mandatory evacuation for its barrier islands in the Gulf of Mexico affecting some 4,200 housing units, while Taylor County, to the east, has a voluntary evacuation order in place for its coastal areas.

Florida Governor Rick Scott, who issued a state of emergency on Saturday in all of the state's 67 counties, said on Sunday that the Florida National Guard had 5,500 guard members ready to be deployed if needed.

"If evacuations have been ordered in your community, do not ignore them," Scott said on his Twitter feed.

The National Weather Service has issued a flash flood watch for an area stretching from Mississippi to North Carolina that is home to millions of people.

Alberto's projected storm track has shifted eastward since Friday, lessening its threat to the active oil production areas in the Gulf of Mexico. Royal Dutch Shell Plc, Exxon Mobil Corp and Chevron Corp have evacuated some personnel from offshore Gulf oil facilities.

A separate storm system on Sunday caused flash flooding in the Baltimore suburb of Ellicott City, Maryland, turning its historic Main Street into a raging river with water reaching past the first floor of buildings and sweeping cars down the road, local news video showed.

Governor Larry Hogan declared a state of emergency for the area rebuilding from a devastating flood about two years ago that killed two people and damaged dozens of buildings.