May 29 (Reuters) - Former subtropical storm Alberto, which threatened to clobber the southeastern United States on Memorial Day, fizzled into a subtropical depression, with winds dropping to 35 mph (56 kph) as it rolled into southern Alabama on Tuesday, officials said.
At its height, it was blasting sustained winds of 65 mph (105 kph) with gusts that packed full hurricane punches of 75 mph (121 kph), said meteorologist David Roth of the National Weather Service.
"It's slowly weakening and it's not regaining any strength," Roth said. "The chances of it spinning off tornadoes now has dropped to virtually zero."
Forecasters had feared the storm could bring dangerous high water to southern coastal states and possible tornadoes.
The National Hurricane Center cancelled coastal warnings and watches. Minor power outages were reported in north Florida, and the state's emergency response team started closing shelters on Monday, citing a "lack of public sheltering need."
Some areas on Gulf Coast barrier islands remained under evacuation orders as authorities weighed the flood risks, officials said.
Alberto will probably weaken through Tuesday as it moves northward to the Tennessee Valley and then into the Ohio Valley, finally withering into a "remnant low pressure storm" by Tuesday evening, with winds around 25 mph (40 kph), Roth said.
The NHC warned it would still deliver heavy, potentially damaging rains of 2 to 6 inches (6 cm to 15 cm), with chances of 12 inches (30 cm) in some spots in north Florida and Alabama through Tuesday night, possibly spurring flooding.
It is expected to dump up to 6 inches (15 cm) of rain as it moves northward to lower Michigan by Wednesday evening, officials said.
Alberto's top winds weakened on Monday to 45 mph (75 kph) when its center made landfall near Panama City, Florida, weather officials said, with a city fire official adding that the wind and rain knocked down trees but did not force any rescues.
Royal Dutch Shell Plc was returning workers to the eastern Gulf of Mexico and Chevron Corp restored some production on Monday after the storm's passage.
Shell plans to restore production at its Ram Powell Hub in the Viosca Knoll area of the Gulf as it soon as the platform can be operated safely, the company said in a statement.
Alberto, the first named Atlantic storm of 2018, spun up days before the formal June 1 start of the hurricane season, complicating holiday travel as it hit on the last day of the Memorial Day weekend.
Authorities in Florida's Franklin and Taylor counties issued mandatory evacuation orders for thousands of coastal residents.
Deadly hurricanes in the United States and the Caribbean last year caused billions of dollars in damage, massive power outages and devastation to hundreds of thousands of structures. (Reporting by Rich McKay in Atlanta; Additional reporting by Ian Simpson and Daniel Trotta; Editing by Clarence Fernandez)