Charleston, South Carolina (Reuters) - Tropical Storm Alberto churned off the coasts of Georgia and South Carolina on Sunday, and heavy rain and dangerous surf were expected even though forecasters said it had lost strength.

Bringing an early start to the Atlantic hurricane season, Alberto reached tropical storm strength on Saturday. By Sunday evening it was about 85 miles south-east of St Augustine, Florida, according to the hurricane center in Miami.

Its maximum sustained winds had dropped to 40 miles per hour, down from 50 mph on Saturday, and a tropical storm warning was lifted for the South Carolina coast.

Alberto was moving slowly west-southwest with tropical storm force winds extending about 70 miles from the center. The hurricane center expected little change in strength for the next 48 hours and its center was expected to stay offshore.

"It's moving very slowly," said meteorologist Brett Cimbora of the National Weather Service in Charleston. "It likes the warmer waters of the Gulf Stream. It's going to try to hug that."

Dangerous surf conditions were predicted along the coasts of Georgia and South Carolina through Monday before the storm takes a slow turn to the northeast and moves along the U.S. mid-Atlantic seaboard before dissipating in about five days.

That would keep it well away from the Gulf of Mexico, where U.S. oil and gas operations are clustered, but the Carolina coast could experience squalls and rough surf.

Most effects will be seen out at sea, Cimbora said. "We'll have some shallow flooding along low-lying coastal areas, where the marshes are, from high tide and the wind pushing on it. There could be light erosion. Nothing like, say, Irene last year," he added.

Winds on Sunday afternoon were 17 miles per hour in Charleston and 23 miles per hour in Savannah, he said. Callers from the beaches have reported waves at three to four feet, Cimbora added, and rip current risk was high.

The Atlantic hurricane season officially runs from June 1 to November 30, but storms outside that time frame are not uncommon. Alberto was the earliest-forming Atlantic storm since 2003, when Tropical Storm Ana formed more than five weeks before the official start of the season, the hurricane center said.