UPDATE 3-Season's 1st Atlantic tropical storm forms off S.Carolina

(Adds tropical storm warning, updates position, strength)

By Jane Sutton

MIAMI May 19 (Reuters) - Tropical Storm Alberto formed off the South Carolina coast on Saturday, bringing an early start to the Atlantic hurricane season, forecasters at the U.S. National Hurricane Center said.

Although the storm was likely to remain offshore, forecasters issued a tropical storm warning for the South Carolina coast late on Saturday, saying there was "too little margin for error" not to issue the warning.

Alberto was in the Atlantic about 110 miles (180 km) southeast of Charleston with maximum sustained winds of 50 miles per hour (85 kph), with higher gusts, the hurricane center said. "Little change in strength is forecast during the next 48 hours," the forecast added.

It was moving slowly southwest and forecasters said dangerous surf conditions were possible along the coasts of Georgia and South Carolina through Monday.

Alberto was forecast to make a slow loop during the next few days and then turn northeast, making its way 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 could bring squalls and rough surf to the Carolina coast.

The Atlantic hurricane season officially runs from June 1 to Nov. 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.

(Reporting By Jane Sutton and David Adams; Editing by Peter Cooney)