MIAMI (Reuters) - A swirling mass of thunderstorms off the South Carolina coast has a 50-percent chance of developing into a tropical depression or a tropical storm and could bring an early start to the Atlantic hurricane season, forecasters said on Saturday.

The weather disturbance was in the Atlantic Ocean about 120 miles southeast of Myrtle Beach, South Carolina, forecasters at the National Hurricane Center in Miami said.

"Showers and thunderstorms have increased near the circulation center. Additional development of this system is possible and it could become a tropical depression or tropical storm as it moves slowly southward or westward over the next day or so," they said.

A southerly course would take it over the Bahamas while a westerly track would take it ashore over North and South Carolina. Neither course would pose an immediate threat to U.S. oil and gas operations clustered in the Gulf of Mexico.

If its swirling winds reach 39 miles per hour (63 kilometers per hour), it would become Tropical Storm Alberto, the first named storm of the 2012 Atlantic hurricane season.

The season officially runs from June 1 to November 30, but storms outside that time frame are not uncommon.

(Reporting By Jane Sutton; Editing by Xavier Briand)