(CNN) -- A large swath of coastal South Carolina is under a tropical storm watch due to Alberto, the first named tropical storm of the 2012 Atlantic hurricane season.

As of 11 p.m. ET Saturday, Alberto was about 110 miles (180 kilometers) southeast of Charleston, South Carolina, and about 155 miles (250 kilometers) east Savannah, Georgia, according to the National Hurricane Center in Miami. The storm carried maximum sustained winds of about 50 mph (85 kph).

The tropical storm watch area stretches from the Savannah River, which separates Georgia from South Carolina, to the South Santee River in South Carolina.

Alberto was moving southwest at about 6 mph (9 kph), and "a decrease in forward speed is expected through Sunday," the weather agency said. It is expected to turn west-northwest and then north and northeast by Monday.

"Dangerous surf conditions are possible along the coasts of Georgia and South Carolina through Monday," the hurricane center said.

Despite being over the warm waters of the Gulf Stream, Alberto is not expected to intensify much over the next couple of days. That is due, in part, to the presence of a cool and dry air mass to the north and west of the tropical storm, CNN meteorologist Sean Morris said.

This year marks the first time in recorded history that a tropical storm has formed in both the east Pacific basin and Atlantic basin before the official start of hurricane season, the hurricane center said. The Pacific storm was called Aletta.

The Atlantic hurricane season officially begins on June 1.