(CNN) -- Tropical Storm Andrea formed in the Gulf of Mexico on Wednesday, marking the first storm of the 2013 Atlantic hurricane season.
The storm was about 270 miles west-southwest of Tampa, Florida, and moving north at 6 mph, said the National Hurricane Center. It had maximum sustained winds of 40 mph.
A tropical storm warning was in effect for the west coast of Florida, from Boca Grande, north of Fort Myers, to the Ochlockonee River, south of Tallahassee, the hurricane center said.
A tropical storm watch was issued for the U.S. East Coast as well, from Flagler Beach, Florida, up through Surf City, North Carolina.
The center of Andea is forecast to reach the coast of Florida Thursday afternoon and move over southeastern Georgia and eastern South Carolina Thursday night and Friday.
Little change in strength is expected over the next 48 hours.