Tropical Storm Alberto had weakened slightly to 45 mph winds by noon Sunday.
At 11 a.m. EDT, the National Hurricane Center said Alberto was about 90 miles south of Charleston, S.C.
Alberto was moving west-southwest at 6 mph but is expected to turn northeast Monday.
A tropical storm watch remained in effect from the Savannah River to South Carolina's Santee River. The Hurricane Center advised people on the coast from Georgia to North Carolina's Outer Banks to monitor the storm and warned of dangerous surf conditions.
"Tropical storm conditions are possible along the coast of South Carolina in the watch area later today or Monday," the Hurricane Center said.
The U.S. Air Force planned to send a Hurricane Hunter aircraft to explore the storm during the afternoon.
Although the official start of the Atlantic hurricane season is June 1, storms have formed in May, the Hurricane Center reports. A total of 18 tropical storms have formed in May since accurate records began in 1851.
Alberto is the earliest-forming tropical storm in the Atlantic since Ana in 2003, which formed on April 21, reports meteorologist Jeff Masters of the Weather Underground.
Alberto is one of only three Atlantic tropical storms to form in May in the past 31 years, he says.
"Formation of an early season tropical storm from an old frontal boundary, like what occurred with Alberto, is not a harbinger of an active hurricane season - it's more of a random occurrence," Masters says.
Contributing: Doyle Rice and Associated Press
By Donna Leinwand Leger, USA TODAY