Overnight, Bonnie drenched coastal South Carolina. The National Hurricane Center
issued a tropical storm warning for the state's coastline. Maximum sustained winds decreased from 45 mph to 40 mph Sunday morning.
Bonnie is close to Charleston and is expected to keep heading north.
The system comes as tourists hit the beaches for Memorial Day weekend.
It's unclear exactly when Bonnie would make landfall, but Sunday looked likely, said CNN meteorologist Jennifer Varian.
"Bonnie has been kind of a fickle system," added CNN meteorologist Karen Maginnis. "We were thinking it would be a little more organized tropical storm, but it is producing kind of a washout in coastal areas in the Carolinas."
However, Maginnis said the storm would "probably not" gain any more strength.
The storm is expected to produce about 2 to 4 inches of rainfall, with a maximum of 6 inches in central and eastern South Carolina stretching south to the Georgia border, according to the Hurricane Center.
The Hurricane Center said a storm surge inundation of 1 to 2 feet above ground level is possible in warning areas during the morning's high tide, and surf and rip current conditions could be "life-threatening."
An isolated tornado is also possible in the early morning over the South Carolina coast, the center said.
Charleston police urged drivers to be on the lookout
for downed trees and power lines, and avoid driving through flooded areas.