A hurricane warning was issued for the South Carolina coast Saturday as forecasters predicted Tropical Storm Gaston will strengthen and make landfall near Charleston on Sunday night.

The National Hurricane Center posted the warning for the South Carolina coast, from the Savannah River to Little River Inlet, as Gaston drifted slowly to the northwest. A tropical storm watch remained in effect from Little River Inlet north to near Wilmington, N.C., and south to the Georgia-Florida line.

Gaston -- the hurricane season's seventh named storm -- was about 95 miles southeast of Charleston, moving westward at about 2 mph with maximum sustained winds near 65 mph. A weather system becomes a hurricane when its maximum sustained winds reach 74 mph.

Gaston, which formed southeast of Charleston on Friday, could bring 4 to 8 inches of rain to coastal areas. A flood watch was issued for parts of South Carolina and North Carolina.

It will be the second time in two weeks that a tropical system has made landfall along South Carolina's coast, which is already saturated from recent heavy rains and Hurricane Charley.

"Charley was a very fast-moving storm. Residents need to realize that Gaston is going to be moving much slower," said Roland Windham, the Charleston County administrator. "They're going to be faced with rains and winds for much longer periods of time."

In the Atlantic, Hurricane Frances strengthened to a Category 4 hurricane with maximum sustained winds nearing 135 mph. Forecasters said it could threaten the Bahamas and Florida by the end of the week.

Frances was moving northwest at about 10 mph, and its center was about 635 miles east of the Leeward Islands in the southeastern Caribbean as of late Saturday. Forecasters predicted that Frances will turn west-northwest by Sunday.