The National Hurricane Center said Tropical Storm Colin formed "rather unexpectedly" near the South Carolina coast Saturday morning.
Colin is expected to move along or just inland of the coasts of the Carolinas, bringing tropical storm conditions inside the warning area into Sunday.
The system is expected to be short-lived, dissipating soon after moving over Atlantic waters.
Tropical Storm Bonnie has emerged over the Pacific Ocean, though life-threatening flash flooding and mudslides remain possible due to heavy rainfall across parts of Nicaragua and Costa Rica.
Bonnie will continue to move westerly away from Central America, and then begin to move in a west-northwest direction by Sunday.
"Although the strongest winds are expected to remain offshore, interests in coastal El Salvador, Guatemala, and southwestern Mexico should continue to monitor Bonnie's track for the next few days, as any northward track adjustments could require tropical storm watches for portions of this coastline," said forecasters.
Some slow intensification is possible, and the NHC believes Bonnie could become a hurricane by Tuesday.