Tropical Storm Bertha, packing winds of 45 miles (75 kilometres) per hour, was located east of the southern Lesser Antilles islands, and was moving to the west-northwest, the Miami-based National Hurricane Centre said in an advisory at 10.00am.
"Bertha is expected to pass near Barbados Friday afternoon (August 1) and move through the central Lesser Antilles Friday evening," the advisory said, adding that residents in the north-eastern Caribbean Sea should also monitor the storm closely.
The forecasters warned of heavy rains and storm conditions in Barbados by midday Friday and continuing along the storm's path to Puerto Rico and the Virgin Islands through Saturday night.
The Atlantic hurricane season began on June 1 and runs through to November 30.
The season is expected to be quieter than usual, the NHC has said, with between eight and 13 tropical storms, of which three to six could rise to hurricane strength.