"Melissa is expected to produce heavy rainfall across portions of the Dominican Republic, Haiti, and Jamaica through this weekend, bringing a risk of significant flash flooding and landslides," the NHC said in an advisory.

The storm is expected to strengthen in the coming days, "and Melissa could become a hurricane on Thursday," it added, noting a hurricane watch is in effect for Haiti's southwest.

Heavy rains in the Dominican Republic, which borders Haiti, have already disrupted traffic and led to the cancellation of sports events.

As of 0900 GMT, the NHC said the storm was about 305 miles (490 kilometers) south-southwest of Haiti's capital Port-au-Prince, and moving at a speed of seven miles (11 kilometers) an hour.

The agency noted that there was "large uncertainty" over the storm's path and intensity forecast.