Tropical Storm Melissa dumped heavy rain on parts of Haiti and the Dominican Republic on Wednesday as forecasters warned of a significant flood risk in parts of the northern Caribbean this week.

Forecasters say Tropical Storm Lorenzo has weakened as it churns over the central tropical Atlantic Ocean and is not threatening land.

Tropical activity is ramping up in the Atlantic Basin. Along with Hurricane Gabrielle, which is currently tracking east of Bermuda, the National Hurricane Center is monitoring two tropical disturbances in the central Atlantic for potential development this week.

Forecasters say PTC Nine could intensify into a tropical storm over the weekend as it tracks through the Bahamas.

The system was located over the central tropical Atlantic as of 1 p.m. Thursday, and forecasters said they expect it to continue moving west at 15 to 20 mph over the next several days. It could develop more once it reaches the warm waters of the Caribbean, forecasters said.

The National Hurricane Center designated a tropical wave in the Atlantic as Invest 98L on Saturday as it slowly crept toward the Caribbean. The term "invest" is used by the NHC to identify weather disturbances they are investigating for possible development into a tropical depression or tropical storm within the next seven days.

The National Hurricane Center is tracking two tropical waves in the Atlantic Ocean. Here's what to know about whether either will impact the US.