(Adds details from Puerto Rico and the Dominican Republic)
MIAMI, Aug 23 (Reuters) - A tropical weather system drenched Puerto Rico and the Dominican Republic on Saturday and is expected to gather strength as it moves over the warm open waters near the Bahamas, the National Hurricane Center (NHC) said.
Forecast models show conflicting paths for the storm, with some bringing it near the Florida peninsula and others showing it drifting toward the U.S. mid-Atlantic region.
Forecasters gave an 80 percent chance of it becoming a tropical depression in the next 48 hours and a 90 percent chance in the next five days.
"The environment that it's going to be moving into later today and the next day should be more favorable to it becoming a tropical depression or storm," said Michael Brennan, a NHC hurricane specialist.
Heavy rain fell overnight and into Saturday in Puerto Rico and several towns were under a flash flood watch.
"We will be affected for the rest of the day and into tomorrow morning," said National Weather Service meteorologist Tony Estrada.
The heavy rains filled up Lake Carraizo and the La Plata River, two of the main water sources for the capital city, San Juan, eliminating the threat of water rationing after months of unseasonably dry weather.
In the Dominican Republic an estimated 790 people were displaced after at least 158 homes were flooded, the National Center for Emergency Operations said on Saturday.
Communication was severed with at least 10 communities by flooded roads and damaged bridges.
Flash flood and mudslide warnings were in place in 17 of the country's 31 provinces.
Federal forecasters in August downgraded their outlook for the 2014 Atlantic hurricane season, predicting below normal activity with seven to 12 named storms, and no more than two expected to reach major hurricane status.
A major hurricane is considered to be Category 3 or above with winds hitting at least 111 miles per hour (178 km per hour).
So far this year two hurricanes - Arthur and Bertha - have developed in the Atlantic. Only Arthur, a Category 2 storm, made landfall, on North Carolina's Outer Banks in early July.
A typical season has 12 named storms, with six hurricanes, and three becoming Category 3 storms.
In its August outlook, the agency said cooler than average temperatures in the tropical Atlantic Ocean would make it difficult for larger storms to develop. (Reporting by Zachary Fagenson in Miami and; Ezra Fieser in Bavaro, Dominican Republic.; Editing by David Adams, Bernard Orr)