(Adds details of latest forecast)
SAN JUAN, Aug 22 (Reuters) - A low-pressure system moved over Puerto Rico on Friday and is expected to veer northeast away from the U.S. East Coast but still has an 70 percent chance of forming into a tropical storm in the next two days, the National Hurricane Center (NHC) said.
The storm is expected to lose intensity as it crosses the mountains of Hispaniola, drenching the Dominican Republic and Haiti before regaining strength over open water near the Bahamas, forecasters say.
"At present there is no credible guidance that takes the system towards the coast of Florida," said NHC forecaster Todd Kimberlain, though he cautioned the outlook could change early next week.
"It's the peak of the hurricane season so it's best to be on your guard," he said.
Officials in Puerto Rico welcomed the rain during a summer of dry weather that has raised the threat of water rationing in the San Juan metropolitan area, but they also warned of flooding expected to begin Friday afternoon.
"We are calling on citizens to remember not to try to cross flooded areas in cars nor try to cross rivers because strong currents can produce unfortunate incidents," said State Emergency Management Agency Executive Director Miguel Rios Torres.
So far this year two hurricanes - Arthur and Bertha - have developed in the Atlantic. Only Arthur, a Category 2 storm, made landfall, swiping North Carolina's Outer Banks in early July.
Federal forecasters in early August downgraded their outlook for the 2014 Atlantic hurricane season, predicting below normal activity with seven to 12 named storms, no more than two of which are 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).
In its August outlook, the agency cited the strengthening of climate conditions as unfavorable to hurricane development, including cooler than average temperatures in the tropical Atlantic Ocean.
A typical season has 12 named storms, with six hurricanes and three reaching major Category 3 status. The hurricane season runs from June 1 to Nov. 30. (Reporting by Reuters in San Juan and David Adams and Zachary Fagenson in Miami, Editing by Bill Trott and Ken Wills)