Subtropical Storm Olga weakened slightly as it lashed the northern coast of Puerto Rico early Tuesday, but forecasters feared its heavy rains could trigger flash floods and mudslides by the time it reaches Haiti.

At 7 a.m. EST, Olga was centered over the waters west of Puerto Rico, about 45 miles east of Cabo Engano in the Dominican Republic. It was moving toward the west at 15 mph, the National Hurricane Center said.

Olga's maximum sustained winds dropped to 40 mph, down from 45 mph earlier Tuesday, and forecasters did not expect the storm to strengthen before it reached the coast of the Dominican Republic. The mountainous terrain of Hispaniola, the island shared by Haiti and the Dominican Republic, was expected to further weaken Olga.

Forecasters said Olga's heavy rains would be the system's greatest hazard, with up to six inches expected to fall over Hispaniola and up to four inches expected over Puerto Rico.

The government of the Dominican Republican issued a tropical storm warning Monday night for the northern coast from Cabo Engano westward to Bahio de Manzanillo at the border of the Dominican Republic and Haiti. A tropical storm watch was issued for the southern coast of the Dominican Republic from Cabo Engano to Punta Palenque southwest of Santo Domingo.

"It is unusual since the season does end officially on November 30," said meteorologist Hugh Cobb. "However, in recent years we have seen quite a few December cyclones."

In 2005, the busiest Atlantic hurricane season on record, Tropical Storm Zeta formed Dec. 30 and meandered over the Atlantic for about a week without approaching land. Zeta was the 28th and final named storm of that season.

Olga will be included in the tally for the 2007 season, which will now have 15 named storms, including six hurricanes. The next season officially begins June 1.