Dominican Republic ? Olga strengthened and became a tropical storm Tuesday, drenching the Dominican Republic and forcing the evacuation of low-lying communities ahead of the rare December cyclone.

The storm had maximum sustained winds of 60 mph, and forecasters warned it could trigger dangerous floods by dumping as much as 10 inches of rain in some parts of the Dominican Republic and neighboring Haiti.

Olga struck nearly two weeks after the official end of the Atlantic hurricane season. It is only the 10th named storm to develop in the month of December since record keeping began in 1851, according to the U.S. National Hurricane Center in Miami.

“It’s not completely unusual to have a storm form in December,” said Daniel Brown, a hurricane specialist at the center, who noted that three named storms have formed after Nov. 30 since 2003.