An encyclopedic resource is the official page, twitter is the latest news:
Yunque park Telephone: (787) 888-1880 or 1810, emergency Dial 911.
Weather - El Yunque will be CLOSED if a hurricane passes near. The peak season is the middle of September, the range is from Aug to Nov, but who nows?
Pay attention to the weather before you come - if there was a lot of rain in the days before, the stage is set for flash floods in some rivers - avoid them. Rio Blanco in Naguabo and Mameyes in Puente Roto are the most prone to flooding. Otherwise it's all safe, but I recommend going in the water with water-shoes due to sharp rocks, feet are very sensitive. If there is a rain-storm near - avoid the park.