Hurricane Fiona struck Puerto Rico's southwest coast on Sunday after causing an island-wide power blackout and threatening to dump "historic" levels of rain.
Forecasters said the downpour was expected to produce landslides and catastrophic flooding, with up to 25 inches (64 centimeters) possible in isolated areas.
"It's time to take action and be concerned," said Nino Correa, Puerto Rico's emergency management commissioner.
Fiona hit about 15 miles (25 kilometers) south-southeast of Mayaguez with maximum sustained winds of 85 mph (140 kph), according to the U.S. National Hurricane Center. It was moving to the northwest at 9 mph (15 kph).