Tropical waves and troughs in Puerto Rico

You do not need a hurricane for it to rain hard in Puerto Rico. Much of the island's flash flooding is caused by tropical waves and troughs.

Tropical waves

Tropical waves are areas of low pressure that move across the Atlantic from Africa during the summer. They bring cloud, downpours and thunderstorms, and some are the seed of future cyclones.

Troughs (vaguadas)

A trough is an elongated axis of low pressure that can stall over the island and unload rain for days. Even when the sky does not look threatening, the ground saturates and rivers rise.

Beware of flash floods

The mountainous terrain sends water downhill fast. When the NWS issues a flash flood warning, do not cross fords or flooded roads: most casualties happen in vehicles.