These are primarily summertime phenomena, and number between fifty and seventy annually. Not all waves bring rain to Puerto Rico, and some may in fact inhibit mesoscale diurnal rainfall effects. Other waves develop into depressions, tropical storms, and hurricanes, which bring copious amounts of rain. These upper-level lows with their direct thermal circulation and absence of dissipative mechanisms can last for several days to weeks. During the wintertime, convective rainfall is often associated with frontal systems from the north.
Tropical convection is notoriously difficult to forecast.
However, the value of such forecasts is large because of the potential for flooding and mudslides. This is particularly important in the tropics where rainfall can be locally intense. A conditionally unstable atmosphere and an abundance of low-level heat the moisture combine with forcing mechanisms, such as sea breeze fronts, to explain the contingency of tropical rainfall. As a step toward developing quantitative precipitation guidance for forecasting rainfall events in Puerto Rico, we provide a monthly rainfall climatology. Daily rainfall totals for stations over the main island were available for the period 1980-89.