The impact of Topography
on Thunderstorm development
and intensity in Puerto Rico


Puerto Rico is an island, territory of the United States located in the Caribbean Sea. It is the smallest and easternmost island of the Greater Antilles Chain. Puerto Rico has tropical climate, so seasonal changes in temperature and precipitation are much more subtle than in the mid latitudes. The Island is topographically diverse. Much of the interior is a mountain range, with its spine running east-west along its length. Foothills compromise the area surrounding this central range, and give way to a costal plain in the northern and southern part of the Island. There exists a division between the Central Range (Cordillera Central) and a small but steep range to the northeast called La Sierra de Luquillo where the Peak El Yunque is located, this important peak acts as an important convection point for rainfall. El Yunque is the Caribbean National Rainforest with an estimated annual precipitation of 240" inches. El Yunque is a strong orographic rainmaker due to the prevailing east winds. Easterly trade winds quickly ascend the steep slopes of eastern Puerto Rico, creating orographic condensation and dropping rain on the forest below.

Tropical convection is notoriously difficult to forecast (Carter 1977). Puerto Rico has a conditionally unstable atmosphere and abundance of low-level moisture and with heat combined with forcing mechanism, such as sea breeze, fronts and mountain uplift. Diurnal convective and sea breeze rainfall events are most prevalent during summer months, African easterly waves create an unstable environment. Easterly trade winds prevail almost the entire year, this wind regime is characterized by two principal factors: diurnal land and sea breezes. Sea breeze is an example of mesoscale front, it is important because it can be a trigger mechanism for afternoon thunderstorms ( Jeff Haby, Hint 11).

During the afternoon temperature difference between land and sea creates a Sea to land breeze, the mountains create the uplift and condensation occurs. But in Puerto Rico the most thunderstorms creator is the mountain range (Cordillera Central) especially during the summer.

Puerto Rico has the greatest recurring threat to life and property due to flash flooding of any state or territory under the jurisdiction of the National Weather Service (Carter, 1977). By the time an underspectect heavy rain event becomes imminent, it is often too late to take protective actions against resultant flash flooding (Krysztofowicz, 1993). During the summertime the island has its maximum precipitation and thunderstorm activity due to daytime heating, sea breezes and orographic lifting. The mountain chain in the Central part of the Island is responsible for the heavy rain events and thunderstorms activity, especially in the western side. The summertime maximum near Mayaguez at the western end of the Island is due to the combined effect of an internal gravity wave on the lee edge of the mountain range and sea breeze conversance. This gravity wave was discovered in a mesoscale modeling project in 1997 (Beneret, 1997). A gravity wave is a vertical wave, a trigger mechanism causes the air to be displaced in the vertical (Jeff Haby, Haby Hints). This particular situation depends on how much moisture and heat is in the atmosphere. The Cordillera Central forces raising motion in to progressively unstable atmosphere resulting focus on the west end nearly stationary and leads to very heavy rainfall totals. The tropical atmosphere over Puerto Rico is nearly always convectively unstable. This instability it's caused by the moist troposphere. The challenge of diurnal rainfall and thunderstorm activity prediction is framed by the moisture content and wind direction, also sea breeze convergence.

In Puerto Rico, western residents know the effects of the mountain range on their weather and they are prepared for it almost every afternoon, but when the wind direction changes it causes floods in areas where the rain is not expected. When winds change from the west, rain and thunderstorm activity occurs at the east and northeast, something not common. During summertime thunderstorm activity is more intense, especially with upper level dynamics like upper troughs or a passing tropical wave. Tornadoes are not usual, but last year Puerto Rico had a F0 Tornado in Aguadilla at the west part of the Island. This phenomenon caused gusty winds and damage to properties and houses. An upper-level trough was the culprit for the Three Kings Day Flood in January 1992 that claimed 23 lives.

According to Rafael Mojica, warning coordination meteorologist at the National Weather Service in Puerto Rico, the bigger challenge is modeling. The models are not accurate predicting in the tropics, especially on sea breezes and prevailing winds. Insufficient wind data creates a forecasting problem, because of the topography the Island low level wind direction it's not the same on every zone. Puerto Rico is a small Island surrounded by the ocean and sea breeze is an important cause of rainfall and thunderstorms. Detecting thunderstorm activity is another challenge, during the morning there is only one sounding from San Juan (North Puerto Rico) also the NWS Doppler Radar is located 3,000 ft. above sea level in the mountain range, some of the circulation patterns and development of the thunderstorm can't be seen by the radar. But models are improving according to Mojica and the NWS works in the improving and development of new models of the future. The model simulations have lead to improve weather forecast and warnings through increase in our understanding of the interaction of the trade winds with the topography of Puerto Rico.


A precipitation forecast experiment for Puerto Rico
M. M Carter, J.B Elsner, S.P Bennet

Monthly Rainfall Climatology for Puerto Rico
Matt Carter and J.B. Elsmer
Florida State University

Convective rainfall regions in Puerto Rico
Mathew Carter
Meteorologist Jeff Haby