Heavy Rain Events on the South Facing Slopes of Puerto Rico

Heavy rainfall on the south slopes of Puerto Rico often results in loss of life and property as water flowing down the steep terrain leads to dangerous flash flooding and mudslides. The serious consequences of these events make it necessary to have an understanding of the meteorological situations that creates them in order to improve forecasts. The major questions to answer in this study are: 1) What are the synoptic situations which favor these events? 2) What time of year do they occur and why? 3) What is the primary precipitation type: convective or stratiform, and does this help determine the spatial distribution of rainfall? 4) What is the role of topography? This study will look at heavy rain events from the past 20 years and use Weather Surveillance Radar-1988 Doppler archive level II, satellite, United States geological survey rain gauge, and the National Centers for Environmental Prediction reanalysis data to answer these questions.

Isohyet analysis of yearly average rainfall indicates that the south side of Puerto Rico is the driest part of the island, with 1000 mm of rain or less. Easterly tradewind flow, which prevails across Puerto Rico 90% of the time, is parallel to the prevailing mountain range of the island, and puts the south part of the island in an unfavorable position for frequent heavy rains. On occasion, a synoptic scale feature will be strong enough to change the prevailing wind direction, and place this side of the island in a favorable position for heavy rainfall. Preliminary results indicate that there are four synoptic situations when this occurs; a tropical cyclone passage, a stalled frontal boundary to the west of the island, a strong mid to upper level low north of the island whose reflection builds to the surface, and the combination of a tropical wave passage at the surface with an upper level trough to the west of the island. Each one of these replaces the prevailing easterly tradewinds with flow that has a more southerly component. This advects abundant tropical moisture over the island, and puts the south side of the island on the windward side. Aside from a tropical cyclone passage, the majority of the events take place during the transition season in October and November.

The rain events can last from two days to five or more days. For the longer events, much of the precipitation is stratiform as southerly flow moistens the entire troposphere, instability decreases, and approaches a moist neutral profile. Shorter duration events were dominated by convective activity. In all cases, orographic lift enhanced the local shower activity, and the flow normal to the main peaks of the island helped dictate the spatial variability of rainfall. However, since instability decreases with the length of the event, the distribution of rainfall in the longer term events is more dependent on orography than the shorter term events as it provides areas of enhanced convergence and lift for the relatively more stable atmosphere. This makes forecasting the spatial distribution of rainfall more challenging for shorter term convectively driven events.

One occurrence of a longer term event was from the 9th to the 13th October 2005, when a large and energetic mid to upper level low, located north of Puerto Rico, helped create an area of surface troughing which spread from the western Caribbean waters northeast into the western Atlantic waters. Widespread rain affected the island for five days, and flash flooding was a common occurrence. As much as 50% of the total rainfall across the island fell on October 11, when a small surface low formed to the west of the island, and provided a prevailing west southwest wind direction. Analysis of the United States geological survey rain gauge data indicates that the heaviest rainfall totals were observed in the municipalities of Ponce, Adjuntas, Penuelas, and Naguabo. The prevailing southwesterly flow placed Ponce, Penuelas, and Adjuntas just windward of Cerro de Punto, which at 1338 m is the highest peak on the island. Naguabo is just windward of the second highest peak on the island (Pico del Este, 1051 m) in southwesterly flow. This indicates that the topographic profile of the island may have influenced the rainfall distribution. By October 11, southerly flow had already moistened the entire troposphere. Instability decreased and approached a moist neutral profile resulting in rainfall which was more stratiform than convective. With less instability, orographic lift provided a fixed lifting mechanism, with the highest peaks likely resulting in areas of enhanced and persistent convergence.