A Baseline Sounding Climatology and a Risk Assessment Tool to Forecast Impact Weather in the Tropical Island of Puerto Rico

The NOAA National Weather Service Storm Prediction Center (SPC) has been responsible for forecasting severe weather events in continental United States since the mid-1990s.  Hawaii and the US Territories (Guam and Puerto Rico) are not included in these severe weather forecasts mainly due to the low frequency of those events in the tropics. However, significant impacts have been observed in Puerto Rico with those rare events. Although a synoptic feature is necessary to produce impact weather, the island's topography serves as a trigger mechanism for these events. The majority of the operational models do not capture the complexity of the island's topography and the sea/land breezes interactions. Therefore, the weather forecasting of these rare events is very difficult. This study introduces a baseline sounding climatology of weather parameters. The data used to create this baseline climatology consists in approximately 22,000 soundings from TJSJ (San Juan, PR). It contains 00Z and 12Z soundings from 1982 to 2012. This baseline climatology is much needed to support certain aspects of operational forecasting. Until now, there was no known sounding climatology for Puerto Rico; as a result forecasters based the severe weather forecasting in their subjective experience or in some model guidance, not in a quantitative analysis of the weather parameters. After the baseline sounding climatology was developed, the next step was classifying the weather reports for each weather hazard. Soundings for those significant weather days were categorized based on flood rain, hail, thunderstorm winds, tornado and waterspout. Specific thresholds were derived after comparing the characteristic soundings of the aforementioned weather hazards with the baseline sounding climatology. Different weather parameters were chosen to discriminate between environments that favor each weather hazard. The calculation of the risk is done automatically by a python script that computed the probability of experiencing impact weather on the island using the latest 12Z sounding for TJSJ. The Risk Assessment Plot (Figure 1) is presented to the forecasters every morning after the 12Z TJSJ sounding is generated by the Weather Forecast Office (WFO) San Juan. The graphical output contains the probability of occurrence of the aforementioned weather hazards and important information about the environment. This forecasting tool has been tested for more than a year and has demonstrated good skill in forecasting the potential of impact weather in Puerto Rico and it is surrounding islands. As a result, forecasters at WFO San Juan are currently using the Risk Assessment tool to prepare and react in case impact weather is developing or occurring inside of the County Warning Area (CWA). Furthermore, they use the Risk Assessment tool to update the Hazardous Weather Outlook (HWO) product to alert the Emergency Managers and the public about the potential of impact weather on the island.