FLASH Guidance Validation: Puerto Rico Study

Flash floods are a year-round hazard in Puerto Rico and the U.S. Virgin Islands. In Puerto Rico and nationwide, the National Weather Service (NWS) issues Flash Flood Warnings (FFWs) to alert the public, emergency management, and cooperating agencies when flash flooding is occurring, imminent, or highly likely. During the issuance of FFWs, the NWS Weather Forecast Office (WFO) evaluates factors such as Flash Flood Guidance (FFG) exceedance and the Average Recurrence Interval (ARI), which are part of the Flooded Locations and Simulated Hydrographs (FLASH) project. Although recommended FFG and ARI values are available in the literature for issuing FFWs, these values were validated primarily at sites within the continental U.S. (CONUS). This research aimed to validate the guidance for locations outside of CONUS (OCONUS), such as Puerto Rico. The study involved retrieving Local Storm Reports from the NOAA Storm Database from October 2020 to May 2024 and comparing them with corresponding FFG and ARI values obtained from FLASH using the Multi-Radar Multi-Sensor (MRMS) interface. The results showed that the recommended FFG values apply to Puerto Rico, as flash flood reports aligned with the expected range. However, the recommended ARI values for issuing FFWs were significantly lower across the island, with nearly 60% of flash flood reports occurring below the suggested 10-year ARI threshold.