National Weather Service Field Testing of the Flash Flood Severity Index: A Retroactive Analysis from 2018 to 2022

Abstract Flash flooding continues to be one of the deadliest natural hazards in the United States. Unfortunately, most flood classification systems fall short in conveying flood threat to the public. An interdisciplinary approach is necessary to properly categorize flash flood impacts to improve forecasting techniques and provide effective messaging of life-saving flood information. Furthermore, understanding how each flash flood event compares to another provides the community with a more thorough frame of reference regarding local flood hazards. To address this, a collaborative research endeavor resulted in a simple, damage-based, postevent assessment tool—the Flash Flood Severity Index (FFSI). The five-tiered index debuted in 2016, and National Weather Service (NWS) field testing began in 2018 at a subset of Weather Forecast Offices (WFOs). The initial analysis showed that most flash flood events can be categorized as moderate or serious (FFSI-2 or FFSI-3). This study outlines the details of the field testing and the results of classifying local storm reports (LSRs) with FFSI. Links between FFSI and flash flood warning (FFW) impact-based warning (IBW) tags were also incorporated. Results show that there are some disparities between the severity of the flash flood based on FFSI categorization and the IBW tags assigned to the associated FFWs. As the inventory of events grows, collaborators aim to further develop the classification system, expand testing across additional WFOs and other climate regions, and explore the potential for improving flash flood forecasting and messaging techniques.