Implementing a Phased Approach to the Evaluation and Distribution of Flood Inundation Maps

The traditional hydrograph format of river flood forecasts provide a wealth of important information to the trained eye, however, one limitation is it’s ability to effectively communicate potential ground impacts. The NEW Flood Inundation Maps (FIMs) generated by the National Weather Service’s (NWS) National Water Model (NWM) provide geospatial extents to simulated river flows allowing for potentially hazardous conditions to be anticipated before a flood occurs. This can be challenging to interpret when there are a variety of options available, but by implementing a four-phased approach (preparedness, pre-event, event, and post-event), forecasters are able to assess the impacts, quality, and confidence partners can expect from each NWS FIM. This approach reduces the area of interest, allowing forecasters and emergency partners to provide more targeted support. During the preparedness phase, initial impacts are assessed and documented for widely available static FIM sites providing a foundation at defined locations to compare dynamic, deterministic extents once they become widely available. When a significant weather event appears on the horizon and dynamic FIM begins to generate, the analysis of available FIM intensifies to include deterministic model outputs and extended reach extents which weren’t previously available. The Hydrologic Ensemble Forecast Service and River Forecast Center Probabilistic Quantitative Precipitation Forecast ensemble model run comparisons are then used together to identify areas which have the highest impact potential enabling more intentional FIM review efforts to occur. This pre-event period is crucial for providing FIM to partners in advance of rainfall onset in order to enhance partners’ decision making for targeted emergency response efforts. Once the rainfall begins, the event phase is triggered where FIM extents can provide guidance to safety and recovery efforts. Decision support graphics are generated highlighting areas of vulnerability, such as transportation infrastructure and residences, which may see flood water impacts. After the event has ended, post event analysis can provide aid to recovery efforts as well as compare previously forecasted extents with ground truth observational data, impact photos, and other flood documentation to verify overall extents and improve future FIM generation. This cycle enables partners to be more prepared and confident in their response decisions rather than reactive once distress calls have already begun to occur.