Simplifying and Clarifying National Weather Service (NWS) Flood Products via the NWS Hazard Simplification Project

For decades, the National Weather Service (NWS) has used the Watch, Warning, and Advisory (WWA) system to alert users of forecasted hazards. In many ways, the WWA system has been highly effective in protecting life and property. However, as we have collected feedback during the NWS Hazard Simplification Project, we have learned that some users find the WWA terms confusing. Also, users are sometimes confused about how to interpret and distinguish among the large number of individual WWA products. Based on this initial feedback, NWS has explored alternatives for more effectively communicating our hazard messages. These include keeping the current WWA system as is, making small to moderate changes (e.g., consolidating and reformatting our WWA products), and executing transformational change (e.g., completely overhauling and revisualizing our current weather and water hazard messaging system).

During fall 2019, the NWS reformatted Flash Flood Warnings (FFWs) to an Impact-Based Warning (IBW) format. This is part of the broader “Hazard Simplification” effort, which has worked to reduce the number of NWS products and improve the product text, and has better aligned FFWs with the short-fused formats used for Tornado, Severe Thunderstorm, and Special Marine Warnings introduced in 2018. The IBW format is a consistent, bulleted format that provides easily readable information describing the hazard (flash flood), the source of the information (e.g., radar, gauge, trained spotter, emergency manager, etc.), and a brief description of the hazard impact. The FFW IBW messages include machine-readable tags to characterize the flash flood damage threat, source information, and causative event. These tags have enabled appropriate limitations to be placed on Wireless Emergency Alerts (WEAs).

Compared with specific numerical or phenomenon-based thresholds for Severe Thunderstorm Warning tagging, characterization of flash flood by impacts is relatively broad, as objective criteria are difficult to define across the country and vary by population, infrastructure, and other factors of local environment and conditions. However, events may generally be categorized as 1) those with the potential for impacts and damage, 2) those which are capable of unusual severity of impact where urgent action is needed to protect lives and property, and 3) those exceedingly rare, violent flash floods which threaten lives and cause disastrous damage when floodwaters are placing or will place people in life-threatening situations by rapidly rising to levels rarely, if ever, seen. Warnings in the first category don’t include a damage threat tag as those flash floods have limited impact. These are envisioned to be about 80% of the warnings issued and will not wireless emergency alert (WEA). Warnings in the second category are tagged “Considerable” due to expected significant impacts to life and property. The tag for these unusual/significant flash floods trigger WEAs. Warnings in the final category are tagged “Catastrophic” and also trigger WEAs, but these types of warnings are very rare. Catastrophic flash flood damage threat tags only appear in FFWs which include flash flood emergency language.

In summer 2020, the NWS plans to continue reformatting and consolidating its flood products. In this context, consolidation means the combining of two or more WWA products for a given hazard into one product. First, the Flash Flood Watch and Flood Watch will be consolidated into one Flood Watch product. The NWS will discontinue the use of the Flash Flood Watch product. Instead, information regarding possible flash flooding will be included in the “What” section of the reformatted Flood Watch product. The Valid Time Event Code (VTEC) phenomena (pp)/significance (s) code FF.A will be discontinued. Second, the five types of Flood Advisories (Urban and Small Stream Flood Advisory, Small Stream Flood Advisory, Arroyo and Small Stream Flood Advisory, Flood Advisory, and Hydrologic Advisory) will be consolidated into the Flood Advisory. Specific information regarding individual hazards will be provided in the “What” portion of the Flood Advisory product. Third, all flood WWA products except FFWs will be reformatted into “What, Where, When, Impacts, Additional Details” format.