Transitioning National Weather Service Flash Flood Warnings to Impact-Based Warning Format to Enable Partner Parsing for Extreme Flash Flood Events

The National Weather Service (NWS) issues Flash Flood Warnings (FFWs) for short-term events when flooding is imminent or likely, and which require immediate action to protect life and property. Such events include dangerous small stream or urban flooding, rain on burn scars leading to flooding and/or debris flows, and dam or levee failures. The intended goal is to give advance notice of the threats from these weather-related hazards to those in the anticipated path, to broadcast and online media that help broadcast these warnings, and to emergency response personnel in these areas. NWS Weather Forecast Offices issue more than 12,000 FFWs each year for events with a range of impacts on lives and livelihoods. Regardless of the certainty or severity of the event, every FFW triggers a Wireless Emergency Alert (WEA) on mobile devices such as cell phones and pagers. Currently, the only way to differentiate the severity of impact of flash flooding is through a “Flash Flood Emergency” sub-headline within the FFW.

In 2019, the NWS plans to reformat FFWs to an Impact-based Warning (IBW) format. This is part of the broader “Hazard Simplification” effort which is working to reduce the number of NWS products and improve the product text. The IBW format provides information on “hazard”, “source” and “impact”. It also, better aligns FFWs with the short-fused formats used for Tornado, Severe Thunderstorm, and Special Marine Warnings introduced in 2018. Moreover, transitioning FFWs to an IBW format will allow for better characterization of flooding damage threat and impact through use of “Considerable” and “Catastrophic” damage tags. These tags would enable appropriate limitations to be placed on WEA alerting

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 limited anticipated impact to life or property, 2) those which are significantly life-threatening and causing substantial damage to property, and 3) those exceedingly rare, violent flash flood events which extraordinarily threaten lives and cause disastrous damage. Warnings in the first category will carry no damage threat tag as those flash floods would have limited impact. These are envisioned to be the majority of warnings issued and will not WEA alert. Warnings in the second category will be tagged “Considerable” due to expected significant impacts to life and property. The tag for these unusual/significant flash floods will trigger WEA alerts. Warnings in the final category will be tagged “Catastrophic” and will also trigger WEA alerts, but these types of warnings would be very rare and would approximate current Flash Flood Emergencies. Including impact statements describing the severity of impacts expected and their locations will facilitate differentiation of less severe events from those that are most damaging in order to improve communication to motivate proper response to warnings.

The IBW format provides valuable information to media and emergency managers, facilitates improved public response and decision making, and better meets societal needs in the most life-threatening weather events.