The Catastrophic August 21, 2021 Flash Flood in Waverly, Tennessee: Decision Support and the Need for Crucial Watch-to-Warning Information

Low probability, high impact weather events continue to be a significant challenge for meteorologists to recognize and message to core partners, decision makers, and the public. On the morning of August 21st, 2021, record-setting extreme rainfall resulted in catastrophic flash flooding in Waverly, Tennessee, resulting in 19 fatalities. A Flash Flood Watch was issued 8 hours before the heavy rain began, but this was largely based on wet antecedent conditions and expectations of 2 to 4 inches of additional rainfall. In reality, a swath of 7 to 10 inches of rain fell, and within that swath, 21 inches was measured upstream of Waverly at McEwen, Tennessee. Convective allowing models provided little help, as they only showed rainfall amounts mainly in the 2 to 4 inch range with some small areas of 7 inches in the vicinity, and little consistency between runs, resulting in low confidence. The watch to warning gap is a crucial messaging period (often 2–8 hours) in which important information regarding timing, location, and severity of hazardous weather impacts can be provided as confidence increases. Future use of tools like the Warn-on-Forecast System (WoFS) storm-scale ensemble with its frequent updates, will allow for a more continuous flow of information to fill the messaging gap. The future National Weather Service (NWS) office will also need to leverage strong relationships with core partners and decision makers so that potentially life-saving information derived from tools like WoFS can be effectively communicated through impact-based decision support services (IDSS). This presentation will detail an August 21st, 2021 retrospective run of WoFS and how it could have impacted IDSS during the watch to warning gap for this historic flooding event.