What Defines a Relatively Normal Tornado Day? Exploring the Relative Risk of Tornadoes Using Practically Perfect Hindcasts for All U.S. Tornado Days from 1950 to 2021

Abstract Risks associated with rare events that occur infrequently but have serious impacts, such as tornadoes, are difficult to communicate. The U.S. National Weather Service Storm Prediction Center (SPC) communicates tornado risks by forecasting the absolute likelihood of tornadoes within 25 mi of a point in their regularly issued convective outlooks. The most common forecast likelihood of tornadoes in these outlooks is subjectively low, at 2% and 5%. Studies of probabilistic risk communication for natural disasters have suggested that normalizing the absolute likelihood of rare events by their baseline rate of occurrence can help users better understand small absolute changes in their risk of negative impacts. This study seeks to develop and investigate the distribution of relative risk, defined as the absolute likelihood divided by the climatological likelihood for tornadoes within 25 mi of a point, across the contiguous United States using the 1950–2021 SPC tornado report database. The analysis reveals that relative risk values vary greatly across time and space, primarily due to the annual and regional climatology of tornado events. Overall, relative risk may be able to provide useful context for tornado risk communication in areas that have higher rates of tornado occurrence, but it can be greatly inflated in regions with low climatological risks. Future work should seek to understand how broadcast meteorologists, emergency managers, and members of the public interact with relative risk information and in doing so identify the types of tornado events where relative risk improves or complicates risk messaging. Significance Statement Forecasts for rare events like tornadoes are difficult to communicate to weather messaging recipients because of their very low forecast likelihoods. However, risk communication literature suggests that relative risk, which compares the forecast probability of a hazard to how likely a hazard is at a given time, can add important context to risk messages. By calculating relative risk for all reported tornadoes from 1950 to 2021, we observe that relative risk values are highest in areas that infrequently see tornadoes, and extremely large values of relative risk can occur when a single tornado occurs at a time of year and place where no others are recorded. Future work should identify how individuals react to relative risk values in tornado forecasts.