Using Average Recurrence Intervals to Identify Intensity Risk in the Day 1 Time-period

The Excessive Rainfall Outlook (ERO) issued by the Weather Prediction Center (WPC) provides guidance on the risk of exceeding Flash Flood Guidance (FFG), which in turn suggests that flash flooding could occur. The WPC uses categorical risks to highlight the chance of FFG exceedance. Tied to the risk definitions is the expected coverage of flash flooding. Therefore, the product itself can be thought of as a way of highlighting the coverage of the impact (aka. flash flooding) from excessive rainfall. However, rainfall intensity is a key ingredient in the flash flood forecast process, though itself proves to be a challenge to forecast. For instance, guidance struggles with the task of predicting thunderstorms, their motion, and thus duration of heavy rainfall. Even so, they often provide valuable tidbits of information about possible location and intensity. Thus, can we use this guidance and other tools to identify where it will rain and possible rainfall intensity?

To explore this idea, the Flash Flood and Intense Rainfall (FFaIR) Experiment, part of the Hydrometeorology Testbed (HMT) at WPC, created an experimental forecast product that uses Average Recurrence Interval (ARI) rainfall exceedances to identify the rainfall intensity risk. The product, referred to as the ARI-based ERO, or the AERO, attempts to highlight regions where the 6-h ARIs at a given yearly threshold are likely to be exceeded. The product was designed to include loose definitions to help assess at what confidence level participants feel comfortable highlighting heavy rainfall risk. This talk will cover the product’s progression over the past three years of FFaIR, focusing on verification results and participant feedback from the 2023 FFaIR Experiment. It will also touch on the early findings of the analysis of the probabilistic space participants preferred for the product.