Storm Displacement Errors in the Warn-on-Forecast System

The Warn-on-Forecast System (WoFS) is a regional, rapidly-updating, ensemble data assimilation and prediction system designed to provide short-term probabilistic guidance of severe and hazardous weather, including individual thunderstorms. As with most convection-allowing modeling systems, the WoFS occasionally produces forecasts of thunderstorms with storm motion biases, which can be due to multiple sources of error within the data assimilation and forecast system. The storm motion biases lead to storm displacement errors during the forecasts resulting in increasingly poorer forecasts. In this study, we investigate storm displacement errors in WoFS forecasts from 2020-2023 using an object-based technique to define and match WoFS and MRMS reflectivity objects. Results from this investigation reveal storm displacement errors in WoFS forecasts generally have an eastward bias, are the smallest 1-3 hours after a thunderstorm develops, and grow the fastest in the first hour. These and other results from different perspectives of storm displacement errors, such as by lead time, maximum intensity, mean wind, and ensemble member, will be presented. By understanding and characterizing the storm displacement errors, WoFS developers will be able to focus attention on possible error sources and preventative measures to further improve WoFS.