Evaluation of Thunderstorm Rotation in 1- and 3-km Versions of the Warn-on-Forecast System

NSSL’s experimental Warn-on-Forecast System (WoFS) provides probabilistic guidance to NWS forecasters of individual thunderstorms and their associated hazards. WoFS uses ensemble data assimilation that consists of 36 analysis members and 18 forecast members. The rapid data assimilation produces a daily, on-demand forecast every 30 minutes with a regional domain of 900 x 900 km with a 3-km horizontal grid spacing (WoFS- 3km). Recently, a 1-km horizontal grid spacing version of WoFS (WoFS-1km) has been under development at NSSL. WoFS-1km has a domain of 402 x 402 km and is nested within the WoFS-3km domain. Recent work has shown that WoFS-1km predicts smaller thunderstorms better than WoFS-3km. This study extends that work by focusing on the ability of WoFS-1km to predict rotation in thunderstorms compared to WoFS-3km. To assess this ability, 23 cases from 2022 and 2023 with various storm modes and environments are verified using an object-based method. Using this method, over 200,000 composite reflectivity (CREF) object hits are determined to exist in both WoFS-1km and WoFS-3km. For each forecast and observed CREF object, mid-level updraft helicity and MRMS rotation objects, respectively, are identified. Rotation object centroid displacement errors and contingency table statistics are computed. Results show that WoFS-1km predicts rotation significantly better than WoFS-3km for reflectivity object areas less than 800 km² and minor axes less than 24 km.Also,WoFS-1km has smaller rotation centroid displacement errors than WoFS-3km.