Short- and Medium-Range Predictability of Warm-Season Derechos. Part II: Convection-Allowing Ensemble Forecasts

This study explores convection-allowing ensemble (CAE) forecasts of a subset of 24 warm-season (May- August) progressive derechos from 2012 to 2022. The derecho cases were stratified into low predictability and moderate predictability based on the performance of Storm Prediction Center's Convective Outlooks. The Model for Prediction Across Scales (MPAS) is used with a global mesh of varying horizontal resolution, with a 60-km spacing over most of the globe decreasing to a 3-km spacing where the derechos occurred. A 10-member ensemble was initialized at 0000 UTC from 5 days before the derecho (D5) to the day of the event (D1). The CAE is evaluated using objective metrics as well as subjective assessments of convective mode, coverage, initiation timing, and spatial displacement of simulated convection. The objective evaluation indicates that the MPAS CAE has skill in predicting severe wind gusts, even in the medium range (3-5 days before). The skill diminishes faster with lead time in low-predictability cases compared to that of moderate- predictability cases. Overall, more than half of the ensemble members generated a sustained, progressive bowing mesoscale convective system (MCS) on D1 in both low- and moderate-predictability cases, but the percentages decrease substantially for forecasts with progressively greater lead time. Furthermore, the environmental differences among ensemble members that produce bowing MCSs, multicell clusters, and supercells are minimal. These results indicate that correctly predicting sustained bowing MCSs prior to observed derechos is challenging for the MPAS CAE for lead times longer than 1-2 days, reflecting convective-scale complexity and predictability limits. SIGNIFICANCE STATEMENT: This research evaluates the forecast capabilities of an ensemble modeling system with convection-allowing resolution in the prediction of derechos, which are convective events resulting in widespread wind damage. Our study reveals that these ensembles have skill in forecasting damaging wind gusts within both shortterm (1-2 days before) and medium-term (3-5 days before) time frames, with the highest skill at shorter lead times. However, a limitation is found in representing the correct convective organization, which is important in derecho forecasting. The small differences in environments leading to diverse convective organizations suggest that storm-scale stochastic and other internal processes play a crucial role in convective evolution. These f indings underscore the potential utility and weaknesses of medium-range convection-allowing ensembles as guidance for forecasters in predicting derechos.