Predictability of Severe Convective Storm Environments in Global Ensemble Forecast System Version 12 Reforecasts

Abstract Prediction of severe convective storms at timescales of 2–4 weeks is of interest to forecasters and stakeholders due to their impacts to life and property. Prediction of severe convective storms on this timescale is challenging, since the large-scale weather patterns that drive this activity begin to lose dynamic predictability beyond week 1. Previous work related to severe convective storms on the subseasonal timescale has mostly focused on observed relationships with teleconnections. The skill of numerical weather prediction forecasts of convective-related variables has been comparatively less explored. In this study over the United States, a forecast evaluation of variables relevant in the prediction of severe convective storms is conducted using Global Ensemble Forecast System Version 12 reforecasts at lead times up to four weeks. We find that kinematic and thermodynamic fields are predicted with skill out to week 3 in some cases, while composite parameters struggle to achieve meaningful skill into week 2. Additionally, using a novel method of weekly summations of daily maximum composite parameters, we suggest that aggregation of certain variables may assist in providing additional predictability beyond week 1. These results should serve as a reference for forecast skill for the relevant fields and help inform the development of convective forecasting tools at timescales beyond current operational products.