Extended Range Machine Learning Severe Weather Guidance Based on the Operational GEFS

Abstract Hill et al. demonstrated promising results for 1–8-day severe weather predictions using a random forest (RF) trained with Global Ensemble Forecast System reforecasts (GEFS/R) and applied to operational GEFS forecasts. However, the skill of the reforecasts may be affected by using fewer members and coarser initial conditions relative to operational GEFS forecasts. Thus, this work builds on Hill et al. by formulating and testing a similar RF model using operational GEFS data for training and testing, instead of reforecasts, to produce 1–15-day severe weather predictions. Prior to training RFs from operational and reforecast GEFS data, feature engineering experiments were conducted for optimization and assessing performance sensitivities. These experiments found 1) there was only modest degradation in forecast skill with decreasing numbers of members; 2) RFs performed best using simple ensemble means as predictors rather than percentiles or individual members; 3) thermodynamic predictors were most impactful to RF performance; and 4) a multimodel RF combining predictors from GEFS and ECMWF’s Integrated Forecasting System improved upon single-model RFs. Using the optimal RF configuration, detailed performance characteristics were presented and four case studies were analyzed. Finally, the optimal RF trained on operational GEFS forecasts significantly outperformed an identically configured RF trained on reforecasts. The superior performance in the RF trained from operational data could be related to operational GEFS forecasts themselves having better performance than the reforecasts and/or the storm report database better sampling severe weather in more recent years. Significance Statement This study builds on previous work that developed automated 1–8-day severe weather forecasts using machine learning, which are widely used operationally. Using the same machine learning concepts, we have developed an improved algorithm and extended the lead times to 15 days. Thus, results from this study are expected to improve extended range severe weather forecasting capabilities.