Convection-Allowing Medium-Range Severe Weather Guidance from a Variable-Resolution Global Ensemble

Convection-allowing ensemble forecasts have traditionally only been produced from limited-area models for short-range lead times. However, with increased computational resources, it is becoming feasible to run global convection-allowing models that may be better-suited for providing medium-range (i.e., 3–5-day) severe weather guidance than limited-area models.

Thus, looking ahead to a future where global convection-allowing forecasts are commonplace, global, 132-h (5.5-day), 10-member ensemble forecasts were produced with the Model for Prediction Across Scales (MPAS) for 35 cases in April and May 2017 using a variable-resolution mesh with 3-km cell spacing over the conterminous United States (CONUS) that smoothly relaxed to 15 km over the rest of the globe. To examine benefits of configuring MPAS with a 3-km mesh refinement region for medium-range hazards guidance, a second set of 10-member MPAS ensemble forecasts was produced using a quasi-uniform 15-km global mesh. Precipitation forecasts from both MPAS ensembles were objectively verified over the central and eastern CONUS, and forecasts of severe weather surrogates (e.g., updraft helicity) from the 3-km MPAS ensemble were also assessed.

The 3-km MPAS ensemble most faithfully reproduced the observed diurnal cycle of precipitation throughout the 132-h forecasts and had superior precipitation skill and reliability over the first 48 h. However, after 48 h, the 3- and 15-km ensembles had similar spread, reliability, and skill. Nonetheless, despite fewer benefits of increased resolution for spatial placement of precipitation after 48 h, 3-km ensemble members explicitly provided potentially valuable guidance regarding convective mode throughout the 132-h forecasts, and verification of 3-km severe weather surrogates compared to observed severe weather reports indicated skill through 132 h.

Collectively, these results suggest both strengths and potential limitations of medium-range high-resolution ensemble forecasts for severe weather guidance.