Evaluation of Real-time, Medium-range, Convection-allowing Ensemble Forecasts Produced for NOAA’s 2023 Hazardous Weather Testbed Spring Forecasting Experiment

Coincident with NOAA’s Hazardous Weather Testbed (HWT) Spring Forecasting Experiment (SFE) in May 2023, two sets of real-time, medium-range (3–8-day) ensemble forecasts were produced with variable-resolution global models that had ~3-km horizontal cell spacing over the conterminous United States (CONUS). One set of ~3-km ensemble forecasts had 10 members, produced 204-h (8.5-day) forecasts, and used GFDL’s “C-SHiELD”, a prediction model based on the Finite-Volume Cubed-Sphere Dynamical Core (FV3). The second set of ~3-km ensemble forecasts had 5 members, produced 132-h (5.5-day) forecasts, and used a variable-resolution configuration of NCAR’s Model for Prediction Across Scales (MPAS). Both ensembles were initialized daily at 0000 UTC from Global Ensemble Forecast System (GEFS) initial conditions over 38 days encompassing the 2023 SFE. These efforts represented the first time convection-allowing ensemble guidance at medium-range forecast periods had ever been produced in real-time with variable-resolution global models.

The forecasts appeared to be useful. For example, HWT SFE participants found that both medium-range ensembles often successfully highlighted areas where severe weather occurred 5 days in advance. Moreover, objective verification of surrogate severe weather diagnostics indicated both ensembles were skillful relative to climatology through at least 5 days. Additionally, 3-h accumulated precipitation forecasts from both ~3-km ensembles were more skillful than corresponding forecasts from NCEP’s much coarser operational GEFS through at least 5 days, further indicating the potential value of convection-allowing ensembles at medium-range forecast periods.

This presentation will discuss these findings as well as challenges that would need to be overcome to justify operationalizing high-resolution medium-range ensembles over the CONUS.