Storm-scale data assimilation and ensemble forecasting with the NSSL Experimental Warn-on-Forecast System for ensembles

The NOAA Warn-on-Forecast (WoF) research project is working toward the development of very short-range (0-1 h) probabilistic forecasts that accurately predict severe thunderstorm hazards. In a future WoF system, storm-scale analyses and forecasts will likely be generated on very-high-resolution (grid spacing ~1 km or less), event-dependent grids, but preliminary work with a WRF-based ensemble data assimilation system has begun on coarser convection-allowing grids. This system—the NSSL Experimental Warn-on-Forecast System for ensembles (or NEWS-e)—will be run quasi-realtime during the NOAA Hazardous Weather Testbed 2015 Spring Experiment, and preliminary findings from these storm-scale data assimilation experiments are presented.

Storm-scale ensemble analyses and forecasts of severe weather events from spring 2015 will be produced on a 3-km event-dependent grid. This storm-scale ensemble is nested within a 15-km continental United States (CONUS) ensemble constructed from initial and boundary conditions provided by members of the Global Ensemble Forecast System (GEFS) forecast cycle starting at 1800 UTC the previous day (as described more fully in a companion abstract by K. Knopfmeier). Around the time of convective initiation, radar and satellite (cloud water retrievals) data are assimilated every 15 min using the ensemble Kalman filter (EnKF) approach encoded in the Data Assimilation Research Testbed (DART). One (or two) 90-min ensemble forecasts will be initialized from the resultant storm-scale analyses each hour of the storm event.

This study is primarily focused on ensemble forecasts launched during or just before the 1 h preceding the onset of storm reports. Preliminary work will evaluate the ability of these forecasts to highlight severe weather hazards, including the low-level rotational characteristics of supercell thunderstorms and tornadic meso-convective systems, thunderstorm winds, and flash flooding.