Storm-scale Data Assimilation and Ensemble Forecasts of the 27 April 2011 Severe Weather Outbreak in Alabama, Part 1

As part of the National Oceanic and Atmospheric Administration's Warn-on-Forecast (WoF) initiative, a multiscale ensemble data assimilation and prediction system is developed using the WRF-ARW model and the Data Assimilation Research Testbed (DART) software. To evaluate the capabilities of the system, short-range probabilistic storm-scale ensemble analysis and forecast experiments are conducted over the northern Alabama region for the 27 April 2011 severe weather outbreak. The initial and boundary conditions for the 36-member multiphysics meso- and storm-scale ensemble are obtained from the Global Ensemble Forecast System (GEFS) at 0000 UTC 27 April. Routinely available observations are assimilated on an hourly basis on both grids for more than a day. Before the onset of the afternoon tornado outbreak, conventional and WSR-88D observations are assimilated every 5 min into the storm-scale ensemble for a 6-h long period, and 1-h forecasts are launched from analyses every 15 min. The system predicts the probabilities of strong low-level mesocyclones of most of the significant tornadic supercell storms with lead times up to 40 min, and the predicted mesocyclone tracks correspond reasonably well with the observed rotation tracks. The forecasts obtained from this continuous 5-min storm-scale update system are very encouraging and show promise for short-range probabilistic forecasting of severe thunderstorm events, which is the main goal of the WoF initiative. The results also motivate future work to reduce model errors (e.g., storm-motion errors and spurious storms) and to design storm-scale ensembles that better represent typical 1-h forecast errors.