Using ensemble probability forecasts and high resolution models to identify severe weather threats

High resolution model data (NAM-WRF) and Ensemble Prediction System (EPS) data from the National Centers of Environmental Prediction's (NCEP) Short-Range Ensemble Forecast System (SREF) are used to predict areas with a severe weather threat. This study illustrates the complimentary roles of high resolution model and EPS forecasts, and demonstrates the value of using SREF forecast products that depict probabilities of exceedance and joint probabilities of variables related to severe weather. Probabilities of exceedance for Convective Available Potential Energy (CAPE), Storm-Relative Helicity (SRH), height normalized (mean) shear, and the Energy Helicity Index (EHI) are examined. Joint probabilities of CAPE, effective shear, and 3 hr. convective precipitation are also considered. NAM-WRF forecasts and SREF probability forecasts are examined for a vigorous severe weather event that occurred across much of the central Mississippi and lower Ohio Valleys on 2 April 2006. This presentation will demonstrate that joint and exceedance probabilities from the SREF make it possible to clearly distinguish areas with the greatest severe weather potential. A forecast strategy is proposed which utilizes 1) ensemble data for assessing the likelihood, mode, and forecast confidence of a severe weather event; 2) climatological anomalies for evaluating the historical context of an impending event; and 3) high resolution model data for determining the magnitude of moisture, the horizontal and vertical extent of moisture, important mesoscale structures, and relevant forcing mechanisms.