The evolution of multi-scale ensemble guidance in the prediction of convective and severe convective storms at the Storm Prediction Center

The challenges central to forecasts of convection and severe convection are attributable to many factors including the sufficiency and accuracy of observational data, imperfect numerical models, and an atmospheric system governed by non-linearities. Together, these factors result in an upscale growth of errors, decreasing forecast confidence, and an eventual loss of predictability. Predictability is first eliminated at the finer scales (e.g., the convective scale), which makes convective forecasting particularly challenging.

For nearly a decade, the Storm Prediction Center (SPC) has addressed the challenge of expressing uncertainty through an extensive suite of probabilistic products. The probabilistic products cut across a wide range of time and space scales, ranging from a few hours (e.g., probabilities specifying the likelihood of specific hazards within severe thunderstorm and tornado watches) to several days (e.g., probabilistic convective and severe convective outlooks from one through eight days). Ensemble guidance has played an increasingly important role in recognizing and assessing the uncertainty of the convective forecast.

This work will illustrate the range of new ensemble guidance in use or under exploration at the SPC. The North American Ensemble Forecast System (NAEFS) and NCEP GFS Ensemble Forecast System (GEFS) provide information on the predictability of the large-scale pattern, and the subsequent potential for extensive convective or severe convective outbreaks. This guidance is especially designed for medium range applications out to eight days. Moving downscale, the post-processed NCEP Short-Range Ensemble Forecast (SREF) helps to evaluate the likelihood of mesoscale environments conducive to convection and severe weather. Additional SREF post-processing provides statistically reliable guidance on the probability of thunderstorms, severe thunderstorms, and tornadoes. The development of a Rapid Update Cycle (RUC), time-lagged Rapid Refresh Ensemble Forecast (RREF) targets the short-term likelihood of convective initiation and severe potential. And an experimental WRF-based high resolution convection-allowing Storm Scale Ensemble Forecast (SSEF) is being tested and evaluated for operational applications in the NOAA Hazardous Weather Testbed as part of an ongoing project between the SPC, the National Severe Storms Laboratory, and the University of Oklahoma Center for Analysis and Prediction of Storms. The SSEF provides a remarkable view of future operational ensembles by providing guidance on explicit storm-scale structures such as updrafts, downdrafts, mesocyclones, and convective lines. SSEF applications will likely extend beyond the SPC into other high-impact, convection-related venues such as aviation flight planning and heavy rain/flash flooding.