One of the extraordinary aspects of the 27 April Super-outbreak of tornadoes over the SE United States was the total path length of tornadoes, which preliminary estimates place near 2000 miles. These long track tornadoes occurred because numerous supercells were able to remain discrete for a very long period of time and thus cover a very large area. By examining total path length of hourly-maximum updraft helicity (UH) – a diagnostic developed for identifying simulated rotating storms – it is possible to examine whether convection-allowing models correctly replicated the long-track nature of rotating storms associated with the outbreak. For this purpose, a 3-dimensional (in space and time) object identification algorithm was developed. This algorithm identifies contiguous regions, or tracks, of UH exceeding a specified threshold and then simply computes the track lengths based on the distance between the beginning and end points of the tracks. Because the time dimension is included in the algorithm, identified tracks can span multiple forecast hours and encompass the entire lifetime of simulated rotating storms. The 3D object algorithm is applied to 19 members of a 4-km grid-spacing CONUS-domain Storm-Scale Ensemble Forecast (SSEF) system run by the Center for Analysis and Prediction of Storms for the 2011 NOAA/Hazardous Weather Testbed Spring Forecast Experiment. The SSEF system forecasts were initialized 0000 UTC 27 April and integrated 36 hours encompassing the entire time period of the outbreak. The UH path lengths are computed for forecast hours 12 to 36.

The majority of the SSEF members simulated very long rotating storm path lengths, even when relatively large UH thresholds were used to define the tracks. Compared to a two-year model climatology of UH computed from a similarly configured 4-km grid-spacing model run daily at the National Severe Storms Laboratory, the majority of SSEF members contained total path lengths above the 99.9% quantile of the distribution computed from the NSSL model suggesting a very extreme event. In addition, the spatial distribution of UH from the ensemble matched very well with the observed locations of tornado-producing supercells. Additional discussion is provided on future applications of this unique verification strategy, as well as potential forecast applications.