Forecast guidance for severe thunderstorms based on identification of extreme phenomena in convection-allowing model forecasts

Convection-allowing numerical models are providing operational forecasters with a seemingly unlimited stream of new forecast guidance. Since these models explicitly resolve and predict features as small as individual convective elements, it is possible that extremes in model fields associated with convection might be useful predictors of observed severe phenomena (e.g. hail, wind, and tornadoes). A variety of unique guidance products for use in operational forecasting are possible if the relationship between observed and predicted severe phenomena is robust.

This concept is fundamentally different from traditional NWP-based assessments of severe weather potential because it relies on identification of explicit convective phenomena rather than environmental conditions that might support severe thunderstorms. For the initial exploration of this idea, we examined a small collection of model fields produced from the 4-km 36-hour WRF-ARW simulation run daily at 00 UTC at the National Severe Storms Laboratory (NSSL) during Spring 2008. Fields based on simulated model reflectivity, vertical velocity, 10 meter wind speed, and updraft helicity (a diagnostic field to identify supercellular storms) were chosen, as preliminary assessments suggested that they were likely to be useful indicators of convection intensity in the model. For each field, “surrogate” severe storm reports were placed at grid points where a parameter threshold was exceeded at any point during a 24-hour period corresponding to the 12 to 36 h forecast period (12 UTC to 12 UTC). A Gaussian smoother was then applied to the grid of surrogate reports to create a density (or probability) field resembling Storm Prediction Center (SPC) probabilistic Convective Outlooks.

The utility of this technique was demonstrated with preliminary applications during the 2008 SPC/NSSL Hazardous Weather Testbed (HWT) Spring Experiment. It proved to be useful on many days in delineating the location and coverage of the severe weather hazard. This study provides a more objective and quantitative assessment of the guidance produced using this procedure, including calibration and verification using similarly derived density fields based on observed storm reports. The analysis includes isolation of the impact from the different input fields to determine those that correspond most closely with observed severe weather. Tied to this process is the selection of the threshold for each field. Thus, guidance produced with a variety of thresholds will be compared. The forecasts will be verified using contingency-table based scores and other common metrics. The potential of extending this technique to higher-resolution models and convection-allowing ensembles will also be discussed.