Exploration of the NSSL Maximum Expected Size of Hail (MESH) Product for Verifying Experimental Hail Forecasts in the 2014 Spring Forecasting Experiment

The 2014 Hazardous Weather Testbed (HWT) Spring Forecasting Experiment (SFE) operated for a 5-week period (5 May – 6 June) at the National Weather Center in Norman, OK. The Storm Prediction Center (SPC) and National Severe Storms Laboratory (NSSL) jointly conduct the SFE every spring to test emerging concepts and technologies for improving the prediction of hazardous convective weather. More importantly, efforts to bridge the gap between research and operations continue as a key component of the HWT with each year designed to build successful collaborations. For more background details, historical summaries of the annual SFE since 2000 can be found in both Kain et al. (2003) and Clark et al. (2012). Objective forecast verification of experimental severe weather forecasts was conducted for the third consecutive year during the 2014 SFE. The next-day subjective evaluations by SFE participants have been found to be more complete by incorporating forecast verification metrics in near real-time as opposed to performing statistical assessments only in a post-experiment fashion. As discussed in Melick et al. (2013), the subjective evaluations from the participants were generally consistent and agreed with the statistical results. The comparisons were facilitated by creating time-matched spatial plots of forecasts and observations for display on webpages linked from the SFE website. Skill scores could then be viewed for each forecast time period with the appropriate images and/or examined via table summaries. Preliminary local storm reports (LSR) have traditionally served as the primary verification dataset when computing objective performance metrics. _______________________________________ *Corresponding author address: Christopher J. Melick, NOAA/NWS/NCEP Storm Prediction Center, 120 David L. Boren Blvd, Norman, OK 73072; E-mail: chris.melick@noaa.gov Subjective assessments of the probabilistic forecast products created by the participants during the 2014 SFE were similar to what had been done in previous years. For the first time, however, individual hazard (tornado, wind, hail) probabilistic forecasts were produced by the Severe Desk led by the SPC instead of a single probabilistic forecast for total severe. This study addresses the performance of the experimental probabilistic severe hail forecasts in exploring additional verification datasets instead of solely using LSRs. Images from multiple observation sources were made available for next-day subjective comparisons during the 2014 SFE. Of these, radarderived maximum expected size of hail (MESH; Witt et al. 1998) from NSSL served as a valuable surrogate to document the occurrence of hail, especially in low-density population areas where there may be a scarcity of LSRs. A key goal is to further explore gridded MESH fields as an alternative dataset to verify the experimental hail forecasts. This also provides the opportunity to compare results in the objective forecast verification to those obtained from traditional LSRs.