SPC Evaluation: Steve Weiss Part 2

• Since 2004, the model has been periodically upgraded, including in early 2007 when NMM physics were standardized, grid length was decreased to 4 km, the domain was expanded, and model run time was optimized.  Most importantly, the early success of the WRF-NMM4 in spring 2004 resulted in the model output being distributed to SPC operational forecasters in real-time, and over the last three years they have evaluated its performance and utility for a wide variety of geographic, seasonal, and synoptic regimes over the central and eastern states.  Most of our attention has focused on the ability of the WRF-NMM4 to resolve smaller scale features such as convective storms through the use of simulated reflectivity fields.  Compared to operational mesoscale models that incorporate convective parameterization, the WRF-NMM4 has been found to provide unique information about details of convective initiation, intensity, evolution, and mode (or morphology), which are critical aspects to severe weather forecasting.  The accurate prediction of convective mode (such as discrete cells, lines, or multicell complexes) is directly related to the occurrence of  tornadoes, large hail, and damaging winds, which tend to occur preferentially with specific convective modes.  In this area alone, SPC forecasters have found that the near-stormscale resolution of the WRF-NMM4 has substantially contributed to more confident and skillful forecasts of specific severe weather threats.  This has been aided by the extraction of gridded information concerning the dynamic structure of model-generated supercell thunderstorms, which are convective storms with persistent rotating updrafts.  The value of the WRF-NMM4 has been especially evident during strongly forced situations that are associated with severe weather outbreaks. (See attached powerpoint file entitled “NWA 2006 WRF-NMM4 Outbreaks” that contains a presentation given at the NWA Meeting in fall 2006.)