Some remarkable supercell simulations from a quasi-operational local-scale model: skill or "shear" luck?

For several years the Forecast Systems Laboratory (FSL) has been evaluating the concept of running a local numerical model onsite at a National Weather Service Forecast Office (WFO). The advantages to locally running a small-scale model include the ability to incorporate local datasets that may not be available at a national center. These data might range from mesonet surface reports from stations operated by state transportation departments to mesonet observations made available through cooperative efforts with local television stations. Of course it is imperative that such observations be quality controlled before their inclusion in a local analysis being used to initialize a local-scale model. For tests at FSL, analyses from the Local Analysis and Prediction System (LAPS) are used to assimilate a wide range of data into a mesoscale analysis at 10-km horizontal resolution. The LAPS analysis has been used to test several different models at 10-km resolution, including the Eta, MM5, and two versions of the RAMS model, with displays available via the Internet.

About a year ago one version of the RAMS model, known locally as the Scalable Forecast Model (SFM), was made available to the local WFO via the AWIPS workstation, enhancing the usability of the model to the forecasters. The SFM is run at 10-km horizontal resolution over an area somewhat larger than their area of forecast responsibility, initialized with LAPS with boundary conditions provided by the Eta model, and run out to 18 h four times per day on a workstation installed at the Denver WFO (now collocated with FSL in Boulder). Usually a complete run takes under 2 hours, with output displayed at the end of each forecast hour. The model uses the Schultz explicit microphysics scheme, which clearly is an issue of some concern for a convective simulations at a resolution of only 10 km.

For 10-km resolution simulations, we believe that the best potential for adding value to a forecast in convective scenarios would include the early stages of events forced by topography (diurnal and/or organized upslope) or some other organized feature. Thus, the forecaster might be able to successfully use the SFM model to predict when and where thunderstorm development might occur, for example, on many days where terrain heating might drive a diurnal upslope flow. As expected, on many days convection in the SFM tended to organize more than would occur given the available vertical wind shear, with larger scale outflow boundaries produced by the model. There were, however, a few remarkably accurate forecasts on a limited set of days when there was sufficient vertical wind shear and instability to support supercell storms. The four days of successful simulations, out to 10 h and beyond in some cases, occurred during the 1999 convective season in the BOU WFO forecast area. In this study these cases will be examined in some detail, including an effort to rerun the SFM at smaller and more appropriate resolution for explicit microphysics to determine whether the solution in fact becomes less accurate. Additional cases will be collected during the upcoming convective season to further determine if we observed real forecast skill with the local-scale model under supercell conditions, or if the solution was just a matter of "shear" luck.