Gridded Model Output Statistics: improving and expanding

For years, National Weather Service (NWS) forecasters have used Model Output Statistics (MOS) guidance as an aid in producing text forecast products issued to the user community. However, the methods that forecasters use to generate forecast products have changed because of requirements to produce forecasts on high-resolution grids in support of the National Digital Forecast Database (NDFD). Forecasters need guidance available on a grid, at a resolution comparable to the grid resolution used at the local forecast office. One method of providing guidance on the required grid is to objectively analyze forecasts at MOS stations. The first set of Global Forecast System (GFS)-based gridded MOS guidance using this method became available in the National Digital Guidance Database (NDGD) in August 2006 over the conterminous U.S. on a 5-km grid. Since the initial implementation, more weather elements have been added and the geographic coverage has expanded to include an Alaska domain. Gridded MOS has become a valuable resource to NWS forecasters for initializing forecasts, particularly in the forecast periods from days 4 through 7. Emphasis is placed in the later time periods on only making changes to the MOS grids when and where the forecasters see an obvious opportunity to improve over the guidance.

The gridded MOS guidance has demonstrated many of the positive attributes of its parent station-based MOS, while including options for tailoring the grid point values to best reflect the terrain. Since the original implementation, gridded MOS has undergone several adaptations to improve consistency as well as regional applicability. Cycle averaging was implemented to reduce the impacts of model flip flops and pulsing in small areas between cycles. MOS developers have welcomed assessments of gridded MOS from local forecasters in order to make adjustments to provide guidance that is both representative of the local area and compatible with the process for generating forecasts for NDFD. Some of the improvements resulting from forecaster feedback include: removal of unrepresentative input data from the analysis, expansion of the grid coverage, and modifications to the land/water mask. As the system has improved and expanded, the guidance has proved to be skillful and reliable. A new procedure was recently implemented to use the GFS-based gridded MOS as the back up provider to fill in holes in the NDFD when NWS Weather Forecast Offices are not able to send their grids for extended periods.

In this presentation, ongoing enhancements to the operational GFS-based MOS guidance are discussed, along with the process used to diagnose potential errors in the analysis. Challenges encountered in gathering and processing the data necessary to support the development on a high-resolution grid are outlined. In addition, current and future development activities are shown.