APPLICATION OF THE NCEP REGIONAL SPECTRAL MODEL TO IMPROVE MESOSCALE WEATHER FORECASTS IN HAWAII

The operational implementation of the National Centers for Environment al Prediction (NCEP) Regional Spectral Model (RSM) in Hawaii is the fi rst application of a mesoscale model to improve weather forecasts in t he Pacific region. The primary model guidance for the National Weather Service Pacific region has been provided by the NCEP Aviation (AVN) r un of the Global Spectral Model (GSM). In this paper, three recent syn optic-scale disturbances that affected the Hawaiian Islands are select ed to demonstrate the potential utility of model guidance produced by the RSM and contrast it qualitatively with that from the AVN. NCEP RSM simulations, with enhanced grid resolution, can resolve convective ra inbands and the interaction between the environmental airflow and the complex island topography, features the GSM cannot capture. RSM model performance in reproducing mesoscale structures associated with the sy noptic-scale systems is encouraging. For the first simulation, a kona low case on 3 November 1995, the RSM predicted a northeast-southwest-o riented rainband that closely matched a convective cloud band in the s atellite imagery and maximum rainfall over Kauai. The second RSM simul ation, a cyclogenesis event on 3 March 1996, shows remarkable agreemen t with observations. Important features such as the heavy rains and hi gh winds over portions of Maul and Hawaii are accurately forecast. The third RSM simulation, a heavy rain event on 13 November 1996, is asso ciated with convergence along a trailing cold-frontal trough. In this case the RSM correctly forecast the timing and distribution of heavy r ainfall on the island of Oahu. Subjective comparisons between RSM outp ut and observations demonstrate the potential utility of the model gui dance for local weather forecasts in Hawaii.