After the simulation domain has been established, REGRID is run to generate first-guess fields for the model simulation by horizontally interpolating a larger-scale dataset (global or regional coverage) to the simulation domain. REGRID interpolates the background fields to the simulation domain for times throughout the simulation period; these files are used to ultimately generate lateral boundary conditions for the coarse-domain simulations (nested domains use lateral boundaries from the coarse domain). REGRID also processes the sea-surface temperature and snow files by interpolating the analyses data to the simulation domain. As a last step, REGRID calculates map-scale factors and Coriolis parameters at each grid point to be used by MM5. REGRID is described by Manning and Haagenson [1992].