An operational system has been developed to produce monthly grids of maximum and minimum temperature, precipitation, and dew point for the lower 48 states approximately 7-14 days after the end of each month. This system was developed in support of US Forest Service wildfire forecasting efforts, but has many applications in the monitoring of drought and climate variations, agricultural planning, and others. Monthly station data from COOP, ASOS, and Snotel networks are supplied by the Western Regional Climate Center shortly after the end of each month. At Oregon State University, spatial quality control is performed on the station data, and PRISM is used to interpolate the data to grids across the conterminous US in three sections, west, central and east. These sections are merged into one complete grid at 2.5-min (~4-km) spatial resolution. Instead of the customary digital elevation grid, PRISM uses high-quality 1961-90 mean climatological grids for each month as predictor grids in the interpolation process. As they are produced, monthly grids and map images are available free of charge via the World Wide Web. These new grids are part of what is now a continuous series of monthly climate grids spanning the period January 1895 to the present. This is a precursor to the product expected to be developed through the WestMap climate mapping initiative, which aims to produce very high-resolution, high-quality monthly climate grids for the western US, with accompanying error assesment statistics.