This data publication contains a gridded historical (1979-2015) dataset for the conterminous United States that has been summarized to the monthly time scale. This monthly historical dataset was developed using the daily MACA observational data (METDATA), also called MACA Training Data (https://www.northwestknowledge.net/metdata/data/). The MACA training data were used to downscale global climate model projections to the 1/24 degree grid scale. This monthly data will be used in the modeling analyses for the 2020 RPA Assessment. Data include eight separate files containing mean daily mean potential evapotranspiration, total monthly precipitation, mean daily minimum and maximum relative humidity, mean daily downward shortwave radiation at surface, mean daily maximum air temperature, mean daily mean air temperature, mean daily minimum air temperature, and mean daily mean wind speed at 10 meters high.