The National Climatic Data Center (NCDC) has recently completed the digitization of Cooperative Observer station records back to the beginning of written records for the continental US (TD3206 product). The availability of these station data (previously available uniformly over large areas only for 1948-present) offers the opportunity to produce high quality gridded data sets, and in turn hydrologic simulations of large rivers back to about 1915. Central in preparing the climatological data for use as driving data for hydrologic models is the need for topographic correction of gridded precipitation and temperature data in mountainous areas. We have previously developed methods for making topographic corrections based on the Precipitation Regression on Independent Slopes Method (PRISM) precipitation maps (and simpler methods for temperature), but temporal inhomogeneities in the precipitation and temperature records have made corrections to the early part of the records problematic. To deal with these problems we have developed methods to remove temporal inhomogeneities from raw Cooperative Observer precipitation and temperature data records by comparing temporally and spatially smoothed gridded data sets based on a) the raw station data and b) a smaller number of high quality stations with long records from the HCN data network (US) and HCCD data network (Canada). After temporal corrections are made, the gridded station data closely reproduce the monthly trends associated with the HCN and HCCD data, while retaining much of the increased spatial information from the larger number of stations in the Cooperative Observer records. Following these corrections, topographic corrections are made as before to produce the final meteorological driving data set. Simulations using the VIC hydrologic model implemented for the Columbia River basin at 1/8 degree resolution, show consistent improvements in hydrologic simulations of streamflow when the temporal corrections are made to the driving data sets before topographic corrections are made.