Studies have shown that assimilation of ground-based GPS observations can have a significant impact on forecasts of humidity and precipitation. This paper presents results of data impact experiments involving the addition of NOAA/GSD GPS-IPW network ZTD and surface weather observations to the CMC regional (North America) data assimilation system (RDAS). The RDAS consists of a three-dimensional variational (3D-Var) data assimilation with first-guess provided by the CMC Global Environmental Multiscale (GEM) regional forecast model. The impact on 48-hour regional GEM forecasts initialized with analyses from the RDAS is evaluated. Experiments are carried out for both summer and winter periods. The assimilation of GPS observations produces a generally positive impact on humidity forecasts in the mid-troposphere for the 0-24h range (based on verification with North American radiosondes), especially for the summer period in the southeast US. Verification of 24h precipitation accumulation forecasts with rain gauge measurements shows an overall positive impact over the entire 48h forecast period.