We examine the relation between δ18O in rainwater collected in southwestern Oregon and climate variables including temperature, parcel trajectory, precipitation amount, and specific humidity. Local surface air temperature at the time of sample collection explains a large proportion of δ18O variability, suggesting that paleoclimatic archives that are related to rainfall δ18O should be useful for qualitative temperature reconstructions. Models of Rayleigh distillation of air masses originating in the North Pacific can broadly constrain the observed isotopic variability in southwestern Oregon. Results from a Hybrid Single-Particle Lagrangian Integrated Trajectory Model suggest that recent parcel pathways have little influence on the isotopic composition of precipitation collected at our site. We also find no significant relation between rainfall δ18O and precipitation amount. Changes in specific humidity along the parcel tracks, however, indicate that the water vapor exchange of the air mass with the underlying ocean influenced the isotopic composition of the most enriched samples.