The species composition and phenology of mosquito fauna vary substantially across the diverse ecological regions of California. Previous publications on mosquito distributions only briefly addressed temporal variations in abundance and were based on reviews of published accounts rather than spatially extensive and semi-continuous datasets of mosquito trapping records. Using surveillance data collected by mosquito control agencies in California, we developed Bayesian Poisson regression models to examine the effects of meteorologic, edaphic, and geographic factors on the abundance and phenology of Culex tarsalis and the Culex pipiens complex, which are important vectors of viruses. We also characterized the spatial and temporal dependence among trap counts and accounted for this autocorrelation in the regression models. Temperature, precipitation, landcover, and human population density were associated with trap counts, but the direction and magnitude of these associations differed between the species. The predictive value of the models and the relative importance of the predictors will be discussed. Effects of these factors on mosquito distribution and abundance have important implications for transmission of endemic mosquitoborne viruses as well as other viruses with the potential for introduction.