Aim: Drought stress, and its effects on the biogeography of vegetation, has focused primarily on water availability during the growing season, thus focusing primarly on summer. However, variation in rainfall continentality (i.e., the continental interior being insulated from oceanic influences) can produce striking vegetation differences. We aim to disentangle summer water balance from the influence of rainfall continentality on winter rainfall, to better understand how climate regulated the distributions of woody plants in the Western USA. Location: Western USA. Time period: Actual. Major taxa studied: Angiosperms and Conifers. Method: We used Redundancy Analysis (RDA) to investigate correlations between rainfall continentality, summer water balance, minimum winter temperature and length of growing season on the distributions of 130 tree and shrub species in 467 plots. Rainfall continentality was calculated using the Gams (1932) index, modified for winter precipitation, and summer water balance with the ratio of summer precipitation to temperature. We estimated Actual EvapoTranspiration (AET), Deficit (DEF), mean annual temperature and rainfall from global gridded datasets and correlated them with RDA axes. Results: Rainfall continentality measured with the Gams index and minimum temperatures best explained the contrast between oceanic vegetation in the Pacific Coast Ranges and continental vegetation in the Intermountain Region and Rocky Mountains. Growing Season Length (GSL) was the second strongest factor correlated with vegetation distributions. Summer water balance, despite being the most widely used climatic factor to assess drought stress in biogeography, was the third strongest factor correlating with vegetation classes of the western US. AET was equally correlated with RDA axes 1 and 3, and, thus, could not discriminate between the contrasts in the RDA. Main conclusions: Rainfall continentality measured with the winter Gams index provides a more precise metric than summer water balance for understanding how the biogeography of woody plants in the western USA is regulated by climate. Broadly integrating the Gams index of continentality into plant distributions may improve our understanding of biogeographical distributions, the evolution of subspecies in species that span coastal to interior regions, and predictions of responses to climate change.