Atmospheric transport of water vapor from the North Pacific is the primary source of moisture for most of western North America. Wintertime precipitation accounts for upwards of 75% of the total along the west coast of the United States. Atmospheric rivers in particular deliver large fractions of this precipitation in high-intensity events. Proxy records from western North America indicate a much wetter environment in Nevada and southern California at the Last Glacial Maximum, which has been interpreted as evidence for a southward shift of the mid-latitude jet stream that steers extratropical storms in the eastern North Pacific. Using climate models (and reanalysis data), I explore the influence of the Last Glacial Maximum and last deglaciation conditions on the circulation and moisture budget of the eastern North Pacific and North America, as well as the climatology of North Pacific atmospheric rivers. I also examine the relative contributions of the mean flow and transient systems to some of these changes, and diagnose the relative importance of thermodynamic and dynamical mechanisms.