Observational evidence shows that spring temperatures over western North America have undergone significant warming over the past half century, while autumn temperatures have shown relatively little change. Low-frequency modes of atmospheric variability for spring and autumn are demonstrated to account for a great deal of the seasonal asymmetry, with trends in spring circulation patterns exacerbating regional warming, and trends in autumn circulation patterns counteracting warming. After excluding warming associated with the primary modes of atmospheric variability, temperature trends in spring and autumn over western North America are similar to one another and in broad agreement with seasonal trends from a multimodel ensemble.