Observational evidence shows that spring temperatures over western North America have increased substantially over the past half century and especially the past three decades. Spring warming has been tied to an advance in the timing of both snowmelt runoff and the blooming of plants, thereby having implications for water and fire management across the western US. Unlike the robust observed trends in spring, autumn temperatures have shown relatively little change across western North America over this same period. In order to better understand the origins of regional scale trends in surface temperature this study examines how surface temperature trends are influenced by the observed changes in the dominant modes of atmospheric circulation. Preferred low-frequency modes of atmospheric variability during spring and autumn are demonstrated to account for a great deal of the seasonal asymmetry in surface temperatures. Positive trends in PNA-like and AO-like modes in spring promote a circulation pattern that exacerbates regional warming over western North America, with approximately 40% of the spring warming attributable to the trends in atmospheric regime patterns. Conversely, opposing trends in these circulation regimes in autumn have likely acted to mask the regional detection of warming. . After excluding contributions from trends in the 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 run under 20th century forcing. Observed differences in spring versus autumn circulation trends are inferred to pertain to seasonal differences in extratropical air-sea interactions. Sea surface temperature trends over the North Pacific are quite different in spring and autumn, and are likely responsible for the driving mechanisms that led to the observed differences in atmospheric circulation trends. By accounting for variability and trends in surface temperature that are associated with atmospheric circulation regimes, we may be in a better position to answer many of the questions regarding climate change detection.