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The Madden-Julian oscillation (MJO) modulates atmospheric river (AR) activity along the West Coast of North America. The forecast skill of ARs based solely on the MJO phase is limited, motivating the identification of other predictive factors. Here we show that low-frequency variability of the Pacific/North American (PNA) pattern is mostly independent of the MJO and the monthly PNA pattern influences the MJO-AR relationship much more robustly than other large-scale patterns such as the El Niño Southern Oscillation, and the quasi-biennial oscillation. During PNA+ conditions, when the Pacific jet exit region is located further east than average, baroclinic wave packets extend downstream toward North America. The clear contrast in wave packets’ zonal scale results in partitioning active and inactive AR phases. The PNA influence is most significant over the Pacific Northwest region of North America, where AR landfall is most frequent, with implied forecasting skill beyond 20 days.