UC Berkeley

Abstract Atmospheric rivers (ARs) influence weather and climate along the North American west coast, yet their effects on Pacific Northwest (PNW) surface temperatures remain unclear across seasons and synoptic configurations. Motivated by the 2021 PNW heatwave—which was preceded by a poleward‐tracking AR—we analyze how ARs modulate daily maximum temperatures ( T max ) during summer (JJA) and winter (DJF) from 1989 to 2024. Using reanalysis data and an AR detection algorithm, we composite integrated vapor transport (IVT), Z500, total precipitation, cloud cover, and T max anomalies for AR detection days and the subsequent 10 days, and classify summer ARs according to the tilt of their IVT plumes as meridionally oriented (MARs) or zonally oriented (ZARs). Winter ARs exhibit a coherent southwest to northeast orientation and generate widespread, statistically significant PNW warming that persists for over a week. In contrast, composite summer ARs produce no consistent T max response due to the cancellation of ARs that track north versus south of the region. Separating JJA ARs by orientation reveals a strong dynamical divide: MARs induce substantial warming (>3°C in the median T max ), associated with a northeast‐tilted ridge that enhances subsidence and poleward heat transport, whereas ZARs generate significant cooling tied to zonal flow and inland advection of cool, moist marine air. These results show that AR seasonality and orientation jointly influence the sign and magnitude of PNW temperature anomalies. Importantly, AR orientation at a fixed offshore detection region provides predictive value for downstream thermal impacts beyond AR intensity and ridge strength, improving heat‐extreme forecasting in a warming climate. Plain Language Summary Atmospheric rivers—long, narrow bands of moisture that often bring heavy rain to the North American west coast—can also strongly affect temperatures in the Pacific Northwest (PNW), but their impact depends on both the season and the direction they take. Using more than 30 years of weather data, this study finds that winter atmospheric rivers consistently warm the region, as they typically transport warm air northward and produce elevated temperatures that can last for over a week. In summer, atmospheric rivers do not show a clear temperature effect unless they are separated by orientation: storms that tilt northward tend to cause substantial warming, while those moving more directly from west to east often cool the region by drawing in marine air. This distinction helps explain why an atmospheric river contributed to warming ahead of the 2021 PNW heatwave, while also showing that atmospheric rivers alone cannot account for such extreme events. Importantly, the results demonstrate that the orientation of an approaching atmospheric river offshore provides useful advance information about whether temperatures in the PNW are likely to rise or fall, offering potential benefits for forecasting heat extremes in a warming climate. Key Points Winter atmospheric rivers (ARs) tend to be meridionally aligned and are associated with persistent regional warming Summer ARs show weak composite temperature signals because different AR orientations produce opposing temperature responses that cancel The offshore orientation of an approaching AR provides advance predictability of downstream temperature anomalies