The Role of an Atmospheric River in Amplifying the June 2021 Western North American Heat Wave

May 29, 2026

How Did an Atmospheric River Intensify the June 2021 Pacific Northwest Heat Wave?

A newly published study in Monthly Weather Review, entitled “The Role of an Atmospheric River in Amplifying the June 2021 Western North American Heat Wave” sheds new light on the unexpected drivers of one of the deadliest weather events in the Pacific Northwest. Co-authored by Gary Lackmann, Lauren Getker, and Jay Cordeira, the paper represents a collaboration between the Center for Western Weather and Water Extremes (CW3E) at the Scripps Institution of Oceanography and North Carolina State University. This research was funded by the State of California Department of Water Resources (CA AR Program) via CW3E, and the National Science Foundation.

This paper is related to the “Extreme Event Attribution and Dynamics” priority area within CW3E’s 2025-2029 Strategic Plan. While traditional heat wave analyses often focus on local high-pressure systems and land-surface feedbacks, there is a gap in our understanding of how upstream, marine and meteorological processes influence downstream weather extremes. To help address this challenge, we investigated how an upstream atmospheric river (AR), a phenomenon typically associated with winter storms and heavy precipitation, primed the atmosphere several days in advance over the Gulf of Alaska to amplify the intense heat that later shattered records in the US states of Washington, Oregon, and the Canadian province of British Columbia in late June, 2021.

Using numerical model experiments and backwards air trajectories, our study demonstrates that upstream latent heat release (the heat energy released when water vapor condenses into clouds and precipitation) played a dual role in amplifying the extreme warmth. Condensation along the AR (Fig. 1) and during orographic lift over coastal Alaska not only strengthened a downstream upper-tropospheric blocking anticyclone associated with the heatwave, but also directly warmed lower-tropospheric air. As the pre-heated air curved anticyclonically around the upper ridge, it experienced compression and downslope warming before descending into the lower atmosphere over the Pacific Northwest. Experimental simulations that artificially weakened the upstream AR resulted in significantly cooler near-surface temperatures in the heat wave region, providing compelling evidence that upstream ARs can be important factors to monitor when predicting extreme heat events. Sea-surface temperatures were anomalously warm to the west of the heatwave prior to the event, and trajectories indicate that some of the hot air in the Pacific Northwest had previously resided in the lower atmosphere in the vicinity of the warm ocean water before rising, being heated by condensation, and then undergoing compressional warming (Fig. 2).

Figure 1. Integrated water vapor transport from WRF control simulation (kg m⁻¹ s⁻¹, vectors, magnitude shaded as in colorbar), and sea level pressure (contours, interval 4 hPa). Locations of back trajectories at or below the 600-hPa level and within ±1 h of the valid time are shown as semi-transparent red circles; valid 0000 UTC 25 June 2021. Figure 5 from Lackmann et al. 2026.

Figure 2. Eastern North Pacific sea-surface temperature anomaly (◦C, shaded as in first legend at right) valid 24 June 2021, with backward trajectories originating below the 900-hPa level between days -3 and -6 shaded by pressure as in rightmost legend. SST anomaly computed from NOAA Optimum Interpolation SST V2.1 using a 1991–2020 baseline. Data source: https://www.ncei.noaa.gov/. Figure 11 Lackmann et al. 2026.

Citation:

Lackmann, G. M., Getker, L. E., & Cordeira, J. M. (2026). The Role of an Atmospheric River in Amplifying the June 2021 Western North American Heatwave. Monthly Weather Review 154(5), 793-811. https://doi.org/10.1175/MWR-D-25-0074.1