The 2021 Pacific Northwest Heat Wave and Associated Blocking: Meteorology and the Role of an Upstream Cyclone as a Diabatic Source of Wave Activity

We investigate the meteorological and dynamical conditions that led to the extreme heat in the Pacific Northwest from late June to early July 2021. The extreme heat was preceded by an upper‐level atmospheric blocking that snatched a warm pool of air from lower latitudes. A heat‐trapping stable stratification ensued within the blocking anticyclone, raising the surface temperatures significantly. An upper‐tropospheric wave breaking and the concomitant surface cyclogenesis off the coast of Alaska initiated the block formation. The regional local wave activity budget reveals that a localized diabatic source associated with this storm critically contributed to an enhanced zonal wave activity flux downstream, whose convergence over Canada drove the blocking. A simple reconstruction based on the observed wave activity budget predicts a 41 percent reduction in strength and a 10‐degree eastward displacement of the block when the upstream diabatic source is reduced by just 30 percent. Plain Language Summary From late June to early July 2021, an unprecedented heat wave enveloped the Pacific Northwest, causing over 1,000 deaths. We investigate the meteorological condition and physical processes responsible for this event. Persistent meandering of the upper‐level jet stream (blocking anticyclone) established a warm, stagnant column of air over the Pacific Northwest, which suppressed convection and trapped heat near the surface. Somewhat counterintuitively, the blocking anticyclone itself grew out of a cyclone that developed upstream (Gulf of Alaska) a few days prior: the heat released during the formation of clouds in this storm played an essential role in strengthening the blocking anticyclone downstream, and the subsequent heat wave. To the extent that the condensation of moisture enhances blocking anticyclones in summer, we can expect them and associated heat waves to intensify as the climate warms and the atmosphere contains more water vapor. Key Points A strong atmospheric blocking preceded the Pacific Northwest heat wave in late June 2021, setting up a heat‐trapping stable stratification An upstream cyclogenesis provided a critical diabatic source of wave activity flux, which converged downstream to create the block When the upstream diabatic forcing is artificially reduced, the reconstructed blocking weakens dramatically and shifts downstream