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.