Abstract

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 block, 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 the block by enhancing the zonal wave activity flux downstream, whose convergence over Canada drove the blocking. A simple model-based reconstruction 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.