The Role of Near Solstice Solar Radiation on the Pacific Northwest Heatwave of June 2021

Abstract In late June 2021, the Pacific Northwest experienced a severe heatwave, shattering temperature records and making worldwide news. The extreme temperatures have been attributed to an anomalously strong atmospheric block, high moisture content of upstream air, and dry soils. We present the first study quantifying the impact of the near‐maximum incoming solar radiation resulting from the event's proximity to the summer solstice. Using the Weather Research and Forecasting model, we contrast the observed heatwave with meteorologically identical heatwaves, but with incoming solar radiation of 30 (+30 days) and 60 days (+60 days) later. Spatially averaged mean heatwave temperatures are reduced by 1.5°C (4.8°C) in +30 days (+60 days), and the spatial area over which records were broken is reduced by 25% (81%). Although our results imply that the heatwave would still have broken 80‐year records even in our +60 days simulation, heatwave timing likely played a significant role in the maximum heatwave temperatures.