The 2021 Western North American Heatwave and Its Subseasonal Predictions

Record‐breaking above‐normal temperatures were observed across western North America in June–July 2021. In this study, our ability to predict this heatwave 2–3 weeks in advance is assessed based on 10 Subseasonal‐to‐Seasonal prediction models. It is found that the above normal temperature in Western Canada during June 28–July 4 was predicted by most models as early as June 10. However, for the forecasts initialized earlier than June 17, not a single ensemble member of all the models is able to capture the magnitude of the observed temperature anomaly. We identify two important processes: an upper tropospheric wave train associated with the boreal summer intraseasonal oscillation in Southeast Asia and an anomalous North Pacific atmospheric river leading to high moisture conditions. Most models are able to predict the wave train across the North Pacific. A realistic representation of moisture transport and its pattern appears crucial for the extended‐range forecast of this heatwave. Plain Language Summary Record‐breaking high temperatures were observed across western Canada and the U.S. Pacific Northwest from late June to early July 2021. The heatwave caused more than 1,400 deaths, and led to numerous extensive wildfires and widespread smoke pollution. An accurate long‐range prediction of such heatwaves is needed and remains an enormous challenge. In this study, we assess our current ability in predicting this heatwave 2–3 weeks in advance. We identify two important processes for this heatwave: influence of the boreal summer monsoon variability in Southeast Asia and moisture transport across the North Pacific. We investigate why some forecasts are more successful than others and how the forecast could be improved. Key Points The ability of current state‐of‐the‐art operational Subseasonal‐to‐Seasonal systems to predict an extreme heat wave 2–3 weeks in advance is assessed Two important dynamical processes for the extreme heatwave are identified A realistic representation of moisture transport and its pattern is important for the extended‐range forecast of heatwaves