Northwestern North America has experienced an exceptional heatwave in late June 2021 with many new temperature records across western Canada, Oregon and Washington states. Here we use a recent atmospheric reanalysis and a conditional approach based on dynamical adjustment to assess and quantify the influence of atmospheric circulation and other driving factors to the heatwave magnitude during the June 28–30 period. A blocking anticyclone, enhanced low‐level moisture and clear‐sky downward long‐wave radiation are shown to be the main factors of the heatwave persistence and magnitude. The heatwave magnitude is mainly attributable to internal variability with climate change being an additional factor (10%). Consequences of a similar atmospheric circulation anomaly in different phases of the Pacific Decadal Oscillations and in a warmer world at different global warming levels (1, 2, 3, and 4°C) are explored based on a single model initial‐condition large ensemble. Plain Language Summary Gathering robust statistics and performing extreme event attribution for very rare heat extreme events, such as the 2021 Northwestern North American heatwave, remain challenging due to incomplete sampling of weather data (∼100 years) challenging the application of extreme value theory and caveats related to the use of imperfect climate models in estimating likelihood changes between worlds with and without human influence. Here we use the dynamical adjustment method to quantify the key factors responsible for the magnitude and persistence of the heatwave. Dynamical adjustment aims to identify the causal factors that led to the heatwave with an approach conditional on the observed atmospheric circulation during the event. We find that natural variability is the main driver of the heatwave extreme magnitude with a small contribution from climate change. We also find that the heatwave spatial pattern mainly comes from the atmospheric circulation‐related component (the dynamic component). We investigate a possible contribution due the strong negative phase of the Pacific Decadal Oscillation observed in June 2021 and find it to be rather small. Finally, we ask whether future climate change can make a future similar event even more extreme. We find that the dynamic component would increase by 4°C in a 2°C warmer global climate. Key Points A blocking ridge, enhanced moisture and clear‐sky downward long‐wave radiation are the main causes of the heatwave magnitude and duration The circulation‐induced heatwave component is the main driver of the heatwave pattern and magnitude with a minor role for climate change A similar blocking event in a 2°C warmer climate would lead to a 4°C increase of the circulation‐induced heatwave component