The 2021 Pacific Northwest heatwave was an extreme event that challenged traditional statistical and climate-model-based approaches to attribution. However, advanced operational weather prediction systems can simulate the detailed physics of such events. This study leverages these systems to show that human influence made this event at least 8 [2–50] times more likely. At the current rate of global warming, the likelihood of such an event doubles every 20 [10–50] years. Given the multi-decade lower-bound return time implied by the historical record, this rate of change is highly relevant for decision-makers. Forecast-based attribution can synthesize conditional event-specific storyline and unconditional event-class probabilistic approaches. If developed as a routine service, it could provide reliable estimates of human influence on extreme weather risk, crucial for effective adaptation planning. The study uses a forecast-based approach to attribute the 2021 Pacific Northwest heatwave to human influence. By perturbing initial and boundary conditions in a state-of-the-art operational weather forecast model, the researchers simulate how the heatwave might have occurred in a pre-industrial or future climate. The model successfully predicted the heatwave, capturing key proximal drivers like atmospheric rivers and blocking patterns. The attributable warming ranges from 0.7 [0.1, 1.3] to 1.5 [1.3, 1.7] °C in 11- and 3-day forecasts, respectively. The relative risk of the heatwave is estimated at 8 [2, 50], indicating a significant impact of human-induced climate change. The study highlights the potential of forecast-based attribution to provide reliable and practical operational services, supporting adaptation planning and risk management.The 2021 Pacific Northwest heatwave was an extreme event that challenged traditional statistical and climate-model-based approaches to attribution. However, advanced operational weather prediction systems can simulate the detailed physics of such events. This study leverages these systems to show that human influence made this event at least 8 [2–50] times more likely. At the current rate of global warming, the likelihood of such an event doubles every 20 [10–50] years. Given the multi-decade lower-bound return time implied by the historical record, this rate of change is highly relevant for decision-makers. Forecast-based attribution can synthesize conditional event-specific storyline and unconditional event-class probabilistic approaches. If developed as a routine service, it could provide reliable estimates of human influence on extreme weather risk, crucial for effective adaptation planning. The study uses a forecast-based approach to attribute the 2021 Pacific Northwest heatwave to human influence. By perturbing initial and boundary conditions in a state-of-the-art operational weather forecast model, the researchers simulate how the heatwave might have occurred in a pre-industrial or future climate. The model successfully predicted the heatwave, capturing key proximal drivers like atmospheric rivers and blocking patterns. The attributable warming ranges from 0.7 [0.1, 1.3] to 1.5 [1.3, 1.7] °C in 11- and 3-day forecasts, respectively. The relative risk of the heatwave is estimated at 8 [2, 50], indicating a significant impact of human-induced climate change. The study highlights the potential of forecast-based attribution to provide reliable and practical operational services, supporting adaptation planning and risk management.