Fig. 3: Return-time diagram of the Pacific Northwest (PNW) heatwave in the operational and counterfactual forecast ensembles.

a–d diagrams for the forecast ensembles initialised at the lead given in the panel titles. Red, grey and blue dots indicate empirical return-time plots based on the ensemble members of the future, current and pre-industrial forecasts respectively. The dashed grey line shows the temperature threshold observed during the PNW heatwave. The black dots indicate the recent climatology, based on detrended ERA5 reanalysis over 1950–2020. The solid grey line indicates the model climatology estimated using detrended hindcasts over 2001–2020 for the medium-range forecast and using detrended and bias-corrected hindcasts over 1981–2020 for the seasonal forecast. The arrow in the left-hand panel indicates, for illustration, the displacement along the log-scaled x-axis equivalent to a 5-fold increase in occurrence probability.