Predicting the unprecedented : forecasting the June 2021 Pacific Northwest heatwave

In June 2021, an unprecedented extreme heatwave impacted the Pacific Northwest of North America, resulting in more than 1000 excess deaths and affecting infrastructure and wildlife. Predicting such extreme events is key for preparedness and early action, and beyond temperature, it is important to consider biometeorological forecasts, accounting for the effects of the environment on the human body. The performance of ECMWF's (the European Centre for Medium-Range Weather Forecasts) temperature and heat stress (Humidex and the Universal Thermal Climate Index) forecasts during this heatwave is explored, and highlights that several days in advance, an event surpassing the maximum of climatology was predicted with extremely high confidence (100% of ensemble members) – successfully predicting the unprecedented.