In June 2021, Canada experienced an intense heat wave with unprecedented
temperatures and far-reaching socio-economic consequences. Anomalous
rainfall in the West Pacific triggers a cascade of weather events across
the North Pacific, which build up a high-amplitude ridge over Canada and
ultimately lead to the heat wave. We show that the response of the jet
stream to diabatically enhanced ascending motion in extratropical
cyclones represents a predictability barrier with regard to the heat
wave magnitude. Therefore, probabilistic weather forecasts are only able
to predict the extremity of the heat wave once the complex cascade of
weather events is captured. Our results highlight the key role of the
sequence of individual weather events in limiting the predictability of
this extreme event. We therefore conclude that it is not sufficient to
consider such rare events in isolation but it is essential to account
for the whole cascade over different spatio-temporal scales.