Numerical weather prediction (NWP) and climate models still struggle to correctly predict and represent atmospheric blocking over the European region (EuBL). In recent years, there has been growing evidence that latent heat release in midlatitude weather systems such as warm conveyor belts (WCBs) contribute significantly to the onset and maintenance of blocking anticyclones. In this study, we show that for the European Centre for Medium-Range Weather Forecast’s IFS reforecasts in extended winter (1997–2017) WCB activity around EuBL onsets becomes challenging to predict in pentad 3 (10–14 days) and beyond. This is in line with the short overall WCB forecast skill horizon of around 10 days and partly explains low EuBL skill in NWP models. However, we also show cases in which accurate WCB and EuBL forecasts are possible even in pentad 4 (15–19 days). These cases are associated with accurate WCB forecasts over the North Atlantic and North Pacific pointing towards a teleconnection between the two. Lastly, we find that WCB activity over the North Atlantic emerges way before the block is established and different pathways into EuBL exist in the reforecasts which are characterised by a westward shift of the main WCB inflow and outflow region compared to reanalysis. We conclude that despite intrinsic limits of predictability there is room to improve forecasts of EuBL onset by improving the representation of WCB activity in NWP models.

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.