Alternative rainfall storylines for the Western European July 2021 floods from ensemble boosting

In July 2021, a cut-off low-pressure system brought extreme rainfall to Western Europe, leading to flooding that caused loss of life and infrastructure damage. Here, we use ensemble boosting to investigate alternative storylines of the event, given the observed dynamical situation. Using a fully coupled free-running climate model, we identify atmospheric flow analogues of the 2021 event in an initial-condition large ensemble. These analogues are re-initialised with slightly perturbed atmospheric initial conditions to generate physically plausible alternative storylines. The storylines are used to investigate how a potentially worse event could have unfolded given the same large-scale dynamics. We identify rainfall events with longer persistence and larger extent, yet the observed event appears to be towards the upper end of what is plausible in the current climate. Such storylines can be used to prepare for possible future events, helping society to imagine dangerous, but plausible, scenarios. The July 2021 extreme rainfall event in Western Europe appears near the upper bound of plausibility in the current climate, though alternative storylines revealing the potential for even more severe outcomes in terms of rainfall persistence, spatial extent, and location – given the large-scale dynamics of the event, according to a boosted ensemble.