Was July 2021 extreme rainfall in western Germany close to the worst possible?

In July 2021 extreme rainfall associated with a cut-off low pressure system led to huge impacts in western Germany, Belgium, and the Netherlands. The event was costly both in terms of loss of life and insurance damages. We use a multi-method approach to examine the event and to assess whether it could have been even worse. Using atmospheric analogues from reanalysis, pseudo global warming simulations, and a boosted ensemble of a dynamically similar event we show that the observed rainfall pattern is highly sensitive to the large-scale dynamics. For example, although good dynamical analogues are found in reanalysis, these do not all show the same hazards – with many showing very little rainfall.  

Our results suggest the magnitude of rainfall experienced was very unusual, perhaps close to the worst possible in the current climate, as small dynamical changes lead to a drastic reduction of the rainfall.