Past and future evolution of synoptic weather patterns leading to extreme precipitation events in Belgium. Linking synoptic scale events to their local impacts.

Cut-off lows (COLs), which are warm season high altitude cold depressions originating from the pole, have lead to the most extreme precipitation events in Belgium in the past decades, notably in July 2021. Their frequency is expected to rise with Global warming due to slowing dynamics during the warm season. On top of this, extreme precipitation events are becoming more frequent, and more extreme due to the increase of atmospheric moisture content resulting from its warming (Brajkovic et al., 2025 in prep.). To understand the cause of this increased frequency, we want to assess the evolution of the frequency of COLs which lead to extreme precipitation events in Belgium.

First, over 1940-2023, using our bias-adjusted high-resolution (5-km) Regional Climate model MAR (Modèle Atmosphérique Régional) precipitation data over Belgium and ERA5 reanalysis 500hPa geopotential data over Europe, we identify COLs which lead to extreme precipitation over the country. We find COLs leading to extreme precipitation all over the period. Their occurence has increased over the last decades reaching a frequency of 1 COL per year. However, we find periods with less COLs like in the 1970s.

Second, MAR is forced 6 CMIP6 Earth System Models over 2015-2100. Four Shared Socioeconomic Pathways baseline scenarios (SSP) are used ranging from low-end (SSP1-2.6) to high-end emissions (SSP5-8.5). Again, using ESM 500hPa geopotential and bias-ajusted MAR precipitation data, we proceed to the same detection over the future. We find that the occurrence of COLs is stochastic and without clearly identified trends. Intense precipitation events occur irrespective of the scenario at timings which are challenging to predict. However, the frequency of COLs reaches 1 COL per year irrespective of the model or of the scenario. This analysis shows that a large amount of the uncertainty over future computed extreme precipitation statistics lies in the occurrence of COLs.