Soil moisture-precipitation feedback is an important factor in the water and energy cycles. But how important is it on the time scale of an atmospheric extreme precipitation event? We are investigating this question using the example of heavy precipitation in July 2021, which led to destructive flash floods in Western Europe. To quantify the importance of land-atmosphere coupling and continental moisture sources for the precipitation, we perform numerical simulations with wet, dry and normal soil moisture conditions over Europe. Ensembles of simulations are performed using a global set up of the ICON numerical weather prediction model with a grid refinement over Europe. To account for both the limited predictability and the delayed response of the atmosphere to changes in soil moisture, we use data assimilation to steer the system’s development toward the extreme event, but only to the extent necessary so that our interventions in the soil are not undone. We find that the moisture supply of the event crucially depends on continental moisture sources. This result is further confirmed using moisture tracking. Conversely, increased soil moisture only leads to slight precipitation increases, since surface moisture fluxes are energy-limited. Moisture is also important for the development of the near surface low pressure system, which had a central role in the event. Our ensemble simulations also show that there is potential for more devastating events, i.e., more precipitation.