Moisture origin for the heavy precipitation event in Central and Eastern Europe in September 2024

In September 2024, cyclone Boris brought intense precipitation to Central and Eastern Europe, causing severe flooding in Austria, Czech Republic, Poland, and neighboring countries. Understanding the processes that led to this event is important for improving the prediction and mitigation of similar events in the future. Here we trace the origin and transport pathways of the moisture contributing to the precipitation during the event using a Lagrangian moisture source diagnostic. The results show that evapotranspiration from land played a more important role than previously thought: most of the moisture came from the European continent, with additional contributions from the Mediterranean, Black, and Baltic Seas. To place the results in a broader context we compare them with a climatology of moisture sources based on a Lagrangian reanalysis dataset for the years 1940 - 2023. This provides additional insight into atmospheric processes driving heavy precipitation events in this region and highlights anomalous patterns associated with cyclone Boris.

Contributions of different source regions to precipitation in Central and Eastern Europe in September 2024. The figure shows the total precipitation from ECMWF (orange line) compared with the precipitation estimated by the Lagrangian moisture source diagnostic (blue line) and the contributions of different regions (defined in the upper left panel) in colors.