The case of flash floods in Montsià county (Catalonia, Spain): from the source of precipitate water to thunderstorm cells

The latest IPCC report (2022) projects an increase in climate risks for all regions of the world, both in frequency and intensity. In particular, on the Spanish Mediterranean coast, catastrophes such as the Gloria event in January 2020, or the tragic floods that occurred in October 2024 in Valencia and Castile-La Mancha, are aligned with these projections. On a smaller geographical scale, flash floods that occurred in the Montsià county (southern Catalonia) in 2018, 2021 and 2023 also point to an increase in frequency in this in this 733 km² region located at the south of the Ebro Delta. This region is a paradigmatic example of a Mediterranean region with a high flood risk. Firstly, it has a high flash flood hazard, as a result of its abrupt orography with steep slopes that favours the existence of numerous steep torrents, as well as the rise of humid air masses from the Mediterranean, especially when they hit perpendicular to the coastline, which helps trigger convection and gives rise to intense rainfall. Likewise, the geographical region in which it is located is favourable to the entry of humid air from remote sources, which contribute to the increase in the intensity and amount of precipitation. Secondly, it has a high flood exposure despite the low population density, but which is multiplied by four in summer and early autumn in some municipalities. Thirdly, it has a high flood vulnerability, a consequence of being divided into three hydrographic basins, managed by three different administrations, which makes coordination difficult, especially regarding flood prevention. This is combined with a low-risk awareness both socially and individually that is joined to the difficulty of predicting and nowcasting the convective events that give rise to the severe flash floods that the region frequently experiences.

During the catastrophic flooding event of October 18–20, 2018, the maximum precipitation recorded in the Montsià region was 312.2 mm, and a daily rainfall of 209.6 mm, with a peak of 30-minute rainfall of 52.4 mm. On September 1, 2021, 251.9 mm were recorded over three hours, with a peak of 30-minute rainfall of 72 mm.  On September 3, 2023, very heavy rainfall was recorded once again in Montsià, with a maximum rainfall of 206 mm/24h and a peak of 30-minute rainfall of 61.4 mm.  In this study we characterize these three catastrophic flash flood events taking into account the complexity that local scale phenomena may have. For this reason, the characteristics of the thunderstorms that gave rise to the catastrophic flash floods are analyzed, to then go on to understand the synoptic and mesoscale context and finish with the search for the moisture source fields at global scale. In order to ascertain whether this increase in frequency in recent years responds to a significant trend, a spatio-temporal analysis in extreme rainfall indicators has been made. To do this, information from multiple data sources has been integrated, including meteorological station observations, weather radar products, lightning detection networks, high-resolution mesoscale model outputs.