Catalonia experienced an extraordinarily rainy early October across its coastal and prelitoral areas, driven by a combination of high sea temperatures and atmospheric instability. According to local weather reports, forecasts failed to anticipate the true magnitude of the rainfall, as computer weather models struggled to capture localized convective storms in the Mediterranean basin.
Why Weather Forecasts Missed the Torrents
Meteorological models performed well in mapping broad atmospheric instability and tracking approaching low-pressure systems, but struggled with the micro-scale dynamics of convective storms. Unlike Atlantic weather systems where precipitation fronts follow predictable paths, Mediterranean weather involves high uncertainty. Small variations in local temperature, humidity, and wind patterns can cause severe storms to shift by just a few kilometers, completely altering rainfall distribution maps. As a result, initial computer forecasts pointed only to scattered showers and thunderstorms rather than the intense, highly localized downpours that ultimately struck.
How Mesoscale Convective Systems Fueled the Deluge
The severe weather was driven by the formation of a mesoscale convective system (MCS)—a large, self-regenerating cluster of thunderstorms anchored over the sea. Sarai Sarroca, director of the Servei Meteorològic de Catalunya (Meteocat), explained during a press conference that the event was neither a DANA (isolated high-altitude depression), a traditional easterly llevantada storm, nor a cold frontal passage. Instead, shifting lines of instability formed successive mesoscale convective systems. During the weekend storms, some meteorological scenarios even raised the theoretical possibility of a medicane—a small Mediterranean hurricane—though it ultimately did not produce. The resulting storms combined high humidity, extreme sea warmth, and slow storm movement to produce exceptional downpours and strong wind gusts.
Torrential Rainfall Triggers Record Flooding Across Barcelona Coast
Rainfall totals between the weekend and Monday reached between 200 and more than 300 liters per square meter across the Barcelona coast and prelitoral zones. Granollers recorded 68 liters per square meter in just 30 minutes over the weekend, pushing its three-day total to nearly 350 liters per square meter. In Barcelona, the Can Bruixa station registered 213 liters per square meter in a single 24-hour period over the weekend, marking a record for the location. These torrential volumes triggered sudden urban floods and river overflows.
The Impact of Over-Saturated Ground
Because the weekend storms had already soaked the soil across the prelitoral and coastal regions, the ground lost its capacity to absorb further water. Consequently, Monday’s secondary wave of precipitation in the Vallès Oriental area—where over 100 liters per square meter fell in a short span—caused immediate flooding and river overflow. Santi Segalà, head of prediction at Meteocat, warned that the underlying atmospheric instability, fueled by an unusually warm summer and high sea temperatures, would persist in the days following the event.
