Heavy rain in Catalonia has activated vigilance in several regions, including Girona, due to the risk of strong storms and sudden floods in rivers and streams. The orange alert has been issued for Barcelona and Girona, while in Lleida and Tarragona the alert is yellow for rain, storm and wind. The key to this phenomenon lies in the presence of warm and humid air from the Mediterranean, which when it meets cooler air at higher altitudes favors the development of these storms. The relief also plays a role, as in the Pyrenees, the Pre-Pyrenees and areas of the interior of Girona and Barcelona, the orography and terrain force the humid air to rise. Once it has risen and cooled, it condenses quickly, which can pour down with force on small basins and narrow streams. In episodes of rain in Catalonia like this, rivers and streams can swell even after it has stopped raining in the specific point. The Civil Protection has asked for caution due to the risk of sudden floods. It is essential, according to Catalan authorities, not to cross low bridges, streams or flooded areas, neither on foot nor by vehicle. In Campdevànol, in the Ripollès, the Merdàs river has flowed with a greatly increased flow after the precipitation. The images show turbid water, strong current and a quick response of the channel, something common when the rain pours down with intensity in mountainous areas.