An atmospheric river, a long corridor of water vapor measuring 5,000 to 6,000 kilometers, is expected to cross France this weekend, targeting the Gulf of Lions, the Cévennes, and the south of the Massif Central. This phenomenon can transport a flow of humidity exceeding twice the discharge of the Amazon, with rainfall accumulations of several hundred millimeters predicted over the Cévennes' reliefs in a fortnight. Given the risks of floods and landslides, it is recommended to avoid flood-prone areas, not to park near watercourses, and to monitor weather bulletins. An atmospheric river refers to a long band of clouds, a real rainy disturbance, which originates from the tropics or subtropical regions before heading towards more temperate latitudes. The term comes directly from English atmospheric river, sometimes nicknamed pineapple express in the United States, when it refers to air masses coming from Hawaii, or rum express in Europe, in reference to its origin in the Antilles. What is most striking is its dimensions: between 5,000 and 6,000 kilometers long, for only a few hundred kilometers wide. After a summer considered the hottest ever recorded, France is just entering a transition phase marked by a gradual return of rain. Models show a clear humid signal around the Gulf of Lions, the Cévennes, and the south of the Massif Central, a sector particularly targeted by these forecasts. The flow of humidity transported by an atmospheric river can exceed twice the discharge of the Amazon river. This mechanism plays a rather beneficial role: it allows the atmosphere to be rebalanced by transporting masses of hot and humid air to temperate zones. In the United States, this phenomenon is even vital for certain regions: California can receive up to 30 to 50% of its annual rainfall thanks to these rivers coming from the Pacific, which has led American authorities to create a scale from 1 to 5 to assess the ratio between benefits and risks of each episode. In France, some simulations already anticipate considerable rainfall accumulations over a fortnight, with local values exceeding several hundred millimeters over the Cévennes' reliefs. The scenario unfolds in several stages. First, an increased risk of strong Mediterranean rains, particularly over the Gulf of Lions, the Cévennes, and the south of the Massif Central. Then, more disturbed weather that could spread to a larger part of the country. This shift is also explained by the awakening of depression activity over the Atlantic: a powerful cyclogenesis has recently been observed off the coast of Ireland, with a depression descending below 960 hPa, a clear sign that atmospheric dynamics are changing rhythm after a particularly stable summer. The simulations for the next two weeks confirm humid signals over the west and south of the country, more pronounced over the Mediterranean regions where rainy and thunderstorm episodes are expected. The northeast, on the other hand, should remain out of reach, protected by a powerful anticyclone installed over Scandinavia and western Russia. As November approaches, a possible strengthening of the ocean current could further intensify these humid flows. Practically, if the models confirm, it is better to anticipate. Here are some useful reflexes to adopt before the passage of this humid corridor: Avoid roads in flood-prone areas, especially in the Cévennes and the south of the Massif Central. Do not park near watercourses in case of possible overflow announcements. Be cautious about the expected rainy and thunderstorm episodes. Monitor weather bulletins by following the progression of the phenomenon in stages. Dress appropriately: October marks a rapid drop in temperatures, up to 4°C less between the beginning and end of the month. This return of rain is generally good news after months of extreme drought. But as often with these phenomena coming from afar, the benefit can quickly turn into a risk, such as floods or landslides, if the accumulations intensify too quickly. This vapor corridor, invisible but powerful, illustrates well the complexity of the sky above us: the same phenomenon can bring the long-awaited relief after the drought, while also raising vigilance against floods and landslides. It remains to be seen whether this transition will mark the autumn durably, or if it will only be an episode among others in an increasingly unpredictable climate.