A moderate intensity Cevennes episode is expected between tomorrow Sunday and Monday morning. If the expected accumulations do not promise to be exceptional, the situation will still have to be monitored because of a risk of locally marked floods – fortunately not extreme – especially between the Gard and the Ardèche, perhaps over a small part of Lozère. This rainy and lightly stormy episode is linked to a vast low pressure system currently positioned between Iceland and the United Kingdom.
This low pressure system will stretch in the form of a thalweg (zone of low pressure which stretches in the shape of a “V”) which, tomorrow Sunday, will extend to the Bay of Biscay and the north of France. Spain, where it will adopt low mobility. This thalweg will slide over the Western Mediterranean basin on Monday morning, between Catalonia and the Gulf of Lion, doubled by a meso-depression (small depression) which is fortunately mobile off the Gard. In short, this is a great meteorological classic for our region.
The first rainfall is expected next night in the Cévennes Héraultaises, Gardoises and Ardéchoises. Tomorrow morning (Sunday): we are witnessing a very clear strengthening of the sea wind on the plains of Hérault and Gard, with gusts from the southern sector that can reach 80 to 90 km / h for several hours, until during afternoon. A very marked orographic forcing is expected over the Cévennes, especially between Aigoual, the south of Mont-Lozère and the Ardéchois plateau where these winds will come up against the mountains, rising rapidly. The cold air present at altitude will promote a very clear condensation of the clouds, which will then produce copious rains in the morning.
Good news: despite very dynamic forcings and a significant precipitable water content of the air mass (saturation), atmospheric instability (the famous MUCAPE which we often talk about in our articles) will be very low. This will clearly limit the risk of thunderstorms and the likelihood of extreme water levels. However, peaks of 30 or 40 mm / 1 hour are possible on the aforementioned axis, on several occasions, which is already significant. Initially, our plains are little affected, although showers are possible on the plains of the center and the east of the Hérault, as well as on the west of the Gard. They are more frequent by approaching the reliefs, rarer by the sea. A summary of what we can expect tomorrow at the end of the morning with the map below (heavy rains in the Cévennes, but crumbs in the plain, hence the term Cévennes episode).
This configuration persists in the afternoon, with rare rains in the plains, but always moderate to strong and persistent in the Cévennes, with maximum intensity in the Cévennes Gardoises and Ardéchoises. Probable paroxysm in the Génolhac / Mas de la Barque sector, with intensities of 30 or 35 mm / h for several hours. At the beginning of the evening, a convergence of the winds set up in line with a low pressure anomaly on the plains of East Hérault and Gard. Atmospheric instability is low but the forcings are such that a pluvio-thunderstorm line should form, sweeping all the plains fairly quickly.
As it passes, water levels of 30 to 40 mm / h are possible, very locally 50 mm / h in the Cévennes where the orographic effect worsens the situation. On the east of the Hérault, the passage is rapid. On the plains of Gard, the line evacuates more slowly in the evening. At the rear, a return of occlusion should take place with new moderate rains that may persist until Monday morning in the Gard, Ardèche and Cévennes Gardoises. Therefore, in 30 hours slippery, not exceptional but significant accumulations are to be expected:
We often expect 120 to 160 mm in the Cévennes, with regular peaks at more than 200/230 mm near Aigoual and more widely from the south of Mont-Lozère (Vialas / Mas de la Barque) to the Ardéchois / Tanargue plateau. The most affected sectors are located near Génolhac (slightly to the north) as well as on the side of the Croix de Bauzon / La Souche (Ardèche) where 250 to 300 mm are expected, very locally 320 mm. On the Génolhac / Mas de la Barque side, it could fall 150 mm in about 4 hours (slightly aggravating factor).
In the plains, there is a strong degradation between the seaside and the interior of the land, as shown in the map above. A doubt is present on the interior plains of Gard, where approximately 50% of the scenarios model accumulations of 60 to 80 mm, locally 100 mm, over a fortunately quite long period (this will depend on the positioning of the Monday morning occlusion) . In short, floods are to be expected on the Hérault river (ordinary) but a little more significant on the Gardons (all loaded at the same time) with overflows in the low and usually exposed points. The Ardèche rivers will also react. The Ardèche flood will also have to be monitored because of the concomitant inflows from the tributaries. Our services will be fully mobilized to keep you informed.