Weather alert: Heavy rain from the Cévennes to the mid-east

From in

Situation

A marked period of rain will be in place until Saturday morning from the Cévennes to the central-east to the central Rhône valley. This situation is explained by the rise of warm and humid air formed by a wide depression that turns over the Atlantic Ocean. In this flow, stormy rains block the Cévennes in the Jura, which abounds in the reliefs of the Cévennes. Then we talk about the Cevennes episode.
The Cévennes in the Gard and especially the Ardèche, as well as the central Rhône valley, are the sectors most exposed to the strongest rainfall accumulation due to the intense and more stationary nature of the rainfall. These accumulations could be between 100 and 150 mm in Ardèche with 200 to 220 mm on the borders of Ardèche and Drôme.

Observation

At 21h, the rainy-storm activity is maintained from the Massif Central north of the Alps to Alsace. Many lightning-thunder lines run through these regions. Dijon and Vesoul suffered flooding under the storms. In 1 hour, 35 mm of rain fell in Dijon and 16 mm in Vesoul. From the Ardèche to the north of Drôme to the south of Isère, a line of stationary thunderstorms remained in this area since yesterday evening, producing significant accumulation of rain. From Thursday 9 pm to Friday 9 pm, 105 mm fell in Deaux (30), 100 mm in Vernoux (07) and 101 mm in La Souche (07), equivalent to 2 to 3 weeks of rain.

At 6 p.m., the rain-storm activity is maintained from the south of the Ardèche to the north of the Drôme ascending towards the north of the Alps with heavy rain that does not change. Between 4:30 pm and 5:30 pm, 26 mm of water fell in Saint-Christophe (26) and 12 mm in Colombier (07). These rains are intensifying ahead of the cold front currently located in the Massif Central and which is moving towards the Center-East, already bringing heavy rain to the Loire where 25 mm of water in Panissières (42) and 21 mm in Feurs ( 42).

At 15h: the main rain-storm axis extends from the north of the Gard to Savoie, while the other concerns the departments between the Doubs and Alsace. Initially, the heaviest and sometimes stormy rains were observed in the last 3 hours in the Ardèche and western Drôme. Rainfall is continuous and locally heavy, with hourly intensities of up to 20 mm. Going up to Savoie, the rains are less sustained, but still regular with an average of 2 to 5 mm/h.
On the ridges of the Jura, the south wind blows in a storm with almost 100 km/h at La Dôle at 1677 m.

At 12: still thundering from the north of the Gard to the south of the Ardèche, up along the Rhône, from Montélimar to Valence. Hourly accumulations, very variable, range from 5 to 15 mm. Since the beginning of the episode, the main accumulations fell in the Cévennes (120 mm), but occasional thunderstorms dumped up to 50 mm in places this morning in Orange and Lapalud (Vaucluse), causing some localized flooding.
In addition, accumulations of 20 to 50 mm were recorded in the mountains, on the borders of Hautes-Alpes and southern Isère.

At 9 am: the storms moved faster than expected in the Rhône valley, so the rain stopped in the Cévennes. This evolution is due to a faster flow that can avoid the stagnant phenomenon of the storm. This development is encouraging, although other storms will develop again during the day and into the next night.
There were accumulations of 120 mm from last night in the Cévennes mountains, and locally 50 mm as thunderstorms passed through this morning in the Rhône valley, between the north of Gard and Montélimar. These storms have reached the Drôme Prealps.

last night :

Stormy rain continued overnight in the Cévennes, with accumulations already reaching the 120 mm mark in the Gard mountains, extreme southeast of Lozère and Ardèche. In the plains, for now, the accumulations are weak.
Thus there was 117 mm in Villefort (48) and 116 mm in La Souche (07), but only 15 to 20 mm on the edge of this stormy zone.

Evolution

Tonight, a lull is observed in the Ardèche before the arrival of new moderate rains. Conversely, sometimes stormy rains continue to affect the east of the Rhône-Alpes region. In the North-East, it sometimes rains heavily from the south of Alsace to Franche-Comté.

Tonightthe Ardèche gets away from the heaviest rain confined from the north of the Drôme to the Savoies.

saturday morning, the axis of heavy and locally stormy rain will continue to stretch between the north of the Drôme and the northern Alps, before an expected improvement in the early afternoon. In the North-East, the situation does not present any concern despite the sometimes rather heavy rains. This improvement will continue, as the sun gradually returns.

Throughout the episode, total rainfall will reach an average of 30 to 50 mm in the central-east, locally up to 60 mm over the Jura, Ain and Northern Alps. This corresponds to 3 weeks of rain within 36 hours. In the Cévennes, the rains will be more abundant with total rainfall of 100 to 150 mm, locally 200 to 230 mm in the Ardèche Cévennes, with the risk of exceeding these values ​​and locally approaching 250 mm . This relatively short episode does not seem capable of causing major general consequences, but the reaction of small fast-flowing rivers such as Gardons, Cèze and Ardèche should be monitored. Stationary storms can cause localized runoff and flooding, with road traffic difficulties.

In this topic, specify that accumulations of more than 200 mm in 24 hours in the Ardèche occur once every 3 years and once every 12 years for accumulations reaching 300 mm. Over 48 hours and for 200 mm, it happened once a year for 60 years. For accumulations greater than 300 mm, it is once every 3 years.

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