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From September 8, 2021 at 7:00 p.m. to September 10, 2021 at 7:00 p.m.
In connection with the rise of a depression from the Bay of Biscay towards Ireland, a stormy disturbance crosses the western part of the country on Wednesday. Another low, which carries hot and very unstable air, evolves in the vicinity of the Balearic Islands before progressing Thursday towards Sardinia.
This development initially favors the development of stormy lines on Wednesday from New Aquitaine to Normandy and the Paris Basin. In the evening and at night from Wednesday to Thursday, it is conducive to the establishment of an episode of intense rains in Languedoc-Roussillon. These heavy stormy rains will gradually weaken in intensity Thursday morning. A lull will then be observed before the return of new thunderstorms in New Aquitaine in the afternoon. These thunderstorms, which generate heavy showers and hail, will strengthen in the direction of the Massif Central and Occitania until the end of the evening. The stormy activity will decrease in intensity during the night from Thursday to Friday, when the storms rise towards Burgundy and the Rhône-Alpes region. In this unstable context, a risk of very localized stationary thunderstorms development in Languedoc cannot be excluded.
At 10:30 p.m., thunderstorm activity remained marked from Haute-Normandie to Midi-Toulouse. The progression of the disturbance is slowed down by the resistance of hot air at the front, which causes long-lasting rains of high intensity, especially in the Lot-Garonne, one of the most affected departments. Thus, 129 mm of rain was recorded in Agen between 7 p.m. and 10 p.m., which corresponds to nearly 20% of the annual rainfall. This is also an absolute daily rainfall record in this city since the resort opened since 1941.
At 21h, rain and storm activity is very important between the east of Aquitaine, the west of the Massif Central going up towards the Loire Valley as well as in Upper Normandy. Under the strongest thunderstorms, rainfall accumulations are very important in a short time, as in Saint-Pierre (18) where it fell between 19 and 20 h 58 mm of water, equivalent to 1 month of rain in 1 time. In Agen (47), 89.8 mm of rain just fell in 1 hour. This is an absolute daily rainfall RECORD since the resort opened in 1941! In Villeneuve-sur-Lot (32), 65 mm of rain just fell in 1 hour, or more than 1 month of precipitation.
Ahead of the disturbance, very active storm cells are rapidly developing from the Allier to the west of Burgundy. Other thunderstorms formed over western Paris and progressed towards the capital.
At 18 o’clock, an active thunderstorm line concerns the north of New Aquitaine and the south of the Center-Val de Loire region with intense rains. Other more diffuse and localized thunderstorms formed over Béarn, Ariège, Allier and Calvados. The most significant amounts of rainfall are observed in Limousin with 37 mm in Brigneul (Haute-Vienne), 30 mm in Saint-Junien (Haute-Vienne), 26 mm in Brantome (Dordogne).
At 15h, we still observed some lightning strikes on the northwestern part of Calvados but thunderstorms were evacuating in the English Channel. Other storms were developing rapidly between Charente, Limousin, Poitou and Touraine. They were accompanied by intense rain with 10 to 15 mm in less than an hour.
At 12, the thunderstorms which affected Ile-et-Vilaine and part of Côtes-d’Armor in the second part of the morning generated maximum accumulations between 20 and 30 mm in 1 h, or the equivalent of 15 days of precipitation .
At midday, the thunderstorms ceased over Brittany before moving up to the Channel and the neighboring departments where they should persist until 2 p.m., but while partially losing their intensity. In contrast, other storm cells can be observed in the Bordeaux region and rapidly ascend towards Poitou.
At 9 o’clock, many thunderstorms developed last night over Brittany and Loire-Atlantique. They generated up to nearly 34 mm at Saffré (44) in 2 h, and more generally 10 to 25 mm. This represents on average 30% of the rains in September.
The storms have now stopped in Finistère and Morbihan and are concentrated between Ile-et-Vilaine and Côtes-d’Armor.
Tonight, It is in the departments of Aude and Tarn that the situation could become critical with the persistence of heavy stormy rains over several hours. The Minervois and the Montagne Noire are particularly exposed to violent thunderstorms with a risk of heavy stationary rains, flash floods and flooding.
Thusday, heavy rains in Languedoc could persist for a few hours in the early morning before a lull from 10 a.m. In the afternoon, the lull will be general over Languedoc-Roussillon, but new thunderstorms will develop between New Aquitaine and western Occitania.
Friday will again be a stormy day in a south-westerly train, but to date the situation is not really extreme.
This situation is favorable
– the establishment of an episode of intense rains in part of the Hérault and Aude, a significant deterioration that should be monitored in relation to a high risk of bad weather with flooding.
– strong electrical activity over 3 days.
– heavy intensities of rain in Occitania, under the most violent thunderstorms causing accumulations of up to 1 month of precipitation.
– heavy hail showers.
List of departments concerned