Weather alert: heavy thunderstorms and heavy rains – La Chaîne Météo

From Wednesday September 8 at 3:00 p.m. to Friday September 10 at 9:00 a.m.

Situation

A low, positioned at the entrance to the Bay of Biscay at 1010 hPa, rises over the south of Ireland before slowly receding Thursday and Friday towards Norway. Its associated and stormy disturbance crosses the west of the country on Wednesday. On the other hand, another depression, which carries hot and very unstable air, forms between Spain and Algeria then heads for the Balearics before spreading Thursday towards Sardinia.
This development initially favors the generalized development of thunderstorms this Wednesday over the western half. It is then conducive to the establishment of an episode of intense rains, between the evening and the next night 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 Occitanie until the end of the evening. This 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 stationary thunderstorms very localized in Languedoc cannot be ruled out.

Observation

Wednesday

At 3 p.m., we still observed some lightning strikes on the north-western part of Calvados but the 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 stopped 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.

Evolution

This afternoon
After the thunderstorms which circulated in the morning over the east and the north of Brittany, it is the north of New Aquitaine and Touraine which is affected in the afternoon by strong thunderstorm cells with a risk that intensities reach 20 to 30 mm / hour. These thunderstorms will then move towards the north of Auvergne, Berry, Beauce and Ile-de-France at the same time that another stormy degradation will take place between the east of New Aquitaine and the western Occitania. This stormy degradation will thus gradually spread on a Midi-Pyrenees / Paris Basin axis. Heavy showers are locally accompanied by hail and violent gusts of wind up to 80 km / h. In this context, the cumulative rainfall could reach up to 1 month of precipitation in the departments placed on orange alert.

In the evening, this activity in western Occitania will extend to the east of the region where heavy rainfall is also expected. On the Paris basin, thunderstorms will be locally strong and sometimes accompanied by hail in the early evening, before weakening on the way up towards Hauts-de-France.

In the night from Wednesday to Thursday, it is in the departments of Aude and Hérault that the situation could become critical with the persistence of heavy stormy rains over several hours. On the outskirts, then going up towards the Massif Central, thunderstorm activity should subside.

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