August 3, 2021

Weather Alert: Heavy thunderstorms in Brittany

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From Friday July 23 at 9:00 a.m. to Friday July 23 at 11:00 p.m.

Situation

A depression is positioned in the immediate west of Brittany, which is quite unprecedented at this time of the year. This low will be located on Friday noon off the Pointe du Raz and will go up to Brest at the end of the day, causing a rotation of stormy rains all around its heart. A marked stormy degradation, with abundant accumulations of rain, then triggered over Brittany and the neighboring departments this Friday in the morning. Two stormy waves will affect this small north-western quarter of France: this Friday morning, then again Friday at the end of the afternoon and evening. A lull will be felt until Saturday noon before the resumption of new very heavy thunderstorms on Saturday afternoon. This special press release will then be extended accordingly.

This degradation is characterized by:

– Significant amounts of rain that can reach 30 to 50 mm in 36 hours, locally 60 mm.
– A risk of flooding by runoff in the low points of the valleys and municipalities concerned.
– Strong electrical activity (lightning strikes).
– Strong gusts of wind up to 80 to 90 km / h, locally 100 km / h.

Observation

The low pressure system begins to form off the Atlantic, in connection with a descent of cold air. It is therefore a new cold drop. This development is currently reflected in a sensitive easterly wind along the western Channel coasts, as well as by the arrival of light high altitude clouds (Cirrus and cirrostratus).

Evolution

The evolution is as follows:

– Friday morning : a first stormy salvo goes up from the Bay of Biscay towards the south of Brittany, which is especially concerned between 9 am and 11 am. Morbihan and southern Finistère could receive heavy thunderstorm rains with sudden accumulations of 30 to 40 mm, accompanied by strong electrical activity. The east wind will be sustained over all of Brittany during this episode. Thunderstorms can also break out, on the fringes, as far as the Pays de la Loire coast.

– Friday midday : the thunderstorms could mark a relative pause, although Finistère, as close as possible to the low pressure core, risks keeping frequent thunderstorms going up towards the north of the department.

– Friday afternoon : from 5 p.m. to 11 p.m., with the rise of the low pressure core, a second marked stormy wave will sweep Brittany from south to north, affecting the entire region. These thunderstorms will give fairly generalized and abundant rainfall accumulations, with, on average, 20 to 30 mm, with locally accumulations of 40 to 60 mm. These water offal may cause runoff and flooding in the usual low points.

– Friday night : this stormy wave will continue to move north towards the British Isles. It will then be the end of thunderstorms over Brittany. However, with the proximity of the low pressure center along the coast of Finistère, the night will remain windy and new showers, sometimes heavy, will again arrive in western Finistère.

The bad weather episode will therefore not be over, with the resumption of very heavy stormy showers accompanied by gusts of wind to 80 km / h on Saturday afternoon: this development will give rise to a probable extension of this special press release.

List of departments concerned