Weather alert: Heavy showers and thunderstorms along the English Channel

By Regis CREPET, meteorologist

Warning for heavy rains from the Channel to the North

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From Thursday, September 8 at 2:00 p.m. to Friday, September 9 at 6:00 p.m.

Situation

A depression is circulating very slowly over England. This is the cold drop which has been turning on itself in the vicinity of Ireland since this weekend and which is progressing very slowly towards the North Sea, which it will reach on Friday evening. It is responsible for unstable and cooler weather over a large northern half of France, with a succession of significant showers along the Channel coasts, mainly from Normandy to Hauts-de-France.

Heavy stormy showers follow one another all day long, locally causing accumulations of up to 20 to 30 mm over the day, sometimes falling over short periods. Accumulations that could approach 50 mm are envisaged in Picardy and the Côte d’Opale.
This situation can lead to runoff and some flooding at low points and on secondary roads. It will be necessary to monitor, until Friday, the possible reaction of the Boulonnais rivers, which could experience an increase in their level.
These sustained showers, sometimes stormy, will continue (during the night and in the morning of Friday north of the Seine. Boulonnais is the sector most exposed to abundant accumulations of rain, which could occasionally reach more than 50 to 60 mm.

At the same time, strong gusts of wind can occur as the strongest thunderstorms pass, with swirling phenomena, such as waterspouts along the coast.

Observation

At 17 o’clock, thundery showers with lightning strikes concerned Normandy, spreading towards Picardy. Some showers are sustained, with hourly accumulations having locally reached 7 mm in Calvados. The strengthening of the showers and their generalization north of the Seine is expected after 6 p.m., in the evening and all night.

At 15h, the showers continue to multiply from the coasts of the English Channel to the Paris Basin to the northern borders. They are sometimes strong and stormy.

At 12, heavy showers affected the Boulonnais and the Côte d’Opale, in the Pas-de-Calais. In addition, the showers tended to develop gradually throughout the northwest, before increasing in intensity this afternoon.

At 10 o ‘clock, showers were relatively light and few. However, a rainy front approached the coast of Pas-de-Calais at the level of Boulonnais. The strongest intensity of these showers is, in any case, expected in the afternoon.

This morning, the depression responsible for this disturbed weather, clearly visible on the satellite images, is located in the Irish Sea at 1000 hPa and begins its slow progression towards England.
After the heavy and frequent stormy showers of the night between the Boulogne-sur-Mer region and that of Calais, which generated accumulations of up to 49 mm in Boulogne, equivalent to 3 weeks of precipitation, new showers approach the coast Atlantic. But a lull set in on the Channel coasts during the morning.

Evolution

Tonight and into the night, the showers will be sustained north of the Seine. The strongest are concentrated on the Opal Coast, the North and Pas-de-Calais. They continue during the night on these departments of Hauts-de-France, with accumulations which can reach a total of 50 to 80 mm on the hilly sectors of Boulonnais. Peaks of almost 100 mm are possible until Friday noon at the end of the afternoon. The small streams descending from the hills could react, but the levels being very low following the summer drought will limit the risk of overflows. We can especially fear the runoff on the axes of circulation.

This Friday, thea Côte d’opale, the Nord and Pas-de-Calais are mainly affected by these repeated heavy stormy showers, especially between 11 a.m. and 5 p.m. At the end of the afternoon and in the evening, the most sustained showers and thunderstorms will break out in the interior: from the Aisne to the Ardennes with a risk of strong gusts of wind and showers of small hail. The new accumulations for this Friday could reach 15 to 40 mm, which could bring the total for the episode between 80 and 100 mm on the hills of Boulogne-sur-mer.
Note the persistence of a moderate to fairly strong westerly wind, between 60 and 70 km/h, locally 80 km/h towards Cap Blanc Nez. The sea is strong, which requires caution if you are at the beach.

List of departments concerned

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