The Ciaran depression, expected on the western coasts of France from Wednesday and which should remain active in France until Thursday noon in the north-western quarter of the country, was classified by Météo-France among the phenomena potentially dangerous”.
Having barely recovered from storm Céline, France will soon have to face Ciaran. With heavy precipitation, gusts of wind and large waves, this depression which is due to arrive in France in the middle of the week should be particularly violent. Météo-France has classified it among the potentially “dangerous” phenomena in his newsletter published at midday this Monday, October 30.
The storm “will affect us from Wednesday evening until Thursday noon,” said François Gourand, Météo France forecaster, during a press conference this Monday afternoon.
“Storm Ciaran approaches the coast of Brittany on Wednesday evening then sinks into the land of the north-western quarter of France,” the meteorological agency indicated in its bulletin.
“Strong gusts of wind are expected from Wednesday evening until midday on Thursday: between 100 and 110 km/h inland and between 120 km/h and 140 km/h on the coasts of Brittany and the Channel with locally peaks at 150km/h”, Météo-France further clarified. The Breton tip and Cotentin should be the most affected by these strong gusts.
Potential “weather bomb”
The phenomenon will take its low-pressure form “in the coming hours off the coast of Newfoundland and the northeastern United States.” By Wednesday evening, it should be propelled towards the south of the British Isles by the “jet stream”, a high altitude air current (around 10,000 meters) which circulates at more than 300 km /h.
The storm is expected to strengthen during this crossing of the Atlantic and “appears to meet the criteria of a meteorological bomb”, according to François Gourand.
In this case “a drop of more than 24 hectopascals (hPa) in 24 hours”. Ciaran’s drop could be 30 hPa, according to the forecaster. “This tells us about the speed of strengthening of the phenomenon but not about its intensity, even if rapid strengthening” suggests “violent winds” in particular.
Gusts, risks of waves-submersion and rain
The forecasts suggesting a potentially violent phenomenon are notably linked to atmospheric pressure which could be particularly low at the time of the storm’s passage over France, with values below 960 hectopascals (hPa). However, the lower the surrounding atmospheric pressure, the more violent the depression phenomena can be.
In France, the post-1950 low pressure record is 951.8 hPa recorded at Pointe de la Hague (Manche), on February 25, 1989. Ciaran should bring values between 950 and 955 hPa according to Météo-France.
From late Wednesday afternoon, the coastal areas of the North-West will experience a risk of a surge (rise in average sea level), which will favor the appearance of very large waves, with disturbed weather offshore which will produce already a significant swell. Météo-France forecasts waves of “between 8 and 10 meters” on the Atlantic coast and “between 6 and 8 meters” on the Channel coast, with, as the main danger, a “risk of waves – very probable submersion” .
The gusts expected on land between Wednesday evening and Thursday also represent a real danger: due to the late summer, most of the trees are still in leaf and therefore have more resistance to the wind, which could encourage their fall. A risk increased by the rains of recent days which have waterlogged the soil, making it loose and therefore less able to retain the roots of trees in the event of a gust of wind.
Precipitation should also be significant but not stationary, which should avoid the risk of flooding.
Series of storms and depressions
Ciaran, which seems to have everything of a future storm, is part of a series of depressions of which Céline was the most recent example, and which could continue in the days to come, favored by a phenomenon nicknamed “rail of the depressions.” This two to three kilometer thick tube marks the boundary between warm and cold air masses.
Starting from the northeast of the United States, it ends more or less close to France, towards Brittany. The closer this area is to the earth, the more violent the depression phenomena are and this on a regular basis. “But in the coming days, this atmospheric circulation will emerge on the outskirts of our country,” writes meteorologist and BFMTV contributor Guillaume Séchet on his site Weather cities.
The sky should therefore continue to be choppy until the end of the week, even if the forecasts still need to be refined.