Ciara from 9 to 11 February, Inès le 13, Dennis this weekend: in recent days, storms have followed one another on the British Isles and the north of France. And the temperatures will be particularly mild for a month of February.
First there was Ciara, which swept northwest Europe between 9 and 11 in February. Storm Inès then briefly crossed northwestern France on Thursday 13 in February. This weekend, it will be the turn of the one called Dennis to hit the British Isles and the north of France. A chain of storms due to the exceptional atmospheric conditions that the North Atlantic is currently experiencing.
A context favorable to storms
In general, atmospheric pressure at sea level is on average around 1 0 13 hectopascals. But in reality, the North Atlantic is divided between two semi-permanent atmospheric systems which oppose as much as they complement each other: in the north, a system of depressions (low pressures) located in the region of Iceland and bringing cold, humid air; to the south, the Azores high pressure (Portugal), synonymous with hot and dry air and high pressures.
Between these two contrasting air masses, serving as a delimitation, is what is called the jet stream, a vast tube of strong winds (200 km / h in general) from the west. This current behaves like a kind of rail along which the depressions (storms) that form in the Atlantic slide. And the higher the pressure difference (the pressure gradient) between the mass of cold air and that of hot air, the stronger the jet stream.
This is exactly what has been happening since the beginning of the month. In the North, intense low pressures (of the order of 930 hPa) persist for several weeks, while in the south, a very powerful high (1 0 030 hPa or more) and very wide (going back to Spain) is maintained, causing very large pressure differences over a short distance (2 000 km). And, sandwiched between the two, the jet stream is therefore particularly strong, with winds up to 400 km / h to 10 km of altitude.
As announced at the end of January with more than 10 due days, the circulation of atmos early February corresponds to extreme values of the NAO + zonal regime. And it's not over! See pressure card scheduled for Sunday 16 / 02 expires +5 days! Rare stability / intensity / predictability https://t.co/KFwg8zHSoC pic.twitter.com/QOvUKAPhtZ
— Christophe Cassou (@cassouman 48)) February 12, 2020
If Météo France cannot explain this exceptional atmospheric context, it results in very depressions (storms) intense over the North Atlantic. “In itself, this is not unusual in winter,” said Marion Pirat, a forecaster at Météo France. But that challenges us because the jet stream, currently curved and markedly reinforced by the very high pressure gradient, projects these storms on the British Isles and the North of France.
Dennis, an exceptional depression
Thursday 13 February, in addition to Inès, two other storms, more important, were being formed in the North Atlantic. The most powerful of them, which generated waves from Thursday to Friday close to 20 meters on the Atlantic, was not named because it will not land. The other was named Dennis by the British meteorological services, the Met Office, because it will affect the British Isles in particular.
Magnificent ballet of 3 depressions on the #Atlantic this 13 – 02 – 2020 # Ines (strong gale, northern France), followed by a powerful # storm which will affect Iceland this Friday. But the real “star” will be #StormDennis, about to experience a memorable explosive cyclogenesis… pic.twitter.com/JM8kKBkMVE
— Etienne Kapikian (@EKMeteo) February 13, 2020
Dennis, who is about to develop this Friday so “Explosive” (notably losing 70 hPa in 30 hours ), already looks exceptional from a meteorological point of view. “The forecasting models predict a very rapid widening of Dennis”, explains Météo France, which estimates that it will lead to Saturday evening “a depression less than 920 hPa, can even approach 915 hPa, an extremely rare level on the North Atlantic for such an extra-tropical storm ”(the record is of 914 hPa and date of 10 January 1993, when Storm Braer passed).
The #Dennis depression, which originated on the east coast of the USA, will intensify under a #jetstream at ~ 400 km / h (strong thermal contrast) with a pressure drop of 85 hPa in 40 h!
At maturity, she will approach 915 hPa Saturday , tututing the North Atlantic low pressure record (914 hPa in 1993) pic.twitter.com/N0QNMahzja
— Etienne Kapikian (@EKMeteo) February 13, 2020
Dennis will travel further north than Ciara, depression – even should not, “initially, directly touch the British Isles, remaining rather offshore,” according to Météo France. “But its very imposing size and the very strong associated pressure gradient will be enough to cause significant bad weather between Ireland and Great Britain”. In France, the northern third will again be exposed, less than during the passage from Ciara, but enough for certain departments to be subject to vigilance by Saturday evening.
“In France, where the south-westerly winds will strengthen from Saturday near the English Channel, It is especially Sunday that the gusts will be strong, possibly reaching 100 at 110 km / h near the coast of Manche and 80 at 90 km / h in the interior “
High temperatures in the Southwest
Winds from storm Dennis will spare the rest of the country , and especially the Southwest, but in its wake, temperatures will (again) be particularly mild for a month of February. The south-westerly winds that accompany Dennis “will bring warm, dry air from France to the Magreb and Spain,” said Marion Pirat.
Consequently, the sun will shine widely and on Saturday and at the best of the day, it will be between 14 and 19 ° C in New Aquitaine. The sweet peak is expected on Sunday. The bar of 20 ° C will still be exceeded in the region and that of 25 ° C could be approached at the foot of the Pyrenees , according to Météo France. The night minimums will also be remarkable, sometimes greater than 08 ° C, even in the north of the country.