The winter of 2020-2021 was generally milder than normal in France. The thunderstorm activity, a little more frank in December, remained relatively modest throughout the season. The northern regions observe a marked deficit of thunderstorms.
Rare thunderstorms in the northern third
Over the three winter months, France counts 45 days of thunderstorm is 4 more days than the 2009-2020 average, and 7 days more than the previous winter. During this winter of 2021, December was the month which, like last year, was the most stormy in terms of frequency, followed equally by January and February.
The Corsica is the region of France having recorded the most days of thunderstorms during these three winter months with 17 at 18 days. Then come the Country (15 days), the Pyrenees-Atlantiques and the Gironde (11 days), the Alpes-Maritimes (9 days), the Where (8 days), the Bouches-du-Rhône and the Hautes-Pyrenees (7 days).
Conversely, 20 metropolitan departments have not recorded no stormy days during this winter. The latter are overwhelmingly located north of the Seine and in the north-east of the country.
We must add Paris but this should be put into perspective given the size of this territory.
Compared to the 2009-2020 average, it is on Country that the positive anomaly is the most marked with a surplus of +6 days thunderstorm. Overall, we observe a positive anomaly on the Alpes-Maritimes, the Corse-du-Sud and the Hautes-Pyrenees (+4 days). The surpluses then become not very marked, at most +1 to +2 days in the southwest or in the Northern Alps.
Conversely, the deficits are significant north of the Seine, where they reach -7 days on the Seine-Maritime, -6 days on the Sum and the Some, -5 days on theYour, -4 days on many other departments extending from Finistère to the Lorraine.
Note that thunderstorms were rarely strong during the winter. In fact, only 5 days with strong thunderstorm and 2 with severe thunderstorm. The criteria were met due to violent gusts in a thunderstorm or from tornadoes to the passage of trains.
Number of stormy days (right) and deviation from the 2009-2020 average (left)
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Considering this winter’s average storm severity index (ISO), the score stands at 0,39, a value much lower than the average of the last 12 winters.-
An unstable but irregularly stormy winter
On a national scale, this winter 2021 present a remarkable surplus of instability of +124%, ie a much larger surplus than the previous winter (+ 27%). In fact, this winter ranks third among the most unstable winters since the 1940s, behind the 1977 and 1979 winters which retain their first and second place respectively. This excess of instability was permanent during the three winter months, with a more particularly marked excess in December.
Even if all the regions were affected by this excess of instability, the situation was more noticeably unstable in the south-western quarter of the country as well as on the outskirts of Corsica. In fact, in these regions, winter 2021 frequently comes in second among the most unstable winters, and this is where the anomalies of days with thunderstorms were the most significant. North of the Loire, the excess is generally more modest, and it has been associated with configurations that are not conducive to thunderstorms; therefore, the excess instability more rarely resulted in thunderstorm activity.
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Latent instability anomaly (MUCAPE) during winter 2020-2021
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What weather regimes have dominated this winter?
On average, during these three months, the synoptic configuration was clearly dominated by low pressure conditions over western Europe. Thus, at altitude, we note the presence of low geopotentials centered on average between Ireland and Brittany (map below left), and high geopotentials set between eastern Canada, Greenland and Svalbard. In this configuration, the disturbed oceanic flow was regularly active, and well supplied with cold air on the middle floor, which caused repeated rainy episodes, sometimes very windy, followed by active trains which bring together most of the days. thunderstorm this season.
If one places oneself at ground level (reduced pressure at sea level, below right), we note a configuration very similar to that observed at altitude. The position of the most marked low pressure anomaly (between the Channel and Ireland) favored on the one hand periods of disturbed and agitated oceanic weather, milder than normal and very rainy; but also temporary intrusions of polar continental air masses from Scandinavia and Russia, when low pressure lows temporarily moved to somewhat lower latitudes. In fact, in these configurations, the north of France is subject to east to north-easterly winds, which have carried, especially in February, dry and cold weather.
Geopotential anomaly at 500 hPa (left) and reduced pressure at sea level (right)
A mild and unstable winter
We notice that excess instability observed in France (orange tint on the map below on the left) concerned the whole of Europe, in connection with temperatures that are often too mild in the low layers and, on the contrary, cooler circulation on the middle floor . On the scale of the northern hemisphere, the situation is however more contrasted, with abnormally stable conditions in Russia and the United States in particular. There are also areas that are significantly more stable than normal over the Horn of Africa or the southern tropical Atlantic.
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If the temperature on the ground was in excess during this winter in France (with an average anomaly close to + 1 ° C), the temperatures at altitude showed negative deviations from the normal on the contrary. Thus, at the temperature of the air mass at 850 hPa (around 1,500 meters above sea level), the map below on the right shows a negative anomaly of about -0.5 ° C. This fresh anomaly also concerned the British Isles, and extended, in a much clearer way, to the Canaries. Unlike winter 2019-2020, conditions are significantly colder than normal throughout Russia. It is nevertheless the mildness that has prevailed over a large part of Europe and Africa, and even more between Canada and Greenland where the seasonal anomaly is remarkable and exceeds + 4 ° C.
Latent instability anomaly (left) and temperature at 850 hPa (right)
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