This week marks the sudden arrival of a noticeable cool spell starting on Wednesday. It will be very cool with temperatures worthy of the end of October on Thursday and Friday. This cool spell will be accompanied by a cold snap in the mountains with snow and a risk of white frost in the plains at the end of the week. Here are all the parameters to watch out for so as not to be surprised.
A notable breath of fresh air for the month of September
This sudden arrival of autumn will be notable for the season, even if, usually, it is in mid-September that the first cold snaps occur in France. This was the case, for example, in September 2022 when the first frosts in the plains occurred on the 18th between Limousin and Ile-de-France. A little further away, September 2017 was 1°C cooler than average, with a very cold spell between September 14 and 20. In all cases, the great mildness had returned before the end of the month. In the plains, the dates of the earliest first frosts under cover are between October 4 and 6, as for Mulhouse and Orléans. In this case, white frosts in the countryside will be possible (i.e. at ground level and on cold surfaces such as car windscreens), but frost under cover (measured in a weather shelter 1.50 m above the ground) should be avoided, except at altitude.
Temperatures 5 to 10°C lower than seasonal averages
Notre thermal indicator indicates that the national temperature deficit will reach 3 to 6°C between Wednesday and Sunday, before slowly rising. The air mass anomaly, calculated at around 1500 m altitude, will reach 10°C from the Pyrenees to the Alps as well as in Switzerland, which highlights the intensity of the cold snap expected in the mountains. It is also noted that the cold air will plunge into the Mediterranean as far as the Maghreb. Due to the wind that will blow very strongly on the French coast (mistral, tramontane and westerly wind), the risk of thunderstorms will be excluded, but Italy risks experiencing new bad weather.
Temperatures worthy of the end of October
The coldest days of this cool spell will be Thursday and Friday. The average afternoon temperature will be 14 to 15°C across France. The 20°C will barely be reached near the Mediterranean. With a maximum of 13°C in Lille and Strasbourg, 14°C in Paris, Lyon and Orléans and 16°C in Bordeaux, temperatures will be 4 to 7°C below normal, at the level of late October. Even in September 2017 and 2022, daytime temperatures had not dropped as low in Paris, but an identical configuration occurred on September 25, 2020 in Lille and Strasbourg, in a roughly similar weather context.
Polar dropout in question
This great freshness is linked to the plunge of a polar air mass that will descend from Iceland to the Mediterranean, passing through France. At altitude, around 1500 meters, the temperature of the air mass above France will be around 1 to 3°C, explaining the lowering of the rain-snow limit. On the plains, showers Hailstorms will then be quite likely in a cloudy sky on Thursday and Friday.
A whole week of refreshing mistral and tramontane winds in the Mediterranean
Due to the deepening of a depression over the Gulf of Genoa, dominated by cold air on a warm sea, a lasting period of mistral and tramontane winds will set in from Monday. This wind will continue until next Saturday. It is therefore a whole week of strong winds that will affect the shores of the Mediterranean.
Snow in the mountains
As this polar air mass will be humid, of maritime origin, it will generate rain and showers. In the mountains, significant snowfall for the season is expected in the Alps and the Pyrenees from 1400 to 1800 meters on Wednesday and especially on Thursday. The highest peaks of the Massif Central could also turn white. In the Pyrenees and the Savoies, we could see around twenty centimeters at 2000 m altitude.
Towards very cool mornings at the end of the week and white frosts
From Thursday evening, the clearings should return, which will favor the nighttime drop in temperatures. The coolness will increase the following nights, especially during the night from Friday to Saturday. The presence of cold polar air at altitude and the clear night sky will accentuate the radiation, especially since the length of the night is now increasing very quickly. As a result, the first white frosts could appear on the plains on this occasion, between the interior of Normandy, Sologne and Limousin. Remember that white frosts can occur in clear skies even if the temperatures under shelter are slightly positive (up to +3°C), because the ground and surfaces cool more quickly than the ambient air.
- La Chaîne Météo
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