The heat wave affecting the whole of France at the beginning of September is unprecedented for several reasons. We take stock of this exceptional episode.
A textbook case of omega blocking
“Omega” blockages are nothing new. However, the one that formed at the beginning of November in Western Europe is a real textbook case and is proving to be very powerful. This Monday, September 4, 2023, two small-scale low pressure areas are present off Portugal and towards Greece. Between the two, a powerful anticyclonic ridge rises from the Maghreb towards the North Sea, bringing up very hot air. As these centers of action stand still (the reason for which we speak of blockage), the mass of hot air can accumulate and remain permanently over France, leading to an impressive heat wave.
Situation on a continental scale this Monday, September 4, 2023 – via polarwx.com
Proof that the air which concerns France comes to us directly from Africa, we noted the presence of sand dust from the Sahara in quantity in the west of France this Monday, September 4, 2023. The concentrations are significant from the south-west to the tip of Brittany, as the impressive satellite image below reveals. These regions experienced whitish or yellowish skies with a pale sun.
Satellite image showing Sahara sand over western France Monday, September 4, 2023 – via Copernicus
A remarkable mass of air
It suffices to look at the temperatures of the air mass to understand the remarkable intensity of the hot air currently affecting France. On this Monday, September 4, temperatures at 850 hPa (around 1500 meters above sea level) exceed 20°C over almost all of France and even climb to over 25°C in the southwest ! These levels are worthy of the major heat waves occurring in the middle of summer. The fact that we are slightly below the heat wave thresholds lies only in the fact that the days are much shorter in September. Such a situation in July would have given rise to a very intense heat wave.
Air masses around 1500 meters this Monday, September 4, 2023 – via wxcharts.com
The map below highlights areas where air mass temperatures are at levels never seen in early September. These appear in pink and reveal that the situation of this Monday, September 4, 2023 is unprecedented in France! Moreover, the air mass is also overheating to remarkable levels over the whole of the British Isles. These are all the regions located at the heart of the omega blockage that are experiencing an extraordinary situation.
Area where the air mass is record (in pink) for the season this Monday, September 4, 2023 – via polarwx.com
Many records broken
This Monday, September 4, 2023, several areas in the south and west of France experienced their hottest night ever observed in September! In Tarn-et-Garonne, Montauban even experienced its hottest night since records began with 24.4°C, beating the 23.9°C of August 2003! Among the monthly records, we note Toulouse with 24.2°C, Gourdon with 23.5°C or Agen with 22.2°C. The north-west was not left out with records in Vannes (21.8°C), Nantes (21.6°C), Île d’Yeu (21.2°C) and even Quimper (20.5°C)!
Minimum temperature records this Monday, September 4, 2023 – Weather Cities
A rain of record heat for the month of September was observed on Monday! He did until 38,8°C in Montmorillon in Vienne and 38.0°C in Issoire in Puy-de-Dôme! Among the exceptional values of the day, we can note 37°C in Poitiers, 36.8°C in Cognac, 36.6°C in Niort, 36.4°C in Bourges or 32.7°C in Vannes. Note that records have also fallen in the Alps with 22.4°C in Tignes in Savoie, a resort located at an altitude of 2084 meters! The national thermal indicator has reached 25,1°C this September 4, 2023. It is the first time that it reaches 25°C in September, proof of the unprecedented nature of the current heat!
Maximum temperature records this Monday, September 4, 2023 – Weather Cities
An unprecedented duration in September
While heat peaks are still frequently observed in September, a lasting heat wave during this month is much rarer. With temperatures that will remain between 30 and 35°C in most French regions until the end of the weekend of September 9 & 10, 2023, the weekly anomaly could reach +6 to +8°C from the center to the northwest of France! Such excesses over normal over a 7-day period are nothing short of remarkable and will make this the hottest September week on record!
Thermal anomaly during the week of September 4 to 10, 2023 – via ECMWF
The example of Paris is particularly telling. The capital has just recorded its third consecutive day at more than 30°C (between 30.4°C and 31.9°C) and the heat will increase further this Tuesday, September 5, 2023 with an expected maximum of 34° VS ! According to forecasts, the 30°C mark will be crossed daily at least until next Monday 11 September. This would mean that Paris would experience at least 10 consecutive days of high heat, which has never happened in September! By way of comparison, the capital recorded “only” 8 in the months of July and August combined!
Temperatures measured and forecast in Paris in September 2023 – via infoclimat.fr & Weather Cities
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