As at the start of each month, we offer you a climatological report of the past month. So let’s take a look at the mapped balance sheet for the month of SEPTEMBER 2023 in terms of temperature, rainfall and sunshine over a panel of 73 stations*. The statistics are calculated under the official climate average for the period 1991-2020.
“20“. It is now the number of consecutive months where temperatures were not below average. A historic series, just like this month of September 2023.
Because in fact, with a national thermal indicator having reached 21.1°C (i.e. a very high anomaly of +3.6°C compared to the 1991-2020 average), it is quite simply the hottest month of September ever measured in France since at least the post-war period (start of reliable readings). This dethrones the old record of September 1949 (20.3°C, anomaly of +2.7°C) followed in 3rd position by September 1961 (19.9°C, anomaly of +2.4°C).
Average national heat indicator in September since 1946 – Infoclimat
A month that can clearly be considered summery, worthy of summer. The first decade was absolutely incredible, also the hottest ever measured (23.98°C average or +5.1°C anomaly), more than 2°C beyond the previous record of a first decade, and with monthly records broken on several hundred stations. THE September 4 and 9the national indicator even reached 25.1°C, the two hottest days ever measured in September. On the afternoon of the 4th, we observed more than 37°C in Poitiers and Angoulême, and even 39.0°C in Le Blanc (Indre) and 39.3°C in La Trimouille (Vienne), barely believable values past the heart of summer (event summary >>).
Temperatures remained well above seasonal values until the very beginning of the 3rd decade for the autumn equinox, then marked by rather cool or even cold mornings (>>). There first frost in the plains of the season is even observed at Ménat (Puy-de-Dome) on the morning of September 24. A timid coolness quickly swept away by the return of the heat at the end of the month (before the start of October which promises to be just as historic…)
Daily temperature anomaly in September 2023 in France – deviation from the 1991-2020 average – Infoclimat
This historically hot month is found in all the cities in our panel which were all above average. Only the South-eastern quarter of the country remained between +1 and +2°Cwhich already remains notable (+1.4°C in Ajaccio and Bastia, +1.6°C in Hyères, +1.8°C in Montpellier). Everywhere else, anomalies exceed +3°C, and even reach frightening values of +4 to +5°C from Charentes to Alsace, via Centre-Val-de-Loire, Ile-de-France, Massif-Central and even Burgundy : we reach on our panel up to +5.0°C in Bourges and Nevers, +5.1°C in Clermont-Ferrand and even +5.2°C in Luxeuil (in the latter city, the anomaly for maximum temperatures even being of the order of… +7°C!).
On the Météo-France secondary network, the anomalies are there too all above +1°C for the whole country (minimum difference of +1.1°C in Toulon (Var), Figari (South Corsica) or Castanet-le-Haut (Hérault)). The maximum anomaly, quite exceptional, climbs up to +5.9°C in Lormes (Nièvre) and Vitry-sur-Loire (Saône-et-Loire)which is a warmer month of September than the usual months of July or August!
As for the rains, although the results are quite mixed, it remains deficit on a national scale of around -17% on our range of stations.
The first decadeunder the high heat and high pressure, had in fact proven to be very dry throughout France. A situation that we found for the very last days of this month. Between the two, we had reconnected with a little restlessness, first of all at the start of the second decade with the return of strong instability and storms putting an end to the intense heat (sometimes strong and even accompanied by hail >>).
The The high point of this agitation nevertheless occurred between September 16 and 18 : first with an iimportant Cévennes and Mediterranean episode in the upper cantons of Hérault on the 16th locally bringing more than 500mm in 24 hours. On the 17th, a large part of the country was affected by strong thunderstorms (31,121 lightning strikes during the day, the most lightning strikes in September since measurements began in 1997), with even one tornado observed in Mayenne. On the 18th, it was the turn of Ardèche to be affected by a rainstorm episode causing flooding in the north of the department (summary of the entire episode >>). A rainy front then brought sustained rains particularly in the West and the North-West on September 20 and 21 (>>).
In the end, it is indeed the Western half which received the most precipitation overall (except the Cévennes sector) with anomalies sometimes greater than +50% in New Aquitaine (+78% in Niort, +84% in Biarritz) and in Pays-de-la-Loire (+70% at Le Mans). If certain sectors have also collected enough rain to exceed their monthly average (Ardennes, Picardy, Val-de-Saône, South of the Alps), it is indeed a good time mostly dry which predominated over the rest of the regions, particularly in South and East where deficits have widened from -40% to -70%. It is in Occitanie, Haute-Corse and the Grand-Est where the deficits were the most significant (-73% in Metz and Saint-Girons, -76% in Millau, -78% in Montpellier, -86% in Bastia).
Despite these two very deficit values in Montpellier and Millau, it is between these two cities where we observed the most remarkable surplus, with +296% (almost 4 times the average) in Les Plans (Hérault) due to the significant rain-storm episode of September 16. On the deficit side, it is Tarascon (Bouches-du-Rhône) which holds the national prize on the secondary network with -97% (almost sec).
These surpluses noted in several cities in the western half result in accumulations often between 70 and 100mm across the entire Atlantic coastreaching 110mm in Niort… and even 218mm in Biarritz (panel maximum). Rain also in the middle of the month in Rhône-Alpes allowed it to exceed 100mm at the main station of Grenoble-St-Geoirs (103mm). For the lowest accumulationsthey are therefore noted in Occitanie, Auvergne, Grand-Est and Upper Corsica with often less than 30mm collected during the entire month of September (23mm in Toulouse, 21mm in Strasbourg, 20mm in St-Girons, 19mm in Montpellier, 18mm in Millau, 17mm in Colmar and only 10mm in Bastia).
On the secondary network, the two extremes are for the same two stations mentioned above: up to an impressive 564.7mm at Les Plans (Hérault) under the episode of September 16, against a very small 2.6mm in Tarascon (Bouches-du-Rhône).
The sunshine also testifies to the unusual and very summery character of this month of September, since theanomaly is excess of approximately +21% on our range of stations. This is mainly explained by an anticyclonic and very sunny first decade (and the very end of the month), and the absence of a real lasting ocean flow (very temporary at mid-month).
The end of the 2nd decade and beginning of the 3rd, which was a little more gloomy and oceanic, made it possible to limit this excess on theWest and especially the North-West of the country, not exceeding +10%. In some places, it was a month almost in line with the average: +3% in Biarritz and Brest, +2% in Rennes and Caen, and even a very insignificant deficit in Lorient (-1%).
A surplus too not very noticeable near the Mediterranean, often between +5 and +15%.
Elsewhere, Pyrénées in the North-East, this month of September can be tagged as very sunny compared to usual. The surpluses are of the order of +15% to +35% between Pyrenees and Massif-Central, but reach +40 to +60% in the Grand-Est (+52% in Nancy and Strasbourg, +58% in Colmar).
In terms of total sunshine duration, this translates into values ranging from 250 to 280 hours accumulated on average in 2/3 of France (East and South-West of the country): up to 284h in Carpentras and 282 hours in Marseille-Marignane for the highest national sunshine levels, but certain cities in the Rhône valley and even in the North-East are not left out and go the distance (278 hours in Lyon, 270 hours in Colmar, 268 hours in Luxeuil).
As during a good part of the summer, it is once again the North-West quarter and in particular the vislands bordering the Channel which remain at the back of the pack (although at the level of season averages), not exceeding 200 hours of sunshine : 174 hours in Caen, 172 hours in Saint-Brieuc, and one national minimum of 163 hours near Brest.
Summary:
* PANEL OF 73 STATIONS
Temperature – rainfall – sunshine:
Agen, Ajaccio, Albi, Alençon, Angers, Aurillac, Beauvais, Bergerac, Besançon, Biarritz, Bourg-Saint-Maurice, Bourges, Brest, Brive, Caen, Carcassonne, Charleville-Mézières, Chartres, Châteauroux, Cherbourg, Clermont-Ferrand, Cognac, Colmar, Dijon, Embrun, La-Roche-sur-Yon, Langres, Le Mans, Le-Puy-en-Velay, Le Touquet, Limoges, Lorient, Luxeuil, Lyon-Bron, Mâcon, Marseille-Marignane, Melun, Millau, Mont-de-Marsan, Montélimar, Montpellier, Nancy-Essey, Nantes, Nevers, Nice, Nîmes-Courbessac, Niort, Orléans, Paris-Montsouris, Perpignan, Poitiers, Rennes, Saint-Brieuc, Saint-Etienne, Saint- Dizier, Saint-Auban, Saint-Geoirs (Grenoble), Saint-Girons, Strasbourg, Rouen, Tarbes, Toulouse-Blagnac, Tours, Troyes.
Temperature – rainfall (partial or total absence of sunshine data):
Abbeville, Bastia, Bordeaux, Lille, Metz, Hyères, Romorantin, Saint-Quentin.
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