We were talking about it a few days ago, and the situation is confirmed: the second half of October will be turbulent and above all very humid ! Unlike the summer-like first part of the month, autumn should set in for the long term, bringing with it its share of freshness and precipitation.
Rains until the end of the month in all regions
Uncertainty still remained a few days ago in the regions north of the Seine with the possibility of drier (and colder) weather at the edge of the Scandinavian anticyclone. This possibility now seems more and more ruled out, with agreement among the main models on a omnipresence of low pressures in our country, and a withdrawal of the anticyclone much further to the North.
Weekly atmospheric pressure anomaly from October 16 to 23, 2023 – CEP / ECMWF model
Therefore, it is our entire territory which will be confronted with this humidity, as evidenced by the two main medium-term models (American CFS model and European CEP model). Above all, this humidity will not be confined to the coming week alone, but probably to the entire second half of October (or even at the beginning of November).
In the two cases presented here, the southern regions and especially the South-Eastern quarter of France would be the most targeted by these heavy rains, particularly over the coming week of October 16 to 22.
Weekly rainfall anomaly from October 14 to 21 and October 21 to 28, 2023 – CFS model via Tropical Tidbits
Weekly rainfall anomaly from October 16 to 23 and October 23 to 30, 2023 – CEP model via ECMWF
It is difficult to accurately quantify the rainfall totals between now and the end of the month in the different regions. However, under such low pressure conditions in a westerly flow, it is very likely that we will exceed 100mm of precipitation over the next two in several regionssuch as the west facade, on the eastern reliefs (Alps, Jura, Vosges), on the Cévennes and locally on certain Mediterranean sectors.
Rainfall accumulation by October 31, 2023 – GFS model via WxCharts
If some can speak of real degradation, we can however reverse this term and evoke a “rainy improvement” in view of the ongoing drought across the country. With a particularly dry summer and early autumn, the most rivers in many countries are at low or very low levels for the season. At the beginning of October, 98% of river measuring stations had a flow rate lower than their average for the period 1990-2020with fairly critical thresholds for this time of year over a large central strip of the country.
Estimated river flow from October 2 to 8, 2023 compared to the 1990-2020 average – SCHAPI data via FranceInfo
Finally beneficial rains in the Mediterranean area
Let us now focus on the Mediterranean rim, which could experience a fairly hectic week. With a flow oriented south to southeast and a low pressure trough over the Iberian Peninsula, it is (at least) a Cévennes episode overflowing into a Mediterranean episode which should be gradually implemented over the coming days.
According to current models, this could be done at least in three times :
A first phase between Monday and Tuesday, mainly Cévennes as well as some possible rain on the Provencal coast; A second phase Wednesday a little more dynamic, with more continuous and abundant rain on the plainsaimed in particular at Alpes-Maritimes;
A third phase to be refined between Thursday and Friday where we could attend a more substantial episode (Cévennes, Provence Côte d’Azur and Corsica seeming more privileged).
L’lull would then seem privileged mainly by models for the weekend. The expected quantity of rain remains very difficult to determine, as the variations are significant as the low pressure system shifts (a few tens of km of differences can cause the total to vary by a factor of three). However, one thing seems more and more certain: Roussillon and western Languedoc could be away from the heaviest rain (example of Perpignan). For the other chosen cities (Nice, Marseille, Montpellier), the accumulations vary with numerous scenarios exceeding 50mm over the week, some even going to 150-200mm for Montpellier.
Rainfall accumulation until October 24 – multi-model comparison for Nice, Marseille, Montpellier and Perpignan – Meteologix
In any case, we can here also speak of “rainy improvement” in view of the drought which is raging in these regions. In three and a half months (since July 1), it only rained between 5 and 8 days on average on this Mediterranean rim, sometimes less (5 days in Marseille-Marignane or Sète, 4 days in Hyères and Alistro, 3 days in Figari). Apart from the Languedoc hinterland, the Vermeille coast, Alpes-de-Haute-Provence and the eastern coast of Corsica, it has not fallen no more than 80mm on average since the start of summer (very low totals in Toulon with 38mm, 35mm in Bastia, 26mm in Béziers-Vias and 24mm in Alistro).
Precipitation in the Mediterranean area from July 1 to October 14, 2023 – Weather-Cities
To follow the evolution of the situation at the national level, you can consult our expert forecasts daily updates on the site Weather-Cities (>>).
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