Of
Thursday October 24 at 5:00 p.m. au
Sunday October 27 at 12:00
Situation
A new episode of heavy, lasting stormy rain is gradually setting in this Thursday and Friday, with an increase in stormy showers between the Cévennes, the PACA region and Corsica. This start of the episode remains quite dispersed and of moderate intensity.
The Cévennes mountains have the highest accumulations until Friday evening, out of all proportion to last week's episode.
But from Saturday to Sunday noon, another much more intense episode will affect the PACA region and Corsica, remaining moderate in the Cévennes. Saturday, and particularly the evening, seems to be the period most at risk.
This stormy rain episode could persist on Sunday and Monday, shifting towards Languedoc-Roussillon. In this case, the heavy rainfall could lead to new floods, but not in the same areas as last time.
The risks linked to this heavy rain are new overflows in the Cévennes, but more moderately than last week, and more to the south of the massif. We also fear flooding during stationary storms in the lower Rhône valley.
Then, we must remain very attentive to the risk of runoff and a very high risk of flooding on Saturday in the PACA region, a region which seems to us to be the most threatened by these bad weather.
Observation
Throughout this Thursday afternoon, heavy showers, sometimes stormy, followed one another from the Camargue coast to Ardèche, in the Rhone corridor.
Scattered storms also broke out in Provence and the Var. Hourly accumulations, very heterogeneous, varied from 3 to 10 mm, which could generate some runoff on the roads.
These rains now tend to settle more on the Cévennes relief. Other stormy showers move up during the night, from the open sea, towards the Côte d'Azur and the south of Corsica.
Evolution
Until the night of this Thursday to Friday : stormy precipitation remains frequent and sustained in the foothills of the Cévennes, mainly between the north of Gard and the south of Ardèche. A few thunderstorms may still occur locally on the Gard plain and the Montpellier region.
This Friday morning : precipitation loses a little intensity except on Corsica and the Côte d'Azur where storms persist on the coast. The most sustained showers are 5 to 10 mm/hour between Gard and Ardèche and sometimes 15 mm/h on the coast of Corsica and the Var.
Friday afternoon and evening : the weather remains unstable from Languedoc to the Cévennes with sometimes sustained stormy showers. The foothills of the Cévennes remain the most exposed to heavy rain, while the intensities are more moderate on the Languedoc plain. Thunderstorms are still quite strong affecting eastern Corsica and spreading again to the Côte d'Azur. These slow moving storms can dump 40 to 50 mm of rain in 1 hour. Depending on the affected area (urbanization, relief), this may lead to runoff.
Saturday will be the day most at risk of this episode, in the PACA region and eastern Corsica. Thunderstorms regain intensity on the Côte d'Azur from the night of Friday to Saturday, coming up from the sea. The storms will follow one another all day long. Stationary but very localized storms should also break out in eastern Corsica, but the precise location of which still remains to be refined.
In this context, watercourses could experience strong reactions (Var and Argens watersheds for example). The risk of flooding will become significant.
During this time, the rains will also continue in the Cévennes but with a much less intensity than in PACA and last week. Rivers may experience significant rises but more spread out over time and lower than last week.
The critical period will be Saturday evening and night from Saturday to Sunday in PACA and eastern Corsica. A fairly general lull should be felt from Sunday morning, but the episode will perhaps not be over. In fact, new storms are expected on Sunday evening in these same sectors. But this Sunday development still remains unreliable and will require confirmation in our next bulletins.
Here are the characteristics of this episode of heavy rain expected between this Thursday afternoon and Friday, initially:
– cumulative rainfall over the entire episode of 80 to 120 mm in the Cévennes, locally 150 mm
– cumulative rainfall of 30 to 50 mm in the Languedoc plain,
– a risk of strong thunderstorms over Corsica and the Var, with marked electrical activity and heavy precipitation over a short period of time.
– hourly intensities which can reach 20 to 30 mm on exposed reliefs
– moderate flooding on the rivers descending from the Cévennes.
– a risk of flooding in low areas, due to soils saturated by last week's torrential rains.
In total until Sunday noon, we can expect very abundant accumulations from time to time:
– from 150 to 350 mm in the Cévennes
– 100 to 200 mm in the plain of the lower Rhône valley
– from 150 to 200 m in the Var and the Alpes-Maritimes on Saturday as well as in eastern Corsica.
