Chance of severe thunderstorms next night and tomorrow Wednesday

Chance of severe thunderstorms next night and tomorrow Wednesday

We are now under the influence of a dynamic trough extending between Ireland and northern Spain. This thalweg quickly evolves into an altitude cut-off. It is a cold low pressure that lasts for about 24 hours between the Bay of Biscay and the west of France. The latter drives a southerly flow aloft in the Mediterranean regions. On the other hand, such patterns are quite unusual this early in the season, and should be noted to occur more often in the fall.

Another highlight: This low pressure system follows a summer heat wave. This afternoon was still very hot, with 32 to 34 degrees Celsius in the region. In short, the level of instability is very high near the Mediterranean. Next night and tomorrow Wednesday, sometimes severe thunderstorms – with the potential for flooding – can be expected from Languedoc to Provence via the Lower Rhône Valley. Here is the evolution of the synoptic context for the next night:

Degradation between Odin and Tarn will be rapid over the next few hours (early evening). In these areas, the storms will be fairly localized, but between Montagne Noire, Minervoise and north of Corbieres, violent outbreaks are possible: heavy rain, hail and wind. In these areas we can have accumulation of 30/50 mm/1h. The storms then reach Hérault (in the western part of the department) and progress to Garde the following night (the second part of the night, a priory).

Both these departments are subject to a virial dynamism because they are located at the left exit (very different) of the branch of the jet stream at high altitude. Also, a low tropopause anomaly will rise from the Gulf of Lion, allowing for a pronounced forcing. On the ground, or at least in the lower layers of the atmosphere, we find a high content of accumulated water (PWAT temporally > 50 mm, which is quite large). In short, developing storms tend to be less dynamic and tend to linger in a line with very noticeable rainfall intensities.

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The second part of the night is stormy in a large part of Hérault and Garde, in storms on the plains can reach up to 50 mm/1 hour, without excluding a water slide of 80 mm/h locally (probability of intense flow in cities. Regions). Accumulations will reach 120/150mm locally within a few hours, but between Herald and Guard. These are clearly local collections. A risky situation because of the potential for a weakly mobile storm system trapped in the flow. Flooding can be caused by runoff, especially since the plains are the most exposed, see seacoast.

The morning may be quiet before a fairly strong second salvo between noon and evening tomorrow. Then we may have new heavy and very rainy thunderstorms. In two salvos, we won’t rule out accumulations of up to 150/200mm somewhere east of Herault, Garde and neighboring Bouches-du-Rhône. These storms may create the risk of hail, strong winds and water bodies/tornadoes. Although 34/30 low plains are foreseen (near Montpellier, Lunell, Nîmes and Arles) the location of the strongest accumulations is imprecise at the scale of a municipality.

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