Why are the bad weather in the Alpes-Maritimes exceptional?

In the Roya valley, it is a landscape of devastation.

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Three dead in Italy, eight people missing in France, destroyed roads and houses, villages in the upper Nice country cut off from the world, and probably for a long time: it is the toll, still very provisional, of the bad weather that struck the Alpes-Maritimes, between Friday and Saturday.

What really happened in the skies of south-eastern France and why it is exceptional.

wanted to know more from Frédéric Nathan, forecaster at Météo-France.

What happened in the Alpes-Maritimes between Friday and Saturday?

"We can speak without problem of an exceptional phenomenon," said Frédéric Nathan, forecaster at Météo-France, 20 Minutes away.

It confirms the feelings of the inhabitants of the Alpes-Maritimes who, at the height of their 40 or 50 years, explain never having seen that.

And for good reason: 500 mm of water fell in just twenty-four hours in the town of Saint-Martin-Vésubie.

That is three months of usual precipitation in this season in the region.

This is a new record for the department.

These torrential rains caused spectacular flooding which washed away dozens of houses and raised fears of a terrible human toll.

But “there were really two different phenomena.

The storm Alex on the one hand which formed during the night from Thursday to Friday and on the other hand the Mediterranean episode, ”insists Frédéric Nathan, the weather forecaster for Météo-France interviewed by

.

“Storm does not mean a Mediterranean episode, there are plenty of storms without a Mediterranean episode and vice versa.

Most Mediterranean episodes do not happen with storms, ”he adds.

We must therefore detail the two phenomena to understand the exceptional nature of the event.

What is a Mediterranean episode?

It is a phenomenon that is well known around the Mediterranean.

We also call that a Cévennes episode if it happens at the level of the Cévennes but it is the same weather phenomenon.

It happens every year, in the fall, "between one and eight per year", specifies Frédéric Nathan.

It happens in the fall because it is a time when the Mediterranean Sea is still very warm and when at the same time masses of fresh air how to arrive "from northern Europe, Iceland and Scandinavia" .

And, we have known since elementary school, when hot and cold meet, in weather, there is electricity in the air.

Literally.

“When we have cold air at altitude which comes to position in the Mediterranean, it gives a lot of instability because of the temperature difference between sea level and altitude.

This causes the development of stormy clouds, cumulonimbus, ”explains the forecaster.

However, the temperature difference is so great between this mass of cold air and the Mediterranean that the condensation is very strong and this causes phenomenal rains.

For example, this would be impossible in Brittany where, at the same time of year, the sea, around 14 degrees, is much colder.

Less condensation, so less water to pour.

What's the storm Alex doing in there?

Storm Alex, which had already hit Brittany and the Atlantic coast earlier, occurs here in three stages.

First, the cold air which allowed the formation of the Mediterranean episode comes from the storm, quite simply.

But it had two aggravating effects.

Firstly, because its winds created a southerly flow.

“The cold air came down on the Mediterranean and started to give a southerly flow towards the Alpes-Maritimes and the thunderstorms got stuck there from Friday at midday and started to spill quantities of 'water that we saw,' notes Frédéric Nathan.

It is a "classic" effect.

Because a Mediterranean episode doesn't need a storm to be strong.

A classic southerly wind can give the same effects: “At that moment, the stormy clouds will block themselves at the level of the reliefs and renew themselves for hours and hours giving stationary thunderstorms with, therefore, quantities of phenomenal water.

"

Second aggravating effect: the force of the winds.

If, as we have said, the wind is not necessary for the formation of a Mediterranean episode, when we arrive at strong winds that changes the situation.

“We had strong winds, up to 160 km / h in the Alpes-Maritimes.

This favored the flood phenomenon since the water could no longer flow normally towards the sea. Because the wind rubs on the water, it causes the sea to rise to the level of the river mouths and therefore prevents the normal flow of water. rivers ”, explains Frédéric Nathan.

It is thus easy to understand how the raw material phenomenon was aggravated in these circumstances.

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