Storm Aurora: has Brittany been hit by a series of tornadoes? – Brittany

In Plozévet (29), a house under construction was crushed. In Plouguernével (22), uprooted trees have fallen in the courtyard of the public school, preventing the reception of children. In Kernascléden (56), a hangar flew away. In Riec-sur-Bélon (29), a twenty-year-old cypress collapsed in a garden. In the evening of Wednesday, very violent and localized wind phenomena accompanied the passage of the Aurora storm in Brittany. In the mouths of witnesses, the same word: “Mini-tornado”.

“This term does not exist”, corrects the Breton meteorologist Stéven Tual straight away. “This type of events taking place in relatively confined areas are either tornadoes or descending gusts”, replies Emmanuel Wesolek, president of Keraunos, the French observatory for tornadoes and severe thunderstorms. This design office specializing in forecasting and risk management has not yet decided between the two hypotheses concerning the Breton events of Wednesday evening. Using information from local contacts, he hopes to “see things more clearly in a day or two, probably.”

Conditions favorable to extreme events

Tornado and downward gust can indeed form under the conditions experienced by the region on Wednesday evening. Tornadoes, because Brittany was bathed in a mass of hot air “almost exceptional for the second half of October, with temperatures of around 20 ° C”, points out Stéven Tual. With the Aurora storm, a cold air front rolled in which generated heavy rains. “This cold air descending to the ground could have been coiled by hot air rising, creating a vortex. On touching the ground, it becomes a tornado, ”continues the expert from Météo Bretagne.

In Plouguernével, an agricultural building was gutted. (Photo Jean-Luc Chevance)

The other scenario, that of the descending gust, could have taken place in Brittany because very violent winds were blowing in all the thicknesses of the atmosphere Wednesday evening. “These winds with a speed of more than 150 km / h, present at an altitude of 2,000 to 3,000 meters, can be blown towards the ground, sometimes over a width of only 50 to 100 meters, when a front passes. and stir the mass of air, ”explains Emmanuel Wesolek.

The swirling winds of a tornado do much more damage than the constant, one-way winds of a storm

The analysis of the damage will make it possible to tip the scales towards one of the two weather phenomena. “For a tornado, it is often obvious. The corridor is generally narrow, particularly rectilinear, as if cut with a knife, that is to say that we do not see any damage on either side. We also observe projections from a distance, ”notes Emmanuel Wesolek. A descending gust has rather a tendency to be a “crushing phenomenon, the damage is relatively little projected and the trajectory can be a little less clear”, notes the president of Keraunos.

After analyzing the photos taken in Plozévet, Stéven Tual is categorical: “It is indeed a tornado, the wind corridor is quite distinct, the houses which overlook the one which collapsed have absolutely no damage” . No video has come to confirm this observation. Anyway, in a very hectic context like that of Wednesday evening, it is difficult to discern the characteristic vortex of a tornado in such a dense mass of rain.

An “outbreak” of returns?

If they are indeed tornadoes, they could, in view of the damage photographed, be classified as EF1 in the Fujita scale (which starts at EF0 and ends at EF5). These are events qualified as “moderate”: “Subject to any reservation, the winds may have been blowing around 150 and 170 km / h”, estimates Emmanuel Wesolek. “The swirling winds of a tornado do much more damage than the constant and unidirectional winds of a storm”, specifies Stéven Tual.

If at least four tornadoes occurred in an interval of a few hours, Brittany would have experienced an “outbreak”. A grouped event, “which goes beyond the norm, without being unheard of”, according to Keraunos. The design office also classifies Brittany among the risk areas in France, but to a lesser degree than Poitou-Charentes, Normandy and Pas-de-Calais.

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