Weather: What is a supercell thunderstorm, like the one that hit Langon on Saturday?

Far from the "thunderclap" at the end of a summer day, a supercell thunderstorm is a special type of storm, considered the most dangerous and devastating in the world . This rare phenomenon can be accompanied when it forms by very strong winds, intense precipitation, large hail, and even tornadoes, and carries a risk of flash flooding.

Its structure distinguishes it from classic thunderstorms: forecasters speak of a "supercell" because this type of thunderstorm forms from a single, immense storm cell (a sort of autonomous core in which all the storm's reactions take place: rising hot air, condensation, cloud formation, rain, etc.). Unlike most thunderstorms, which rarely last more than an hour, the supercell is organized, stable and long, and can live for several hours while traveling hundreds of kilometers.

At the heart of a supercell thunderstorm is also a mesocyclone , a column of air that rises and spins like a huge top. This rotation makes the storm both powerful and unpredictable.

A thunderstorm, whatever it may be, always arises from the same process: warm, humid air at ground level rises into the atmosphere, encounters colder air at higher altitudes, which causes condensation, the formation of clouds (often cumulonimbus, the large anvil-shaped clouds), then precipitation and sometimes thunderstorms.

But in the case of a supercell, three additional ingredients must come together:

• Very strong atmospheric instability : the faster the warm and humid air rises, the more powerful the storm will be.
• Significant wind shear, i.e. a change in wind speed and direction with altitude (for example, the wind can blow both from the south at the ground, and from the west at 2,000 meters above sea level), which causes a rotation of the rising air column, the famous mesocyclone.
• Sufficient humidity and favorable atmospheric dynamics to fuel the storm over a long period.

As Météo-France explains, "the supercell is the only type of storm in which a persistent rotation (the mesocyclone) develops."

Supercells are capable of producing several types of extreme phenomena at the same time:

Hail : Strong air currents rising within the cloud allow the hail to grow considerably before falling. Hailstones sometimes reach more than 5 centimeters in diameter , making them capable of breaking windshields or damaging roofs.

Tornadoes : Some supercells can spawn tornadoes, particularly when the internal rotation of the winds is very pronounced. In France, they are rare, but not non-existent. In 2022, a tornado originating from a supercell hit Bihucourt, in Pas-de-Calais.

Very strong winds : Supercells can generate downward gusts, called downbursts, with winds that can reach up to 150 km/h.

Flooding : Due to the slow movement of some supercells, very large quantities of water can fall on the same location in a short time, which causes rapid rises in water levels on saturated ground.

Forecasting a typical thunderstorm is already complex, but anticipating a supercell is even more so. Meteorologists analyze weather radar images, satellite images, and numerical forecast models in real time, looking for certain characteristic signs. Keraunos notes, for example, that a supercell thunderstorm "appears very clearly on Doppler radar by the juxtaposition of winds from opposite directions," which is manifested by the presence of a hook-shaped echo "on the southeast flank of the storm."

Forecasters sometimes can issue an alert a few dozen minutes in advance, but this accuracy remains limited. This is why Météo-France's weather alerts are important to follow, especially on days predicted to be unstable.