According to specialists, the storm that rages over the United States a few days before the Christmas holidays only happens “once in a generation”.

The heavy snowfalls and polar temperatures are brought on by a weather phenomenon called a "low pressure bomb", caused by a depression that is deepening very quickly but whose intensity is made historic by freezing arctic air.

Several cold records have thus been broken: -53°C in Western Canada, - 38°C in Minnesota, but also, further south, where temperatures are usually milder at this time of year. , -13°C in Dallas or -8°C in Houston.

Its causes are however quite classic, namely a low pressure system, caused by a powerful conflict between two masses of air: one very cold from the Arctic and the other tropical from the Gulf of Mexico.

What makes the current case extraordinary is that the atmospheric pressure this time dropped very quickly, in less than 24 hours.

Felt temperatures down to -55°C

It is the same phenomenon which had been at the origin, in France, of the storm of 1999 remained in the memories, or of Xynthia in 2010.

“This led to the formation of extreme storm conditions near the heart of the depression, with particularly harsh conditions”, marked in particular by very strong winds, blizzards and snowfall, indicates Cyrille Duchesne, meteorologist at the Chaine Weather report.

For him, the rarity of this storm does not come so much from its causes but from its intensity and the extremely low temperature level.

Quebec will be affected this weekend

What is this due to?

Quite simply to the fact that the storm causes a plunge in the polar vortex, that is to say a mass of particularly cold air coming from the Arctic, towards the temperate latitudes, carried by the undulation of the jet stream (current of altitude) and favored by anticyclonic conditions in the Pacific.

Result: sometimes vertiginous drops in temperature have been observed.

Combined with the blizzard and the snow, the feelings on the plain can sometimes reach -55 degrees.

"A cold of this magnitude could cause frostbite on exposed skin within minutes, as well as hypothermia and death if exposure is prolonged," warned the US National Weather Service.

And this makes any movement “dangerous or even sometimes impossible”.

For this weekend, the depression, currently located at the level of the Great Plains and the Great Lakes, should move towards Quebec.

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