What is the historically intense “low pressure bomb” hitting the US?

The US is facing a historic cold snap on Christmas Eve with temperatures that could drop below -50 degrees. In question: a meteorological phenomenon called “low pressure bomb”. Explanations

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The heavy snowfall and cold temperatures hitting the US as Christmas approaches is due to a weather phenomenon known as the “low pressure bomb”, caused by a depression that deepens very quickly but whose intensity is made historic in freezing Arctic air. This type of storm does not intervene only once per generation according to the US National Weather Service in Buffalo.

We are dealing with a ” historic cold “, explains Cyrille Duchesne, meteorologist at the Weather Channel. Several cold records have thus been broken: -53ºC in western Canada, -38ºC in Minnesota, but also further south, where the temperature is usually milder at this time of the year, -13ºC in Dallas or -8ºC in Houston.

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

“Explosive Genesis”

A depression is a system of low atmospheric pressure, often synonymous with bad weather: its dynamics lead to upwelling that causes clouds and precipitation.

What makes the current case extraordinary is that this time the atmospheric pressure dropped very quickly, in less than 24 hours. This is called an “explosive genesis”. This is evident when the pressure drops more than 20 hectopascals (hPa) in 24 hours in mid-latitudes. It is the same phenomenon that had been the cause, in France, of the memorable storm of 1999, or Xynthia in 2010.


In the case of the United States, atmospheric pressure suddenly dropped by 40 hPa in 24 hours, according to Yann Amice, meteorologist at Weather’n’co. ” This caused extreme storm conditions to form near the heart of the depression, with particularly severe conditions “, which is particularly characterized by very strong winds, blizzards and snowfall, states Mr Duchesne.

For him, the rarity of this storm comes not so much from its causes – a ” depression bomb, it happens regularly – but by its intensity and the extremely low temperature level. ” That’s what makes it extraordinary. »

Risk of death in case of prolonged exposure

What causes this? Quite simply because the storm leads to a dip of the polar vortex, that is, a particularly cold air mass coming from the Arctic, towards the temperate latitudes, carried by the wave form of the jet stream (altitude current) and favored by anticyclonic conditions in the Pacific Ocean. Result: sometimes dizzying temperature drops have been observed, as in Denver, where the thermometer dropped 33 degrees on Thursday in just 7 hours, registering -31 degrees its coldest temperature since 1990.

Combined with the blizzard and the snow, the feelings on the plain can sometimes reach -55 degrees. ” Cold of this magnitude can cause frostbite on exposed skin within minutes, as well as hypothermia and death if exposure is prolonged. “, warned the US National Weather Service. And so does every trip dangerous or sometimes even impossible “.

For this weekend, the depression, currently level with the Great Plains and Great Lakes, should move toward Quebec, which risks ” to be very moved ” with particularly strong winds and heavy snowfall, and to a lesser extent towards the north-eastern United States, states Mr Duchesne.

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