We’re dealing with a “historic cold wave,” explains Weather Channel meteorologist Cyrille Duchesne. Combined with the blizzard and the snow, the feelings on the plain can sometimes reach -55 degrees.
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The heavy snowfall and polar temperatures hitting the United States as Christmas approaches is due to a weather phenomenon called the “low-pressure bomb,” caused by a low-pressure depression that deepens very quickly, but whose intensity has been made historic in freezing Arctic air.
“Historic Cold Wave”
This type of storm occurs “only once in a generation,” according to the US National Weather Service in Buffalo. We’re dealing with a “historic cold wave,” explains Weather Channel meteorologist Cyrille Duchesne.
Thus, several cold records have been set: -53°C in western Canada, -38°C in Minnesota, but also further south, where the temperature is usually milder at this time of year., -13°C in Dallas or -8°C 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.
A depression is a system of low atmospheric pressure, often synonymous with bad weather: its dynamics lead to updrafts that cause clouds and precipitation.
“Explosive Genesis”
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 resulted in the formation of extreme storm conditions 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. »
What causes this? Quite simply to the fact that the storm causes a dip in the polar vortex, that is, a particularly cold air mass coming from the Arctic, towards temperate latitudes, carried by the jet stream waveform (high altitude current) and favored by anticyclonic conditions in the Pacific Ocean.
Result: dizzying temperature drops have sometimes been observed, as in Denver, where the thermometer dropped 33 degrees Thursday in just under 7 hours, registering -31 degrees its coldest temperature since 1990. Combined with the blizzard and snow, emotions were sometimes in plain sight reach -55 degrees.
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