On a relevé plus de 25 °C au Wyoming et au Montana début décembre, des États connus pour leur temps habituellement glacial en décembre. © Pixabay

un mois de décembre historique



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Since 1er December, which corresponds to the beginning ofwinter meteorologically, the United States is affected by temperatures well above seasonal norms: up to 20°C above the average for season all over the American West! Of the anomalies thermal events that occurred in the mountainous areas of the Rockies of the northwest, known for their climate freezing in winter. Multiple records of heat have been beaten for about ten days, making December 2021 a historic month in terms of climate.

25 to 37°C recorded at the beginning of December

meteorological winter started with a bang in the USA with series heat records in the most unusual states: Montana, Wyoming, Washington and North Dakota, but also in Canada (British Columbia). For these states, it has never been this hot on a December day since records began weather report. Recorded temperatures are historic in these northwestern states: 25.5°C in Jordan, Montana and Buffalo, Wyoming on the 1er December, 21.5°C in Hettinger, North Dakota. For these cities, the average is around 0 to 3°C at this time of the year at the best of the day, a difference of 17 to 20°C with the temperatures recorded! In California, it was up to 37°C at the beginning of December in Palm Springs (against an average of 21°C usually in December), beating the previous record of 1949.

On the other side of the border too, the situation is the same: in Canada, with 22.5°C recorded in British Columbia on 1er December, this is the highest temperature ever recorded in the whole country for a month of December, beating the previous record of 1982. Across the entire western United States, United States like Canada, this month of December 2021 resembles to be mistaken for a month of July!

A dome of heat like in the middle of summer

The reason for the unprecedented heat at this time of year has to do with a weather-locked situation, with a strong anticyclone trapping the heat in place for over a week. This is a real heat dome present in the upper atmospherelike the one that already affected western North America last summer, favoring gigantic fires in Canada of the month of July. A weather blockage that follows an already historically hot summer: the hottest on record in the United States, nationally. The presence of this heat dome would be linked to the jet stream circulation which traveled further north than usual. The repetition of these heat domes, in summer as in winter, could be one of the direct consequences of global warming.

This heat dome is accompanied by very calm and very dry weather, since the high pressure blocks the lows. This situation favors the west winds which pass over the Canadian Rocky Mountains of Alberta, the famous winds chinook, which then descend from even hotter and even drier reliefs on the valleys, to Montana.

The consequences: fires and parasites

Exactly like in the middle of summer, all the weather conditions come together to cause fires. In Montana, these extreme temperatures, combined with drought, favored the development of prairie fires, an exceptional situation in the middle of December. In Denton, Montana, these fires spread to a farm and about twenty homes were devastated. The drought that has lasted for ten years throughout the American West would be the worst recorded on these regions for 1,200 years.

Apart from the recreational and economic consequences, on the increasingly shorter ski season, in Montana, Wyoming and Colorado in particular, nature also has to adapt to this hotter and drier climate. The snow deficit has a direct impact on the water level of rivers and rivers on which biodiversity totally depends. The late onset of cold weather also allows the “mountain pine beetle” to do much more damage: it is a small beetle which ravages the pines of the American West in the summer and which is put to sleep by the cold.

According to the latest forecastsit would seem that, despite some temporary cold snaps and several snow storms, the month of December is prolonged in an abnormally hot way. For the rest of the month, heat anomalies are expected to affect more southern and southeastern states (Florida, Texas, Alabama, Georgia…). In this case, it is possible that December 2021 will ultimately position itself as the hottest December on record nationwide.

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