Heat records in the USA: a historic December

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

25 to 37 ° C recorded in early December

Meteorological winter got off to a resounding start in the USA with serial 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 so hot on a December day since the start of the surveys weather report. Temperatures recorded are historic in these northwestern states: 25.5 ° C in Jordan, Montana and Buffalo, Wyoming on 1^er 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, i.e. a difference of 17 to 20 ° C with the temperatures recorded! In California, it was recorded as high as 37 ° C in early December in Palm Springs (compared to an average of 21 ° C usually in December), breaking 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 1^er December, this is the highest temperature ever recorded in the entire country for a month of December, breaking the previous record of 1982. Across the American West, both United States and Canada, this month of December 2021 looks like to be mistaken in a month of July!

A dome of heat like in the middle of summer

The reason for this unprecedented heat at this time of year is linked to a weather lockdown situation, with a powerful high pressure system trapping the heat in place for over a week. It is a real dome of heat present in the upper atmosphere, like the one that already concerned western North America last summer, promoting 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, for its part, be linked to the jet stream circulation which traveled further north than usual. The repetition of these domes of heat, in summer as in winter, could be one of the direct consequences of global warming.

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

The consequences: fires and parasites

Just like in midsummer, all the weather conditions are there to cause fires. In Montana, these extreme temperatures, associated with the drought, favored the development of prairie fires, an exceptional situation in the middle of December. In Denton, Montana, the fires spread to a farm and around 20 homes were devastated. The drought that has lasted for ten years throughout the American West would be the worst listed in these regions for 1,200 years.

Apart from the recreational and economic consequences, on the increasingly shortening ski season, in Montana, Wyoming and Colorado in particular, nature too must adapt to this warmer and drier climate. The deficit of snow has a direct impact on the water level of rivers and streams on which biodiversity is totally dependent. The late arrival of the cold 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 summer and is lulled to sleep by the cold.

According to the latest forecasts, it would seem that, despite a few temporary colds and several snow storms, the month of December is prolonged in an unusually hot way. For the rest of the month, the heat anomalies are expected to affect more the 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 across the United States.