The Lower 48 states have seen record-shattering warmth so far this December, with temperatures running as high as 35°F above average for this time of year. The warmth has been so pronounced that during the weekend, brush fires broke out in a snowless, unusually mild Denver metro area.
The big picture: The jet stream, which is a river of air that rides at about 30,000 feet along the temperature contrast between air masses, steering storms as it goes, has been stuck in a position well north of the contiguous U.S., keeping storms and cold weather at bay.
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The prevailing jet stream has also helped direct a series of potent atmospheric river storms into the Pacific Northwest and British Columbia, where flooding devastated communities like Abbotsford, to the east-northeast of Vancouver.
Yes, but: There is a cold side to the jet stream, and it's been bottling up the frigid air in Alaska and parts of northern Canada. Some locations in Alaska were at least 20°F colder than average during November, with unusually cold temperatures forecast to continue through much of December.
According to the National Weather Service, several locations in southwest Alaska saw a top 5 coldest November.
This included King Salmon, where the average temperature for the month was just 4°F, setting a monthly record, and the aptly-named Cold Bay, which also set a record for its coldest November.
November was a cold month across much of Alaska – especially in southwest Alaska. These location all saw a Top 5 coldest November. Both King Salmon and Cold Bay recorded their coldest November on record! 🥶 pic.twitter.com/pv3LZMnDgF
— NWS Alaska Region (@NWSAlaska) December 1, 2021
On Dec. 4, a weather observer in Chicken, Alaska, recorded a temperature of minus-52°F.
To make things even more strange, a "Kona low" stalled northwest of Kauai has brought a surge of tropical moisture to Hawaii, which is falling in the form of wind-blown snow at the state's highest mountain peaks, including Mauna Loa and Mauna Kea.
This has meant that the longest duration blizzard warnings in effect for the past three days in the U.S. have been outside the Lower 48, in Hawaii and Alaska.
The bigger weather story in Hawaii, however, has been flash flooding at lower elevations, which is threatening heavily populated areas.
Context: Winter is the fastest-warming season across the U.S., and while a climate change trigger for the contiguous U.S. warmth has not yet been identified, the overall temperature trend does make such events more likely.
So too do cycles of natural climate variability, such as La Niña in the tropical Pacific, the intensity of the polar vortex, which is an ultra-cold low pressure area swirling about the North Pole at high altitudes, and the North Atlantic Oscillation.
Right now, all the climate variability cycles, from the Pacific to the North Atlantic, favor unusually mild winter weather during December across the Lower 48 states.
By the numbers: Four states tied or broke records for their hottest December temperatures: Washington, Montana, Wyoming and North Dakota, according to weather historian Maximiliano Herrera.
During the seven days ending on Dec. 3, there were 2,657 daily record highs set or tied in the U.S., compared to just six daily record lows, according to a NOAA database.
On Dec. 1, British Columbia saw its highest temperature ever recorded during the month of December, with a high of 72.5°F recorded in Penticton, which is 250 miles east of Vancouver. This tied the country's all-time record high for the month.
In Denver, Colorado, which often picks up heavy snow at the start and end of winter, it had not yet snowed as of Monday morning, setting a record for the latest measurable snowfall.
The temperature on Sunday was in the mid-70s. Combined with strong winds and dry air, this sparked multiple fires in the area, which is almost unheard of for this time of the year. The region has since cooled down considerably, though no snow has fallen, meaning the fire danger could flare up again.
What's next: Computer models are nearly unanimous in projecting a warmer than average December across the Lower 48 states, along with continued cold in Alaska.
Looking beyond this month, there are hints this pattern may continue if the polar vortex stays strong, which would keep Arctic air bottled up in the Far North.
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