Axios

A weather station mounted on a tide gauge on the picturesque island of Kodiak, Alaska, recorded an air temperature of 67°F on Dec. 26, which if verified would become the state's record high for the month.

The big picture: Such astonishingly mild conditions in America's Arctic state come at the end of a month that largely featured extreme cold across much of the region. The warm spell, which reached inland areas, turned snow into freezing rain that made roads sheets of ice early this week.

Why it matters: This freak temperature reading in a state known for its cold, snow and winter darkness puts a capstone on a year filled with extreme weather events in the U.S. and worldwide, many of which were made worse by global warming.

The weather station at the Kodiak tide gauge reached the 60s again Monday afternoon, and up to 55°F on Tuesday morning. "In late December," tweeted Alaska climate scientist Rick Thoman, "I would not have thought such a thing possible."

• Thoman also pointed out that the temperature at Unalaska Airport rose to 57°F at noon local time Monday, and only fell to 50°F overnight Sunday into Monday morning.
• "That's higher than the 1991-2020 normal daily low temperature for ANY day of the year," Thoman stated.

How it works: The warmth is due to a major, albeit relatively short-lived, shift in the jet stream, which is a river of air flowing at high altitudes that helps steer weather systems.

Instead of dipping south of Alaska and carving out a frigid trough of low pressure, as was the case during parts of November and much of December, the jet stream buckled in such a way that allowed mild, maritime air to surge in from the south.

• Other locations also saw record warmth on Dec. 26, including areas near the Bering Sea. Cold Bay, whose name does not suggest tropical weather conditions, hit 62°F, setting a new monthly record and smashing its daily record by 18°F, the National Weather Service tweeted.
• In response t0 the building Alaskan ridge of high pressure, the downwind jet stream across western Canada and the western U.S. has buckled southward, carving a sharp dip, or trough, in this region.
• This has caused extremely cold air to set in across British Columbia, with readings comparable to cold snaps not seen since the 1960s. The weather pattern is also repeatedly steering storms into the West Coast, with heavy snows measured in feet falling in the Sierra Nevada Mountains during the course of a week.

What's next: Computer models indicate that once the heart of the frigid air seeps into the U.S., another batch is likely to reload across Alaska and make its way southeastward as well.

• A large dip in the jet stream across Alaska and western Canada during the first week of January could lead to widespread unusually cold conditions across this region, based on computer model projections.
• In other words, residents of Kodiak might not want to get too attached to temperatures in the 60s.

California's ski resorts are digging out from under 5 to 9 feet of snow that has fallen over the last several days from a series of storms. And more snow is on the way.

The big picture: A large dip, or trough, in the jet stream across the West is allowing frigid conditions and a relentless series of storms to bring record-breaking snowfall into the Sierra Nevada Mountains. The snow has closed major interstates and even forced the shut down of several ski resorts.

Heavy snows and frigid temperatures were plaguing the West Monday, forcing several highways in Northern California and Nevada to close, with travel warnings issued in the snow-lashed Sierra Nevada.

Threat level: The National Weather Service said significant snowfall hit West Coast mountain ranges and the Intermountain West, with record cold weather in the Pacific Northwest. Meanwhile, the U.S. South was experiencing "unusually warm temperatures," with dozens of cities in the lower 48 states on track for their warmest December on record.