An intense winter storm driven by Arctic air from the polar vortex is set to bring record low temperatures and powerful winds to much of the US between 22 and 24 December.
The cold front started “diving” south across the Central Great Plains on Wednesday, 21 December, rapidly bringing temperatures of -10 to -20°F (-23 to -29°C) in mountain states and on the High Plains, to the National Weather Service. Gusts as high as 96 kilometres per hour are expected to cause wind chills of -40°F (-40°C), with some places reaching -70°F (-57°C) amid blizzard-like conditions.
As the cold front moves further east on Friday, temperatures in the Midwest aren’t likely to rise above 0°F (-18°C), with much of Texas and the Gulf Coast still below freezing. Every state except Hawaii is forecast to see (-7°C) on Christmas Eve.
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The storm is expected to lead to the coldest recorded temperatures in some parts of the country and the coldest in 30 years in others; Casper, Wyoming, hit its coldest temperature on record, reaching -42°F (-41°C) Thursday morning. The storm has already led to school closings and thousands of cancelled flights during one of the busiest travel weeks of the year, with more than under a winter storm warning as of Wednesday night.
The cold is also dangerous for livestock and wildlife, and could strain the energy grid as people crank up the heat. In Texas, the freeze will test the effectiveness of  since its during another, colder storm in February 2021.
In general, human-caused climate change has made cold extremes like this one warmer and shorter, says at the Breakthrough Institute, a California think tank. “In the metrics that look at cold extremes, it’s not getting colder,” he says.
But some researchers have proposed that warming in the Arctic can cause disruptions to the polar vortex that lead to more frequent cold extremes like this one in North America.
The idea revolves around the fact that the Arctic is warming faster than lower latitudes, a phenomenon known as “Arctic amplification”. A warmer Arctic reduces the temperature gradient between the freezing polar vortex and warmer air further south.
The reduced temperature gradient in turn weakens the jet stream, the fast-flowing west-to-east current of air that forms on the boundary between the Arctic air and warmer air.
One compelling but less well-supported idea is that the weakened jet stream then becomes “wavier”, bringing air from the polar vortex further south more often. “We’re seeing this warming overall in the winter,” says at the Woodwell Climate Research Center in Massachusetts. “But we’re also seeing these big cold-air outbreak events happening more often.”
Arctic amplification can also cause disruptions to the polar vortex as changing ocean temperatures and snowfall increase the “waviness” of the jet stream, says at the Massachusetts Institute of Technology.
