Declining sea ice in the Arctic is leading to more snowfall in Siberia, which is arguably one of the coldest places on earth. Temperatures in Siberia through the end of December have been flirting with 50 below zero, it's these temperatures that eventually make their way into the US.
Snow depth in Russia has been increasing significantly over the past 20 years, which is ultimately leading to colder arctic outbreaks in the US. During the winter, the mainland gets most of its cold from Alaska, Canada, and Russia. Russia has been coined as the "refrigerator of the north", due in large part to some of the coldest temperatures being recorded there.
As more snow piles up in Russia, colder air is essentially harvested in the region until a kink causes the cold to spill out. It's this cold that eventually makes its way to the US, where we can see a large swath of the US seeing subzero temperatures. This usually occurs when there is a kink in the jet stream, which is forecasted to occur through the first week of January.
The Climate Prediction Center has a majority of the US, excluding Florida, under a decent chance of seeing below average temperatures. Here in Mid-Missouri that would be highs below 38 degrees, much like we saw nearly 2 weeks ago. While it will be short-lived, after the atmosphere recharges you can bet that more blasts of arctic air will arrive in Mid-Missouri.