North American winter snow coverage is an excellent proxy for how far south Arctic air is penetrating into the United States. During years when the cold air pushes further south, the snow line pushes further south, causing a larger area of snow coverage. Winter snow cover has been increasing in a hockey stick since the very warm winter of 1995, and is now at record levels.
The animation below correlates North American winter snow cover vs. raw and adjusted US winter temperatures. As you can see, the correlation with raw temperatures is better than the correlation with adjusted temperatures, indicating that the adjustments degrade their accuracy. All that the adjustments do is add noise and diminish the correlation.
The raw temperatures show a net winter cooling since 1970 in the US, but the NCDC adjusted temperatures show a net warming.
Every test I have made on USHCN adjustments shows that they are complete crap. Their only purpose is to turn cooling into warming – for political purposes.