Using UK Met data from here, I noticed that Tmax for Decembers are cooling at -.45C / decade for 30 years. Thats a lot!
And then I wondered why? A theme I have returned to over and over is that hours of bright sunshine has increased because of cleaner air. Could more sunshine in December lead to cooler Decembers since clear skies can lead to colder temperatures in winter?
Guess what. Sunshine is up an average of 5.5 hours per year in December for the last 30 years. And compared to 1988 sunshine is up 80% in December. Instead of 28 hours of sunshine in December, the average is closer to 48.
Click on the graphs to enlarge.
sunshine hours 
That is consistent with more southerly jet stream tracks.
They are tending to be over the UK during the summer but south of the UK during winter so cloudier wetter summers and cooler sunnier winters.
I blame wind turbines myself. 🙂
Check my paper Max Temp vs Sunshine Hrs Central UK 1930 – 2010, at Tallblokes Talkshop. A year ago.
You have reference for data for contiguous US? Sunshine vs Max daily temps, maybe midnight temps?
USCRN has recent sunshine. But nothing much earlier than 2006.