Why is it so warm this Winter?
Today is exceptional, but the pattern has been warm for a while now. La Nina appears to be a reason, because it's helping to setup a pattern with the jet stream much farther North, so we miss the typical waves of Arctic air this time of the year. In this graphic, click here, you can see the polar jet forced way North.
This is just a general characterization of the jet stream during a La Nina influenced Winter, which leads to warmer temperatures for St. Louis and occasional but not frequent cold spells.
Wait a minute...Last year we had a La Nina too, but it was cold! That's because our climate is driven by several factors and last year's La Nina was trumped by the Arctic Oscillation in a strong negative phase. I know, I know....you're thinking...he's making this stuff up! Arctic Oscillation? Yep, it's real. And this year, the Arctic Oscillation is a big factor in why it's been so warm.
Arctic Oscillation, North Atlantic Oscillation, Pacific Decadal Oscillation and El Nino (also referred to as the Southern Oscillation) are all large scale patterns that develop and truly have a big impact on our climate. And those are the ones we know about. The problem is that while we have some skill in forecasting a La Nina/El Nino, we cannot do that (yet) with the Arctic Oscillation.
But we can measure the Arctic Oscillation (and the North Atlantic oscillation) and it's in a strongly positive phase, helping along La Nina in making for a warmer than normal Winter...so far.
Here's a link for more on the Arctic Oscillation, click here.
By the way, I hope I didn't lose you with the explanation above, but it goes to show that weather is so complex that there is rarely one easy answer.