You may think I'm lazy (ask my wife) but I have copied and pasted a blog I wrote a month ago on why our Winter has been so warm so far. But the reasoning still holds and people are still asking. I have added some stats I dug up from the record books too.
Why is it so warm this Winter?
First, here's some stats...
8th warmest start to Winter through Feb. 1st
3.8" of snowfall (last year we had over 26" at this time and we average about 17.5")
13th warmest December on record
13th warmest January on record.
La Nina appears to be one of the reasons, 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 (it's complicated) 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 has also been a factor in why it's been so warm.
Arctic Oscillation (AO), North Atlantic Oscillation (NAO), Pacific Decadal Oscillation (PDO) and El Nino (also referred to as the El Nino/Southern Oscillation or ENSO) 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 past a week or two with forecasting the Arctic Oscillation.
But we can measure the Arctic Oscillation (and the North Atlantic oscillation) and it's in been in a strongly positive phase much of Winter, helping along La Nina in making for a warmer than normal Winter.
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.