In the past ten days we've had 3 days with severe storm warnings, an 8" snow storm, 2 days with highs in the 70s and 3 days with highs near 30. It's been a wild ride and if you caught our forecast tonight, we don't have any major storms in the next 5 days, though we do have a high of 50 Friday and then a low of 12 by Sunday morning.
So what's causing this extreme weather? There are so many variables involved in daily and weekly weather that's it's truly not realistic to point to one factor as "the cause" of extreme weather. However, it's worth mentioning that we are in a La Nina right now and that certainly could be one of the larger forces in helping to set us up for some of this extreme weather.
La Nina is the opposite of El Nino. Specifically, it's a cooling of the Pacific Ocean water in the Eastern equatorial Pacific (think off the coast of Peru and Ecuador). More importantly, this abnormally cool large pool of ocean water is thought to influence atmospheric winds which influence our climate and weather.
Below is a general characterization of how La Nina impacts North America during the Winter. Notice how abnormally warm (and moist) gulf of mexico air can work it's way North into the Midwest and Deep South. This is not typical for our winter, instead the cold Canadian air usually dominates. This means that part of a La Nina influence for St. Louis should be warmer than normal temperatures, at least on average for the Winter. At the same time, it could explain how we can have unusually warm temperatures in the 70s, and the next day the typical cold Canadian air takes over and causes a high of 30. Also, as the warm tropical air advances unusually far North for the Winter, it moves closer to the sharply cold Canadian Air. This clash of extreme airmasses is a general set up for thunderstorms and in the Winter the moisture can also feed snow storms.
I don't think it's accurate to attribute wild weather to one influence like La Nina. That's because there is so much we still don't understand about our atmosphere and just can't explain. For instance, there are other cyclical patterns that influence our climate and weather similar to the El Nino/La Nina but are less popular in the media (like the Arctic Oscillation and the North Atlantic Oscillation). Scientists continue to research these influences and trends, but we're just not at a point where we can say with confidence that a week of wild weather is due to any one factor. But I think it is fair to say that La Nina could be playing a role.