Arctic Air Aiming at Stateline

It has been a mild winter thus far across Northern Illinois and Southern Wisconsin as a strong El Niño has been present. However, there is a slight blip in the road setting up for next week in the form of very cold air from Canada. This will likely be the coldest air of the season, and it will last for several days before we likely return to slightly milder temperatures. On the map to the left, the blues and greens are indicative of where the forecast models believe colder than average temperatures will be from January 9th thru 14th. This particular map only shows that time frame, but the colder air will likely hold on until the 18th-20th. This arctic air will plunge south behind the storm system that will affect the region late this week into Saturday.

The reasons behind the upcoming stretch of cold air boil down to a few things, but the most important are two factors that meteorologists look at to determine temperature patterns switching from positive to negative. The graph to the right shows what is called the Arctic Oscillation (AO). Notice the trend from positive (above the black line in the middle) to negative as we head into January. That is highly correlated with temperatures that are colder and below average.

The other variable meteorologists look at is referred to as the North Atlantic Oscillation (NAO). The same trend can be noticed here. The values were positive during December, but they are expected to continue diving negative as we head through January. Again, much like that AO, a negative NAO value usually points to colder weather for the Eastern United States. An important question that cannot be answered quite yet is: "Just how cold will it get?"

At this time, putting specific numbers out there just isn't feasible due to the likelihood that things will shift around a little bit once the models grasp a better idea of what will happen. However, high temperatures in the teens--even lower teens--and low temperatures in the single digits to near zero are looking increasingly likely.