Peoria, Ill. – Call it what you want, the polar express, a polar coaster or an Arctic blast, multiple rounds of cold air will impact Central Illinois through early next week.

The weather pattern remains in an active northwesterly flow thanks to ridging in the eastern Pacific and a blocking high-pressure system over Alaska. This pattern diverts cold air that would otherwise be in Canada into the contiguous United States.

Round 1
The first round of cold air is expected to move in Thursday behind a cold front that could bring a wintry mix to Central Illinois Wednesday night into Thursday morning. As of this writing, snow and ice accumulations look to be minimal across the area, though higher impacts are possible south of I-72 depending on how fast the cold air arrives.

Temperatures on Thursday and Friday are expected to be well below average with highs in low to mid-30s and lows Friday morning in the teens. Temperatures will return to the 40s on Saturday ahead of an even stronger front that will move in on Sunday.

Round 2
The second round is expected to be even colder than the first round, sending daytime temperatures into the lower 30s and upper 20s and overnight temperatures into the teens. Once again, some light snow could accompany the cold front Saturday night but any accumulations would remain light.

Why the early season cold?

As mentioned earlier, these cold air outbreaks can be attributed to blocking ridges over the eastern Pacific Ocean and Alaska. This ridging could be caused, in part, by unusually warm water in the northeast Pacific Ocean dubbed “The Blob” by researchers. It’s not clear what led to this anomalously warm water but the loss of Arctic Sea ice and variations in the Pacific Decadal Oscillation likely play a role.

The combination of upper-level ridging over the eastern Pacific and “The Blob” can lead to wild swings in temperatures and weather conditions over the lower 48. With a ridge of high pressure blocking colder temperatures from moving into Alaska, the colder air is diverted elsewhere and often ends up east of the Rockies.

This is particularly true when the Polar Vortex enters a weakened state, allowing the jet stream to become very wavy and sending frigid air into the U.S. The variations in this pattern is known as the Arctic Oscillation (AO). When the AO is negative, very cold air is diverted into the Contiguous U.S., the only question is where does it go? With ridging over the west, there’s only one place it can go….east of the Rockies and across the Midwest.

The AO is currently negative and is expected to trend even more negative through the start of the next week. While this particular pattern is only identifiable a few weeks out, there are signs that we can see a pattern change that would bring warmer than normal conditions to Central Illinois for the latter half of the month.

The Arctic Oscillation looks to trend even more negative through November 15th.