Peoria, Ill (WMBD) — We are currently in the midst of a rare triple dip La Niña, a phenomenon determined by below average sea surface temperatures in the eastern Pacific. While La Niña patterns result in near average to cooler than average Winters across Central Illinois on average, it’s not unusual for the Month of December to be mild.

You can look at the past two Decembers as an example, both were part of a La Niña winters and both ended up with above average temperatures. Eventually, the warmer starts to winter would be offset by much colder Januaries and Februaries.

However, while Decembers have been historically mild, there are signs that this year may be a bit different in that December 2022 may actually end up cooler than normal. The signs I’m talking about are short term climate patterns called teleconnections. Teleconnections such as the Arctic Oscillation (AO), North Atlantic Oscillation (NAO), and Eastern Pacific Oscillation (EPO) can dictate what kind of weather patterns we experience over North America. I won’t bore you with the science of each oscillation here, but you can scroll to the bottom of this article for a deeper dive on them if you’d like.

It’s no secret that winters in Illinois can be highly variable, often swinging from fall like temperatures to bitterly cold in a matter of hours. While details of these drastic swings are often not known until a few days ahead of time, we can look at the evolving weather pattern over Greenland and over the north Pacific to get a sense of what the prevailing pattern will be for the next 2-3 weeks.

Let’s take a look at what’s coming down pipe…

Long range model projections suggest that the Arctic Oscillation (AO) will become sharply negative in the first week of December. A negative phase of the AO indicates that polar vortex is in a weaker state and allows arctic air to spill south to lower latitudes. The question then becomes, “where does that cold air go?” This is where the North Atlantic Oscillation (NAO) and Eastern Pacific Oscillation (EPO) come in to play.

Models suggest a strong area of high pressure will build over Greenland. It’s what we call a “Greenland Block”. The Greenland Block is a feature of the negative phase of the NAO and causes the Jet Stream to buckle over the North Atlantic. This in turn creates troughing over eastern North America usually resulting in cold and unsettled weather across the eastern U.S.

The next piece of the puzzle comes from the Pacific Ocean. While not as strong as the blocking high over Greenland, we’re anticipating periodic ridging near Alaska, what we call the “Alaska Ridge”. This ridge is associated with the negative phase of the EPO which usually results in ridging over the western U.S. and more troughing east of the Rockies.

When negative phases of the NAO, the EPO, and AO all occur simultaneously, it steers cold arctic air southward into the central and eastern U.S. and often results in a more active weather pattern east of the Mississippi.

December Outlook

The Climate Prediction Center’s December Outlook shows a pattern that one might expect given the negative phases of the AO, NAO and EPO. They are calling for a greater chance of below average temperatures across the northern U.S., including in Central Illinois. The CPC’s outlook also calls greater chance of below average precipitation across Central Illinois. This doesn’t necessarily mean less snow though. On average, December is our 3rd snowiest month of the year, with January and February taking the top two spots respectively. It’s quite possible we end up in a situation like we did in November with below average precipitation but above average snowfall.

Deeper Dive in to the Various Teleconnections

The Arctic Oscillation (AO)

The Arctic Oscillation is a short term teleconnection that determines how much cold air will impact the eastern U.S. Unlike ENSO, the predictability of the Arctic Oscillation only extends a few weeks out. The phase of the AO is characterized by the strength of the winds circulating counterclockwise around the north pole, a phenomenon known as the Polar Vortex.

  • Positive Phase – This phase of the AO indicates that the Polar Vortex is strong. This produces stronger westerly flow around the Arctic which confines colder air to the Arctic Circle.
  • Negative Phase – The Polar Vortex is in a weakened state and the jet stream becomes highly amplified and sends arctic air south. In Central Illinois, a negative phase of the AO often results in more frequent arctic air masses and increased storminess making for some rather uncomfortable stretches of winter weather. 

North Atlantic Oscillation (NAO)

The North Atlantic Oscillation is based on sea-level pressure difference between the Azores High and the Subpolar Low. Similar to the AO, the NAO is only predictable a few weeks out.

  • Positive Phase – This phase of the NAO means that there is a significant difference in pressure between the Azores High and the Subpolar Low, in other words, both the high and the low are strong. This makes the jet stream winds over the north Atlantic more westerly which then leads to warmer temperatures across the eastern U.S.
  • Negative Phase – The NAO is in this phase when the difference in pressure between the Azores High and the Subpolar Low is small and both systems are weak. This leads to a wavier jet stream and a blocking ridge over Greenland. This leads to warmer temperatures pushing north towards the North Pole over the Atlantic while colder temperatures move southward over the eastern U.S.

Eastern Pacific Oscillation (EPO)

The Eastern Pacific Oscillation is based on the flow of the atmosphere over the eastern Pacific and Alaska.

  • Positive Phase – This phase results in a strong northern Pacific jest stream that brings mild Pacific air into the western U.S. This pattern results in heavy rain and snow across the western U.S. and cold temperatures across Alaska. The pattern also results in warmer weather east of the Rockies.
  • Negative Phase – This phase leads to high pressure building over the eastern Pacific and Alaska, what we call and Alaska Ridge. This ridge leads to warmer than normal conditions over western parts of North America while a large scale trough often results in cooler than normal conditions in the east.