Thanks to a few early season snowfalls in October and November, the 2018-2019 winter season has begun and it’s looking like it may be a cold one!
There are several factors that come into play when making a seasonal forecast. Looking at everything from historical similarities between the past and present, to the current states of various teleconnections such as the El Niño Southern Oscillation, and long-range model data can help paint a picture of what lies ahead. Here’s how I see things playing out this winter season…
What is the El Niño Southern Oscillation (ENSO)?
ENSO is one of the most important climate phenomena on Earth and has the ability to change and impact the weather across the globe. It’s often highlighted in seasonal forecasts such as this to help determine how the weather across the U.S. will be impacted. There are three phases to ENSO that are determined by monitoring sea surface temperatures and wind patterns over the Pacific Ocean.
The three phases are:
1. El Niño – This phase occurs when sea surface temperatures across the central and eastern Pacific are above average. This often results in less rainfall over Indonesia and increased rainfall across the central and eastern Pacific. Shifting winds near the equator will carry tropical moisture north into North America. This typically leads to cooler and wetter seasons across the southern U.S. while warmer and drier seasons occur across the northern U.S.
2. La Niña – This is the cooling phase of the El Niño Southern Oscillation. In this phase, sea surface temperatures across the central and eastern Pacific are below average and rainfall often increases across Indonesia. The sub-tropical jet stream weakens and polar jet becomes the main driving force for storms across the U.S. and results in dry and warm conditions across the southern U.S. while the northern part of the country sees cooler and wetter seasons.
3. Neutral – In this phase, sea surface temperatures across the central and eastern Pacific Ocean are near average. This tends to bring warm and wet conditions to the southeast U.S. while colder conditions linger across the northern U.S.
This year El Niño conditions are favored as sea surface temperatures across the central and eastern Pacific are above average. Looking at this, you might assume that we are in for a mild and dry winter season. Not so fast!
“El Niño can be a major player in our winter weather, especially when we’re looking at a moderate or strong strength to the El Niño pattern…This year it’s looking like we’ll have a weak El Niño, so the impacts will be much less in our area.” – Meteorologist Chuck Schaffer, NWS Central Illinois.
Every El Niño is different and the strength of the El Niño can make a big difference. When you look at strong El Niños you find what you might expect in an El Niño year, warm temperatures across the northern U.S. with cooler temperatures across the south. Stronger events also favor wetter conditions across the south and the Midwest while the Northern Plains and the Ohio River Valley end up drier.
Weaker El Niño events were much different with temperatures across the eastern half of the U.S. below average while warmer temperatures were in place across the western U.S. When it comes to precipitation much of the U.S. sees below average precipitation with slightly wetter conditions across the southwest, the southeast, and New England. With a weaker El Niño expected this winter, I’d expect the El Niño to look more like this.
Interestingly enough, even though precipitation amounts were below average an analysis of the top ten weakest El Niños found that snowfall amounts were actually higher across the Midwest. This actually makes a lot of sense! Cooler air masses hold less moisture than warmer air masses, the colder the air the drier the air will be. With colder air in place during weaker El Niño events snow would contain less water.
So what brings the cooler air to the eastern United States?
Since the impacts from El Niño are less in Central Illinois during weaker events, it opens the door to greater impacts from other, smaller scale, teleconnections called the North Atlantic Oscillation and the Arctic Oscillation.
What is the North Atlantic Oscillation (NAO) and the Arctic Oscillation (AO)?
These two teleconnections are closely related to one another and unlike ENSO, can only be projected a few weeks out at a time. Depending on the phases of the NAO and AO, colder temperatures and stronger storms can impact the eastern U.S.
North Atlantic Oscillation – The phase of this phenomenon is based on the fluctuations of atmospheric pressure in the North Atlantic Ocean. Differences in sea level pressure between the Icelandic Low and the Azores High controls the strength of the winds and the location of storm tracks across the North Atlantic Ocean.
A positive phase of the NAO indicates a greater difference in pressure between the Icelandic Low and the Azores High. This creates a stronger west to east Jet Stream across the Atlantic Ocean and leads to wetter and warmer conditions across the eastern U.S. A negative phase of the NAO indicates a smaller difference in pressure between the Icelandic Low and the Azores High. With a negative NAO, colder and drier conditions are often seen across the eastern U.S.
Arctic Oscillation – 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.
A positive phase of the AO indicates stronger westerly winds around the Arctic which confines colder air masses to the Arctic Circle. When the AO is negative, westerly winds circulating around the North Pole become weaker and more distorted which then allows the colder air to penetrate southward into lower latitudes.
When the Arctic Oscillation and the North Atlantic Oscillation are both in negative phases, the eastern U.S. can end up seeing significantly cooler temperatures. In Central Illinois, negative phases of the AO and the NAO at the same time will often result in more frequent arctic air masses and increased storminess making for some rather uncomfortable stretches of winter weather.
So How Are Things Looking For This Upcoming Winter Season?
My winter forecast differs a bit from some other winter outlooks that have been issued so far. I based a lot of my forecast on what has happened in the past during weak El Niño events and put less of an emphasis on long-range model projections.
Temperatures – The impacts from El Niño will be felt the most across the western U.S. where I am expecting to see above average temperatures. The eastern U.S. will see less of an impact from El Niño and a greater impact from negative phases of the of the Arctic Oscillation and the North Atlantic Oscillation. This should lead to temperatures below average over the eastern U.S.
In Central Illinois, while temperatures have been well below normal so in November, temperatures should warm up a bit in December. Temperatures should then trend near to below average for the start of 2019.
Snowfall & Precipitation – I feel above average precipitation will be likely across the southern U.S. while below-average precipitation occurs across the northern U.S. When it comes to snowfall, I feel less snow will fall across the Northern Rockies and the Pacific Northwest this winter season. Thanks to multiple clipper systems diving out of Canada, the Colorado Front Range into the Central High Plains and the northeast U.S. should see above average snowfall.
In Central Illinois, I think we will be looking at drier snowfalls due to the presence of multiple arctic air masses. This should leave us with below average precipitation (liquid or melted snow) but near average snowfall. After some early season snows in November, the pattern should trend less snowy in December before near to above average snow returns in early 2019. On average, Peoria received roughly 24.0 inches of snow in a snow season. After some early and late season snowfalls, I think Central Illinois will receive 21.0 to 27.0 inches of snow.