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CHICAGO – The Chicago area has been enjoying a rather mild November so far with most days finishing near or above average. We’ve had a few chilly days here or there, but it’s looking like we could be in for a true cold snap as arctic air builds into the region around Thanksgiving.
So, why the sudden change in temperatures? It appears we’ll have both the jet stream and something called ‘Sudden Stratospheric Warming’ to blame. Sudden Stratospheric Warming sounds like it would be something that would bring warm weather, but in fact, it does the opposite.
A sudden stratospheric warming event is a disruption of the stratospheric polar vortex due to large-scale atmosphere waves, called Rossby waves, getting pushed higher into the atmosphere. If the waves are strong enough, it causes cold air to sink and warm rapidly, displacing or splitting the polar vortex and sending that cold air southward into the mid-latitudes.
Now, when you hear the term “polar vortex,” you’re probably thinking of sub-zero or single digit temperatures. While that can sometimes be the outcome in January or February, the temperature drop won’t be that extreme this time around.
November is unusually early for an SSW event to occur, and the polar vortex isn’t as cold as it would be in the middle of Winter. So, while we’re forecasting arctic air to make an appearance in the Chicagoland area around Thanksgiving, high temperatures will likely still make it into the 30s with lows in the 20s.
The average high temperature for Thanksgiving is 43 degrees and the average low is 29 degrees.