Some forecast models are pointing to an unusually early disruption of the polar vortex that could influence weather patterns across the Northern Hemisphere.
The polar vortex is a large circulation of very cold air high above the Arctic, extending through the stratosphere. Meteorologists commonly monitor it around 100,000 feet above Earth’s surface.
Sometimes atmospheric disturbances cause that circulation to weaken, stretch or even split apart. This can be accompanied by a dramatic rise in temperatures high above the Arctic, known as a sudden stratospheric warming event.
What’s especially unusual about the current forecast is the timing. These events typically occur during the middle of winter, not early November.
The National Oceanic and Atmospheric Administration has acknowledged the potential for an unusually early warming event. If it meets the criteria for a major SSW, it could be the earliest documented on record, going back to the late 1950s. The earliest widely recognized major event occurred around Nov. 28, 1968.
Could this bring colder weather to Chicago?
It’s possible, but a disruption of the polar vortex doesn’t automatically mean Arctic air will plunge into the Midwest. It can take weeks for changes roughly 20 miles above the ground to influence weather at the surface. We also don’t know where that Arctic air could spill south.
For Chicago, the developing disruption could increase the likelihood of colder weather during late November or December. If the effects persist, that colder pattern could last several weeks.
Does this change our warmer-than-average winter outlook?
Not necessarily.
We’re also experiencing a very strong El Niño, which historically favors warmer-than-average winters around Chicago and the Great Lakes.
NOAA’s latest seasonal outlook continues to favor above-average temperatures across the Great Lakes for December through February, even while acknowledging the potential polar vortex disruption.
An early disruption also leaves time for the stratospheric polar vortex to recover later in winter. If larger atmospheric circulation patterns shift again, warmer weather could return during January or February.
It’s important to remember that one cold stretch doesn’t determine an entire winter. Chicago could experience significant Arctic outbreaks and still finish the season with above-average temperatures.
Also, keep in mind the air isn’t nearly as cold in November as it is in January, when polar vortex disruptions are more common.
For now, a colder period in late November into December remains a possibility, while the overall winter outlook continues to favor above-average temperatures.
The NBC 5 Storm Team will continue to watch for any changes to the forecast and keep you updated.
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