DENVER — Bitter cold is in the forecast for Colorado.
The high temperature Thursday will struggle to surpass zero degrees, and wind chills could plummet as low as 50 degrees below zero in some parts of the state.
It will be a dangerously cold 24 hours. If you haven’t read up on how to prepare yourself – including preparing your home, car and more – we urge you to stop here and read that story first.
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Now that you’re braced for the arctic blast, we can turn to the question you may well be asking: How and why in the heck is it going to be colder in Denver than it is in Anchorage?
The answer lies in what’s known as the polar vortex.
According to the National Weather Service, the polar vortex is a low-pressure system that sits over each of the earth’s poles. We’ll focus on the North Pole, as that’s the source of the bitter cold headed for Colorado.
The “vortex” refers to the counterclockwise circulation of air around that system typically keeps the cold air contained over the North Pole. However, that vortex can expand during the winter and send some of that arctic air south toward the United States.
We spoke with Dr. Judah Cohen, a climatologist with Atmospheric and Environmental Research, to better understand these meteorological phenomena.
“When you get a disruption or weakening of the polar vortex, you tend to get an increase in the probability or risk of severe winter weather,” he told us on a video call.
Dr. Cohen likened this weakening of the polar vortex to the slowing of a spinning top. As it slows down, the top begins to wobble.
“The same thing happens with the polar vortex. When it's a nice, tight rotation, the cold is really kept close to that center of circulation over the Arctic,” he said. “When it slows down, that area kind of just expands outwards.”
The resulting expansion leads to what’s called the “southern lobe” – a U-shaped weather pattern that plunges southward, stretching like a rubber band. Air that usually resides over Canada can make its way into the U.S.
In extreme cases, like what is likely happening this week, we see something known as “cross-polar flow,” in which air from Siberia (where temperatures can dive as low as 88 degrees below zero) can literally cross over the North Pole and stretch all the way to places like Colorado.
“At least a piece of that air is getting transported from Siberia, across the Arctic Ocean, into Canada and down into the US,” Cohen said. “[It’s] unusually cold air, certainly for this time of year.”
Dr. Cohen told us these weather events are not unheard-of. In 2017, a similar expansion of the polar vortex brought a prolonged deep freeze to the northeastern United States. Like it did in 2017, these events can last for a week or longer.
Fortunately for those of us in Colorado, while this week’s arctic blast will bring some of the coldest air the state has ever seen and is expected to remain in the U.S. through the weekend, subzero temperatures should only last about 24 hours here.
Mild temperatures return to Colorado for the Christmas holiday.