The Polar Vortex - Winter is Coming

Monday, November 10, 2014

12:39 pm

I think the above meme explains all you really need to know about the weather this coming week. But since I am a kind and merciful weatherman, I shall oblige your curiosity and explain the meteorological situation that has led us to our first real chill of the winter.

But first let me explain what a "polar vortex" is. This catchphrase became quite the buzzword last year as all sorts of media dished it out like free miniature Bibles. Professor Rush Limbaugh took it upon himself to define the polar vortex, seemingly settling the hysteria once and and for all.

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"So, ladies and gentlemen, we are having a record-breaking cold snap in many parts of the country. And right on schedule the media have to come up with a way to make it sound like it's completely unprecedented. Because they've got to find a way to attach this to the global warming agenda, and they have. It's called the 'polar vortex.' The dreaded polar vortex."

"Do you know what the polar vortex is? Have you ever heard of it? Well, they just created it for this week."

"Now, in their attempt, the left, the media, everybody, to come up with a way to make this sound like it's something new and completely unprecedented, they've come up with this phrase called the 'polar vortex.'"

And here's my favorite part.

"Whatever it is that keeps the polar vortex vortexed in the Arctic Circle is vanishing, and that cold air is coming to us. Normally it stays up there. But now it's down here. How did it get here? That's the deepening mystery. That is the crisis. That is what is man-made. Man is destroying the invisible boundaries that keeps that air up there.

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"Polar vortex vortexed." Good one Rush. The fact is, the polar vortex was not just created in the last week. In fact, the term has been alive and well for over 50 years. 

The polar vortex is a persistent cyclone that encircles either one of Earth's geographical poles in a cyclonic fashion (in the same direction as low pressure systems; counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere). They are usually less than 1,000 km in diameter, and their strength depends on the temperature gradient between the poles and midlatitudes. As such, the vortices are stronger during the winter than the summer, as the temperature gradient between the poles and midlatitudes is greatest at this time. 

It can be a little bit tricky to explain just in terms of words, so hopefully the picture below help you understand.

Northern Hemisphere Polar Vortices. Source: NASA Earth Observatory

Vorticity is an advanced concept, but it can be thought of as the tendency for a an air parcel to spin, with higher values responding to higher tendencies. Therefore, the higher the the vorticity, the stronger the polar vortex. The diagram above shows how a singular polar vortex over the North Pole weakens and breaks down into two separate vortices. When this happens, one of these vortices can slide south and bring a whole bunch of cold air that had been previously been stored up near the poles with it. 

The below diagram gives a general idea to how a part of the polar vortex gets cut off from the main circulation by the pole, bringing frigid air southward with it.

Schematic of the formation of an arctic outbreak in the Northern Hemisphere. Source: Wikipedia.

So, now, we've got a general idea of what the polar vortex is, and how it works. So what's gonna happen this week?

It turns out we are actually going to see an event like the one illustrated in pictures a, b, and c above. Take a look at the model chart below, and you'll see what I mean.

Retrieved from Supercell Weather

The colors represent the "thickness" in decameters of the atmosphere, which is a function of the overall density of the atmosphere and therefore the temperature via the ideal gas law. Take a look at how much lower the thicknesses are over much of the United States than areas much further north, with Southern Canada having particularly low thicknesses. This, right here, is the "polar vortex" that has been cut off from its 'parent' circulation up by the pole. Some of this cold air is actually infiltrating into Washington and even Seattle, and we will see our first freeze of the region Tuesday night. Models had been flirting with snow in the Portland region, but for now, it looks like any snow will be confine at and east of the Cascades, particularly near the Columbia Gorge. 

In the extended, we'll have a ridge of high pressure off our coast that will close the door to the the Pacific storm train that has been open for so long. We look to be relatively dry for the foreseeable future. Enjoy the break!

Charlie