ROBERT SIEGEL, HOST:
As with much of the country, we here in Washington, D.C. are in for a taste of winter unlike anything we've felt for a couple of decades, and it's all thanks to a phenomenon called the polar vortex, which is the subject of today's lesson in meteorology from Jason Samenow, weather editor of The Washington Post. Hi, welcome to the program.
JASON SAMENOW: Thank you.
SIEGEL: And a vortex suggests a cyclone and I gather this is a very big kind of cyclone.
SAMENOW: That's exactly right. We're talking about a huge sprawling area of circulating cold air originating from the North Pole. It's a low pressure center, and typically during the winter months it resides up there. At times some tentacles of it will slip southward and bring cold air outbreaks into the U.S., but this year we're seeing a huge chunk of it, most of it descending into the U.S.
SIEGEL: So there always is a polar - I gather both an Arctic and an Antarctic vortex working. It's just how far from the pole it goes.
SAMENOW: That's exactly right. In fact, the Antarctic polar vortex is stronger and more impressive than the North Pole vortex due to the fact that Antarctic is landlocked and so it's colder down there. But even in the North Pole, this vortex is a powerful feature. It's there all the time during the winter months and there even in the summer months at a weaker intensity.
SIEGEL: Well, compared to other winters, how different is the polar vortex this year from in typical years?
SAMENOW: It's fairly typical in terms of its behavior, but what's happening this week is rather unusual in that this huge piece of it is descending southward into the U.S. and that's really a result of the configuration of weather patterns. Right now we have a huge area of high pressure over Alaska so it's actually warmer in parts of Alaska than it is in much of the upper Midwest.
And there's also an area of high pressure over Greenland and just to the west of there, and so basically that's allowing the jet stream to dive south over the U.S. and also for this polar vortex to drop south with the jet stream.
SIEGEL: But judging from the forecasts, this particular cold snap that the vortex is scheduled to bring to Washington, D.C. this evening and tomorrow morning is very intense but very brief; 24, 36 hours later it's gone.
SAMENOW: That's right. We don't have this year what we sometimes have, which is called blocking patterns, which are basically roadblocks in the atmosphere steering flow which can trap some of these cold air outbreaks over the U.S. for an extended period of time, so that's the good news. We're not talking about several days or weeks of historic cold.
We're talking about 24 to 48 hours in most areas. Around Chicago, they may see about 60 hours of sub-zero temperatures, which would be close to a record there. But for most of us, this is somewhat fleeting.
SIEGEL: And where does it flee to? I mean does it just gradually go back up toward the Arctic again?
SAMENOW: That's exactly right. You know, the atmosphere's a fluid and you can think of it sort of as a bubble circulating through the flow, just sort of dipping down and then floating back up to the North and the East.
SIEGEL: Last time we had a polar vortex that came this far south, when would that have been?
SAMENOW: Well, as I mentioned, a lot of times we'll see tentacles of the vortex or little pieces of it dip down into the U.S. Most winters that happens. The last big one was in '94. In '96 we had a fairly sizable polar vortex-type event occur as well, and there were several in the mid '80s. One of this magnitude where we're seeing this large of a piece of the vortex drop into the U.S., it's about a once a decade or so event.
So this is fairly extreme, but fortunately because of the configuration of the weather patterns and the overall setup, it will be short-lived.
SIEGEL: Jason Samenow, thanks for the lesson.
SAMENOW: You bet.
SIEGEL: So Jason Samenow, weather editor of The Washington Post, describing the cause of our imminent discomfort in Washington, D.C. and in much of the country this week, the polar vortex. Transcript provided by NPR, Copyright NPR.