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What’s Causing This Warm Christmas In The Eastern U.S.?

The map looks like it was misplaced from a different season: 73 degrees in Washington. 76 in Raleigh. 65 in Louisville. 73 in Little Rock. And here in New York, 73 degrees, two degrees cooler than it was on July 4. But those forecasts are for today, Christmas Eve, creating a disconcerting backdrop to the holiday lights and street-corner pine-tree stands. What is going on with this weather, and should we be worried about its implications?

We called together two of our sharpest weather minds: Harry Enten, a staff writer who is as obsessed with outdoor forecasts as he is with politics, and Matt Lanza, a professional meteorologist and freelance writer who works in the energy industry in Houston. David Firestone, FiveThirtyEight’s managing editor, moderated the discussion. The transcript below has been lightly edited.

David: It’s Christmas Eve, and people on the East Coast are walking around in shorts. As New York worriers, should we feel guilty about this, Harry?

Harry: I see no reason to feel guilty about this. I’ve heard a lot of talk about how this is linked to climate change, and while it is certainly possible that climate change is making these temperatures slightly more extreme than what might have been 50 years ago, this warm-up is mostly because of normal weather patterns. Mainly, El Niño, which is essentially unusually warm temperatures around the equator in the Pacific Ocean. Of course, there are other things going on too, which I hope we get to in this chat.

David: Absolutely. So Matt, do you agree that we’re allowed to enjoy this bizarre phenomenon?

Matt: Without question! But I live in Houston, so shorts are a hot commodity year-round. Seriously though, after the last couple winters you’ve endured in the eastern U.S., this might feel great to a lot of folks.

Harry: I should point out, as Matt says, he is in Houston, and there are parts of the country which aren’t actually experiencing above-average warmth — mainly the western part of the country, where there are plenty of winter weather advisories and warnings.

David: Good point. How widespread is this thing? Is anyone in my hometown of Kansas City wearing a windbreaker?

Matt: So Kansas City has been pretty warm this winter too. They’re about 10 degrees above normal for the month, but if you go further west, Las Vegas has had a fairly average month in the temperature department, as have parts of Southern California. But with very few exceptions, the entire country has been warmer than normal lately.

David: Will it stick around, or can the Midwest and East Coast plan to go back to parkas in a few weeks or months?

Harry: Well, if you look at the longer-term models, there is going to be a shot for wintry weather in the beginning of next month. Part of that will have to do with the Pacific North American (PNA) pattern going into what’s called the positive phase over the next week. It’s currently in a negative phase, which is associated with a deep trough in the western U.S. and a ridge in the eastern U.S.

December Month to Date Rankings

This map, from the Northeast Regional Climate Center at Cornell, shows how unusual December temperatures have been. A “1” indicates places where the temperature is the warmest on record.

David: So that raises the question of the science behind this warmth. We’ve heard a lot about troughs and El Niños, etc., but most of us still don’t really understand what’s happening. Matt, can you explain the forces that are delaying an entire season?

Matt: Everyone’s heard so much about El Niño this year, and to be fair, it’s definitely one of the big reasons December doesn’t feel like winter in the East. In fact, historically, during El Niño events of this magnitude (one of the top three in the historical record), December has been a very mild month. But one thing a lot of people don’t realize is that one of the key players this month has been none other than our friend from a couple winters ago: The polar vortex!

David: Aiee!

Matt: Exactly. And you can take a crude measurement of the strength of the polar vortex by looking at something called the Arctic Oscillation (AO. We love acronyms in meteorology.) If you look at how that index has behaved this month, it has been extremely positive, like 4-5 standard deviations above normal. To put it simply: When the AO measures that positively, it means the polar vortex (or the bulk of winter’s extreme cold) is wound up tight and located over the North Pole. When that index shifts negative, it’s a sign the polar vortex can drop south, closer to the U.S., Europe, etc. Just to be clear, it’s not the only factor causing the warmth or leading to cold, but it’s certainly a key player.

David: Maybe this is a good moment to remind people of what happened a few years back when that vortex did drop down.

Harry: Some really freaking cold temperatures is what happened. That was back in the winter of 2013-14, and it even had something of a return in July of that year, right around the Major League Baseball All-Star Game. The possible return to a negative-AO for a short period early next year is another reason we might get some snow in the East Coast. You’ll also note how Matt and I both made reference to key indices that tell us a lot about our weather pattern (PNA and AO). Another one, the North Atlantic Oscillation (NAO), is forecast to be quite positive for the foreseeable future, which means low pressure over Greenland. That historically has meant warm temperatures in the East.

David: The Obama administration secretly controls the vortex, right? For its own political purposes? Or if not, what does?

Matt: Of course! I always love when people blame the meteorologist for the weather instead of the forecast. If only we could control it! But in all seriousness, what will change the orientation or location of the polar vortex is what I like to call a complex ballet of large-scale weather changes around the planet, both on the ground and many miles above our heads. Among other things, there are a number of theories floating around about how the weather in the stratosphere can manipulate the strength and orientation of the polar vortex. A lot of that is science that’s in its early days. We don’t have all the answers to explain what forces the chain reaction to begin (let alone fully implement them into a forecast). But rest assured, anyone who has a remote interest in weather forecasting (or is thoroughly snow-starved this winter) is looking for that answer, and we’re certainly making progress.

Harry: This is one of the most interesting things about weather forecasting, right? We’ve made a lot of strides over the last few decades, but there’s a lot that we don’t know. That’s among the many reasons long-term forecasts are imperfect. We saw a number of forecasts calling for above-average snowfall or colder-than-average temperatures this year for the East Coast. That may turn out to be true, but it really cannot be known yet. I’ll further note that the most interesting thing to me about weather is that all it really takes is “one time.” So I think a lot of the reason we have thought the winter has been so “lousy” (or great depending on your angle) is the lack of snow. But in an El Niño pattern, there is typically above-average precipitation in the East. So all it will take is a short window of cold air (which will come eventually) meeting up with that moisture, and BAM! You’ve got some of the white stuff. [Editor’s note: Can you tell how much Harry loves — and desperately wants — snow?]

Matt: Keep an eye on the forecast next week. We may get one of those brief windows, especially in New England!

David: I love your “complex ballet” metaphor, Matt. But to extend it — if the theater gets too warm, the ballet dancers don’t behave as normal. Today’s momentary phenomenon might not be linked to global warming, but if we start to see this kind of thing happening regularly, shouldn’t we start to get a little nervous?

Matt: That’s a good point, David. And I think it’s true to some extent. While El Niño is certainly going to take home most of the accolades this winter, you’re going to see numerous records for warmth get broken (heck, obliterated in some cases) this month. When attribution studies are performed on this warm pattern (likely in the coming months), it is likely that they’ll conclude that it would have been exceptionally warm without the effects of climate change. It’s also likely, though, that they will conclude that climate change added a certain component to the warmth that would have otherwise been absent. So maybe we still would have broken records this month, but we were able to do so with a little extra padding because of climate change.

Harry: I’m always very careful to point to specific events because as I said all you need is “one time.” A lot of climate change skeptics like to say, “See, global warming isn’t real” when it’s snowing outside. That, of course, is foolish talk. But I certainly agree with Matt’s point that climate change exacerbates already occurring phenomena. What we know is that regardless of what we are feeling in our own backyards, the Earth as a whole had its warmest November on record last month. That to me is quite telling, even if it isn’t as sexy.

global weather


David: But even those of us who are not Republican senators can get confused by the information we’re hearing. We’re told that glaciers melting are the result of climate change, while the California drought might not be. How should ordinary people know how to interpret the phenomena we see around us, Matt?

Matt: It’s so difficult today to cut through the noise and occasional agendas that exist. From a scientist’s standpoint, it’s frustrating that people have to be so cautious about the information they hear, but alas, that’s 2015 I suppose. [Editor’s note: Just wait for 2016!] What I tell friends and family is that if you really, truly want to know how to interpret reports and attributions regarding climate change and changing weather, you need to research the sources yourselves. If the studies are published in reputable, peer-reviewed journals (i.e., Nature or various publications from the American Meteorological Society), they’re valid studies.

David: Or FiveThirtyEight, of course.

Matt: Of course. But you may hear conflicting information sometimes because it’s science. By definition, science is a work in progress. We learn new things daily, so you’ll occasionally see studies refute previous studies. It’s no different than having a study in the medical field determine eggs are bad for you, followed up by one saying they are good in moderation. Science and our understanding evolves with time. One other point I need to make is that, as Harry points out, there’s a huge difference between “weather” and “climate.” They are truly two very, very different things and while we want “the answer” as to what causes a weather event to be significant, I caution people to be careful about what timescale you’re witnessing the event in.

Harry: It’s sort of like climate is the political environment and weather is a special election. Sure, the climate influences the individual event, but the special election can go off in all sorts of weird directions that are unexpected and have nothing to do with climate.

David: I should have known Harry would find a way to bring this back around to the campaign, since it’s been so lacking in heat. Thanks to you both, and have a climate-neutral New Year’s.

Harry Enten was a senior political writer and analyst for FiveThirtyEight.

Matt Lanza is an operational forecast meteorologist in the energy industry in Houston.