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Posted at 04:55 AM ET, 01/07/2012

A pattern change, probably yes, but what it means for D.C. uncertain

(originally posted Friday afternoon)

Several meteorologists have been alluding to a potential pattern change that would flip our weather conditions from unseasonably mild to more persistent cold. I also think that there will be a pattern change but not as radical as some may lead you to believe.

Does the pattern look better for snow than it did during December? Yes. We should have more shots of cold air but it still remains to be seen whether the pattern change can lead to a storm track more favorable to snow.

So what are some of these meteorologists touting a big pattern change saying?

On Twitter, WeatherBell’s Joe Bastardi (@bigjoebastardi) exclaimed: “10mb [stratospheric] warming on European [model] over the pole by 360 hrs reaches 50C!!!! This is a huge event and will have [northern] hemisphere cold implications…By that I mean, something major is about to happen.”

Henry Margusity (@Henry_Margusity) of Accuweather also is anticipating a major change stating: “This is such an amazing weather pattern evolving. I told all the [meteorologists] that we have a lot of winter ahead of us.”

Finally, Dave Tolleris of Wxrisk made a more nuanced statement about a possible pattern change. He stated: All this being said this does NOT mean the winter of 2011-12 is about to turn Nasty for everyone and or the Northeast is going to see BIG snows or noreasters. …It might turn that way but we dont know that yet. BUT the old winter pattern of 2011-12 that has featured sustained warm/ mild dry pattern IS going to end soon. And whatever the new pattern … won’t be the same as what the last 45-60 days have been.

Much of the excitement by Bastardi and Margusity probably stems from a strong stratospheric warming event that is now showing up on the model guidance and how it might lead to more blocking and a negative Arctic Oscillation (AO). Remember, a negative AO means a cold and sometimes snowy weather pattern over the eastern U.S.

The pattern change ideas stem from research by Baldwin and Dunkerton published in a 2001 “Science” article which documents how there is coupling between the stratosphere and troposphere and that sudden stratospheric warming events can have an impact on the AO (by helping modulate the strength of the polar vortex), flipping it negative.

That research notes that once the warming propagates down sufficiently to produce a weather pattern favoring a negative AO (lots of high latitude blocking), the pattern tends to last for about two months. That probably explains why some forecasters think that if the AO goes negative it might stay that way through much of the remainder or the winter.

However, the Baldwin and Dunkerton article also documents that in December 1998 there was a strong sudden stratospheric warming event that did not impact the troposphere. And a strong polar vortex and positive AO developed which dominated pattern that month. The Baldwin/Dunkerton article also says that switches in the stratosphere and the AO sometimes occur independently and that no one understands the mechanics of the linkage between the two. Their research suggests that not all stratospheric warming events act the same way and that just because one is forecast, be cautious in using it to forecast your pattern change.

Other research has tried to identify the ingredients that are present when a stratospheric warming event might impact upon the tropospheric polar vortex. Right now it’s not clear to me that those conditions are going to be met. Stu Ostro (@stuostro) of the Weather Channel recently tweeted “Methinks stratospheric warming events can be overrated, i.e. they’re just one factor, & they don’t all have the same outcome.”

In my view Stu is right. This stratospheric warming event may indeed end up being the genesis of a sustained pattern change but right now there is no way of knowing whether it will or not.

However, changes still may be afoot. The potent polar vortex event and positive AO that has been going on for over two months already appears to be on the wane with or without help from the stratosphere. This strong vortex event has lasted longer than most such events so the forecasts of it weakening may have some merit.

However, on the figure below, which forecasts the future state of the AO, note how much spread there is in the individual ensemble members (red lines). Some show the AO index rebounding and going positive while others (the majority) have it trending negative and ending below zero.

Observed Arctic Oscillation (AO) phase since September (positive mostly) and forecast of the AO over the next two weeks. Many of the model forecasts predict the AO will trend negative. (NOAA)
Even if the AO does switch from positive to negative, there is no easy way to tell whether the switch will be permanent. Also, even with a negative AO, that does not always mean that the blocking will be located over Greenland and the Baffin Bay regions, especially if the AO is only weakly negative. The current ensemble mean forecasts from the GEFS or European model do not show blocking over Greenland. Without that blocking, most storms will still track to our north placing us on the warm side of the storm.

500 mb (18,000 ft) height anomalies during December. Yellow/oranges/reds indicate warm and blues/purples indicate cold. (NOAA)
Let’s look at how the pattern is forecast to change by looking first at the 500 mb (18,000 feet) height (or pressure) anomalies for December (to the right). Note how the heights from western Europe across Greenland, Canada and Alaska are below normal (blue and purple shades) and that the heights to the south across North America into the Atlantic are above normal (warm colors). Such a pattern is a classic positive AO pattern which favors warm temperatures across much of the country.

Now compare that pattern to the 240 hour forecasts of the European (left panel) and GFS (right panel) ensemble mean patterns (below). The forecasts show a significant change across Alaska and the Bering Strait area where the below normal heights of December give way to a strong ridge (warm colors) that extends northward towards the pole.

500 mb (18,000 ft) height anomalies forecast by European model (left) and GFS model (right) in ten days.
That change causes the flow across western Canada to be from the polar regions rather than from the Pacific (see black arrows). Western Canada, which for most of December was warmer than normal, now has a pattern that facilitates the building of cold air and high pressure systems. Both ensemble means show the same general pattern.

However, both still maintain below normal heights across eastern Canada, Greenland and Iceland with above normal heights to the south. Essentially the north Atlantic Oscillation (the eastward extension of the AO) remains positive. The lack of much of a southern stream and the positive NAO probably means that most of the stronger storms will stay to our north and/or will be fast moving. However, more frequent shots of cold air opens the possibility of a so-called overrunning event somewhere down the line. Right now, I don’t see one on the horizon but that could change.

Despite the similarities of the ensemble mean patterns shown above, the individual members of the GEFS for the same time period show significant differences among the members suggesting that the pattern may be less predictable than one can imply from the charts above.

Two consecutive 11-15 day forecasts from the GFS model for temperatures at 850 mb (5,000 feet). Green shades are warm, blue/purple shades are cold
Forecasts of the 11-15 day 850 mb (5,000 feet) temperature anomalies from two consecutive GFS model runs illustrate this uncertainty (to the right). One run (top panel) shows an extensive area of below normal temperatures across much of the county with the cold (blue shades) extending into the Southeast. The other run (bottom panel) has warmer than normal temperatures (warm shades) across much of the South and has the cold limited to the Pacific Northwest and the Northern Plains. The differences in the two forecasts below stem from where the models aim the cold air. Subtle differences in the pattern can make big differences in our sensible weather. My best guess is the true answer is somewhere in between but either is a possibility.

The bottom line is the pattern is changing but the extent of the change and what it means for our area is far from certain. We should get more frequent shots of cold air but relative to normal the coldest temperatures will probably remain to our north and west.

While I’m a little more optimistic about our snow chances than in my previous article , the lack of blocking in the North Atlantic means that the timing between systems will have to be perfect to get a decent snowstorm and that a major snowstorm remains unlikely though our chances of lighter events probably has risen some. If blocking develops over Greenland, then I’d be more excited about our snow chances.

By  |  04:55 AM ET, 01/07/2012

Categories:  Latest, Winter Storms

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