Wednesday, December 26, 2012

Polar Vortex Breakdown Will Lead To Frigid January-February

Confidence is growing that the polar vortex will sustain enough damage to break down in some levels of the atmosphere and lead to a pretty darn cold January into February.

We'll start out with observations. Above is the observed temperatures in the 70mb layer of the stratosphere, known as one of the lower layers in the stratosphere. We are looking on the far right side of this picture to see current values. If we do so, we see that two warming events have already taken place. The first warming event brought temperatures well above normal in the 70mb layer, and there is an ongoing warming event taking place that is leading temperatures to skyrocket in the lower stratosphere.

[Animation of 50mb]

This animation of observed temperatures in the 50mb layer also shows such recent warming (for those unfamiliar with the millibar scale, lower numbers = higher height in the atmosphere). Both warming events previously described on the first graph are shown in this animation, with the first warming event propagating from East Asia into the North Pole. That warming event then died down and slipped back into East Asia. From there, that warm body of air moved across the North Pacific and is now in Canada, where it may be trying to make a move into the Arctic Circle. If such an event happens, this warming will become much more significant than the last.

The main subject of this post is something called the 'Polar Vortex', or PV for short. The Polar Vortex is a low pressure system stationed over the Arctic, with varying strengths. The PV is actually what drives the well-known Arctic Oscillation. When the vortex is weak, the AO is negative. When the vortex is strong, the AO is positive. We'll talk more about the AO later on. But it is important to note that the polar vortex is weakened when significant warming events, like the two shown above, happen.

Now, let's go back to the first chart. See how we are already well above normal for this time of year? That means that the polar vortex is theoretically weaker than normal at the moment. If that warming in Canada continues and/or strengthens, the polar vortex will only get weaker and weaker. As long as those above normal temperatures continue, the polar vortex is indeed vulnerable to collapse, if the right parameters come together.

There is something else used to see how the stratosphere is being impacted by warm air, and it is called the Eliassen-Palm Flux, or EP Flux for short. The EP Flux measures the strength by which air from the lower atmosphere is being forced into the stratosphere. Considering the word 'flux' means movement, one can think of the EP Flux as showing the strength of movement of warm air into the stratosphere. Shown above is an 8 day observation of the EP Flux. In recent days, we have seen the EP Flux strengthen, as the second warming event continues to hold its ground. Note the new presence of colorful arrows in the area above the number '10' on the lefthand side of the Dec. 24 image. This means that warm air is now penetrating the upper stratosphere, effectively infiltrating the entire stratosphere. This is a major blow to the polar vortex, as warm air is now pushing into the entire vortex, not just the lower part.

The ECMWF forecast for EP Flux is at the bottom of this multi-image forecast. Looking at that EP Flux, we see that the European model is forecasting the flux to rapidly strengthen as the New Year approaches. This would mean warm air from the troposphere would be shooting into the stratosphere at a rate more than triple the strength we saw with our first warming event in early December. That first warming event is shown as a cluster of arrows above the word '1DEC' in that same bottom image. Should such a forecast verify, it would be the highest EP Flux values we have seen this winter season, and would certainly induce very heavy stratospheric warming- even more than we have seen to this date. If any further warming occurs (and if the EP Flux forecast shown above verifies), we have a shot at breaking the warmest value in the 70mb stratosphere on that date (shown in the first graph as the top light gray line)- something that means chaos in the polar vortex.

Continuing with observations, we visit an index called 'Mountain Torque'. The Mountain Torque, also called MT, is a fairly challenging index to decipher. However, I have come to understand it as a component that can help warm the stratosphere and weaken the polar vortex. In high values of mountain torque, it is expected that the stratosphere will warm rapidly in a phenomenon called the Sudden Stratospheric Warming (SSW). (The Sudden Stratospheric Warming got its name due to the rate at how the stratosphere warms, hence the word 'Sudden'. There is also a weaker sudden warming event called a Major Stratospheric Warming event.) Considering the mountain torque values are at values not seen since late fall, I find it very plausible that the next 5-15 days will bring another good warming to the stratosphere, a crippling blow to the vortex as the current warming continues.

Now we get to the forecasts of what the models say will happen.

We start with the GFS model and what it says about warming in the stratosphere. Forecasters commonly use two types of graphs when dealing with the stratosphere: Temperature forecasts and pressure forecasts. The former option shows warmings in the atmosphere, the latter shows the strength of the polar vortex (remember, the polar vortex is a low pressure system just like ones we see in the US, just bigger and more permanent). We'll start with the temperatures. Below are 5 images of 5 layers of the stratosphere; I'll annotate each image appropriately.

1 millibar 10 day forecast

2 millibar 10 day forecast

5 millibar 10 day forecast

10 millibar 10 day forecast

30 millibar 10 day forecast
You might be wondering why you should care about a bunch of reds and blues and fancy colors. Well, in each of these images, significant warming in the form of very warm colors is shown next to very cool colors, a.k.a. the polar vortex. Anytime you see those reds and grays, that defines very warm air finally reaching the upper stratosphere.

Something very interesting I saw in those 5 images, something you may have noticed as well, is the steady westward progression of the warm colors with each image. If you can picture 3-dimensional things in your head, you may have thought of a spiral, in the sense of the warmest areas of each respective stratospheric level rotated as each image showed a lower level of the stratosphere. If you want to compare it to material objects, think of rotini pasta or how you get bubblegum from the gumball machine; how it goes down the spiral to your waiting hand.

That spiral analogy leads us into the next significant forecast piece- displacement. You may hear this word and think back to 8th grade chemistry and water displacement, or even high school, but this is a different displacement. This is the displacement of the polar vortex.

100 millibar 10 day temperature and pressure forecast

1 millibar 10 day temperature and pressure forecast
Shown above are two pressure and temperature forecasts from the ECMWF model. The letter 'L' defines the center of the polar vortex for each level in the atmosphere, and the letter 'W' shows the warmest values in each respective layer of the atmosphere. If you notice a plus symbol, that is the center of the Arctic, quite literally the North Pole. And if you happen to see the letter 'H', that symbolizes the highest pressure for that atmospheric level.

I said we were looking for displacement here, and that is what we will do. We are trying to find how far apart the letter 'L' is from the 1mb image to the 100mb image. If we look in the top image (100mb), we see that the polar vortex is centered just to the south of the North Pole, a.k.a. just north of Europe. The 1mb image on the bottom has the polar vortex just east of Greenland, and high pressure building in over Eurasia.
While this is not a classic example of extreme displacement, it is apparent that there is some good displacement. The top image shows two apparent centers of low pressure- one with the 'L' that I described in the above paragraph, and one not marked but still shown in the top right corner of the image in what appears to be East Asia. On the 1mb chart, high pressure has built in right over that second low pressure area that I told was in East Asia. As for the deepest centers of the polar vortex, they are relatively close to each other, meaning the displacement is not that extreme, but far enough apart for me to introduce my Jenga analogy:

Think of the game Jenga. You must try to take out pieces of the tower to get as many pieces out, but still have it standing. The more pieces you take out on one side, the more unstable that side becomes, and eventually the tower falls down because of the difference in stability of the tower (a.k.a. where more blocks are placed). In a similar situation, when you have the polar vortex far apart in different levels, like what is forecasted above, the polar vortex becomes more unstable and weakens, possibly to the point of collapse.

One more analogy for you: Think of the polar vortex like a cylinder full of cold air. If you cut the cylinder in half and move one half away from the other, the cold air will sink. That is exactly how the stratosphere. If you have displaced parts of the stratosphere, the cold air that is held in the polar vortex will be released and flow down into lower latitudes. If the PV is displaced enough, it may collapse altogether, leading to an icebox solution over parts of the world.

The GFS model shows a similar situation as far as that high pressure/low pressure difference in Asia, but has a lot more displacement in the actual polar vortex as shown below:

1mb 10 day pressure forecast

100mb 10 day pressure forecast
The GFS dislocates the polar vortex far away from itself in between these two levels, which theoretically would enhance the probabilities of a weaker polar vortex and thus higher potentials for a strong cold snap in January and February.

So, we have big warming in different levels of the atmosphere per the GFS big pressure differences in the polar vortex in the GFS, as well as good displacement of the vortex in the ECMWF. But there is something else showing up in the GFS- complete collapse of the upper stratosphere polar vortex.

1mb 384 hour pressure forecast
5mb 384 hour pressure forecast
10mb 384 hour pressure forecast
This is where you will need to read these images closely. We'll start with the top image at the 1mb level. In this top image, we see a large circle covering much of the Arctic. If you read closely, you will see that the numbers go up as the lines get smaller, meaning that this is a gigantic high pressure system. The polar vortex has collapsed in the 1mb layer at Hour 384! The 5mb layer shows a big high pressure system over the Bering Sea splitting up the polar vortex into two pieces, and a similar forecast resides in the 10mb image.

You weather aficionados are most likely wondering why I'm showing forecasts at Hour 384, the longest of the the long range, and the high point for forecast failure rates. The reason is simple: Trends. I went back across several GFS runs for the 1mb forecast and found that the high pressure system was still covering the Arctic in as many as 5 other forecasts. That said, my faith in this forecast increases dramatically, and, while it's still quite a ways out, I am encouraged by this forecast of a polar vortex collapse in the very upper stratosphere.

Remember how I said the polar vortex is the driving force behind the Arctic Oscillation? Let's keep that front and center. This is a multi-model (and ensemble) forecast of the Arctic Oscillation. In this forecast, we see that the GFS, GFS Ensembles and GGEM Ensembles project the AO to be in the positive range come the 2nd week of January. Based on all the evidence I reviewed above, do you think that will actually happen?

This 8-10 day 500mb height anomaly forecast from the ECMWF (left) and GFS (right) shows no striking presence of a positive Arctic Oscillation, which would be shown as below normal height anomalies across the Arctic Circle. While we do see some low pressure anomalies creeping into the North Pole, nothing defined is showing up, meaning I believe this positive AO forecast is bogus. Considering the warming already happening, the warming forecast to happen, and, potentially, the collapse of the upper stratospheric polar vortex, I have a hard time believing the models are correct with a +AO in the future.

In summary:
-Significant warming is already ongoing in the stratosphere.
-EP Flux values are now pushing warm air into the entire stratosphere.
-EP Flux forecasts show dramatic and significant warm air being forced into the stratosphere.
-Mountain torque highlights more stratospheric warming in the future.
-Model forecasts support heavy damage being sustained to the polar vortex.
-The positive AO forecast does not look correct in my opinion.

All in all, what I have described here today bodes well for a cold episode that may span the months of January and February. If the polar vortex has a collapse, severe cold could very well overtake the nation. Prepare for cold times ahead.


New Year's Potential Storm

The New Year's potential storm system continues to be on track, but a bump in the road may mean it won't go up the East Coast.

Currently, there is a forecasted positive North Atlantic Oscillation (NAO), meaning that low pressure exists over Greenland. Because of this low pressure system over Greenland, storms that travel along the South US will not get picked up and brought north along the East Coast. If that positive NAO forecast does verify for this timeframe, I find it likely that the system will shoot offshore.

Something that should be watched in this GFS forecast shown above is that high pressure system off the coast of the Southeast US. It is possible that the storm system could be forced north into the Ohio Valley or even New England area if that high pressure system holds its ground against that storm system in the Gulf Coast, or it could go out to sea.

Nonetheless, a good severe weather situation should set up across the South in response to this storm system in the area. Already shown in the 3 hour precipitation forecasts is a linear precipitation forecast, meaning a squall line could be forecasted by the GFS. It's too far out to tell, but that is certainly a forecast typical of squall line formation.

The ECMWF says a very similar situation for this storm system, as in the system goes offshore.

This is a pretty sparse post. I have been VERY busy behind the scenes constructing a major post that will be out later today concerning the stratosphere.