Stratospheric Polar Vortex Showing Long-Range Rumblings

 Good evening, everyone!

As has been the case in previous attempts to re-start this blog, my stay here will be brief - I am blessed enough to be transitioning to a new job that will continue to allow me to forecast the weather, but for about the next month or so, the proprietary covers are off, if you will, allowing me to get back to where we first started!

Attached above is a look at the current state of the 10-millibar part of the atmosphere, corresponding to the upper levels of the stratosphere and - more importantly - the slice of the atmosphere most relevant to tracking the stratospheric polar vortex. Here, blues and greens represent below-normal geopotential height anomalies (in this case, indicating the polar vortex is stronger than normal) while oranges and reds represent above-normal geopotential height anomalies (here, meaning the polar vortex is weaker than normal).

As the graphic shows, the stratospheric polar vortex is currently pretty well consolidated, with broad below-normal anomalies across the Arctic Circle and minimal (if any) attempts at bringing warmer air towards the North Pole. This is a primary driver behind my expectations for a warmer than normal December across most of the United States, perhaps save for the Pacific Northwest in line with traditional La Nina climatology. But things begin to change as we move towards the middle of December:

The 10-day forecast from the latest ECMWF model shows a notable change by December 9th, as a broad ridge begins forcing its way poleward from western and central Canada. A secondary, weaker ridge is also evident in Eurasia, making for what looks on paper like a wave-2 vortex-splitting attempt but in reality ends up shoving the vortex into Siberia and not really splitting at all.

Temperatures also jump considerably over North America at this level of the stratosphere during this same time as this ridge formation...

All of this is good and well, and we do see some support from the GFS ensembles (not shown), but it doesn't look very promising to me. For one, this warming event appears to originate in North America; my preferred origination point for a polar vortex disruption is in the Bering Sea, as this area (in my experience) has tended to bring about the more powerful polar vortex disrupting events. Second, if this really is a vortex-split attempt instead of a vortex-displacement attempt (a la the weak secondary ridge in Eurasia), and if we take this forecast verbatim, there's no way we see this transpire. 

Think of it this way: a vortex-disrupting SSW involves a single, massive ridge that brings blowtorch-level warmth into the Arctic Circle. A vortex-splitting SSW involves two somewhat-strong ridges that pinch the polar vortex somewhere around the North Pole, cutting the vortex into pieces. Here, we see a single moderately strong ridge. That doesn't inspire confidence to me, and as a result I think this is a flash in the pan: a good sign for future SSW opportunities, but not enough to result in the risk of a severe Arctic outbreak at the turn of the new year.

To Summarize:

• The stratospheric polar vortex is currently stronger than normal, raising the risk of above-normal temperatures across the U.S. through December.
• There are some indications of a warming event around the middle of the month, but from what I see, this doesn't appear likely at this time to significantly disrupt the polar vortex
• Consequentially, the chances of an SSW by mid-December appear low

Andrew

Upper Stratospheric Polar Vortex Warming in Short Term

(Note: This post discusses the stratospheric polar vortex in the short term (out to Day 10). The other stratospheric post discusses expectations next month.)

The upper stratospheric polar vortex is forecasted to experience some intensive warming, in the atmosphere's attempt to get this winter started.

FU-Berlin

The image above shows a graph of the forecasted Wave-1 temperature outlook from the ECMWF model, 10 days from today. While daunting at first glance, it can be explained.
The different levels of the atmosphere are listed on the left-hand legend. We can see the graph covers the atmosphere from the 1000-millibar level up to the 1-millibar level. Note the warmest colors centered right around the 3-millibar line. The bottom legend shows lines of longitude.

Let's depart this forecast for a moment and discuss different modes of stratospheric vortex disruption. There are two primary modes of disruption: Wave-1 and Wave-2. In a Wave-1 stratospheric polar vortex disruption event, a single ridge / body of warm temperatures forms aloft and singlehandedly tries to displace the polar vortex. This singular body of warmth / ridging is why it is called a Wave-1 event- Wave-1 events are also the strongest type of disruption event. Similarly, Wave-2 events involve two bodies of ridging / warmth trying to squeeze into the Arctic Circle, usually to split the polar vortex into two vortexes. This is weaker than a Wave-1 event, but still wields significant power in the atmosphere.

This temperature chart above shows a temperature spike at the 3-millibar level. Remembering this chart specifically identifies Wave-1 events, we can then deduce that the ECMWF model is expecting a Wave-1 disruption attempt at the 3-millibar level of the atmosphere. We can look at this on a 3-millibar forecast map below, valid for the same timeframe as the graph above:

FU-Berlin

Note how we see a single body of warm temperatures on the right-hand side of the hemisphere (if you look closely, the warmth is centered over Eurasia), visually showing that Wave-1 pattern I discussed earlier.

Why is this important? It means that the atmosphere is applying pressure to the stratosphere to try and make the pattern more conducive for wintry weather here in the troposphere, something that's been lacking this December. Now, vortex disruption at the 3-millibar level won't do much down here at the surface, but if it can expand to lower levels of the stratosphere (ideally 30, 50, and/or 100-millibar levels), the influence becomes greater on us here at the surface.

To summarize:

- The upper portion of the stratospheric polar vortex looks to experience warming in the next 10 days and beyond.
- While insignificant at that height of the stratosphere (3-millibar level), it could indicate a pattern more conducive for wintry weather may set up down the road (i.e. into next month).

Andrew

Upper Stratosphere Experiencing Minor Warming

The upper stratosphere appears to be undergoing some minor warming, although it is unlikely to set up a polar vortex disruption event.

CPC

The above animation shows temperature anomalies at the 10-hPa level of the atmosphere, in the upper levels of the stratosphere, over the last month. Beginning around November 20th, we started to see increasing temperatures just east of Japan, which really amplified in strength around November 27th. They aren't really moving in a particular direction, although a slight northward push has been noted over the past couple weeks.

CPC

This warming is likely thanks in part to a substantial increase in north poleward eddy heat flux values since about the last two weeks of November, right at the time when we started to see that warming commence in the 10-hPa layer. Typically, increased eddy heat flux values reflect an increased movement of warm air from the lower latitudes to the poles. Higher values can precede or coincide with stratospheric warming events, which is likely why we're seeing warming at the 10-hPa and 30-hPa levels.

CPC

The 30-hPa level, just a bit higher than the middle of the stratosphere, is showing some warming, albeit deflected a bit further east than the 10-hPa warming. In addition, this 30-hPa warming is showing an eastward push, rather than a poleward push. The latter would be more favorable for a polar vortex disruption event / stratospheric warming, so winter weather fans may not be too encouraged to see this.

To summarize:

- Minor warming is being observed at the 10-hPa and 30-hPa levels of the stratosphere.
- This warming is not strengthening quickly and is not making a strong poleward movement, so a stratospheric warming event is not expected over the next several days.

Andrew

Long Range Outlook: Dynamic Pattern Unfolding into February

This is the Long Range Outlook, covering the period from mid-January (present day) to around the middle of February.

Tropical Tidbits

The above image shows 500mb geopotential height anomalies across the West Pacific, projected from the GFS ensembles and valid on January 13th. Notice the strong ridge forcing itself north to the west of Japan, likely bringing substantial warmth along with it. If we apply the Typhoon Rule to this situation, which states weather phenomenon occurring in Japan is reciprocated 6-10 days later in the United States, we should expect a period of warm weather commencing around January 15th to 17th, evidenced by a gradual warming trend unfolding in short range forecasts here at home. The Typhoon Rule supports such warmth lasting for about 3-5 days, before a storm system ushers in another wave of cool air. From here, we go to our impending cold blast.

Paul Roundy

A look at the hovmoller diagram above will help us diagnose the atmosphere. We see our current Madden-Julian Oscillation (MJO) wave moving eastward to the 180 degree longitude mark by around the 18th of January. From here, typical MJO phase space diagrams (not shown) show the index moving into territory too weak to identify the phase, but that's just because another MJO wave is forming out by the 50E longitude line, around January 21st. This positioning around the 50E line puts us in a Phase 1 MJO event, as seen below.

BOM

This chart shows typical alignment of outgoing longwave radiation (OLR) values by MJO phase in the west-central Equatorial Pacific. Notice that we see negative OLR anomalies (blue colors) in the hovmoller diagram around January 21st near that 50E longitude line, which matches up with Phase 1 (top-left panel) in this graphic immediately above. This, along with some Typhoon Rule support, should enable a sharp Arctic blast to round out January, possibly to begin February. I do have concern that this wintry return may be weaker than what is currently advertised, as stormy weather over Japan only lasts about 24 hours, if that. This should be a cold event lasting anywhere from 2-5 days for the USA.

As we move ahead into February, things get a little dicey.

JMA

We saw a notable stratospheric warming event strike to kick off the New Year, with temperatures at the 30hPa level only now settling back down to normal levels, as the graphic above displays. Let me please clarify: This was not a Sudden Stratospheric Warming (SSW) event. In order to deem an event a true SSW, wind direction in the stratosphere must either significantly slow or even reverse, in addition to intense warming of temperatures. We saw the latter, to some degree, while the former wasn't as prevalent as we needed. For this reason, it's referred to as just a notable warming event.

The concept right now is that this warming event will lead to more wintry weather about 2-4 weeks down the road from when the warming occurred, which brings us to that late January-early February period. With unfavorable indications from Japan, though, I worry that the cold (while intense) may not stick around for long. In order for that to happen, we may need another warming event, ideally a true SSW. However, even that prospect is quickly fading, as we see a strengthening of the upper polar vortex in coming days and weeks.

Let's dive into February a bit deeper.

Kyle MacRitchie

The above image once again shows forecasted outgoing longwave radiation (OLR) anomalies, but now forecasted well out into the spring. Due to such low confidence, we're only going to look at the long range OLR forecast as an indicator of what may come, not necessarily as something that's "set in stone". Notice how we see our ongoing MJO wave fading near the 21st of January with the dissipation of negative OLR anomalies, as well as our second wave forming well off to the west. As the Paul Roundy hovmoller showed, this second wave slowly shifts east as we move into February, but then note what this forecast predicts in the middle and late stages of February. We see the MJO stalling out around the 160E longitude mark. Going back to our MJO OLR composite image earlier in this post, we see this correlating to Phase 6, also known as the warm weather phase. Here's a graphic displaying a Phase 6 MJO event in February, to give you an idea of what we could see if this forecast verified.

Meteonetwork

We see a strong ridge unfolding across the East US into the Ohio Valley and Great Lakes during this February Phase 6 MJO event, with stormy weather impacting the Pacific Northwest. Ridging is shunted south and east across the Northeast Pacific, into the Southwest US. This should be particularly concerning for winter weather fans in the East, because this set-up does have additional support. Model guidance is expecting the atmosphere as a whole to dive into a La Nina-esque pattern, which is shown like the image below.

NOAA

During a typical La Nina pattern, we tend to see strong high pressure across the Gulf of Alaska, before the jet stream buckles south and delivers cold weather into the North Plains and Northwest. Ridging then is provoked in the East US (the reason why cold weather fans in the East see La Nina's in their nightmares). This, of course, is not a perfect correlation, but you can make key connections between the typical La Nina environment, as well as the projected Phase 6 MJO event in February. This would not bode well for a cold and stormy end to winter for most east of the Rockies, unfortunately.

On a side note, many have asked why I am using the MJO in my forecasts, since we are currently in the pro-warmth Phase 6, but cold weather is prevailing. I continue to use the MJO because it usually provides a good idea as to where we are going in the future. The MJO's effects were being overruled by a very strong ridge along the West Coast; remember that the MJO is not the sole player in this atmosphere, and that's being exhibited well. We are currently seeing warmth return to the US as that ridge breaks down in accordance with the warm MJO phase.

Even when the MJO seemed to be failing, it still managed to reflect itself well in the atmosphere. Shown below is a side-by-side comparison of the last few days of January 500mb geopotential height anomalies (left), as well as typical 500mb height anomalies during a January Phase 6 MJO event, in an ENSO atmosphere reflective of this one. The comparison isn't perfect, by any means, but it's there.

ESRL and Meteonetwork

To summarize:

- Warmer than normal weather is expected around a January 15-23rd period.
- A storm system may impact the country around January 21-25.
- Cooler than normal weather is expected in the final days of January, possibly into February.
- There is increasing potential for a warming trend in the middle of February back to above-normal temperatures.

Andrew

New, Successful Analog Guidance Predicts Warm December, Cold January

A new system of analog guidance I composed in recent days, showing success in the limited number of trial runs I have conducted, is forecasting a predominantly warm month of December, followed by a cold January.

ESRL

The image above shows analog guidance's temperature forecast for the month of December. As the image shows, millions across the country would average out over a degree (Celsius) above normal, primarily in the Plains and Midwest. Only the Southwest would see a colder than normal start to winter.

Now, how accurate is this analog guidance? I just got this system up on its feet last night, and these are the first results of the forecast. Some development is still needed, but that will come with time. I've noticed the analogs seem to understand the general pattern (i.e. negative Arctic Oscillation, positive PNA, etc), but still do struggle with less-synoptic features. For now, I'm presenting the data here, allowing it to verify and examine how it does later in January.

ESRL

In January, my analogs see the mass of cold air being stored in Canada on the last image bulldozing southward, bringing below-normal temperature anomalies to much of the nation, especially the northern Plains. The South then looks to see slightly above normal temperatures. Oddly enough, this pattern is surprisingly La Nina-like, even though I made sure to factor in El Nino analogs.

ESRL

The 500mb height forecast anomalies for January show sustained ridging over the Arctic, forcing what appears to be the brunt of the Polar Vortex south into western North America. This then bleeds east, allowing for those chilly temperature anomalies in the Central US.

I'll end this post with a look at the analog forecast for the stratospheric portion of the polar vortex, since this image only shows the tropospheric version.

ESRL

If you look closely, you'll see the polar vortex has been nearly obliterated...

To summarize:

- A new set of successful analog years is predicting a warm month in December, followed by a cold month in January.

Andrew

Polar Vortex Split Occurring; Arctic Outbreak Possible in Mid-December

The stratospheric polar vortex has experienced a split as a result of its second interruption by warm air this season, and it appears some cold weather may be on the way as a result.

CPC

The chart above shows heat flux values over the 2014 calendar year, from January 1st to present day. In this graph, shifting our attention to November and December, we find that flux values have skyrocketed to near-record levels, only now beginning to drop down a bit. According to the Climate Prediction Center (CPC), strong episodes of eddy heat flux events to the poles may result in a Sudden Stratospheric Warming (SSW) event. You may recall that SSW events occur when wind direction in the stratosphere reverses from low pressure-favoring winds to high pressure-favoring winds. This decimates the polar vortex, and usually unleashes near-record intensity cold weather on whoever is in the line of fire about 2-4 weeks after the SSW event occurs. Considering we are seeing an eddy heat flux episode of this magnitude, it's no surprise the polar vortex is undergoing some serious damage.

FU-Berlin

The ECMWF forecast of height contours and temperatures (shaded) about 96 hours from now, at the 50 millibar level, shows a Wave 2 stratospheric intrusion event. For some clarification, let's remember that the stratosphere exists primarily from the 1 millibar to the 100 millibar level. Therefore, I'm using the 50mb level as a happy medium, per se, so we don't receive any tropospheric influences that may occur at the 100mb level, but also keep away from vortex events in the far upper stratosphere that will ultimately have little to no effect on the troposphere.
As for the Wave 2 event, stratospheric polar vortex split events are classified into a few 'Wave' numbers. Take a look at this image to see the difference between a Wave 1 and Wave 2 event.

NASA

This image here looks complicated to those who may not be as enthusiastic about the weather as others, so we will focus on the middle column. In that middle column, we essentially are seeing areas of low pressure in orange, and areas of higher pressure in blue. On the top row, a normal polar vortex form is displayed.

Wave 1 events indicate a very strong warming event or body of high pressure is forcing the polar vortex off the North Pole, while still keeping it in one piece.
Wave 2 events are seen on the bottom row, where a weaker, but still intense warming/high pressure event occurs, but now splits the vortex into two vortices.

Using that knowledge back on the ECMWF image, we can confirm that a Wave-2 stratospheric split has occurred.

Now, bearing in mind that it usually takes 2-4 weeks for stratospheric intrusions to result in cold air outbreaks on the surface, we flash forward to a potential chilly period in mid-December.

CPC

In the long range forecast from the Climate Prediction Center, the Arctic Oscillation (AO) looks to turn negative in about two weeks, right when the timeframe for any stratospheric consequences opens up. Many signals support the warm start to December ending around the middle of the month, and this stratospheric event only strengthens this potential.

To summarize:

- A stratospheric intrusion may result in a cold air outbreak in the middle of December.

Andrew

Thanksgiving Potentially Significant Winter Storm

This is an update to yesterday's post, discussing a potentially significant winter storm in the November 24-30 timeframe.

Tropical Tidbits

The image above shows the ECMWF 500mb geopotential height values in color, with superimposed mean sea level pressure (MSLP) values and appropriated high/low pressure marks. In this image, valid for November 8th over the West and North Pacific basins, we see an incredibly strong extratropical storm circulating over the far western Aleutian Islands, with a minimum SLP value of 920 millibars.

To give you an idea of how strong this reading is, if this forecast from the most prestigious weather model in the world were to verify, we could very well break the record for the lowest extratropical minimum SLP value ever recorded (the lowest SLP value ever belongs to Typhoon Tip, with 870 millibars, but that was a tropical system). But the most intriguing thing?

It could actually happen.

This whole process looks to begin with the remnants of Typhoon Nuri, currently offshore Japan, which will race northeast and quickly strengthen as it approaches the Bering Sea. While storm systems tend to strengthen in the Bering Sea, this one may strengthen much faster and even stronger than most due to that tropical component from Typhoon Nuri.

But wait! Why should we care about this? What about the Thanksgiving storm? The potential for a significant winter storm evolves out of the Bering Sea for this forecast. The method, referred to as the Bering Sea Rule, takes the occurrence of high and low pressure events in the Bering Sea, and expects a similar weather phenomenon to occur in the US about 17-21 days later. This is almost exactly like the Typhoon Rule I commonly discuss when analyzing winter storm potentials, except now the area to watch is the Bering Sea, and the timeframe from occurrence in the Bering Sea to reciprocation in the US is now 17-21 days. Therefore, if we take the storm in the image above (valid on 11/8) and extrapolate it out using the BSR guidelines, we come up with a potentially significant storm in the November 25-29 period, give or take a day or two.

Tropical Tidbits

This next graphic above shows the same 500mb height values and superimposed MSLP values over the West Pacific, like the ECMWF model forecast, but now from a different model. This is the GFS-Parallel model, rumored to be the next-generation forecasting model that will soon replace the GFS model. For the time being, this is an experimental model, and won't become operational for another month or two, but it's still worth using for examination purposes.

Here, we see in this forecast for November 8th, the minimum SLP is down to 914 millibars. That's about 6 millibars stronger than the ECMWF forecast, and indicative of a likely-record-breaking storm, but the premise remains the same. Both model guidance systems are showing a very strong storm system in the Bering Sea in this November 8th timeframe. If you're a true analyst, you might notice that the GFS-parallel forecast is just a bit east of the ECMWF forecast. That does make a difference in this forecast, and I'll explain it a bit later in this post.

Tropical Tidbits

In the spirit of including the old with the new, we'll go over the currently-operational GFS model and its forecast. Here, we see the projection for November 8th calls for a 923 millibar cyclone to hit the Bering Sea. This is the weakest of the three forecasts we've analyzed thus far, but that's a pretty odd thing to say when the weakest member of the three is "only" at 923 millibars. Despite the slightly weaker outlook, the consensus is still for a very strong, possibly record-breaking storm to hit the western Bering Sea.

Tropical Tidbits

Lastly, to show I'm willing to include all angles and not, as some may accuse, only the strongest projections, we'll go over the ECMWF ensemble set. The ECMWF ensembles, commonly called the most accurate ensemble set in the world at this time, are forecasting a minimum SLP of 947 millibars for this storm system on November 8th.

If you aren't sure how to interpret this, I'll sum it up: WOW!

Ensemble guidance is composed of a number of 'members', or versions of the original model that have been slightly altered, intentionally, to produce a 'spread', or variation, in the forecast. This then adds accuracy to the forecast. It's very surprising to see a set of ensembles - from the ECMWF, no less - to be calling for a 945mb storm to strike the Bering Sea. This average of 52 - yes, fifty-two separate ensemble members - indicates that certainty for such a strong storm to hit the north Pacific is on the rise. It's quite plausible we see this minimum SLP from the ECMWF ensembles drop in future runs.

But we're not just watching for a powerful winter storm- we're watching for potentially significant cold!

CPC

The animation above shows temperature anomalies at the 10 millibar level, in the far upper reaches of the stratosphere, over the past month or so. According to this animation, we recently saw a very early stratospheric warming event overtake the Eurasia and North Pacific basin. If we remember that the warmth in the stratosphere typically results in very cold weather at the surface about 2-4 weeks later... put the pieces together, and we could very well see a strong body of cold air intercept this potentially significant winter storm system, making for one heck of a situation that could unfold.

You might have noticed a combination of rather ominous language about the storm, as well as more than a few caveats. Both are well warranted, as we'll have to wait and see if this storm ends up being as strong as projected in the Bering Sea.

To summarize:

- The potential exists for a significant winter storm around Thanksgiving.

- With stratospheric-induced cold air possibly intercepting this storm, winter weather may be a serious issue to contend with.

- High uncertainty still exists, but appears to slowly be waning.

- As of now, the Central and East US look to be affected by this storm (i.e. Midwest, Great Lakes, Ohio Valley, Northeast...).

Andrew

Weak Stratospheric Warmings Suggest Turbulent Winter Ahead

The first substantial stratospheric warming event appears to be underway in the Northern Hemisphere, and could be a signal for what's ahead this winter.

CPC

The animation above shows temperature anomalies at the 10 millibar level of the stratosphere across the world over the past month. The Southern Hemisphere has been on the receiving end of more than one powerful sudden stratospheric warming (SSW) event, but we'll focus our attention on the Northern Hemisphere for now.

Take a look at what's been happening over Canada and into Greenland in recent days. We've been seeing warmer than normal temperature anomalies push northward from Canada into Greenland, and now across the Arctic Circle. In the winter, when these events happen, outbreaks of cold air may be expected at lower latitudes approximately 2-4 weeks after the event. This warming is pretty minor, so I wouldn't expect any significant cold to come from it, but there's more to this whole process.

CPC

If we look at the same animation, now at the 50 millibar level of the stratosphere (generally considered the middle of the stratosphere), we still see that swath of warmth pushing north from Canada into Greenland, and actually extending westward into the Pacific as the warmth is shoved into the Arctic. The presence of this warmth across the upper and middle stratosphere means that this isn't just a fluke warming, but in terms of an early-season stratospheric warming event, this one's pretty substantial.

All of this may be hinting at what kind of winter we'll see this season. If the stratosphere is already rather vulnerable to warm air invasions, it could mean that the polar vortex, which spans the troposphere and stratosphere, may be weakened significantly this winter, possibly leading to more frequent wintry outbreaks. The chance of this polar vortex weakening is raised by the expectation for stratospheric winds to be unfavorable for the formation of a strong polar vortex.

More on this will be analyzed in the near future.

Andrew

Stratospheric Final Warming Occurring; Sustained Warmth Just 2-4 Weeks Away

In an update to an earlier post I wrote, where I evaluated the state of the stratosphere and possibilities for the final warming occurring, I believe we are indeed seeing the final warming event, meaning spring weather could be as little as two weeks away.

*Note: In previous posts, I addressed the potential of seasonal temperatures returning in just a handful of days. That forecast still stands- I am looking solely at the stratosphere in this post.

The image here shows a black line, which tells us observed temperatures in the 30 millibar level of the atmosphere, and also shows a superimposed gray line, which depicts the average temperature at the 30 millibar level at a given time. If we look at observed temperatures right now, where the black line stops, we see that they are well above the gray line, as much as 20 degrees above normal. The 30 millibar layer is located very high up in the atmosphere, especially when you consider we live on the surface level, located at the 1000 millibar, and considering planes usually fly at 35,000 feet, or about 250 millibars. Now, this massive warming event is not an odd occurrence in the stratosphere. Every winter, we usually see a few instances where the stratosphere suddenly warms up, which we refer to as a sudden stratospheric warming (SSW), or major stratospheric warming, which is a weaker SSW. However, at the end of winter, the stratosphere flips from having persistent low pressure dominate the upper millibars (we call this the polar vortex (yes, it's actually located very high up in the atmosphere, NOT on the surface)) to persistent high pressure, as the stratosphere basically shuts down for the winter. This flip from low to high pressure is natural, and happens every year, usually in the early spring months.

How do we know when this flip occurs? We look for a big SSW event that reverses wind direction in the stratosphere. In the Northern Hemisphere, positive zonal winds are also known as 'westerlies', as they move to the east in a counterclockwise formation. That is why we look for areas of positive zonal winds to identify the polar vortex in the stratosphere, because the polar vortex is essentially just one big low pressure system. In the same sense, negative zonal wind anomalies define 'easterly' winds, as they blow towards the west. Recall that high pressure winds spin to the west in a clockwise motion, providing the reason why we look for negative zonal winds to tell if the polar vortex has weakened.

Let's apply that rule on zonal winds to this observed chart of 10 millibar winds. Oranges show positive zonal winds, or westerly winds, while blues define negative zonal winds, or easterly winds. We observed strong westerly winds in the upper stratosphere in December and into January, indicative of a strong polar vortex. However, as we began to move into February, we began to see weakening of these westerly winds, and perturbations of easterly winds, the result of brief sudden or major stratospheric warmings. These have usually seen the polar vortex weaken but then recover. However, at the bottom of the image, we see that there has not been a recovery yet. Based on the previous SSW events, the polar vortex should have recovered by now. Despite this, we see no resurgence of westerly winds. Considering it's now late March, it's time to face the facts: we're probably looking at the Final Warming event, thanks to the strong 30mb warmings and lack of a resurgence of positive zonal winds.

We can also take a look at past and present zonal winds across the whole stratosphere and troposphere, not just the 10 millibar layer. The image above shows a cross-section of latitude-by-height zonal winds, where positives are in oranges and negatives are in blues. They go from least recent on the top left, to most recent on the bottom right. You can also identify the images by the dates just above each image. Note how in each image, since March 16 to March 21, we have seen the lack of positive zonal winds in the upper stratosphere, shown at the top of each panel. We have also seen a weakening of positive zonal winds that still remain across the stratosphere, and even into the troposphere. Once again, the lack of positive zonal winds coming back after the SSW event looks to have ended tells me not only has the polar vortex essentially collapsed in the upper stratosphere, but the final warming is occurring.

This can be further confirmed by the presence of dominating high pressure at the 1 millibar level of the stratosphere. Note the lack of a sustained polar vortex...

To summarize:
• We are looking at the Final Warming occurring in the stratosphere.
• This event signals the end of winter in the stratosphere.
• Since the effects of stratospheric events take 2-4 weeks to propagate to the surface, we should see a cold shot followed by potentially sustained warmth roughly 2-4 weeks from now.

Andrew

Stratospheric Polar Vortex Collapsing; Final Warming Occurring?

We're going through a collapse of the upper stratospheric polar vortex at this moment, and it's now possible that we are seeing the Final Warming event occurring.

The image above shows a six-panel graphic, each marking a day's observation of zonal wind anomalies by latitude, as the legend on the bottom shows, and by height in millibars, as the legend on the left shows. Going from the top row of panels to the bottom row, from least to most recent, we see a notable and rather sudden drop in positive zonal wind anomalies in the upper right-hand corner. Looking closer, we find this drop in anomalies over the 1 to 10 millibar level, in the highest reaches of the stratosphere (remember that we are based at roughly the 1000 millibar level, so 1 millibar is far higher than even what planes cruise at). Now, this transition from orange colors to blue colors in the top right hand corner of the most recent observation, marked under March 15 2014 means that the winds have reversed. In the Northern Hemisphere, positive zonal winds are also known as 'westerlies', as they move to the east in a counterclockwise formation. That is why we look for areas of positive zonal winds to identify the polar vortex in the stratosphere, because the polar vortex is essentially just one big low pressure system. In the same sense, negative zonal wind anomalies define 'easterly' winds, as they blow towards the west. Recall that high pressure winds spin to the west in a clockwise motion, providing the reason why we look for negative zonal winds to tell if the polar vortex has weakened. In this case, rather than the positive zonal winds just weakening a bit, it looks like they completely reversed in the far upper stratosphere, marking a collapse of the polar vortex at that level. Also note the lack of strong zonal winds in the mid and lower stratosphere, also indicative of a very fragile polar vortex.

An analysis of the 1 millibar level's temperature and height contours tells the story. In this view, covering the Northern Hemisphere (if you look closely you can see the continent outlines in white), we see high pressure duking it out with low pressure-- and winning. The high pressure has taken over the Arctic Circle, pushing the upper stratospheric polar vortex to the south. The swath of warmer than normal temperatures is also helping to weaken the polar vortex to the point that those zonal winds have reversed, meaning the polar vortex has fallen.

Let's take a look at another multi-panel graphic to get an even better handle on this situation. On the top panel, we see something we're already familiar with- the turquoise line denotes zonal wind values at the 1 millibar level, and the prominent dashed line indicates the separation line between positive zonal winds (upwards from the dashed line), and negative zonal winds (downwards from the dashed line). This graph tells us that as of late, zonal winds at the 1 millibar level have been sustained at negative levels, confirming this idea that the winds have reversed in the upper stratosphere, leading to a failure of the polar vortex. The panel just below that one shows us zonal winds for the 10 millibar level (blue) and the 30 millibar level (red). While these two levels are not seeing negative zonal winds, they're awfully close to zero on that graph, and that tells me something else is going on here. I'm led to believe that we are now under a final warming warning, if you will.

In the winter, there are sudden stratospheric warming (SSW) events that see massive quantities of warm air shoved up into the stratosphere, disrupting the polar vortex and sending frigid air down to lower latitudes, such as Eurasia and North America. However, in late winter or early spring, we see a massive warming event that reverses zonal winds to negative values for the summer. This means that we see the polar vortex disintegrate for the summer, replaced by dominant high pressure. This process is natural and happens every year. Now that we're in March, and we're seeing a stratospheric warming event that looks like it's dragging down positive zonal winds across the stratosphere to zero or negative values, it looks to me that this could very well be the Final Warming for the winter, heralding the end to the winter of 2013-2014 (at least in the stratosphere).

The 10 day ECMWF zonal wind forecast, the same format as we saw in the six-panel image at the top of this post (except valid 10 days from today), gives a prognosis that the Final Warming is either occurring or will occur soon. Even 10 days from now, we see slightly negative zonal winds now extending from the top of the stratosphere down to the 20 millibar level. Yes, we see a spot of positive zonal winds centered around the 30 millibar level on the right side of the graphic, but no big rebound to a strong polar vortex in 10 days' time tells me the stratosphere is shutting down for the summer.

This means, in layman terms, that winter could be over in just 2-4 weeks. It might seem like a long time, but it's better than not knowing if it will end at all, right?

Andrew

Sudden Stratospheric Warming Commences in Northern Hemisphere

The winter's most sizable sudden stratospheric warming event to date has commenced.

The animation above shows us temperature anomalies in the 10 millibar level of the atmosphere, which is located way up in the upper stratosphere. We see how the stratosphere has been rumbling since mid-January with attempts at a stratospheric warming event, but it is only as of the last few days that the stratosphere saw a massive warming event, titled a sudden stratospheric warming (SSW) event. To see how big this event is, let's head down to the 30 millibar level, located further down in the atmosphere compared to the 10mb level.

The 30mb temperature anomaly animation above shows how the stratospheric polar vortex has been under attack for a longer period of time compared to the upper stratosphere. We see a large swath of warmth had been developing and manifesting itself in Eurasia in January into February before finally bursting into the upper latitudes. It is now clear that the sudden stratospheric warming event has extended itself into a decent chunk of the stratosphere. Let's check up on the 50 millibar level next.

The 50mb level is also seeing significant warming in the upper latitudes, which only confirms the idea that this is indeed a sudden stratospheric warming event, quite possibly the first one we've seen this whole season.

Why is this a big deal? The sudden stratospheric warming phenomenon allows the polar vortex to weaken, and this enhances the possibility of Arctic cold outbreaks not unlike those we saw earlier in January. I'm not saying we will see January 2014-level cold again, the prospects of cold outbreaks rise roughly 2-4 weeks after the sudden stratospheric warming occurs. This would place cold weather prospects in the February 18-March 4th period. It's already looking like we'll see a cold blast to kick off March, and this could be related.

The ECMWF model sees this intrusion on the stratosphere continuing, forecasting continued elevated geopotential-Wave 1 activity at the 10 day forecast period. We see the elevated values maximized around the 3mb to 30mb levels, with lesser influences downwelling as low as the 500mb level. It is likely that this will be the biggest stratospheric intrusion this season, as we are just about to wrap up the meteorological winter season, and the "final warming" (where polar winds reverse to signal the end of the SSW season) isn't too far away.

Andrew

Stratospheric Trifecta Signals Brutally Cold February Ahead

It now appears that the stratosphere will kick off a number of indices that deem February to be the coldest prolonged period of winter thus far. In today's post, I will address multiple indices that favor a significantly colder than normal February.

Part I: The Stratosphere

•First Warming

In the opening days of January, we saw a stratospheric warming event occur. This event was minor, as the chart above (depicting temperature anomalies in the upper latitudes) shows. Nevertheless, this first warming event, coupled with the ongoing warming event is helping to destabilize the polar vortex in the stratosphere. So far this winter, the stratospheric polar vortex has been resistant to any and all attempts to be put down. We can attribute this to the positive phase of the Quasi-Biennial Oscillation (QBO), which is unfavorable for a weak polar vortex. These two first warmings are just tastes of what is to come. While the first warming was minor, it seems to have shaken up the vortex at least a bit, and this ongoing warming event right now is most likely helping with some slight (at the very least) destabilization of the polar vortex.

•Second Warming

There looks to be a second stratospheric warming event in the making about 5 days away from today. The GFS model indicates we will see a spike in stratospheric temperature values on January 17th, centered in northern Russia. If we take a look at how this warming is angling itself at the polar vortex, so to say, it is clear that this is going to be a Wave 1 event. There are two major types of wave activity- Wave 1 and Wave 2. In Wave 1 activity scenarios, the stratosphere undergoes distress as a major ridge attempts to displace the polar vortex from the Arctic, while keeping it fully intact. With Wave 2 activity, the polar vortex is experiencing ridging across the upper latitudes, making the vortex want to split into two vortices. These vortices can then shift to lower latitudes and bring extreme cold weather to the regions affected. The Wave-1 activity is usually seen as the strongest stratospheric intrusion out of the two, as a significant push of ridging/warm air is made to displace the vortex. With a Wave 2 pattern, you have two areas of high pressure, making it relatively easier to break up the vortex. Considering we see the polar vortex (depicted here as the very cold air mass) being pushed off to one side, with a swath of warm temperatures to the west, it does appear that this would be a Wave-1 stratospheric intrusion. Interestingly enough, this warming is one of the trifecta of warmings that could ultimately significantly damage the polar vortex...

The ECMWF model system is in agreement on this Wave 1 activity, and we can see how it projects temperature and geopotential height values to spike in Wave 1 denotations, as seen in the dark blue lines on the first two panels from the top, and the dark red lines on the third and fourth panels from the top. The top image shows Wave 1 and Wave 2 forecasts for temperatures at the 10 millibar level, and the spike in the dark blue line there confirms what the GFS is already telling us about the chances of a Wave 1 intrusion. The same is true for the second panel, where we see geopotential height forecasts for Wave 1 and Wave 2 activity at the 10 millibar level. Again, the anomalously high Wave 1 values confirm the idea that this second stratospheric warming event would come as a result of strong Wave 1 activity. The same ideas are portrayed in 30 millibar projections of temperatures (third panel from top) and geopotential height values (fourth panel from top)- the consensus is that this would be a Wave 1 intrusion.

•Third Warming

On January 23rd, the GFS starts another stratospheric warming event in roughly the same location as the warming event we just analyzed above. The difference from the second warming event and this third event is that the polar vortex is now well displaced from its usual domain in the Arctic Circle. The polar vortex is now elongated and is being pushed down towards Eurasia as the stratospheric warming events are now having significant impacts on the vortex. The ECMWF does not forecast past 240 hours, and since this forecast is valid at hour 276, we'll have to rely on the GFS. The effects these significant Wave 1 events can have are quickly shown by these two projected stratospheric warming events, and if they're true, we could be dealing with a pretty weakened polar vortex by the time we get around to February.

•Fourth Warming?

The long range GFS indicates that we will see a fourth, even stronger warming event in the closing days of January. We see the polar vortex has now reconsolidated itself from an elongated vortex to a more compact version. Despite this effort to regather itself, the polar vortex remains significantly displaced from its usual spot in the Arctic Circle. Based on temperature anomalies, it looks like this stratospheric warming event would be the strongest one so far in this series of warmings, though because this is just before the end of the GFS forecast time (forecast valid for roughly 16 days away), we can't really guarantee that this event will happen. However, if it did, there is a strong likelihood that it would be another crushing blow to an already-distressed stratospheric polar vortex.

Part II: The Bering Sea

This is the GFS Ensemble 500mb height anomaly forecast for January 18th over the Northern Hemisphere. We can see the highly meridional (wavy) jet stream, as is shown by the significant ridging and troughing over North America. The big story we will focus on here is the Bering Sea. If we look over towards the Bering Sea, we see a plethora of deep negative height anomalies, with the spaghetti charts on the right highlighting that body of water as being the epicenter of at least a decent-sized portion of the polar vortex. According to this forecast image, the Bering Sea would be encountering the polar vortex on January 18th. The reason I'm showing this is because of the Bering Sea Rule.The Bering Sea Rule states that by watching storm systems make their way across the Bering Sea, one can identify where and when a storm system will show up in the United States. The general timeframe is 17-21 days after a storm appears in the Bering Sea, a storm will appear in the United States. In this case, it looks like instead of watching for a storm, we might just have to watch for the polar vortex to enter the United States. If we extrapolate January 18th out 17-21 days, we arrive at a February 4-8 time frame. But that isn't all. The Bering Sea looks to be stormy from January 14th and onwards, meaning we could kick off February with some very cold weather- and stay that way. Time will tell, but this Bering Sea Rule signal is looking mighty cold for the US.

Part III: The Teleconnections

Long range Madden-Julian Oscillation forecasts from the University of Albany's Kyle MacRitchie call for the MJO to enter Phase 7 to kick off February. A common bit of knowledge for those familiar with the MJO is that the latter and early phases of this phase space (particularly phases 7, 8, 1 and 2) tend to favor more wintry weather over the central and eastern United States, whereas the middle phases aren't as supportive of snow and cold in those areas. This CFS Ensemble forecast image takes the MJO into Phase 7 for the first week or two of February, before it starts to swivel down into Phase 8. If the MJO can enter Phase 8 and stay there for the remainder of February, I can easily see the chances of significant cold weather in the US rising.

Temperature composites centered on February for each phase of the Madden-Julian Oscillation show how the big cool-downs start in Phase 8, and continue through Phase 3. While Phase 7 may look warm, I'm not that confident in the idea that the MJO would be able to overpower the other indices (i.e. PNA, NAO, etc) and create a warm environment. Regardless of just how the MJO plays out, it is apparent that we will see a shift towards those latter, wintry phases of the MJO. That should be enough to continue the early February cold through the month. The intensity may vary, but the idea of a cold February will most likely remain intact, especially after accounting for the stratosphere and the Bering Sea.

The second teleconnection to discuss is the atmospheric angular momentum index, or the AAM.

This AAM forecast, from Nicholas Schiraldi, shows the GFS projection of AAM anomalies on a time/latitude graph on the top panel, as well as an overall AAM anomaly forecast on the bottom panel. In the short and medium range we see a relatively neutral AAM, but towards the longer range, the GFS has been consistent in its idea that the AAM will be tanking well into negative territory as we approach February. If you were to take a look at the AAM phase space, the AAM is projected to hit Phase 1 before beginning to cycle through the low AAM phases, as you can see below.

If the AAM does cycle through these early, low AAM phases, one could expect an anomalously cold February. The reason? I took the time to look up February mid-level geopotential height anomalies when accounting for the January AAM values. The result is below.

This map shows correlation values over the Northern Hemisphere. Basically, this chart was constructed by taking January AAM values and looking at the next month's (February) 500mb height anomalies. If the AAM in January was positive, one could expect high pressure in Canada and the Northern US because there are positive correlation values shown. In the same sense, if the AAM were positive in January, February 500mb anomalies would ne negative, due to the negative correlation shown in the Southern US. In our case, we look to average out in slightly negative AAM values for the month of January. If we look at the chart above, we could anticipate seeing unusually cold and stormy weather in the North US, because the positive correlation means a negative AAM produces negative height anomalies. It's even possible that the polar vortex would come down to Canada again, but it's way too far out to speculate about that. As far as the South, it's possible we see some high pressure due to the negative AAM showing a negative correlation there (that equals positive height anomalies: high pressure). I don't think we would see that significant of ridging in the South, mainly because the other parts of the pattern support much of the East being chilly, not just the Northeast. Regardless, this does bode well for cold weather prospects for February in the US.

Part IV: The Forecast

I believe we will see February start out with brutally cold temperatures, as the Bering Sea argues. I have relatively high confidence in this cold really hitting the US hard to open February. Beyond the opening week of February, I anticipate we will see a continued cold pattern, but it will likely fluctuate. As any stratospheric influences begin to affect the US, we may or may not see enhanced cold weather- it will depend on the synoptic pattern around North America that will dictate where the core of the cold will go. Beyond mid-February, we should see the MJO enter Phase 8, which, if the forecast is correct, will allow for continued cold weather, specifically focused on the East US. Naturally, as time goes on my forecast confidence lowers, but the point is, there is definitely potential there for a very cold February.

Andrew