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

Positive QBO Set to Strengthen Polar Vortex in Winter 2019-2020

The emergence of a strong positive-QBO state in the stratosphere in recent months looks set to persist into the coming winter, with a likelihood of a stronger-than-normal stratospheric polar vortex as a consequence. This, of course, will feed into the broader winter outlook for the United States as a result. Click any image to enlarge it.

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As a side note, thank you to all of those who sent well-wishes for my wisdom teeth surgery the other day! The surgery went very smoothly and the recovery has been remarkably painless, which lets me get back to writing these posts quicker!

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Of course, before we dive into the juicy stuff, we need to first briefly review what the QBO phenomenon is.

The Quasi-Biennial Oscillation, or QBO, in a nutshell measures the direction of winds in the stratosphere located over the equatorial regions. The QBO has two phases: a positive phase and a negative phase.
In the positive phase of the QBO, stratospheric winds over the equator are westerly (from the west, to the east). This has the effect of strengthening these "westerlies" in the stratosphere in the upper latitudes of the Northern Hemisphere, in this case promoting a stronger polar vortex as the stronger westerlies strengthen the vortex.
The negative phase of the QBO sees these stratospheric winds over the equator turn easterly (from the east, to the west), which has the effect of slowing down the westerly flow in the stratosphere of the upper-latitudes of the Northern Hemisphere. Consequentially, this weakens the stratospheric polar vortex, all else equal.

Let's take a look at the recent history of the QBO to see what we're working with heading into the summer and fall months.

Screenshot of recent wind values to illustrate the QBO. Positive (negative) QBO wind values are shown in gray (white) shaded areas.
Source of screenshot: Free University of Berlin
Source of annotations: Author

The QBO is readily seen in this time-series view of stratospheric wind values and directions over the Equator, with the positive phases of the oscillation laid out in gray shading and the negative phase shown in white shading.

I've annotated this image to point out the recent emergence of westerlies over the Equator, displayed on the screenshot by the swath of gray shading and positive contour values. As of the latest observations, the westerlies had propagated down to about the 70-millibar level over the Equator, meaning this wave of westerly winds is well on its way and we are firmly entrenched in a positive QBO regime.

Of course, this has notable implications for the coming winter. As this blog has discussed extensively in previous winters, the stratosphere can be used to glean extensive information about the broader forecast, whether on a scale of days or months. In this case, to identify what a positive QBO winter may entail (holding all else equal, of course), I've gone through the Climate Prediction Center's 30-millibar dataset of equatorial stratospheric wind speeds and direction and sorted each calendar month in a descending order. Then, for the three primary winter months (December, January and February), I pulled the years of the five strongest positive QBO values for each month. Given that the QBO is a prolonged oscillation (whereas something like the North Atlantic Oscillation changes in a matter of days), this method pulled out only eight distinct winters. I then created composite images of different atmospheric variables during the winter months for these eight years to give an idea of what a positive QBO environment generally entails.

First, let's see what a 'typical' positive QBO brings in terms of the Northern Hemisphere's stratosphere.

50-millibar geopotential height anomalies over the Northern Hemisphere for the December-February periods of eight positive-QBO winters (with the year of the LAST month in the DJF period shown).
Source: ESRL

As the above image shows, the aforementioned strategy produced the winters of 2016-2017, 2013-2014, 2015-2016, 2010-2011, 1990-1991, 1982-1983, 2008-2009 and 1992-1993, in no particular order. I first produced a graphic showing the geopotential height anomalies over the Northern Hemisphere for those winters at the 50-millibar level, right about at the mid-section of the stratosphere. This level was chosen to give a more-arbitrary view of the stratospheric polar vortex, as there can be issues preferring to look at the 100-millibar level as well as the 10-millibar level.

Creating this composite image confirms the explanation given to what happens in a positive QBO environment: the stratospheric polar vortex in the Northern Hemisphere strengthens. Indeed, the strong negative anomalies centered over the Arctic Circle in this composite show the enhanced westerlies providing strength to the polar vortex, thereby keeping the cold air locked up in the Arctic. It's no wonder that those who enjoy winter weather are firmly in favor of the negative phase of the QBO!

Next, I took these same years but changed the height level of the atmosphere from 50 millibars to 500 millibars.

500-millibar geopotential height anomalies over the Northern Hemisphere for the December-February periods of eight positive-QBO winters (with the year of the LAST month in the DJF period shown).
Source: ESRL

The story isn't too different when moving down to the 500-millibar level, which is where meteorologists look to identify the features that more directly impact you and me. For example, warmer colors on this map illustrate ridges of high pressure, while cooler colors identify low pressure areas. Given the stronger polar vortex, it's therefore not a surprise to see a swath of below-normal height anomalies extending from Greenland into Canada, symptomatic of a relatively-stronger tropospheric polar vortex.

I do want to briefly discuss another feature here, however, and that's the extreme positive height anomalies in the Bering Sea. A sharp-eyed weather enthusiast may recognize that swath of well-above-normal geopotential heights as an almost-textbook negative phase of the West Pacific Oscillation, or WPO. In contrast to the QBO, which records changes in terms of months, the WPO changes on a daily basis, making it more useful for daily & weekly forecasts as opposed to seasonal forecasts. Still, this is important, and especially-so for fans of winter weather.

Composite image of 500-millibar geopotential height anomalies (top panel) and surface temperature anomalies (bottom panel) in the positive phase of the West Pacific Oscillation.
Source: ESRL

The composite image above shows what typically happens when the WPO is positive, so for our discussion of the *negative* phase, simply view the anomalies as opposite for a rough idea of what a negative WPO entails. All else equal, the negative phase of the West Pacific Oscillation involves a strong ridge over the Bering Sea, like what was shown in the positive-QBO composite image right above this. That strong ridge tends to try and force itself further north into the Arctic Circle, which then pushes the jet stream further north. As a consequence, downstream of that ridge, a trough over Canada is coerced into shifting south and even a little west, not dissimilar to what the positive-QBO composite image shows with negative anomalies in southwest Canada extending into the Pacific Northwest. This then can encourage a ridge to build in the eastern U.S., however, which prohibits the cold air from making it too far east.

In other words, the positive phase of the QBO tends to create a strong ridge in the Bering Sea, holding all other variables equal, in a pattern that is then classified as the negative phase of the WPO. These two oscillations may be different in definition but their impact on the contiguous United States is similar: colder and stormier weather in the West U.S., the potential for a ridge in the eastern or southeastern U.S., and the threat for storm systems to ride the jet stream northeast across the Plains into the Midwest and Great Lakes.

Let's see if that explanation holds up to the 'typical' surface temperature anomalies in a positive QBO event.

Surface temperature anomalies over North America for the December-February periods of eight positive-QBO winters (with the year of the LAST month in the DJF period shown).
Source: ESRL

The surface temperature anomalies in positive-QBO environments does seem to match up with what was described immediately prior. In these winters with a positive QBO, colder than normal temperatures are more predominant in the northern third of the country, maximized in the North Plains and northern Rockies into western Canada. Conversely, warmer-than-normal temperature anomalies tend to be more likely in the southern third of the country, with a particular preference for warmer temperatures along the East Coast as that ridge I mentioned seems to try and make an appearance.

250-millibar vector wind anomalies over North America for the December-February periods of eight positive-QBO winters (with the year of the LAST month in the DJF period shown).
Source: ESRL

Just to push the point home a bit more, above is a composite image of vector wind anomalies over North America at the 250-millibar level (roughly the jet stream) for those eight positive-QBO winters. In other words, this gives an idea of how the jet stream is oriented during winters with a positive QBO, all else equal.

In a positive QBO winter, we tend to see the strong ridge over the Bering Sea force the Pacific jet stream to the north, as previously discussed, before it buckles south as it crashes into North America. From there, things get a little tricky. It seems plausible that the jet stream oriented in this fashion wouldn't be sufficient enough to pull storm systems into the Southwest (not as an overarching theme, at least), but would be sufficient to likely come onshore somewhere in northern California or Oregon and then ride a rather-zonal path to the east before turning northeast somewhere over the Plains. This kind of storm track would keep the Northern Plains most pointedly under the gun for winter weather threats, but the hint of a subtropical jet stream along the Gulf Coast suggests this storm track would not necessarily be the only track. That's a positive thing for those in the East U.S. who are hoping for a snowy winter.

To Summarize:
- The Quasi-Biennial Oscillation (QBO) is currently in its positive phase, and is likely to remain this way into the coming winter.
- Typically, a positive-QBO winter brings cooler than normal conditions to the northern third of the country and warmer conditions to the southern third and East Coast.
- A positive QBO also seems prone to supporting a negative-WPO state.
- Most importantly, however, a positive QBO supports a stronger stratospheric polar vortex, which restricts the flow of cold air to lower latitudes.
- As such, a positive QBO this winter may temper the likelihood of colder than normal conditions for the United States as a whole.

Andrew

Stratospheric Vortex Under Attack, But Holding Strong

The stratospheric polar vortex looks to come under attack in the next several days, and possibly beyond, but all indications are that this attack will be weak, and the polar vortex aloft will remain strong.

We'll begin with a current look at the stratosphere.

CPC
(Refresh page if animation stops looping)

This animation above shows temperature anomalies at the 30-hPa level of the stratosphere, just above the middle part of the stratosphere. Note how over the past month we've seen a few instances of minor warming occurring at 'lower' latitudes, but never reaching a strength adequate enough to be an actual stratospheric warming event. Consequentially, the stratospheric polar vortex has remained pretty much undisturbed. There is some warming commencing over the Himalayan mountain region, and this could incite another minor warming event to spread over the north Pacific, as has happened before in the last 30 days on this animation. Whether it builds into anything substantial remains to be seen, but as the analysis below shows, I'm not optimistic.

FU-Berlin

The first image we'll analyze is a longitude-by-height chart, forecasted by the ECMWF model for five days' time. This chart is forecasting the presence of a geopotential Wave-1 event in the stratosphere. As the phrase 'Wave-1' indicates, the forecast here is for the risk of a single body of high pressure (hence Wave-1) forming aloft to try and displace the polar vortex. The legend on the left indicates this risk is highest from the 1-millibar to 10-millibar region, where geopotential values are highest and the warm colors are most prominent. This is a pretty strong Wave-1 event, but because it is centered above the 10-millibar level, and it will weaken shortly after this time period five days out, I'm not expecting any long-term damage to the stratospheric polar vortex.

FU-Berlin

Another tool we can use are time-by-height charts for different levels of the atmosphere to see how much, if any pressure is being applied to the polar vortex. It looks a little intimidating at first, but we'll break it down below.

The top panel shows us the forecast for Wave-1 (explained above) and Wave-2 temperature attacks at the 10-millibar level. The Wave-2 event is where one body or more of high pressure / warm temperatures form aloft (in this case, at the 10-millibar level) and try to split the polar vortex into two vortexes, hence the '2' in Wave-2. Notice the forecast for a Wave-1 event peaking at that 5-day forecast period, which we analyzed above, but then that Wave-2 line starting to rise up towards the end of the forecast period, ten days away. That could be something to monitor, but when we consider Wave-2 events are weaker than Wave-1 events, I'm not particularly encouraged by it.
This description of that top panel also applies to the third panel from the top, except now valid at the 30-millibar level. In this case, however, notice a sustained elevated Wave-1 attack throughout the forecast period, but also a strengthening Wave-2 parameter towards the end of the forecast period. Again, perhaps something to watch, but I'm not exactly keen on it impacting the vortex significantly right now.

Panels 2 and 4 from the top are nearly identical to the first and third panels described above, but these new panels show us Wave-1 and Wave-2 attacks from a geopotential (ridges instead of warm temperatures, although in essence they tend to occur together) standpoint. Note in Panel 2, showing us the forecast at the 10-millibar level, a sustained Wave-1 attack, and a pretty strong one at that, throughout the forecast period. We also see a strengthening Wave-2 event at the end of the forecast period.
The situation in the fourth panel, showing geopotential values at the 30-millibar level, is different than the 10-millibar level. We see a pretty strong Wave-1 event, but a weakening Wave-2 episode. This piques my interest, just because I want to see how that evolves more than anything, and may be something to watch down the road.

To summarize:

- Minor warming has occurred in the stratosphere over the last month, but has had very little impact on the polar vortex.
- Additional minor warming is forecast to occur over the next several days, again with little impact in the lower stratosphere.
- Long-range guidance indicates some items of interest beyond Day 10 forecasts, but I'm personally pessimistic as to the chance of a stratospheric warming event.
- This may keep the chances for a large-scale, significant cold weather event relatively low, speaking strictly from a stratospheric viewpoint.

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

Stratospheric Polar Vortex Expected to Weaken in January

The stratospheric polar vortex is expected to exhibit weakening in the near future.

Garfinkel/Hartmann Publication
(Image obtained from Eric Webb)

The above image, from the Garfinkel / Hartmann publication, 'Tropospheric Precursors of Anomalous Northern Hemisphere Stratospheric Polar Vortices', shows precursors in the tropospheric mid-levels to weakening of the stratospheric polar vortex. In layman's terms, the images above (particularly the left panel) give us a glimpse at what the weather pattern should look like a short period of time before the stratospheric polar vortex weakens.

In that left panel, showing geopotential height anomalies at the 500-millibar level, we see strong negative anomalies (troughs / storm systems) in the Bering Sea extending into the Arctic Circle, even a bit into Greenland, and positive anomalies (ridges / high pressure) over Canada, as well as in the north-central Pacific. We also see ridging over Europe. These are indicators that the stratospheric polar vortex could be weakening down the road.

Let's compare these indicators to the long-range forecast from the GFS Ensembles.

Tropical Tidbits

Attached is the 500-millibar height anomaly prognostication for the 11-15 day forecast period (Dec. 30 to Jan. 4). Here, we see negative anomalies across the Bering Sea into Siberia, all the way across the Arctic Circle into Greenland. We also see weak ridging in Canada and the East US, and ridging in the north-central Pacific. Most notable is a strong ridge over Europe.

If you compare this forecast graphic and that left panel earlier in this post, you'll find that they're incredibly similar, almost identical. In sum, long range model guidance is telling us that we should see the stratospheric polar vortex weaken in January. This is good news for snow and cold fans who have been suffering through this first month of 'winter' (more like an extended fall with these warm temperatures)- even though a weakened polar vortex does not necessarily mean cold and snow, it does raise the chances of more cold air intrusions and snow events for North America.

To summarize:

- Long range model guidance is indicating the stratospheric polar vortex will weaken in January.
- Consequentially, the chances of cold and snow may be on the rise for 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

Sunspot Numbers Plummeting, Stratosphere Warming

The number of sunspots has been anomalously low as of late, the lowest trend since roughly July 2014.

SOLEN

The above image shows sunspot numbers from NOAA over the past many months. Higher sunspot numbers (red) indicate that the sun is more active than usual. In the last couple of months, the number of sunspots has been dropping steadily from average values earlier in the year into 2014, interestingly enough in the midst of the strongest solar storm unleashed on the Earth in this solar cycle, per some reports.

What does this mean for our weather? The influence of the sun on our weather is still somewhat murky, but one big derivation we can make is that the stratosphere tends to cool down when the sun is active, and warm when the sun is quieter.

Berlin-FU

We are already seeing the stratosphere warm up, with more warming expecting in the future. The panels above show observed temperature values at different parts of the stratosphere in color, with the forecasted values in dashed lines. Notice how the ECMWF model expects the 10hPa level (top panel) to heat up notably, with a similar story in the 30hPa level (2nd panel from top). Although this warming won't be sustained, it's quite possible we see additional warming as the stratosphere exits its winter phase and enters its summer phase.

To summarize:

- Sunspot numbers have been steadily decreasing in the last few months, as we begin to end the current solar cycle.
- Stratospheric temperatures are responding by warming up, signaling and end to the winter phase.

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

Above-Normal Stratospheric Temperatures May Signal Blocky Winter Ahead

Warmer than normal temperatures in the lower stratosphere are raising the possibility of a "blocky" winter ahead.

CPC

The above image shows a pretty daunting image of the stratosphere, so let's decipher it. The red line shows observed temperatures at the 70 millibar level of the stratosphere, between the 65N and 90N latitude lines. The dashed green line illustrates average temperatures for any given time, while the gray outlines give an indication of the record high and low stratospheric temperatures for any given time period over the past few decades of records.

Gazing over the image above, specifically around the most recent records of temperatures, we find ourselves on the above-normal side, with that red line bursting up into above-normal territory more than once in the last few weeks. This year's warmth at the 70mb level looks to be a bit more prevalent than that of last year, as you can see just to the left of the center of this image.

Why is this important to the coming winter? Above-normal temperatures in the stratosphere allow for a higher threat for persistent high pressure to form over the Arctic Circle, and general upper-latitude area. The polar vortex, a strong low pressure system of cold air located across the troposphere and stratosphere, can be strengthened during times when the stratosphere is colder than normal, and weakened when warmth prevails.

When this warmth prevails, strong bodies of high pressure can punch north from the lower latitudes into the Arctic, disrupting the polar vortex. When this happens, fragments of the vortex can break off and be sent to the lower latitudes (as was seen last winter), or the whole vortex can be shunted down south. If the high pressure sticks around in the Arctic for long periods of time, it can be referred to as "blocking" high pressure, for the way it "blocks" the pattern from flowing east to west around the globe, since the high pressure remains stagnant and backs everything up.

DMI/COI

The risk for sustained high pressure is increased as we see above-normal temperatures affecting the surface of the Arctic. The graphic above shows us observed temperatures north of the 80th parallel from the first day of the year to present day. Over the summer, we saw sustained below-normal temperatures, as evidenced by the red line being below the green average-temperature line, and I had discussed this as showing a potential cold air build-up for the upcoming winter.

Things certainly have changed since then! In the past couple of months, temperatures north of the 80th parallel have stayed predominantly above-normal, only reaching the average line a few times. The most recent observation shows temperatures diving, but the anomaly is still above-normal for this time of year.

For winter weather fans and warm weather fans, this is a double-edged sword.

On one hand, the lack of colder-than-normal air means not as much cold air may be available for the coming winter, barring a regime shift that would cool down the upper latitudes. Such a scenario could then mean a general 'warm-ish' feel for the coming winter, where cold weather would still occur (since it's the winter season), but the intensity of cold air may not be that extreme.

On the other hand, the warmth both at the surface and into the lower stratosphere suggest a pretty elevated risk of that blocking high pressure this coming winter. All of this warm air in the Arctic has a pretty decent chance of depleting the polar vortex this winter, and this risk is increased even further per some items that will be discussed in our Official 2014-2015 Winter Forecast on October 11th at 12:00 PM Central Time.

To summarize, above-normal temperatures across both the stratosphere and troposphere appear to be raising the risk of blocking high pressure in the upper latitudes this winter, which could then raise the risk of a cold winter in North America.

Andrew

The Story Behind The Polar Vortex

Ever since last winter, the polar vortex has taken over as the go-to thought as soon as the prospect of cold weather is brought up. Let's take this time today to go over what the polar vortex actually is, and disprove some inaccuracies I've seen swirling around the world.

1. The Polar Vortex Exists Year-Round
The phrase 'polar vortex' is used to describe a strong low pressure system (vortex) that is semipermanently placed over the Arctic region (polar). In the summer, the vortex significantly weakens, and is replaced by high pressure. During the fall months, however, the vortex gains strength yet again and low pressure dominates the Arctic as the polar vortex gears up for winter.

2. The Polar Vortex Extends into the Stratosphere, Not Just the Troposphere

NCEP

The image above shows the 10-day forecast from the GFS model in the stratosphere, at the 30 millibar level. That's pretty high up the sky when you consider us humans are at the 1000 millibar level. In this forecast, we see relatively weak low pressure establishing itself over the Arctic Circle, with weak high pressure surrounding it. That low pressure body is the "birth" of the polar vortex for this cold season, as it begins to come back to life and strengthen for the winter.

Some people assume the polar vortex is no different than your average storm system that brings snow to your neighborhood. The vortex is actually quite different. In order for it to be the polar vortex, it must exist at both the tropospheric and stratospheric levels. That's how big and controlling it is in the atmosphere.

3. There is only ONE Polar Vortex

This is something I've been hearing ever since January 2014, when this polar vortex business really took off. The belief is that there is more than one polar vortex in the atmosphere. This might stem from the aforementioned idea that some believe the polar vortex is no different than your average storm system, but regardless of its origins, the belief is false.

ESRL

Take a look at the image above. In this image, we see a view of 500mb height anomalies across the Northern Hemisphere on January 7th, 2014. Warm colors correspond to high pressure, which brings warmth and generally quiet conditions. Blues and purples indicate the presence of low pressure, which permits stormy and chilly weather. Just taking a glance over this map, we see one strong upper level low that seems to be the strongest one in the entire hemisphere. That upper level low, seen scraping the northern US and inundating Canada, is the tropospheric version of the polar vortex. As you can see, there is only one low as strong as the one in North America, hence there is only one polar vortex. Those other low pressure systems are just disturbances compared to the polar vortex.

ESRL

To give you a better idea of how there's only one polar vortex, let's take a look at the stratosphere on January 7th, 2014. In this image, we see what appears to be two strong areas of negative height anomalies, being pinched by two bodies of ridging/high pressure on either side of the globe. If you thought that this image shows two polar vortexes, you are incorrect!
What's actually happening is the two bodies of high pressure are trying to tear apart the polar vortex, something that happens rather often each year. The difference between other years and this past year is that the ridging forced the main part of the polar vortex into North America; that other area of negative height anomalies in Asia is just a weaker part of the single polar vortex that was torn off of the main body. Some of you more experienced weather enthusiasts may know this type of set-up as Wave-2 stratospheric activity, as two bodies of high pressure push into the Arctic.

To close out this post, here's a stellar graphic made by the folks at the National Weather Service office in New York in early January, when the hype over the polar vortex was in full swing.

NWS

To summarize:

• The Polar Vortex exists year-round, varying in strength. It does not (and will not) "come back" for the winter, as it's already present and cannot "come back".

• The Polar Vortex exists across both the troposphere (where we live) and the stratosphere (where Felix Baumgartner jumped from his capsule in October 2012); it does not exist at the surface.

• There is only one polar vortex; our world would be much different if more than one polar vortex existed.

Additional points of summary are included in the NWS graphic above.

Andrew