Long Range Outlook: Colder Pattern Developing Dec. 20 - Jan. 10; Polar Vortex Under Duress

 Good afternoon, everyone, and happy first day of meteorological winter!

The other day I wrote how the outlook appeared to be favoring above-normal temperatures for the month of December into early January, as the stratospheric polar vortex (SPV) is currently stronger than normal and model forecasts for that period had been aggressive with keeping the vortex strong. Since then, however, we have seen a shift in model guidance that bring the SPV under attack to a stronger degree, and a broader shift in the global weather regime may favor a round of sharply colder Canadian air in the December 20-30 timeframe.

Previous forecast model runs anticipated the development of a single, weak ridge of high pressure way up in the stratosphere that would appear to be a nuisance towards, but not disrupt, the stratospheric polar vortex. Consequentially, all stratospheric signals seemed to be aiming in a warmer direction for U.S. weather for the balance of December and into January.

Latest model guidance has changed its tune. The GFS ensembles are shown above, with the 10-millibar forecast of geopotential height anomalies shown in color and pressure contours juxtaposed on top. We see that the GFS ensembles are showing a vortex splitting event to occur around December 7-9, with two distinct ridges of high pressure (one in Eurasia and another in the Pacific) pinching the SPV across the North Pole so that one lobe of the vortex breaks off and settles over Greenland, while the second lobe drifts into Siberia. 

This isn't the most impressive vortex splitting event you'll ever see, but it's enough to disrupt the flow of the stratospheric vortex and, in turn, open the door for a round of Arctic air about two to three weeks later. This puts us roughly in the December 20th - January 5th timeframe, and this will be the period to watch for a potential round of much colder than normal air in North America.

So we know that the stratospheric polar vortex will come under pressure over the next week or two, perhaps splitting in the process. Supposing a cold air event may become more likely two to three weeks later, how will the overall pattern look around then? We start with an analysis of tropical convection.

The chart above (from weathermodels.com) is complex at first blush, but can be easily broken down. This shows forecasted thunderstorm activity along the Equator (in a band between the 15 North and 15 South lines of latitude) from the long-range ECMWF model across all lines of longitude, from today through January 15th (y-axis scale). The variable being measured here is upper-level velocity potential anomalies, which is a fancy parameter for judging how conducive the atmosphere will be for thunderstorm activity. 

Here, yellows and oranges represent above-normal velocity potential anomalies, which suppress thunderstorm activity. On the flip side, greens and blues indicate lower VP anomalies, which then enhance thunderstorm activity.

We can then ascertain what phase the Madden-Julian Oscillation (MJO) is in by seeing when those greens and blues pop up across different parts of the world. For example, over the next few days, we see blues and greens across the islands northwest of Australia and southeast of India, which correspond to MJO phases 3, 4 and 5. This feature has recently helped to enhance above-normal temperatures in the United States, as Phases 3-5 encourages warmer weather across the continent. How do we know this? The composite image on the right (from the Climate Prediction Center) shows temperature anomalies during the Nov-Dec-Jan period for each of the eight MJO phases.

By the same token, when enhanced Equatorial thunderstorm activity develops northeast of Australia in the open Equatorial Pacific, the MJO tends to be in Phases 7-8, which correlate colder for most of the country. The whole process starts over again with enhanced thunderstorms in the Equatorial Atlantic and over Africa, which is MJO Phase 1 territory.

Returning to the ECMWF forecast image above, after our MJO Phase 4 influence dies down by December 10th, we are expecting to see a robust resurgence in tropical convection in the Equatorial Atlantic, which I've highlighted in the graphic. While not explicitly shown, thunderstorm activity around those waters and into Africa correlates to a more-or-less MJO Phase 8 or 1 type of pattern, and will tend to lean cooler for most of the United States as a result. This enhanced thunderstorm activity in the Atlantic kicks off by December 10th and may stick around through early January, with ebbs and flows in the intensity of this thunderstorm activity.

Rather than rely on subjective interpretations of the MJO to decide where the oscillation is heading, we can also make use of an objective MJO phase-space analysis, shown above for the same long-range ECMWF model we were just subjectively analyzing. In this objective analysis above (from the CPC), the MJO is seen meandering (and strengthening) in Phase 7 from December 17th through the end of the forecast period, December 30th. This is a little earlier in the phase progression from the Phases 8 and 1 I had been expecting, but no matter - the general theme of a global pattern shift in favor of colder weather for the last ten days of December and into early January remains intact.

The stratosphere will be coming under some duress in the next several days, which may raise the risk of a cold air event in the U.S. for the end of December and into early January, while the MJO is shifting into pro-cold phases for the last ten days of December and perhaps onwards. Now that we see what kind of environment may be setting up, let's see if model guidance is latching on to any cold weather threats.

Beginning with the American CFS model, we take a look at 10-day temperature anomalies across North America from December 16th through the 26th. The pattern looks generally warm for the U.S., but the real thing I'm watching is that reservoir of colder-than-normal air building in western Canada, and similarly anomalous above-normal temperatures in the Arctic Circle. This suggests a shift in Arctic air to the south, displaced into western Canada and further east into Greenland. While not a necessity for cold air events, a reservoir of cold Canadian air in the western part of the country is traditionally a good sign.

Moving ahead to the December 21st - 31st timeframe, we see what that cold Canadian air reservoir does: it spills south and east. With continued warm air displacement into the Arctic Circle, below-normal temperatures have free rein to grind south and east into the United States and deeper into Canada. Above-normal temperatures are still anticipated to linger in the South and along the Eastern Seaboard, but this may be something that happens earlier in this period followed by colder weather later towards the end of the year.

Our last analysis of the CFS model comes for the December 31st - January 10th timeframe. It appears as though the CFS takes that MJO Phase 7 temperature composite to heart and continues to displace cold Arctic air southwards into Canada and the northern U.S., with much colder than normal temperatures seen across the continent. 

Even with this colder pattern, above-normal temperatures are still evident across the South, symptomatic of a jet stream that refuses to dip too far south (perhaps a consequence of implied storminess over Greenland; a +NAO regime for you weather enthusiasts).

All in all, I see the CFS model as pretty conducive for a cold air event in this December 20th - January 10th timeframe, particularly with such a formidable reservoir of Arctic air in western (and eventually all of) Canada. As our last bit, let's turn our attention to the long-range ECMWF model and see what it says.

Unsurprisingly, the long-range ECMWF is less keen to bring about a much colder weather regime to North America, but it does take the intriguing step of developing and intensifying that reservoir of cold air in western Canada. With anomalies seen deeper here than what the CFS shows, it does open the door for the risk of a more intense cold air event if we took this forecast verbatim. Of course, that would be too easy, and the ECMWF instead brings cold weather to the far north-central U.S. but maintains warmth in the South and East for the balance of the year and into early January.

To Summarize:

• The stratospheric outlook has changed since earlier this week, and it now appears more likely that the stratospheric polar vortex will be disrupted to some degree in the December 5-10 period
• Combined with changes in the global weather pattern (via the MJO) to a state more conducive for below-normal temperatures in the U.S., the chances of a colder last several days of December & colder opening days of January are rising
• Model guidance contains differences, but with both main models showing a formidable reservoir of frigid air accumulating in western Canada by December 25th, I am feeling optimistic that future forecasts for the northern third of the country (and potentially further south) will continue to trend colder
• More on this to come

Andrew

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 

Long Range Ensembles Predict Incoming Spring-Like Weather

Long range ensembles are indicating a much warmer weather pattern is on the way for much of the country that has been held hostage by brutal cold in the last month.

PSU
Click to enlarge

The above image shows 500mb geopotential height anomalies from the GFS ensembles, projected over the Northern Hemisphere and valid for March 1st. In this graphic, we see a massive ridge of high pressure, the same one which has allowed for such a cold and snowy February, meandering westward over the north-central Pacific, as opposed to the northeast Pacific and Gulf of Alaska. This comes as a strong trough drops into the Southwest and Baja California. In response, with the strong upper level low previously positioned over Greenland, now over Europe, a ridge is able to form over the Central and East US. It looks like this ridge will be suppressed a bit, but warmer weather is on the way for those in the Plains, Midwest, Ohio Valley, and Northeast, among other areas.

PSU

By March 7th, ensembles are indicating the ridge remains in the north-central Pacific, but an upper level low has now developed in the Gulf of Alaska. Unfortunately, this then results in a ridge forming over the Western US, as the graphic above shows. Consequentially, the semi-permanent Arctic long wave trough (better than using 'polar vortex', apparently) is being pushed back into North America, setting up another period of cold weather for the Central and East US. This would likely last for a handful of days, right between February 5-10 or so.

But... finally... finally!... relief comes.

PSU

In the long-long range, to March 11th, ensembles see our ridge forcing itself north in the north Pacific into the Aleutian Islands, along the jet stream, where the "polar vortex" is stationed just off to the north. The net result for us is another trough setting up over the western section of North America. You weather enthusiasts know this as a negative Pacific-North American (PNA) pattern, with a trough in the west and warmth in the East.

This looks to be our new pattern, going strictly by these ensembles. Stormy weather in the West with predominantly warm weather out east.

To summarize:

- A brief warm-up is expected in the final days of February and early days of March.
- Another cold blast can be expected around March 7th.
- The pattern may then shift to a warmer set-up for the middle half of March.

Andrew 

Second Round of Harsh Arctic Air Flowing Southward

The second round of intense cold air in under a week is on the way for millions in the Northern United States.

Tropical Tidbits
Click images to enlarge

The above image shows the latest GFS model forecast for Monday morning. On this chart, we see temperatures in the negative-teens across Wisconsin, Illinois, Minnesota, Iowa, Michigan, Indiana, and Ohio. The most intense cold is projected to hit northern Wisconsin and southwestern Michigan, but that doesn't take away from the cold in the other aforementioned areas.

Tropical Tidbits

The GEM model, with its notorious cold bias, agrees with the GFS model. Here, we see a swath of bone-chilling cold in southern Minnesota and Iowa, where temperatures are forecasted to approach -30 degrees. This is more than likely overdone, but the general trend of very cold temperatures similar to the GFS stands in Wisconsin, Illinois, Michigan, Indiana, Ohio, and even into the Dakotas.

Other model guidance, mainly the NAM model, is favoring a warmer solution than the two depicted above. It comes out to the GFS/GEM models and their ensembles, against the NAM/SREF models/ensembles. The National Weather Service appears to be favoring the NAM model / warmer forecast, even though morning observations in the Dakotas and Minnesota support a more GFS-like evolution. Regardless of what happens, another very cold morning is in store tomorrow.

To summarize:

- Another bout of harsh Arctic air is on the move south.
- Temperatures could drop into the negative-teens in some spots.
- Uncertainty still exists with how cold it will get.

Andrew 

Anomalous Upper Level Low to Deliver Intense Cold in East US

An anomalous upper level low connected to the swirling mass of cold air in the Arctic looks to deliver a punch of brutally-cold air, impacting the East US the strongest.

PSU
Click images to enlarge

The above image shows the GFS ensemble mean spread forecast for the 500-millibar field on the top panel, with increased spread/uncertainty among ensemble members shown by shaded colors. That spread can also be seen in the individual member colored lines. The bottom panel shows anomalies for the 500-millibar field, with cool colors indicating negative anomalies, and warm colors depicting positive anomalies.

In this image, we note a strong upper level feature pushing south from Canada, with anomaly values reading 3.27 units below normal, maximized in western New York into western Pennsylvania as the purple colors show. Judging by the numerical denotations on that bottom panel, 500-millibar values could flirt with the 500-dm benchmark along the US/Canada border, indicative of a very strong (and very cold) upper level low.

Tropical Tidbits

Forecasts from computer models suggest sub-zero temperatures will be the theme throughout a significant swath of the North US. Latest guidance suggests temperatures in west New York and Pennsylvania will easily drop below -10 degrees Fahrenheit, with some locations getting very close to -20 degrees F. Locations further to the west, particularly in the Midwest and Great Lakes, may also come close to -15 degrees, with some areas dropping down to that -20 degree F benchmark. All in all, this is looking dangerously cold, potentially life-threatening.

To summarize:

- The first part of a multi-wave cold blast looks to impact the North US this weekend.
- Cold weather will be maximized in the Northeast, where temperatures could reach lows of -20 degrees Fahrenheit.
- Travel is strongly advised against due to the life-threatening nature of this cold air.
- Make sure to seek out assistance now for neighbors, friends, and/or family that may have trouble sustaining heat in their homes, or could be adversely affected by this cold.

Andrew

February 16-20 Potential Arctic Outbreak

There appears to be the risk of an outbreak of cold air between a February 16-20 period.

Tropical Tidbits

The image above shows 500mb geopotential height anomalies over the West Pacific, forecasted by the GFS model and valid on the morning of February 10th. In this image, we see a very strong upper level low displaced just north of Japan, with contours telling me this is a sub-500dm upper level low- a very strong storm. When we use the Typhoon Rule, which states weather phenomena occurring in Japan is reciprocated in the United States 6-10 days later, a strong upper level low could approach the United States from the north in a February 16-20 period. It's too early to tell definitively, but this could correlate to a lobe of the tropospheric polar vortex plummeting south towards the mid-latitudes. Again, it's too early to tell right now.

ESRL

Long range ensemble forecasts for the middle of this timeframe are already beginning to see strong upper latitude blocking leading to a new wave of cold weather for the Central and East US, as those blue colors shown over North America. We will need to watch upcoming forecasts to see how they change.

ESRL

For comparison, here's the chart of 500mb geopotential height anomalies over Japan on December 27th, 2013. We saw a very strong upper level low drop south into southern Japan, which then correlated to the infamous cold blast which originally made the phrase 'polar vortex' go viral. Forecasts above don't see the impending upper level low dropping as far south, which is why I'm thinking this upper level low will approach the region, and not necessarily induce a period of incredibly frigid temperatures, like we saw last winter.

To summarize:

- A strong upper level low looks to drop south to scrape northern Japan in the next week.
- As a result, we could see a period of below-normal, possibly well below-normal temperatures between February 16th and February 20th.

Andrew

Tropospheric Polar Vortex May Deliver Record Cold on January 7-9th

Nearly one year to the day after the tropospheric polar vortex was sent into southern Canada to bring record-breaking cold into the United States, model guidance is unleashing another round of record cold to kick off 2015.

Before we get started, please read this infographic from the National Weather Service to understand what the polar vortex is and is not.

NWS

*Note: The controversial phrase 'polar vortex' is used in this article for purposes of communication to the public, instead of 'Massive Arctic Semi-Permanent Upper Level Low', which would not only be too long to type over and over again, but is also not as effective at communicating this threat to others. 

Tropical Tidbits

The above image shows the European model ECMWF's forecast of 500mb geopotential height anomalies, for the evening of Wednesday, January 7th. Note how we see strong high pressure forcing its way into the Arctic from the Bering Sea. This dislodges the tropospheric version of the polar vortex, and seemingly splits it into two bodies; one just east of Greenland, and another in the East US. That dark blue area represents the lobe of the polar vortex, and if you closely observe the 500mb height contours on this chart, you can see that the lobe gets as far south as the border of New England and Canada before shooting east. The GFS model agrees on this polar intrusion.

So... what can we expect for this latest cold blast?

Tropical Tidbits

Now switching to the American GFS model, as ECMWF temperature products are pay-to-view and may not be posted on this website, we see the cold really begins to set in on the morning of January 7th. We see temperatures on the surface of -25 degrees Fahrenheit over the Dakotas, with major cities like Des Moines, Chicago, Milwaukee, and Minneapolis all sub-zero. Similar to the Arctic air outbreak last January, this is pure Arctic air. This is dangerous and potentially life threatening, IF current forecasts are correct.

Tropical Tidbits

By Thursday morning, the cold recedes from the Plains to some degree, but hits the Great Lakes hard, as Chicago goes sub-negative-15 degrees on January 8th, with similar fates meeting the Quad Cities, Madison, and even Gary, Indiana. It's very possible that any snow cover put down by the January 3-5 winter storm, as well as the January 5-6 clipper, may intensify this cold blast further than models are predicting.

Although I can not post temperature forecasts of the ECMWF model, here are a few key points that I can paraphrase for you:
- Temperatures by midday Wednesday are sub-zero north of a line from Springfield, IL to North Platte, NE.
- Temperatures on Thursday morning bottom out around -22 degrees F in central Wisconsin, with widespread double-digit sub-zero temperatures around the Midwest and Great Lakes. Madison, Minneapolis, and Chicago all hover around -15 to -20F.

To summarize:

- Model guidance is in agreement on a record-breaking cold blast to hit the United States on January 7th and 8th.
- Temperatures, if forecasts are correct, will plunge below -20 degrees Fahrenheit in the North Plains and Upper Midwest, with double-digit negative temperatures widespread across the Midwest, Plains, and Great Lakes.
- High uncertainty still exists with this event, but confidence is gradually rising.
- Residents across the United States must be watchful of future forecasts, as this will be a deadly cold wave if it comes to fruition.

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

Encounter With Polar Vortex On The Horizon in Early-Mid December

It now appears much of the United States will undergo its first encounter with the polar vortex in the second week of December.

Tropical Tidbits

The image above shows 500mb height anomalies over the Western Pacific on the evening of December 1st. In this image, we see what is very likely a lobe of the polar vortex passing just north of Japan, occluding and cutting north as it does so. Minimum values here at the 500mb level in this image, off the GFS ensembles, appear to drop close to 490 dam, an indication of either an incredibly strong upper level low, or the more-likely polar vortex lobe.
This situation appears similar to the case we had last winter, where we saw a very strong upper level low crash into Japan to round out the year 2013, which then led to incredibly cold weather in the first week of the new year.

ESRL
December 27, 2013

If we recall the Typhoon Rule, which states weather phenomena occurring over East Asia is reciprocated in the United States about 6-10 days later, we could expect a bout of  substantial cold here at home in the December 7-11 timeframe. Should this verify, which is becoming increasingly likely given model consistency, the Central and parts of the East US may expect a bout of intense Arctic air to hit in the middle of December.

But it doesn't end there...

Tropical Tidbits

The image above once again shows 500mb height anomalies from the GFS ensembles, now valid on December 6th. Notice that we still see below-normal heights over Japan. This means Japan has been under a strong upper level low's influence for nearly a week, and still going. Building off of the first graphic, it's quite possible that the United States may also undergo a harsh beating from cold weather for nearly a week straight. The degree of this cold air is still under examination, but if it's anything like the upper level low expected to trek north of Japan to open December, watch out.

To summarize:

- Japan looks to undergo severe cold weather to start off December, likely leading to a prolonged Arctic air intrusion in the US in the middle of December.

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

New Climate Model Outlooks Fix Errors; Cool, Snowy Winter Possible

Long range climate model projections of the upcoming winter season, the same ones we discussed a little while back, have been updated. The latest models still retain errors, but two models have come in with drastic improvements.
Bear in mind the typical caveats with long range forecasts still apply.

CPC

The image above shows a compilation of multiple long range outlooks, projecting the El Nino-Southern Oscillation phenomenon from the present to next spring. In this chart, we can see the majority of models favoring an immediate commencement to the El Nino expected to form later this year. The issue here, which we elaborated on in the link above, is that these models want to make the El Nino start immediately, something unlikely to happen. Complicating the situation further is the fact that these models want to bring the El Nino to Moderate (+1º to +1.5º above normal) or even Strong (over +1.5º above normal) levels by this winter. Again, this is unlikely to happen. Consequentially, we have been forced to discard these forecasts, as the incorrect SST projections then ruin the remainder of the forecasts.
The new update to these models came in to me today, and I was surprised to find two models that have changed their tune. In the chart above, instead of all models going up and up with the El Nino, the GFDL (turquoise colored line) and GFDL_FLOR (beige line) prefer either an ENSO-Neutral situation, where the El Nino is unable to form, or a weak El Nino, which is what my preference is for this winter. It finally seems as if we have two models that may stand a chance at verifying this winter.

Without further ado, let's review the forecasts from these two models for this winter.

CPC

 The first model we will review for the temperature outlook is the GFDL_FLOR. The image above shows projected temperatures across the United States for December-January-February 2014-2015. In this graphic, we see that the model keeps the majority of Alaska and western Canada warm, which can be indicative of persistent ridging over the area. Whether that is the case, we don't know, but such a feature would help chances for a cold Central/East US winter. Now looking at the United States, we find the majority of the nation experiencing average temperatures for the winter, with the Plains experiencing slightly below normal anomalies.
This is a rather substantial shift from earlier forecasts of a blowtorch (excessively warm) winter for the nation, which was due to the incorrect SST forecasts. It can be expected that models may project normal anomalies, because these forecasts are being made for months out. However, with this being the first forecast that is not retaining major SST flaws, things are looking up for those wishing for a cold winter.

CPC

Turning our attention to the GFDL model, we find a slightly different temperature forecast in store. Much of the Central and Western US is experiencing average to slightly below average temperatures, while the Great Lakes, Southeast and East US in general experiences a warm winter. We see this warmth extend into northeast Canada and towards Greenland, which makes me think that we may need to watch for ridging building off the East Coast if this forecast is correct (which, as we know by now, is not a given).

CPC

We now turn our focus on the precipitation outlooks from the two models. The GFDL model, pictured above, shows a wide swath of above-normal precipitation extending from Texas to south-central Canada, bringing the heaviest anomalies to the southern Midwest and western Ohio Valley regions. Wetter than normal conditions are also observed across the Eastern Seaboard, while dry weather prevails into the West Coast.
I'm not willing to say much on this outlook just yet, but since this is the first forecast that doesn't have that dreadful SST flaw, this sort of precipitation pattern may bear watching for this winter. If it did verify, it could spell disaster for California and other drought-affected regions.

CPC

Taking a look at the GFDL_FLOR model, we see a somewhat similar outlook as the one portrayed by the GFDL system. In this outlook, the December-January-February period features a very dry West US, remarkably drier than the GFDL model outlook. Slightly above normal precipitation anomalies extend from the Southern Plains and Gulf Coast into the Midwest and Great Lakes. The Central Plains is also included in this wet trend.
Again, though there's not much else to say here, it could bear watching.

CPC

Lastly, we'll take a look at the model-projected upper air pattern for the coming winter. The image above shows the Z200 outlook over December-January-February from the GFDL model. We want to maintain focus more on the contour lines than the color shades for this post. Taking a look around North America, we find a ridging signal over the West Coast and into Alaska. As we've discussed extensively, persistent above-normal water temperature anomalies in the Gulf of Alaska may support such a scenario, which could then lead to another chilly winter. Looking downstream, we do see a slight suppression in the contour lines, potentially indicative of some stormy weather. We saw a similar outlook in the months leading up to last winter.
Something else to note is the suppressed contour lines over Japan. This could indicate stormy weather there, and if we apply the Typhoon Rule, it could then affect us here in a substantial way.

CPC

Finally, shifting to the Z200 outlook from the GFDL_FLOR guidance system, we still see a cold signal across North America. Eerily similar to outlooks from last year anticipating the winter of 2013-2014, we see strong ridging over the West Coast and into Alaska. This ridge looks to be of similar strength to the one observed last winter, which was the mechanism responsible for the polar vortex scraping the northern US. Once again, we see suppressed contour lines in Japan, again indicative of stormy weather. The most interesting feature, by far, is the depression of contour lines over eastern Canada and into the United States. This is definitely a cold weather signal, dare I say the same signal that eventually brought the polar vortex south. I'm not saying that whole episode will happen again, but if this model is right, cold weather becomes a significant possibility.

All of this looks pretty supportive of a cold and snowy winter, but when it comes down to it, remember that this all rests in the realm of possibility, not certainty. Things do look good from these error-corrected models, but we will have to wait and see if these solutions stick with the updated model forecasts next month.

Andrew