Friday, December 10, 2021

December - January Outlook: Colder Pattern Expected in North; Mild in East, South; Frigid Canada

As I discussed on this blog's Facebook page yesterday, this is my outlook for the end of December through the month of January. We'll be using four primary components in this outlook: the stratosphere, the Pacific Decadal Oscillation, the La Nina, and the Madden-Julian Oscillation. I'll describe each component below, closing this article with my forecast.

I. Stratosphere

As things stand at this writing, the stratospheric polar vortex (SPV) is a bit battered & bruised, but overall in good shape. Attached on the right is a look at the 70-millibar pattern over the Northern Hemisphere, exemplifying something we saw develop in the last couple days: a split in the SPV at the 50-millibar level and lower. Indeed, the two distinct closed-off height contours in this image reflect a split vortex at the lower levels, while at the upper-stratosphere (10-millibar level), where it really matters for sudden stratospheric warming (SSW) purposes, the vortex remains intact.

You may ask, what's the difference if the vortex splits at lower levels or upper levels? If were to see the SPV split up into two pieces throughout the entire stratosphere - not only the 70-millibar level but up through the 10-millibar level as well - it would signify that the stratospheric polar vortex is really in trouble, and greatly increase the probabilities of Arctic air being unleashed to lower latitudes of the Northern Hemisphere. Because the vortex is still intact at that key 10-millibar level, I would consider this something worth briefly discussing, but not something that will have a massive influence on the outlook.

In fact, the only influence I'm drawing from the stratosphere is quite a warm influence; model guidance over the next two weeks suggests the stratospheric polar vortex will remain around normal or above-normal strength for this time of year, which should help to lock that frigid Arctic air up in Canada. This is confirmed in the latest Arctic Oscillation (AO) forecast from the long-range ECMWF model, shown below, with the average of these 51 ensemble members having the AO stay almost entirely positive during the period, and may even be substantially positive for January:

For the month of January, then, the stratosphere adds warm risks across most of North America.

II. Pacific Decadal Oscillation

Another key component to my January outlook is an index called the Pacific Decadal Oscillation. This is a longer-term oscillation, on the order of months and years compared to regimes like the Arctic Oscillation and North Atlantic Oscillation (AO and NAO, respectively) which work on the order of days to weeks. Below I've attached a look at the PDO not using sea surface temperature anomalies, as is normally the case, but judging how the atmosphere is determining the PDO's phase:

The above image shows an index of the Pacific Decadal Oscillation using atmospheric measurements, as opposed to the alignment of sea surface temperature anomalies. This matters because, while one can see how certain water temperature anomalies line up, what really matters is if the atmosphere "picks up" on that signal and responds accordingly. After all, if water temperatures say our PDO should be negative but the atmosphere doesn't act like it's in a negative phase, is it *truly* negative?

In any event, the above image shows the Pacific Decadal Oscillation in its most negative state since the late 2000s, and at its second-most-negative reading since the late-1970s. This matters because, as the image on the left shows, the PDO materially impacts the Northern Hemisphere during boreal winter. For some more context, the image on the left shows the correlation between the mid-level pattern and the PDO index. If values are negative, it means a positive-PDO results in stormy/cold weather, or a negative-PDO results in warm/calm weather during the Dec-Feb period. Similarly, if correlation values are positive, it means a positive-PDO results in warm/calm weather over that area, while a negative-PDO results in cold/stormy weather over that area.

We can confirm this by looking at the correlation between the PDO index and surface temperatures during the winter months, as in the image directly on the left. Indeed, given that we're in a negative-PDO, the positive correlation values in the West suggest this negative-PDO results in colder weather, while negative correlation values in the East U.S. imply a warmer pattern from a negative-PDO.

Given how strong the PDO is currently seen in the atmosphere, and how strongly negative it is, my outlook anticipates persistent colder and stormier weather across the West U.S., equivalent to a negative-PNA index (resulting in milder risks across the East U.S.).

III. La Nina

We remain steadfastly in a La Nina environment, with water temperatures in the Equatorial Pacific comfortably colder than normal and the Southern Oscillation Index (SOI; a shorter-term indicator to identify whether we are in a La Nina or El Nino) staying in the positive-teens, indicative of a La Nina reflecting in the atmosphere as well (instead of just showing up in ocean water temperatures).

The probability-based forecast of the ENSO phenomenon from the Climate Prediction Center and Columbia University is shown on the right, and portrays very high confidence in the ongoing La Nina persisting through meteorological winter into meteorological spring. We see that the La Nina (negative-ENSO state) correlates positively to below-normal temps during Dec-Feb across the Northern U.S. as below...

... while the La Nina tends to provoke above-normal temperature anomalies in the South U.S. as the Pacific jet stream is shunted northwards to deliver storm systems to the Pacific Northwest, rather than the Desert Southwest. Combining the negative-PDO with the La Nina, and accounting for constructive influence between the La Nina and negative-PDO as discussed in Rao et al. 2019, below-normal temperatures seem quite likely across the West U.S. and especially into western Canada for January 2022.

IV. Madden-Julian Oscillation (MJO)

Perhaps the most important part of this outlook, the Madden-Julian Oscillation (MJO) looks to come back to life over the coming weeks, prompting a shift in the global weather pattern.

Before diving into this component of the outlook, let's review what the MJO is and why it matters. In a nutshell, the MJO is based on the positioning and intensity of enhanced thunderstorm activity across the world, along the Equator. The location of this enhanced thunderstorm activity along different parts of the Equatorial prompts different global weather regimes; indeed, the MJO is often seen as the most important sub-seasonal oscillation currently known.

The MJO is said to be in one of eight phases at any given time, depending on where enhanced thunderstorm activity along the Equator is located. If there is minimal thunderstorm activity, then the MJO is simply too weak to have a material impact on the global weather pattern. This is all summed up well in the phase-space diagram below:

This chart might look intimidating at first blush, but we can break it down to make it easier to understand. As I discussed, the MJO can be broken into eight different phases, depending on where thunderstorm activity is most focused along the Equator. Each of those eight phases is labeled in this phase-space diagram, and the rough location of where that enhanced thunderstorm activity is during any given phase is written on the axes of the chart. You'll also notice number ticks on both axes - that tells us the intensity of the MJO at any given time. For instance, if the jagged line goes further away from that middle circle and towards larger-magnitude numbers on the x- and y-axis (whether positive or negative), it means the MJO is becoming stronger, and the global weather pattern should increasingly reflect the 'typical' pattern that develops when the MJO is in that given phase.

Examining the chart further, we find a black jagged line which then turns into a multi-colored thick line, surrounded by a whole spaghetti mess of thin yellow lines. That solid black line shows the progression of the MJO over the last five weeks or so - we see that the MJO began November in that middle circle, meaning the MJO was too weak to be placed in a specific phase / too weak to influence the global weather pattern on a material level. The MJO then spent almost all of November in a weak Phase 4-5 state, bouncing in and out of that middle circle. Since December 1st, however, the MJO has woken up and surged into Phase 6, where we now find ourselves.

The red, blue and purple lines represent the average, bias-corrected forecasted state of the MJO from the latest long-range ECMWF model; here, it's the forecast from December 8th through January 9th. To that extent, each thin yellow line is one of the 51 ECMWF long-range ensembles. 

And now that we're through with all of that, we can finally get into the forecast part!

The ECMWF has been consistent in taking the MJO through Phase 6 until December 18th or so, remaining in a rather strong state through the entire timeframe. Referring to typical temperature anomalies in each MJO phase on the right, we see that a Phase 6 event in meteorological winter corresponds to much warmer than normal temperatures in the eastern half of the country, while the West U.S. experiences below-normal temperatures and generally stormier weather.

This does look to verify, with temperatures set to soar across the eastern half of the country in the coming days (not shown).

By December 20th, the MJO is projected to push into Phase 7, still at a rather strong level, and the index remains in Phase 7 through the first several days of the new year (albeit at a weaker magnitude). Phase 7 still encourages warmer weather in the East U.S., but we now also see colder weather pushing south and east from western Canada into the northern U.S. Rockies and north-central U.S. Put together, it seems likely that the remainder of December will be warm across the eastern half of the country, though with a shot of cold air between December 25 - 31 as the MJO's trip into Phase 7 inspires increased chances for cold Canadian air to develop in western Canada and eventually push into the U.S. for a brief period. A return to warmth is expected thereafter.

January 2022 Outlook

With our four components examined above and the end-December outlook written in the previous sentence, we can get to work on what really matters: the January 2022 outlook.

I am expecting the month of January to feature below-normal temperatures across the West U.S., extending into the Rockies and especially in western Canada. It seems likely that temperatures will be much colder than normal in western Canada, a consequence of the typical January La Nina pattern, negative-PDO and MJO in Phase 7 - all of which encourage colder weather in western North America. 

Temperatures should go through quite a bit of back-and-forth in the Plains, a consequence of creeping cold from Canada bumping up against formidable warmth in the East. A strong polar vortex should allow warm weather to win out over colder weather on average, and I expect most of the Central to be on the warmer side of normal.

The East U.S. is likely to lean firmly warmer than normal, with these warm signals showing up in the MJO, La Nina, negative-PDO and strong polar vortices at the tropospheric and stratospheric levels. Confidence in the East Coast being warmer than normal is higher than normal. Indeed, confidence in this forecast is higher than normal, and it's been some time since I've made an outlook with forecast confidence at this high of a level.


Thursday, December 9, 2021

Long-Range ECMWF ENS Control Member Shows Frigid January; Will it Verify?

 The latest long-range ensembles from the European forecasting agency came in the other day, and while on the whole they produced a generally milder outlook, we did see a surprise forecast from the 'control' member of these ensembles - a remarkably frigid January.

Before we get into the good stuff, let's review what that means.

The ECMWF agency is known for their namesake weather model, but they also provide a critical forecasting tool with their ensemble system. In an ensemble forecast, an initial forecast model is taken, tweaked ever so slightly, and then re-ran. That's one ensemble 'member'. Then, they take that initial data again, tweak it ever so slightly in a different way, and re-run it again. That's another ensemble member, and it continues. The ECMWF's ensemble system has 51 different members - they are all different versions of the ECMWF weather model, but each one has slightly different starting conditions in order to produce a different forecast

You might ask why one would do that, since it sounds a lot like all we're doing is intentionally changing the initial conditions and letting it run free. That doesn't sound very productive, does it? In reality, the value comes from how similar all of these 51 members are at each point in time. For example, if all 51 ensemble members showed a winter storm in the Midwest ten days from today, that would be a big boost to forecast confidence. Why? Because if all 51 members have that storm happening, it means no matter what the weather conditions are right now (i.e. even if they're tweaked in all kinds of ways), we still end up with a winter storm on the horizon. As a result, ensemble forecasting is a potent part of any forecaster's toolbox.

But each ensemble set - whether it's the ECMWF ensembles, the American (GFS) ensembles, or any other set - has one member that is different from the rest. This member's initial conditions are the best possible data we have, and all the other members are those slightly-tweaked versions of this one. We call this special ensemble member the control. Indeed, the control member is the raw forecast taken strictly using initial data. Nothing is tweaked, nothing is altered, just the raw initial conditions. Every other ensemble member takes that control member data, tweaks it a bit, and re-runs it, like I described above. 

One may argue that this makes the control member more attention-worthy, since it only uses the best initial conditions data we have and isn't artificially tweaked. Whether that argument has merit is something I have yet to verify on my own.

In any event, now that we understand why an ensemble's 'control' member is worth monitoring, let's get down to the good stuff: the reason why this post was made.

Attached above from WeatherBell (all images on this post are from WeatherBell) is a look at the seven-day average temperature anomaly from the ECMWF ensemble control member over the December 25th - January 1st timeframe. We see that the control member anticipates a deep and formidable reservoir of below-normal temperatures in western Canada, which begins bleeding south and east into the United States as December turns to January. Indeed, seven-day average anomalies of more than 25 degrees Fahrenheit below normal are nothing to shake a stick at!

The outlook continues into mid-January below:

From January 11th - 18th, temperatures are shown by the ECMWF ensemble control member to be much colder than normal across almost all of the country, particularly east of the Front Range and especially in the Ohio Valley. With seven-day average temperatures seen at almost 20 degrees F below normal in parts of Ohio, Indiana and Kentucky, it's clear that the ECMWF Control member expects the Arctic gates to burst open in the first month of the new year.

Teleconnections accordingly show a breakdown in the tropospheric polar vortex as judged by a collapse in the Arctic Oscillation in the blue line (control member's forecast)...

... and a similar plunge in the North Atlantic Oscillation (NAO) from the control member allows that frigid Canadian cold to plow eastwards...

Now, that's all good and fun to look at, but this post wasn't written to get all the cold weather fans out there hyped up. Indeed, as the above two teleconnection charts show, the control member is an extreme outlier in the late-December and early-January period, and the average of all ensemble members just barely turns the NAO negative at the end of this month. The mean doesn't even turn negative at all for the Arctic Oscillation!

My takeaway is this: we're seeing some noteworthy cold signals for the end of December and start of January from the ECMWF control and the CFS ensemble system (a different analysis of that coming later today), but we can't rely on models that far out. To me, it's just not smart forecasting to use them for anything aside from large grains of salt, if that makes sense. Instead, I'm focusing on a changing global weather regime as the Madden-Julian Oscillation changes into colder phases for the end of December and through January. That, I feel, is where the real intrigue lies for Arctic outbreak risks as we enter the new year.

Stay tuned for that aforementioned post later today.

To Summarize:

  • The ECMWF ensemble's 'control' member - often seen as the most accurate member of the ensemble group - is expecting much-below-normal temperatures from the end of December through much of January in the U.S.
  • At this stage, while it's intriguing, I would not take this signal as anything more than a large grain of salt
  • There is more value in monitoring the changing global pattern as the new year kicks off, and a post discussing that will be published a little later today.


Monday, December 6, 2021

December 10-12 Potentially Significant Winter Storm

 There is increasing potential for accumulating snow across the north-central and Upper Midwest in the coming days, as a strong low-pressure system moves northeast through the Central Plains and Great Lakes. Note that there are two disturbances in this timeframe - one affecting the Midwest, and another affecting the East Coast. This post covers the first region.

Because of relatively large variance between model guidance, I will first examine the GFS model, followed by the ECMWF model. A summary of my own thoughts will conclude.

GFS Model Analysis

Attached above is a look at 3-hour accumulated precipitation (by type) and sea level pressure contours, per and per the 12z GFS, valid at 12pm Central Time on December 10th. The GFS anticipates a strong surface low (989 millibars) to eject from the Rockies into the Kansas / Nebraska area around this time, tossing out a band of snow well ahead of the low stretching into South Dakota, Minnesota and northern Wisconsin. Snowfall intensifies in SD as the low approaches and allows for the deformation zone to form.

Moving ahead to 12:01 am on December 11th, we find the storm to be a quick-moving one, with the surface low now in the Dubuque, Iowa vicinity and developing into a mature cyclone (evidenced by ongoing rain and thunderstorm activity in the South and a continued band of moderate snows north and northwest of the low). Accumulating snow looks to overspread Minnesota and northern Wisconsin at this point in time.

The primary issue for those hoping for snow appears to be an aggressive warm front associated with the storm system. Indeed, a warm antecedent air mass (a.k.a. relatively warm weather in the Central U.S. in advance of this storm system exiting the Rockies in the first place) will allow the warm front to keep precipitation type as liquid well north and east of the surface low, stretching past Michigan and into south-central Canada!

Taking a look at projected accumulated snowfall for this event (remember, NOT including the East Coast disturbance around this same time) shows a relatively impressive snowstorm for parts of the Upper Midwest and Plains. Assuming a 10-to-1 snow-to-liquid conversion (meaning 10" of snow can be melted down to 1" of water), a swath of South Dakota is at risk for snows exceeding 6", while portions of Minnesota and especially northern Wisconsin could see a run at more than 12" of snow as the disturbance associated with this storm system matures and intensifies. Indeed, taking the GFS verbatim, some locations in northern Wisconsin may have a shot at 18" of snow!

It's also possible these totals are a little under what they may actually turn out to be, as surface temperatures look to be firmly in the mid- to lower-20s during this event, turning that 10-to-1 snow ratio more into a 15-to-1 ratio.

Now that we've gotten a look at what the GFS model wants, let's peek at the ECMWF's interpretation.

ECMWF Model Analysis

We'll be examining the same points in time here as we did in the GFS analysis: for instance, this first graph shows forecasted precipitation type and sea level pressure contours at noon on December 10th. The ECMWF places this surface low somewhat to the south of the GFS, resulting in a southward shift of the moderate snow band. Indeed, rather than South Dakota receiving the moderate snows from this system as in the GFS, we find much of Nebraska (but not Omaha!) under the gun for accumulating snow. The overall precipitation presentation in the eastern Rockies also looks more robust in the ECMWF, implying that the system may put down more snow than what the GFS showed us.

Moving ahead to 12:01am on December 11th, we find the ECMWF placing the surface low pretty close to where the GFS showed it, but the big difference is the marked increase in moisture available immediately north and west of the surface low. While the GFS has snowfall coming to an end not too long after the surface low passes a given line of longitude, the ECMWF allows moderate to heavy snowfall to continue in Nebraska even as the surface low is near Dubuque!

The result is a much snowier storm system:

As depicted above, via WeatherBell, the 12z ECMWF delivers over 12" of snow to the northern tier of Nebraska, even laying down several inches of snow on the back end of the storm system in parts of eastern Nebraska. A sharp gradient near the surface low likely results in a modest icing threat in the Omaha metro area, while continued snow accumulations exceeding 15" track north and east through northwest Iowa, southern Minnesota and northern Wisconsin.

My Thoughts

A look at how each model handles the mid-level pattern gives some insight as to why these two forecasts are markedly different. 

The first thing that is complicating this outlook is the fact that we're dealing with two separate pieces of energy in making this winter storm happen. I've highlighted these two pieces of energy in a 500-millibar relative vorticity image on the right, valid at noon on December 9th. That might all sound a bit complicated, but in a nutshell, the more colors you see, the more likely it is that there is a 'piece' of energy present that can develop into a storm system.

Both models agree that these two pieces of energy will make landfall on the West Coast in the Pacific Northwest and in California, respectively. It's how they mingle and interact when they meet up in the Rockies that is throwing models for a loop. 

The GFS model wants to hold the core of this energy back in the Rockies a little longer than the ECMWF, making the system positively-tilted at the mid-levels (a development that keeps the storm system relatively immature, and making it tougher to get those heavier snow totals). In contrast, the ECMWF allows a core of this energy to track north and east with the surface low and tilt negatively as it does so, amplifying the surface low and resulting in a more enthusiastic snowfall outlook across the Central.

The current strongly-positive state of the Arctic Oscillation (shown above) helps mold the broader weather pattern by making it less likely that we get these big storm systems with big snow totals, and does lead some credibility to the weaker, more progressive GFS model. At the same time, the Pacific-North American index (PNA) is poised to be strongly-negative at this point in time, which helps fire up a ridge of high pressure in the East U.S. and gives any storm systems in the Central a "ramp" in the mid-levels on which these pieces of energy can tilt negatively and mature into powerful storms.

Putting all these pieces together, I am in favor of the GFS' solution right now, but I do think it's pretty realistic we could see a southward adjustment like the ECMWF shows, in which case we would also see a stronger storm system and higher snow totals. The overwhelming negative state of the PNA drives that conclusion, with multiple GFS ensembles joining a majority of ECMWF ensembles in playing that southern shift out and making for a stronger storm.

To Summarize:

  • A potentially significant winter storm is expected in the December 10th - 12th timeframe across the Central U.S.
  • At this time, South Dakota, northwest Iowa, southern Minnesota and northern Wisconsin may see snowfall totals of 4-12", with those higher amounts much more likely in NW Iowa / S MN and especially northern WI
  • Given regional teleconnections, it seems quite plausible that future forecasts shift this snow axis a bit south and increase snowfall totals, as the ECMWF model projects
  • It is too early to tell for sure, but there may be a minimal icing threat along the freezing-temperature line. Material ice accretion is not expected from this storm.

Wednesday, December 1, 2021

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 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


Tuesday, November 30, 2021

December 4-6 Potential Winter Storm

A disturbance is forecasted to track eastwards from the Pacific Northwest & Southwestern Canada across the northern Plains, bringing accumulating snow to the region before intensifying as an additional piece of energy interacts with this original system around and east of the Great Lakes.

This event begins during the day and evening on December 4th, with an upper-level disturbance riding east-southeast along a band of tight upper-level confluence and spawning a surface low-pressure system in the Dakotas. The above image shows forecasted precipitation types and sea-level pressure contours as of 12pm Central Time, December 1st. While the bulk of the precipitation is seen in the Rockies, the eastward progression of this system begins at this time as an arm of light snow pushes eastwards into the Dakotas.

By 6am Central Time on December 5th, the surface low has pushed east in accordance with the eastward progression of this upper-level system. Some moisture fetch from the South U.S., evidenced by weak rainfall in MO/AR/TX, adds to the system and helps gently boost snowfall totals in Minnesota and Wisconsin. Accumulations are expected to be in the 2-4" range across the Dakotas, northern Minnesota and northern Wisconsin over this December 4-5 period.

Things become much more interesting overnight December 5th into the morning of December 6th, as our surface low pushes eastwards and the upper-level piece of energy finally gets an opportunity to strengthen after being squeezed by a progressive and prohibitive jet stream back in the West and Central. As shown above, the latest European model run sees moderate to heavy snow in southeast Canada and into extreme northern parts of New England, turning into a mixed precipitation event as a surge of warm Southern air brings rain to most of the region.

The GFS model is a little better with snowfall chances in New England (1-3" in western and central New York), but in general we should see the best shot for over 4" of snow in southern Canada. The latest ECMWF snow accumulation chart for this event is shown below, and there is some question as to how strong this system could actually get. I don't think the ultra-aggressive totals shown by the ECMWF will come to fruition, but it seems plausible that some areas along or just north of the U.S. / Canada border in New England could hit 6".

To Summarize:

  • A winter weather event is expected to impact the northern Plains, Great Lakes and New England in the December 4-6 timeframe
  • Snow accumulations of 1-3" appear possible in the Dakotas, northern MN and northern WI during the December 4-5 window
  • Some heavier accumulations nearing 6" may be possible in extreme southeast Canada, but questions remain as to how notably this storm system will intensify as it nears the Atlantic