The weather models are running just as quickly and as often as they can. Outputting nothing. Outputting snow. Outputting rain. Back and forth. Life and death. Snip snap! The fingers are typing and posting on Twitter even faster, if that’s even possible. But yes, actually, there are four “systems” that may produce snow on the East Coast next week, and one of them looks rather delectable, especially from the interior / northern Mid Atlantic into New England.
A quick note on how this pattern is coming to be…
The combination of a negative East Asian Mountain Torque event (lower pressure east of the mountains) and piece of the tropospheric polar vortex dropping into Asia is encouraging a somewhat retracted Pacific jet stream (after it was quite strong / extended to end November and start December) and wave breaks over the northern Pacific. This is allowing heights to rise in the northern Pacific, dropping the EPO and WPO. This is greatly increasing polar air in North America after it lacked to start December and prevented last weekend’s storm from being snowier. And by snowier, I mean producing more than 0 snowflakes in New Jersey.
Tropical forcing is centered near 120E right now, registering as a phase 5 MJO, projected to move into phase 6 next week:
The RMM plots have had a hard time tracking this current MJO as it’s constructively interfering with lower frequency forcing near 120-150E. However, it seems there’s still a coherent MJO in there that’s currently in phase 5 and may get into phase 6 or 7 next week. You can see how the lift really intensified over the Indian Ocean last month as the MJO approached the low frequency forcing near Indonesia…and how it seems the MJO is now emerging on the other side towards the western Pacific:
The CFS (above), EPS and GEFS all have some uptick in 200mb upward motion as far east as the Dateline over the next 5-7 days. A phase 5-7 MJO in mid-December strongly suggests blocking in both he EPO and NAO domain:
This period of potential blocking, aided strongly by tropical forcing along with a recent +East Asian Mountain Torque that really beefed up the sub-tropical jet, has been talked about many (including myself) since November. I personally was not as sold on legitimate NAO blocking, but that does seem likely to occur this weekend through next week.
It’s quite possible that a disruption to the stratospheric polar vortex, causing it to get split in the lower stratosphere for a time, with a lobe dropping into Asia and another into North America, with ridging / weaker flow in between over the north Pacific and north Atlantic, is encouraging this tropospheric blocking.
One can definitely see on the 3D vortex plot from stratobserve.com how the tropospheric and stratospheric blocking across the north Atlantic may be somewhat coupled, and also how the lobe of the tropospheric polar vortex dropping into North America is tied to the stratospheric vortex as well. This activity within the polar vortex is complimenting what the tropical forcing suggests quite nicely…and this disruption is likely tied to a recent strong MJO progression across the Indian Ocean and strong East Asian Mountain Torque event.
I guess this is a long way of saying high latitude blocking, including in the NAO domain (2020 baby!) is quite well-supported next week. Though, do note that phase 5-6 tropical forcing in late December supports troughing over western North America and a Southeast Ridge, and as the MJO eventually weakens and the low frequency forcing takes over later in the month troughing very well may try to slide west. But let’s not skip to that, let’s break down next week’s potentials…
Dig polar jet, dig! Ah that’s been the rub against the Monday – Tuesday threat, the lack of ridging out west would keep the polar jet too bottled up to get enough stream interaction for a storm to deepen and come up the East Coast. Is there hope on that front?
There is quite a bit of blocking over the Arctic and north Atlantic. The polar vortex is over Hudson Bay! There isn’t a 50/50 low yet, but the Atlantic side is not inhospitable. The Pacific side is a little more…eh? If ridging off the West Coast is a bit stouter, or if there is just a bit more “traffic” over the Atlantic, perhaps the northern piece can dig a little more.
The trend to raise heights a bit over the Davis Strait and nudge the PV a bit west…and trend on the West Coast to have a little more ridging between shortwaves…may just force the northern stream to dig a bit more. That would result in a better chance at a phase and a more robust coastal. The rub here is there’s no 50/50 low yet (or “traffic”), so a stronger storm would try to cut a bit more. So, I-95 would require a perfectly timed phase for legitimate snow, but inland is in a better position if we trend towards a better phase.
Both the EPS and GEFS runs at 18z like this slightly more phased idea for whatever reason…seems it’s the northern Mid-Atlantic most likely to get snow, especially NW of I-95, with some potential this gets into NY or New England if we see more wholesale changes to slow down / dig the polar jet shortwave more.
There are a number of EPS members that are stronger than the mean, though most are northwest of the mean, and the strongest ones are well northwest of the mean. This is evident on both the spread (all to the north/northwest of the mean) and map of the member low locations:
My verdict on this one is to watch for the polar / northern jet shortwave to dig more. If it can, it opens the door for more snow, but may also shift that snow NW of I-95 pretty quickly if we aren’t careful.
Wednesday – Thursday:
This is currently the big-ticket item generating most of the images getting posted on weather social media. A primary low moves towards the Great Lakes, as it runs into confluence over southeast Canada the energy gets shunted east and sparks a coastal (Miller B) off the northern Mid Atlantic or New England coast. With reasonably cold air in place to the north, this may produce a good amount of snow for someone…
The pattern is pretty delectable on the Atlantic side. A strong -AO, -NAO, and 50/50 low. The -NAO and 50/50 low coax that strong confluence over southeast Canada, supporting a surface high pressure that forces the Miller B coastal to form. The tropospheric PV and 50/50 low, which was a lobe that broke off from the PV, mean there’s actual cold air available to tap, unlike last weekend when a robust Nor’easter produced more rain than snow. The Pacific pattern isn’t ideal…the ridge axis is a little southwest of preferable and the polar vortex is a little west/northwest than an ideal location. So, this would likely act to limit any threat in the Southeast and lower Mid-Atlantic. But, the Atlantic blocking and cold air over Canada being so robust would pretty much guarantee snow from much of PA and northern NJ into NY and New England, assuming there’s a storm.
The trends on the EPS over the last few days for this time period are remarkable. The NAO has trending much more negative, with that -NAO also trending more west based, with the strongest anomalies over the Davis Strait! There has also been a trend for a much stronger and more optimally placed 50/50 low. These trends are a result of trends with the Monday system for a more amped northern stream and potentially stronger coastal. The AO blocking has also trended stronger, and perhaps also as important, ridging over the Aleutians has been trending towards mainland Alaska. One more trend in that direction and all of a sudden the PV is nudged a little to the east, which would be a better spot.
Right now, it’s quite a good set-up north of the Mason-Dixon line, especially inland, but if the Pacific side trends a bit better could become something interesting in the lower Mid Atlantic or Southeast.
These are impressive ensemble mean snow swaths for over a week out for the EPS and GEFS this afternoon / evening. The greatest signal is from eastern PA through NYC and Boston, initially favored NW of I-95 but then right along it. There is considerable spread on both sides through. Also note how there is some potential for snow with the primary low over the Ohio Valley too on the other side of the Appalachians.
While there’s great agreement on a general low track off the East Coast next Wednesday – Thursday, there is still a good bit of spread in both track and speed, and intensity at all points. This likely has a lot to do with how well this shortwave interacts with the polar stream, and how strong the Monday system becomes, since it turns into the 50/50 low.
My thoughts on this one are that the Atlantic side is stellar, and the shortwave ejected east across the country is strong. There should be a decent Nor’easter with snow for at least part of the Northeast or Mid-Atlantic. If the Pacific side isn’t as ideal, with the PV centered a bit to the west and the ridge axis southwest of what’s preferable, though some improvements may still occur there based on current trends. These improvements could really up the ante for snow somewhere if they occur.
Otherwise, how the Monday system pans out will also impact this second threat. A stronger Monday system may force this system farther south, turning it into more of a Mid Atlantic problem and perhaps limiting how far north any snow can get. A weaker Monday system could weaken the confluence over Canada enough to allow the primary to cut a bit more, which would limit the threat on the southern extent by pumping in more warm air before the coastal tries to form.
A storm seems likely for someone, but a lot can change where and exactly how much. Wow, I know, I’m such a prophet.
Next Friday and Sunday:
The Atlantic pattern remains decent through next weekend and the AO stays negative…however, the EPO rises and the PNA tries dipping, which will try to flood the CONUS with Pacific air and make it harder to see enough northern stream interaction for a phase or good winter storm. How the Wednesday-Thursday storm plays out will also impact the pattern for Friday’s storm, and Friday’s storm will have an impact on the Sunday storm. See a pattern? This is fun.
If the Pacific side holds onto a more neutral pattern just a bit longer that could really make one of these a more plausible storm. Is that possible? My gosh these questions are why I should be sober when I write. I ask fewer weenie questions while sober.
After the ongoing negative East Asian Mountain Torque, we’ll have a positive torque develop much of next week into the following weekend. This will act to extend the Pacific jet again. There will be a brief window next week where this may encourage a stronger low near the Okhotsk Sea (NW Pacific). As some of this momentum gets dispersed into the sub-tropical jet over the East Pacific, this may combine with the Okhotsk low to try to wedge a ridge between that low and the PV edging into the EPO domain from western Canada. That’s a long way of saying it’s possible it takes the EPO a bit longer to rise than the current ensembles, though it seems inevitable to eventually happen.
My impression is that unless the Wednesday – Thursday storm falls apart that it will make the Friday system unlikely to happen. Or, it will shove it so far south it’s mainly a Mid Atlantic or even Southeast event. The hostile Pacific pattern may limit the potential on next Sunday, though if there is enough polar influence it may again threaten the Mid Atlantic or southern New England.
The takeaways to all of this are that a lot supports high-latitude blocking next week, and we have a few systems that can take advantage. The Wednesday – Thursday is the most likely to be a major storm for the greatest number of people as Monday, Friday, and next Sunday’s threats all need a bit of work. But all are close enough not to rule any out. That’s to say, let the model watching continue…
It’s past November’s midpoint, so I’m clearly late to the party with this more technical write-up on what I think will happen this winter. My thoughts to date have been posted by me at times on weather forums and hinted at on Twitter, and my employer has also posted a couple of YouTube videos…but, this is the first time I’ve been able to fully sit down and write everything down. Given how late it is, my thoughts have evolved some from October.
This write-up will have 3 sections of discussion and then finish with the actual forecast. I’ll go through the drivers heading into the winter and what they may mean, some analogs based on said drivers, and also the recent / current pattern, how that may move forward and how that compares to some of the analogs.
Since I understand some people just want the maps, I will post them first, but the fun obviously comes from the write-up 😉
Please note that I expect a week or two of normal to below normal temperatures in the Great Lakes, Northeast, and perhaps Ohio Valley and Mid Atlantic, along with windows where snow is possible, despite the overall mild temperatures in December.
Total seasonal snowfall (including what’s already fallen, where applicable):
We’ll start with the moderately strong La Nina event across the Pacific. This developed over the summer and has strengthened through the fall, and the atmosphere has clearly responded. The La Nina is currently moderately strong and will likely be at moderate to borderline strong strength for the winter.
A recent weakening of the trade winds has allowed for both the surface and sub-surface to warm slightly, however, most guidance suggests another period of strong trade winds developing the rest of this month and into early December, which likely will result in further cooling.
Despite a little bit of recent warming, it is still quite cold below the surface across much of the Pacific, particularly east of the Dateline. Another period of enhanced trades would result in upwelling and notable surface cooling into December. Various models depict more strengthening into early-winter and suggest that a strong La Nina tri-monthly peak may still be in play, though the modest warming this month will probably make that difficult to pull off. Most guidance shows the La Nina peaking in intensity in December and then beginning to weaken:
So, let’s start with the La Nina…here’s what the last 10 have gotten us when averaged together:
Gee thanks for nothing La Nina. Except for 10-11 and 17-18. Those are kind of OK. The last 10 Ninas, on average, fit the stereotype of starting colder than they end for the eastern U.S.:
The last 10 moderate-strong La Ninas only are similar for the winter as a whole, though are a little uglier in the NAO and EPO domains:
There were a couple of fairly good winters in there, and even some of the duds had brief but intense cold shots…though on the whole they’re not great.
In general, La Nina winters feature a stronger stratospheric polar vortex and limited high-latitude blocking, with the signal stronger in moderate to strong events with a +AO, NAO, and EPO on the above composite. There are a few exceptions, even with moderate to strong events, but with all else being equal they are not more supportive of high-latitude blocking than a neutral or warm ENSO.
With that said, where the coolest water with the La Nina is associated can make a significant difference. This paper examines different response to East Pacific La Nina events and Central Pacific events:
The tl;dr here is that “East Pacific” La Nina events tend to have a slightly more poleward north Pacific ridge, and more notably, a modest -NAO…as compared to “Central Pacific” events that have a flatter north Pacific ridge, and a +NAO.
The question is, is this event central, eastern, or more basin-wide?
From the same paper, above is the typical SST departures with time in East Pacific, Central Pacific, and hybrid La Nina events. The key difference between eastern and central events is that in eastern events, the cooling originates near South America and propagates west, while in central events the cooling is centered near 150W. Central events also tend to be stronger, though there are exceptions both ways.
Here is this year’s evolution. It’s clearly NOT central Pacific based…and the drift west in SST anomalies is typical of east based events. You can argue that for a time in late-summer, it appeared more centrally based, though it’s been hybrid or east-based this fall. Per the referenced paper, La Ninas don’t switch from EP to CP based (or vice versa) from fall to winter, so I don’t think this event is “becoming central Pacific based”…though you can argue there are some hybrid characteristics, which per the paper can lead to some tendencies of both with regards to the pattern response. This does suggest, at first blush, that we don’t have the “worse” flavor of La Nina and may maintain some hope for the event weakening during the second half of winter.
The mechanism for East Based La Ninas producing a more -NAO is not really explored in the paper. A couple of thoughts are that the low-frequency response is slightly closer to “neutral” conditions in the EP events, as the cooling isn’t as far west. Other research (that I’ll reference later) suggests that La Ninas in general don’t have as robust of a blocking response to the MJO as El Nino / neutral ENSOs do. Another thought is that the MJO may be more active into the western Pacific during EP events than during CP events. The paper did present rainfall rate anomalies in CP and EP events:
The CP events (B) have a stronger negative precipitation anomaly that is centered farther west, focused at or just west of the Dateline. The EP events have a weaker anomaly centered slightly farther east, closer to neutral years, perhaps muting the effect La Nina has on dampening the MJO into the western Pacific somewhat and allowing for more blocking to result. In a somewhat concerning trend, most of the seasonal models, that overwhelmingly have a +NAO for December-February and in some cases have a flatter north Pacific ridge, all have precip anomalies for the winter that look more like a CP event:
And the CFS:
This has been consistent on the majority of seasonal model runs for the last few months. This could be a red flag that despite the SST profile clearly not behaving like a CP event, we may still have a heightened risk for a +NAO (and EPO) for December-February as a whole, due in large part to the response to this La Nina. Or, it could mean the seasonal models are a little too +NAO and warm…hmm.
Other SST Areas of Interest:
1 is La Nina
2 is the Indian Ocean. Last year saw a strong positive Indian Ocean Dipole event, with cool waters near Australia and warm waters closer to Africa. This contributed to convection/forcing frequently sitting over the western Indian Ocean during the fall of 2019 and into the first half of winter, with a dearth of forcing over the Pacific, especially in the critical period when the polar vortex began wrapping up in December and early January. This fall, the IOD is neutral to perhaps slightly negative. This argues for the low frequency forcing centering farther east than last winter, but in tandem with the much stronger La Nina doesn’t necessarily promise that the MJO frequently gets far into the Pacific.
3 is the Western Pacific warm pool…as has often been the case in recent years, it’s quite warm and expansive. This, along with the neutral to weakly negative IOD, does argue for convection getting into the western Pacific and perhaps helping to amp the north Pacific ridge due to enhancing Rossby Wave Trains across the Pacific. As referenced above, if the convection doesn’t wander close to the Dateline, this still may not dump the cold air in where the East Coast wants it on a consistent basis.
4 is the North Pacific…it’s quite warm across the board, but this is a negative Pacific Decadal Oscillation. The warm waters near Japan and cooler waters near the southern Alaskan and western Canadian coast are characteristic of a -PDO, and the short-medium range pattern argues for these anomalies worsening. A –PDO tends to be associated with a weakened Aleutian low and a more negative PNA, which causes troughing to be biased towards the Pacific Northwest.
A note that the off-equator SSTs are quite warm over the central and eastern Pacific (a positive Pacific Meridional Mode, or PMM). This is one of the stronger +PMM events observed during autumn during a La Nina. This may add a bit of kick to the sub-tropical jet over the next couple months.
5 is the northern Atlantic. The SST pattern here has been used, especially more so in the past, to predict the winter NAO. The configuration this spring – now has been slightly more conducive to a -NAO than recent years, with cold waters south of Greenland/Iceland and somewhat “warmer” waters closer to Greenland and Iceland. While not as unfavorable to a -NAO as most recent years, it still is displaced north of where it is prior to -NAO winters, and doesn’t really move the needle in the face of other factors if they support a +NAO winter.
The QBO is a typically nice downwelling of westerlies and easterlies in the stratosphere. This is shown on the image above. However, the QBO basically “skipped” the easterly (negative) phase in 2016. After one and a half normal oscillations from 2017-2019, we had an extremely anemic easterly QBO late 2019-early 2020, and will be in a westerly (positive) QBO for the winter.
An easterly QBO tends to favor a weaker stratospheric polar vortex, which can lead to more high latitude blocking in the troposphere, while a westerly QBO tends to favor the opposite. Yes, there are easterly/negative QBO years with limited high-latitude blocking, and westerly QBO years with blocking…and yes, sudden stratospheric warming events (SSWs) can still occur in a +QBO and lead to a weaker polar vortex and increased blocking potential. In fact, per this graphic posted by Dr. Amy Butler on Twitter (@DrAHButler), half of westerly QBO La Nina winters featured a sudden stratospheric warming event (filled circles had a SSW):
This may be a significant wild card this winter, as otherwise, the combination of a La Nina and positive QBO (along with our current, absolutely putrid pattern) favor a strong stratospheric polar vortex, which is less favorable for tropospheric high-latitude blocking. This was a major culprit in the extremely mild winter of 2019-20. I will circle back more to this topic when tying all the drivers into our current / short-term pattern later.
Aside from the typical PV and blocking implications discussed each year, a phenomenon (that I frankly did not notice on my own when I started dabbling in seasonal forecasts) that has gotten increased attention in recent years is the QBO’s apparent influence on the north Pacific ridge in La Nina winters. In general, +QBO winters have a more poleward ridge, while -QBO winters have a flatter ridge (interestingly, EP La Ninas have a more poleward ridge, while CP La Ninas have a flatter ridge).
Here is a look at La Nina winters since 1980, that per a combination of the CPC’s 50mb monthly QBO data the QBO diagram above had a +QBO at or below 50mb through the winter. The most debatable inclusions are 17-18 and 11-12, but both had a positive anomaly at 50mb through at least December per the CPC data and held onto it just below that level through at least February.
Here are the -QBO La Ninas:
And the difference:
The +QBO La Ninas DO have a more positive AO and NAO than the -QBO La Ninas, but the differences in the north Pacific are beneficial for the Northeast U.S. Keep in mind, La Nina strength, location, and other factors are ignored for this exercise and many of these anomalies may not be statistically significant given the somewhat small sample size, but it’s food for thought, at the very least, and has worked more times than not in La Ninas over the last 15 years.
We are coming off of another deep minimum in the 11-year sunspot cycle. Many papers (here’s one I quickly pulled up, though there are more https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2005JD006283 ) have found a relationship between the solar cycle and the stratospheric polar vortex strength and tropospheric high-latitude blocking in winter. In general, solar minimums correlate with a weaker SPV and increased blocking, while maximums correlate with the opposite. I’m not going to spend a ton of time on this as I think other factors are already in control, but it was a factor weighed in the analogs.
Low Frequency Forcing:
We’ve clearly had a La Nina response to the low frequency forcing this summer and fall, with rising motion over the Indian Ocean and sinking motion over the Pacific. It can be argued the forcing drifted east some over the course of the summer and fall. Another way to visualize this is on a map showing velocity potential from September – November 15:
This shows a similar idea, though do note that the low-frequency forcing is strongest near Indonesia (near 120E). This signal was focused closer to 40E (much farther west) in fall of 2019.
This does suggest that convection may be most amplified this winter in MJO phases (3-6) that are not cold over the eastern U.S. (phases 5-6 are OK in early-mid December, which I will get to later). However, keep in mind that the MJO getting into the western Pacific can set off a wave train that eventually leads to blocking, especially if it continues propagating. Here are lagged composites for phase 5 of the MJO in DJF, which are quite warm with a strong SE ridge when the phase 5 occurs and in the short-mid range, but turn much blockier a few weeks later:
This low frequency forcing IS a warm signal/risk for the bulk of winter, but likely isn’t as bad as last winter’s in terms of potentially encouraging MJO propagations that can lead to temporary breaks in the milder weather down the line.
The location of this forcing was a factor weighed when determining analogs.
Atmospheric Angular Momentum:
Atmospheric angular momentum is, extremely basically, how much westerly flow there is across the globe. Typically, El Ninos feature a stronger / extended Pacific jet due to increased forcing over the Pacific, and higher AAM. La Ninas typically feature the opposite.
Above is the seasonal correlation to AAM in DJF. The correlation is weak in much of the U.S., but in general higher AAM features a +PNA pattern with higher heights over Canada and lower heights over the southern U.S…basically a typical El Nino pattern. If you inverse the colors (use imagination) and emulate the correlation for a -AAM, it looks much more like a La Nina with higher heights off the West Coast and across the southern U.S. and lower heights over much of Canada.
The AAM over the last year or so has been interesting. It was quite low in the fall of 2019, likely due to increased Indian Ocean forcing and limited Pacific forcing, which contributed to a very ridgey pattern in the mid-latitudes (in particular, across the Pacific) and limited disruption to the stratospheric polar vortex, allowing it to get quite strong by early 2020. It rose later in winter, though was distributed poorly and didn’t shake-up the +AO pattern. The AAM fell again over the summer as La Nina developed and strengthened. It has risen this fall and isn’t dropping back off yet. This atypical AAM for a La Ninia may explain why we’ve had cold weather over western North America and warmth nearly everywhere else in November (and quite possibly December, more later), which is not how La Ninas typically play out (usually, La Ninas start colder than they finish in the eastern US).
The fact that the AAM is kind of high for a La Nina isn’t necessary bad, but how it’s currently distributed is poor for winter weather fans in the eastern U.S. and Europe to say the least:
Momentum in the tropics and especially sub-tropics is quite weak. The mid-latitudes are rather unremarkable, but momentum (westerly flow) is high north of 60N, indicative of a +AO and lack of blocking. This distribution was seen at times in January-March of 2020 when all attempts to develop blocking failed.
Part of the reason we couldn’t shake the anomalous westerly momentum in the high latitudes in early 2020 was because the stratospheric polar vortex was very strong and coupled to the troposphere. Nothing was moving that, apparently…despite the increase in AAM and more active MJO during the second half of last winter, and despite a lot of model hints that we would eventually move that. Fortunately, the stratospheric polar vortex, while becoming strong again, is not yet coupled to the troposphere (note how zonal wind anomalies below the white line are still comparatively weaker than higher up). If this doesn’t change in the short-medium range, it may provide for some hope for an eventually blockier pattern if other factors can align.
Now that we’ve more than thoroughly discussed some of the potential drivers that are evident this summer-fall, let’s start diving into the analogs. Factors I weighed while searching for possible analogs included:
La Nina, with preference for first year events and moderate to strong hybrid or east-based events
The Indian Ocean, with preference given to years with a neutral or weakly negative IOD in the fall
The fall PMM, with preference given to years with a higher PMM in the fall
The PDO, with preference given to years with a somewhat negative PDO
The QBO, with preference given to years with a westerly QBO, and some tolerance for years with descending easterlies, but westerlies below 50mb through the winter
The October teleconnections (AO, NOA, PNA, and EPO)
The September-October low frequency forcing
The general hemispheric pattern and AAM in September and October
The speculative November pattern
The speculative November MJO when that data is available
Years with a large / strong West Pac warm pool
The solar cycle (minimum or coming off a min)
There weren’t many analogs that check all of those boxes to an acceptable degree, but if I had to give a few tiers:
Other years evaluated: 54-55, 55-56, 67-68, 70-71, 71-72, 74-75, 83-84, 84-85, 85-86, 98-99, 00-01, 05-06
Here are some composites when using the top 3 tiers with weights:
It fits the general La Nina theme, and looks more like the +QBO La Ninas with more poleward ridging in the north-central Pacific, but also a more +AO overall. December is generally the coldest month, with -EPO and some -NAO potential, with January likely quite up and down but with the AO, EPO, and NAO all trending more positive and the cold backing west. The February look has a robust SE ridge, though the deep vortex near Hudson Bay would probably be deceptively chilly / snowy from the Great Lakes into New England.
Unfortunately, most of these years were horrible matches to the pattern we’re seeing this November. The few years that made an analog tier higher than “also ran” that had a November pattern that was even somewhat palatable compared to this November were: 64-65, 73-74, 88-89, 99-00, and 11-12. Here was their composite November pattern:
This is not by any stretch perfectly like what this November will look like after the second half of the month, but is somewhat tolerable overall, especially compared to the whole analog package’s November pattern (not shown). Arguably, 1988 and 2011 are the best matches to this year. For reference, here is what this November has done through mid-month:
For some idea of how the second half of the month may change this, the next 10 days on the most recent EPS run as of this writing on Wednesday:
Not perfect by any stretch, but still somewhat OK overall. Here is what those years did in December, and then the rest of the winter:
Overall, this is the shittier end of the larger analog pool. However, without giving the rest away yet (dramatic effect!), I do not think this December’s pattern will end up like the composite pattern in the above Decembers.
With that said, a select couple of those years did end up turning much colder in December and / or January:
64-65 is actually one of the higher scoring analogs…the only thing that really held it back from being a tier higher is a lack of MJO data, distance back in time, and a marginal November pattern match.
88-89 was one of my highest scoring analogs.
Not the greatest analog, but was similar in enough areas to be rated somewhat high.
Most of the winters with similar November patterns to this year were not good winters overall, though more than half featured a legitimately cold period at some point in December or January, and January of 2000 did pull off a pretty good East Coast snowstorm. It is worth noting that 64-65, 88-89, and 99-00 all had at least some similarities to this year WRT the September-October hemispheric pattern/AAM, low frequency forcing, QBO, and in 88-89 and 99-00, some similarities to the November MJO (MJO data not available back to 1964). Worth noting that these analogs have several similarities to this year, are a decent match to this November’s pattern, and have notable cold and/or snow over the eastern U.S. in either December or January. It is possible…
Out of all of the analogs, the only ones that had some similarity to this month’s MJO, the QBO, the low frequency forcing in fall, and the hemispheric pattern / AAM in September-October are 88-89, 95-96, 99-00, 08-09, and 17-18. I will throw 64-65 in there, as despite a lack of MJO data it is a good match in the other areas. However, the November pattern in 1995, 2008, and 2017 was not like this year. WRT the 95 and 08 MJO, they’re not perfect matches, but may have a similar move towards phases 4-6 as some models have over the next two weeks, but a couple of weeks earlier in those years. Given everything, a final blend of analogs includes those 6 years with weighting…an interesting note that 4/6 of these winters featured a SSW.
My final maps don’t look exactly like these composites, mainly because we’re far enough into this fall to speculate about the December pattern and its possible ramifications without analogs, but some takeaways are that December into January may offer the best shot at blocking and colder weather into the eastern U.S. February is the greatest risk to see a strong SE ridge. While March is mild on the mean, it’s somewhat muted compared to February, and the pattern February into March would keep Canada cold, so if the SE ridge isn’t dominant it could allow for an opportunity to turn chilly and try snowing in March in the Midwest and Northeast.
Ongoing – Medium Range Pattern:
As has been the case much of November, the pattern is not conducive for cold / snow in much of the central and eastern U.S. The poor distribution of AAM is evident, with ridging in the mid-latitudes pushing anomalous westerly flow to 50-60N, resulting in a +AO. There is a small block near Alaska, that will dump enough cold into Canada that a couple of systems may produce some thread-the-needle snow from the Midwest/Great Lakes into parts of the Northeast through the weekend after Thanksgiving, though otherwise for now a southeast ridge will keep mild air dominant over the rest of the central and eastern U.S.
Convection is very active across the Indian Ocean right now; a combination of the low-frequency base state, a Kelvin Wave, and the MJO appear to be causing this. The MJO is progressing through phase 2 at a somewhat respectable amplitude, contrary to some prior model predictions. This Twitter thread by @griteater features a nice loop showing the increase in convection and upper-level divergence as the MJO arrived in the Indian Ocean (definitely a good Twitter follow for those interested in mid-long range forecasting, he also posts on some of the common forums such as AmericanWx and 33andrain).
This analysis on Dr. Mike Ventrice’s website shows similar, with the MJO and Kelvin wave constructively interfering right now. There may be brief subsidence behind the Kelvin wave, though I think the robust nature of the MJO and low-frequency forcing will offset that and allow the convection to continue propagating east. Yet another plot, this from Carl Schreck, shows the current constructive interference, and the potential for the low-frequency forcing centered near 120E (as discussed earlier) and warm SSTs to keep convection going to at least 120-150E into early-mid December:
In general, the GEFS has consistently been more bullish on getting the MJO into phases 6-7 than the Euro and CFS. Again, given the low-frequency forcing centered near where the MJO enhances convection in phases 5-6 and the warm waters in those areas, I do think convection becomes active between 120-150E (and perhaps a bit farther east) later November and into early December. It is possible that the MJO itself doesn’t make it out into the Pacific, as the low frequency forcing will dampen the MJO’s signal in the short term, and that same LF forcing suggests suppressed convection once farther east into the Pacific. However, convection between 120-150E is not bad through early December, given wavelengths still shorter than mid-winter, and a compelling argument can be made that the MJO is a significant driver at the moment. A few various tools illustrate this I think:
The lagged October-December phase 2 composites on the CPC’s website (each lag is 5 days) shows a similar pattern in the first few pentads to what most ensembles suggest occurring over the next 1-2 weeks, with troughing over the northern Pacific, Alaska and near the West Coast, along with ridging over the central and eastern CONUS. That then gives way to a more +PNA pattern. The anomalies in the later pentads are generally not statistically significantly, so although that’s still not a great pattern it’s not particularly concerning if other factors suggest differently. An interesting note is that the strong low across the northern Pacific that floods much of North America with warmth in the shorter term does not necessarily persist in these composites.
Another tool, from Paul Roundy (http://www.atmos.albany.edu/facstaff/roundy/waves/), shows the typical MJO response at much finer scales than three-month periods. In transition seasons, when the wavelengths typically change quicker than a 3-month mean will capture, these regressions add quite a bit of additional value.
The phase 2 regression for the week of November 17th shows a number of similarities to the pattern we’re actually seeing…and, moving the regressions forward through phases 3-6 with time shows a fair amount of similarity to the pattern showing up on many of the medium range ensembles in late November, followed by an increase in blocking near the West Coast and in the NAO domain, should we get increased convection to near 120-150E by early December.
The CPC October-December Phase 5 lagged composites show similar as well:
Both Paul Roundy’s tool, and the CPC composites, show a much warmer response to Phase 5-6 with a Southeast ridge during winter, so as the wavelengths get longer, this teleconnects to what people traditionally think it does in winter…warmth for the eastern U.S. But since wavelengths are still shorter, that’s not yet the case, and it can actually be helpful for winter weather fans in the central and eastern U.S.
There should be an increase in AAM in the tropics and sub-tropics in late-November as the MJO propagates east towards the far western Pacific, and as a strong East Asian Mountain Torque event occurs…
There has been a fairly persistent signal among the GEFS and EPS for a block/ridge near and east of the Ural Mountains to dump cold air and high pressure into eastern Europe for a prolonged period of time in late-November into the beginning of December, leading to a prolonged, significant +EAMT event:
This will lead to an acceleration of the east Asian-Pacific jet (the tropical forcing moving east towards 120-150E will also contribute to that). There’s also some minor signal on the EPS for an increase in West Pac tropical cyclone activity around the beginning of December (the GFS and GEM have had a number of false alarms while the EPS has remained much more bearish thus far).
This will add momentum in the sub-tropics, which may eventually improve the distribution of momentum across the mid-high latitudes and lead to a regime more favorable for high-latitude blocking, and also will encourage a change in the Pacific pattern in late-November and early December. With convection most active well west of the Dateline, this strong jet will want to break somewhere over the central or eastern Pacific (the +PMM and active STJ may initially cause this to occur farther east than it otherwise would in a La Nina). This will cause a significant attempt at raising heights over the western U.S. and into Alaska, which may increase the PNA and drop the EPO.
You can see this on Thursday’s 12z EPS, which is more bearish than the GEFS in getting convection into the western Pacific (also note the sub-tropical jet enhancement from the positive PMM east of Hawaii):
The wave breaks want to happen somewhere over the central or eastern Pacific, though initially the TPV over Alaska may be stubborn. The stratospheric PV will also briefly swing towards Alaska around Thanksgiving, which doesn’t help this cause:
So, we likely won’t see an immediate rise rise in heights over Alaska, but if the SPV moves away from Alaska and the TPV backs west…which the EPS and GEFS consistently show both at the moment…the wave breaks would eventually have a much easier time trying to raise heights on the West Coast and into Alaska. Initially in late November and early December, the lack of height rises over Alaska will result in the PNA rising, but the EPO won’t drop immediately. The op models consistently have a very progressive, wavy pattern with sub-tropical support to end November and start December with a +PNA look. The ensembles don’t look very stormy yet in this regard, but their mean pattern by the end of November / early December is too smooth:
(A note that this is a couple runs old now as of this posting, and it has trended a bit sharper with its ridge and trough axis)
The wavelengths in early December aren’t long enough to support a trough from 160E to 100W, and a ridge from the West Coast of the U.S. to eastern Canada. While it’s likely it’s generally troughy and generally ridgey in those areas, the pattern will probably be quite a bit wavier than an extended ensemble mean implies, which will allow for a bit more cold air to be available than modeled, and along with the active sub-tropical jet may bring snow potential when / if there’s enough polar jet influence. Given the STJ involvement, if marginally cold air can get displaced well into the CONUS any snow threats, in a more Nino-like fashion, may not favor the northern tier.
December will start quite mild across most of the CONUS with a progressive, but active pattern with a somewhat enhanced sub-tropical jet. That’s pretty baked in based on with the very +EPO to end November. But it very well may be stormier than the smoothed ensemble mean implies, with at least some transient cooler shots too. And, once the TPV starts sliding west of Alaska, it’s quite possible heights rise along the West Coast and towards Alaska quicker than the smoothed means suggests too. A day 12-13 51-member ensemble mean will not resolve something like a wave break at all.
While I think there’s legitimate potential for at least a temporary -EPO developing in early-mid December, I’m not as confident in Atlantic side blocking following suite. It is possible, as both the Roundy and CPC plots show a -NAO response to phase 5-6 forcing in the late-fall, and there have been hints of it on longer range guidance (the ECM monthlies have consistently had a weak -NAO for December. Let’s point out what, other than the Euro monthly, may point to a negative NAO:
An MJO propagation into the western Pacific (in particular, phases 5-7) in late November or early December, as mentioned, can favor the NAO dropping into early-mid December. In addition, Rocky Mountain Torque will remain weak if not somewhat negative into the mid-range (it shouldn’t increase until we start seeing EPO-induced cold shots, early December at the earliest). This favors a weaker jet from eastern North America into the western Atlantic.
With an active STJ late November / early December and a +PNA, the potential is there for strong cyclogenesis in the central or eastern CONUS, which may help raise heights / perhaps cause a wave break over the northwestern Atlantic. This, along with some (but not consistent) hints on guidance of a dropping NAO into early December, and the tropical forcing, does give multiple ways to get the NAO to drop. A couple of sources for pause:
In general, the MJO does not produce as robust of an increase in blocking during a cold ENSO as it does during a warm ENSO in the winter. Note how phase 7 with a warm ENSO has a strong, statistically significant correlation to a -NAO. There are hints of a lowering in the NAO following a phase 7 with a cold ENSO, but it’s weaker and for the most part not statistically significant. Note than there is a temporary increase in blocking along the West Coast and into Alaska shortly following a phase 7 during a cold ENSO.
This suggests that while a -NAO isn’t impossible if we get the MJO to propagate into the western Pacific over the coming couple of weeks, the cold ENSO may otherwise lower the chances of it delivering the goods compared to otherwise. This does, however, further point to height rises along the West Coast being a possibility.
My other concern:
After the PV gets shunted away from AK and allows for a better opportunity for the wave breaks to raise heights there, it moves towards the eastern North America – northeastern Europe sector. While it’s not strongly coupled to the troposphere right now, this may discourage prolonged blocking in the North Atlantic, or at least make it so everything needs to go just right for it to occur.
My overall thought on the NAO over the next few weeks is that while I think we will see attempts for it to go negative…and at times, the op models will do it in a big way…I’m not confident we pull it off. A legitimate -NAO ups the ante along the East Coast in December, though one may not be needed for something if the Pacific side improves as I think it may. Basically, I lean narrowly towards a +NAO for December, with perhaps a couple of short dips, with a lower potential for a wave break in just the right spot to pull it off. Not impossible, but not really predictable at this stage…perhaps I’m just bitter from watching most -NAO attempts fail the last 7 or 8 winters, with the notable exception of March of 2018.
Thoughts on tropical forcing deeper into December:
There is a divergence among guidance, with the GFS in particular keeping forcing active between 120-150E for much of December, while the EPS and CFS, along with the JMA, generally appear to focus it farther west in the Indian Ocean by mid-December, causing the attempt to raise heights on the West Coast into Alaska to be brief and be quickly replaced by a pattern similar to what we have now with the TPV frequently settling near AK. Even if convection does remain active near Indonesia and spark a change to a pattern we’d more typically expect in a La Nina December, as the wavelengths look more like winter later in the month that does teleconnect to a Southeast Ridge. Although I wish other guidance showed it, the low frequency forcing has frequently flared up around 120E this fall, which may point to the GFS solution having some validity. This would somewhat prolong cold risks, probably focused around the second week of the month, before trending milder late in the month.
The CFS weeklies have consistently appeared to develop a robust, eastward propagating MJO for mid-late December that may get into the western Pacific in early January. While the EPS mean VP anomalies focus mostly over the Indian Ocean on the most recent weeklies, a majority of members have a coherent MJO signal at some point beginning mid-late December. The low frequency forcing amplifies this signal over the Indian Ocean and there’s quite a bit of disagreement on timing at this point, so the ensemble mean looks like a standing wave over the Indian Ocean with no MJO activity, but that likely isn’t realistic. An amplified MJO moving across the Indian Ocean would bring a warmer signal to the eastern U.S. as it occurred, but could lead to a cool-down at some point (likely towards mid January) for the eastern U.S. as it attempts to emerge over the Pacific. This would have ramifications on the January forecast.
Thoughts on the polar vortex into December / beyond:
As referenced above, the polar vortex is quite strong, but not well-coupled with the troposphere yet, and is occasionally getting knocked around and stretched a bit. So, while it is currently a signal for mild weather, it’s not an entirely hopeless proposition for it to weaken at some point as it was last January – March. The pattern showing up somewhat consistently on guidance the rest of November into early December is likely a more favorable pattern for disrupting the polar vortex than what we saw early this month.
The CFS is shown, though the EPS and GEFS have occasionally had the look of an Aleutian low and Scandinavian-Ural high as well. These are both features that stand out on composites of the precursor tropospheric pattern to SSW events:
A is the proceeding pattern in the 45 days prior to a vortex split, C is the pattern in the 45 days prior to a displacement. Both similar, though the Aleutian low and Scandinavian ridge are more amplified proceeding the splits. Another set of maps from the same paper shows similar
The polar vortex will again be quite strong in the short-medium term, but runs of some longer range guidance over the last week or so has slowly opened the door to some weakening mid-late December…shown are the CFS, GEFS and EPS:
My thought is the PV will weaken at least some in mid-late December, though I don’t think a SSW is likely before the New Year unless we set off more tropospheric blocking in the shorter term. I do think that the ridge across the Urals supporting continued opportunities for +EAMT in December, and the possibility of another more robust MJO between mid-December and mid-January, may give a window for a SSW to occur in January. I think after that, typical Nina forcing would favor the PV strengthening into February if it isn’t significantly weakened by mid-late January. A 2018 outcome where the La Nina weakens late in the winter and the MJO becomes active would then be the only hope for a PV disruption later in the season.
So, I see two clear opportunities to weaken the PV in December into the first half of January. If this successfully occurs, an MJO propagation into the Pacific in January could provide for a temporary colder and blocky pattern to start 2021, with the +PMM leading to some continued STJ activity. After that, the PV may very well get strong and coincide with a return to Indian Ocean forcing to provide for significantly milder weather heading into February. This is followed by an outside shot at a prayer to end the season if we can get one more coherent MJO as the La Nina begins to fade.
Putting it all together…the forecast:
A +PNA to end November and start December leads to a very mild / warm start to the month across the CONUS (slightly cooler over the Southeast / East Coast)…however, an active STJ and a wavier pattern than the ensemble means will show at this distance may allow for enough polar influence for an opportunity or two at a winter storm across the central or eastern U.S. Given the +PNA and active STJ, this may occur in areas such as the Ohio / Tennessee Valley, Appalachians, Southeast or Mid-Atlantic, farther south than what you may expect in a La Nina and more typical of an El Nino. A +PNA, split flow, and active STJ are features that are frequently seen in an El Nino!
This window is brief, the first week or so of the month and with a questionable amount of polar air to work with, so it may not work, but there should be a bit to track on the models as this comes into range. Thereafter, the SPV moving away from Alaska, persistent +EAMT, and convection getting into the 120-150E area (and likely persisting a bit more as the GEFS has due to the lower frequency forcing in that region this fall) likely allows for a window of -EPO, and a pattern more typical of a December La Nina. I think there’s still enough momentum in the Pacific jet that this dumps into the Midwest and Northeast as opposed to the Rockies and Plains as typically occurs with EPO shots.
This trends towards cold edging towards the Northwest / northern Rockies into late December, with a Southeast Ridge cropping back up due to forcing becoming more focused on the Indian Ocean, and due to Indonesian convection beginning to teleconnect to a Southeast Ridge in later December.
With the start of end of December quite possibly warm for a large portion of the country, the month as a whole will be mild across the CONUS.
While I think the NAO likely ends up positive for the month, if we take advantage of the potential to develop a -NAO in early or mid-December, it would up the ante for snow in the Northeast and Mid Atlantic in that window, and may slow the warm-up later in the month. Even without a -NAO, I think there are opportunities for some snow in December in the central and eastern U.S. (even the southwest may have a window with the STJ, the NW and Rockies will be active and trend colder later in the month as well).
January may start cold in the Northwest U.S. and warm across the southern and eastern U.S. However, I think the MJO has another opportunity to propagate east during the first half of the month. I believe the PV will be weaker and more receptive to the MJO producing a blocking response than it is in early December. This may cause another period of -EPO, -NAO, and a somewhat active sub-tropical jet for a couple of weeks starting the first or second week of January. This will bring another window of winter farther southeast towards the Ohio Valley, Mid-Atlantic, and perhaps even the Southeast.
I think we start trending towards a Southeast ridge later in January as the MJO fades and Indian Ocean forcing crops back up. How much blocking develops in the first half of January will of course influence how quickly this warm-up occurs.
This is generally the warmest month in the analogs, with an amped central Pacific ridge dumping cold into Alaska, western Canada, and the northwest / north-central CONUS. If we have persistent Indian Ocean forcing and a strengthening PV to open the month, that will probably be the case again this year. If there’s cold available in Canada, the +NAO may lead to confluence east of New England that leads to high pressure over eastern Canada that can cause cold air to seep down into New England and perhaps the Great Lakes in February, causing some wintry threats in these areas…however, the pattern I envision for February is not snow-friendly in the Ohio Valley, and especially the Southeast and Mid-Atlantic.
Cold air likely remains available in Canada, so if the MJO becomes active again in late-February or March it may shake-up the pattern enough to bring it south and bring one last shot of winter in March. If this does not occur early enough, the Southeast Ridge dominates much of March and keeps the cold over the Northwest, Plains, and Midwest / northern Great Lakes / northern New England.
AO: Generally positive, but may go neutral or negative briefly in early-mid December, and perhaps a bit more legitimately in January
NAO: Generally positive, may briefly go negative in early-mid December, with greater potential for a couple-few weeks of a variable / negative at times NAO in January
EPO: Generally somewhat positive, but will dip at times. Mid December and perhaps mid-late January may offer windows.
It’s early, but some scattered thoughts based on some of the looking I’ve done into this winter…
Given a La Nina, westerly QBO, climate trends, and majority of long range guidance at this point, a warm winter has to be favored across the southern and eastern U.S. at this point, especially compared to the 1981-2010 normals which are almost obsolete at this point given how much warming we’ve continued to see this decade. I don’t think that can be ignored and is the logical starting point. Warm in the southern U.S. overall, mild up the east coast (but not as constantly warm farther north), and cold across the northwest-north-central U.S.
With that said, La Ninas with a westerly QBO tend to feature more ridging near Alaska and generally more cold farther southeast into the CONUS than easterly QBO La Ninas. This has been pointed out by many people (who are generally smarter than me) over the last decade or so. Also, this La Nina is currently centered over the eastern Pacific, which tends to open the door for a colder outcome compared to La Ninas that are more basin-wide. An extraordinarily warm and large western Pacific warm pool and negative IOD may encourage an active MJO this winter over the western Pacific, and the persistent western Indian Ocean standing wave that severely limited Pacific forcing last winter through this summer may finally be weakening (not 100% sold on that yet, but there appears to at least be some more variability in the near future). Warm water near the Maritimes can be dangerous, as sub-seasonal forcing in that area is warm in the winter for us (and remember, I think the baseline here is a mild to warm winter across the southern and eastern U.S.), but the warm West Pac may encourage an active enough MJO to give us windows of opportunity. While the PDO is rather close to neutral, waters are currently quite warm over the NE Pacific…we’ll see if that changes in October though with more storminess in that area. The warm waters over the NE Pacific and extensive western U.S. drought are a result of a persistently ridgy pattern there this summer…does that continue into the cold season, or will outside forces coerce a change?
So, off the cuff, there’s an urge (especially after last winter) to forecast an all-out torch-fest. There are several reasons for optimism…and the pattern in the short-term, with an amped Pacific jet resulting in an amped west coast ridge and hints of Scandinavian blocking, would be beneficial both short and long-term if it ends up being a semi-regular occurrence over the next few months, as it is not warm as it occurs and also isn’t favorable for the stratospheric polar vortex strengthening uncontrollably. Is this just a brief mid-fall blip, or is it a sign of things to come? Remember, we had a hell of a blip / head fake last November and early December…I bit on it myself.
Looking at some analogs, there are some very warm winters that show up in here, actually a couple of block-busters, and a few OK years. The first set focuses on west QBO La Ninas that reasonably match in at least 2 of the following 4 areas: Indian Ocean (negative IOD), east based La Nina, warm West Pac, and Warm NE Pacific. As a note, 1973-74 would have made this set, but it was an exceptionally strong La Nina so I chose to exclude it.
The DJF 500mb composite and temperature/precip anomalies:
Temps with 1981-2010 climo:
Went back and forth on which climo period is best to use given 55-56 was in a noticably cooler climo, so for completeness here is the same map with a 1951-2010 climo:
The bulk of the cold in these winters is frontloaded, with November and December both colder than normal for much of the northern and eastern U.S:
January is a more mixed month, with February featuring an incredible warm signal over most of the southern and eastern U.S. March is more mixed and leans cooler over the NE.
A second set of years I played with focuses less on SST distribution and more on QBO and early-mid fall patterns and tropical forcing. There were few that matched all areas (though, 95-96, 10-11 and 17-18 were at least reasonable in all fields analyzed, even if not perfect). They show a similar (but slightly muted) signal overall for the whole winter, and still show a preference for any cold being more front-loaded with a strong warm signal for February:
(because all but 1 analog are after 1980 and this is a larger set, will not include the longer climo base period map…but it would make it appear warmer compared to the climo as it did to the other set)
Some overall takeaways…yes, a mild winter is favored (temperature wise) over the southern and eastern U.S., but there are ways to get cold air at times. Analogs are not as warm as you’d think (a disclaimer about ENSO-based analogs generally being too cold the last few winters is needed here, though last winter was exceptional due to the record wind speeds and cold observed with the stratospheric PV between January and March), and there is a strong signal for early cold and snow potential over the Great Lakes, Ohio Valley, Northeast and Mid-Atlantic among the analogs. Given the global pattern setting up heading into early October, I think the idea of an early blocky / cold pattern is there if we can get sub-seasonal forcing into the western Pacific between late-October and some point in November. However, for as surprisingly chilly as the analogs may be early, they’re more mixed in January and are exceptionally warm overall for February (and mixed / leaning cool for March in the Northeast), underscoring this type of winter does have “warm risks” that may be substantial. We’ll probably have a better idea on how the start of the winter will go in a few weeks when we see what actually happens heading through October.
So, I’m not overly pessimistic or optimistic at this point. I am curious though. 2017-18 is a decent analog (that winter held onto a +QBO below 40-50 mb all the way through), and I can see how an overall similar evolution to that winter plays out (though quite possibly without the SSW and hence, a more typical March).
Some nice nocturnal storms at present as a shortwave glances the edge of an EML advecting into the northeast.
There’s higher end potential with Wednesday’s action, though uncertainty still exists.
It seems like there are two mechanisms to watch for thunderstorm development and severe weather:
Remnant MCS or MCV from tonight’s activity over the Great Lakes over PA/NJ during the morning and into the afternoon
The southward advancing cold front during the afternoon and evening
A shortwave tracking from the Great Lakes towards northern New England will cause modest 500mb PVA and height falls during the afternoon hours, along with some right-entrance support in the upper-levels. This, along with the front, provides “okay” forcing in central/northern PA and NJ, though forcing is weaker the farther south you go.
What happens with number one is uncertain, and will affect number two…
Let’s discuss the environment briefly:
We have an unusually untapped EML for the Northeast advecting in tonight into Wednesday…as dew points increase into the low-mid 60s on Wednesday, moderate MLCAPE somewhere on the order of 1500-2500 J/KG will develop by early-mid afternoon where sufficient heating occurs. The NAM dew points seem overdone. The mid-upper level speed max glancing the region will contribute to moderate to strong deep-layer shear of 30-50 knots (increasing to the north) across the northern mid-Atlantic. Some mid-level dry air will contribute to over 1000 J/KG of DCAPE in the warm sector, with mixing to near or above 850mb where heating occurs yielding decent low-level lapse rates and inverted-V soundings.
These ingredients will support large hail and microbursts with any cellular activity, along with swaths of any damaging wind with any lines or clusters, with locally significant wind damage with any bows.
Let’s try to take a stab at the convective evolution now:
The MCS over the Midwest/Great Lakes as of this writing is the big wild card. It’s running a bit faster than guidance, but is gradually becoming less organized and isn’t diving south of guidance yet. There are a few ways this MCS can go into Wednesday:
It runs slightly faster than guidance and moves into eastern PA late-morning and into NJ by the noon hour. It holds together and brings showers/storms, with perhaps some severe threat developing by the time it gets into NJ…likely not a high-end solution though. If this occurs, it likely nudges the frontal convection south, from parts of central/northern IN/OH into central/southern PA, perhaps northern MD, DE, and southern NJ. This would to an extent limit the threat compared to what the environment suggests is possible.
It doesn’t run faster than guidance and weakens considerably by morning. However, it leaves an MCV and likely contributes to differential heating, which encourages new storm development by late-morning across central or northern PA that tracks into NJ early-mid afternoon. Given the environment, MCV-aided fresh development a little later in the day could yield significant severe in eastern PA or NJ…with frontal convection from central and northern IN/OH into southern PA, MD, and DE.
It does whatever the 18z NAM did and completely dissipates in the morning, with unabated heating along the front into northern PA and NJ, with frontal convection firing along I-80 early to mid-afternoon.
Option 1 seems most likely, though option 2 isn’t off the table if the MCS falls apart/slows down some by morning…option 2 has a fair amount of model support, but since it’s running on the fast side of guidance now I’d slightly favor option 1. Option 3, aka the 18z NAM, will likely not happen.
The frontal convection from the Ohio Valley into the Mason-Dixon area will be in an environment of weaker forcing and more moderate, mostly uni-directional shear but with strong instability beneath the EML. The weaker forcing may somewhat temper the coverage of severe weather, but splitting cells with a large hail / microburst threat are initially favored given the moderate but uni-directional shear. Despite the weak forcing, the sloppy storm mode and flow parallel to the front does argue for fairly quick upscale growth into small lines and clusters…any lines or clusters that bow could produce corridors of severe wind given the high DCAPE, low-level inverted-V and sufficient shear. Where exactly the northeastern extent of the frontal convection is over eastern PA/NJ will depend on how the MCS plays out.
The tornado threat generally isn’t much across the board, but if we see the MCV play out the right way over eastern PA/NJ (ie, later timing so it destabilizes in front of it), there could be a non-zero QLCS-style tornado risk. Otherwise, I’d be rather surprised if there was a tornado with the cold frontal convection.
I can see two areas where the SPC considers enhanced risk probabilities or higher…one is E PA into NJ IF the leftover MCS/MCV is slow enough. This could warrant 30 or 45% wind probabilities along with “hatching”, but, is highly conditional and I’m not sure we get it out of the gate at 6z. The other is perhaps eastern OH or western PA into southern PA and perhaps northern MD or DE if there’s high enough confidence in scattered strong convection along the cold front here. Despite weaker forcing and somewhat less shear, the thermodynamics are very favorable for this region, and scattered strong convection would yield enough severe coverage to justify a 30% wind risk in that region. Given a fairly messy storm mode it’s hard to expect more than a 15% hail risk being justified, but if there are any more persistent discrete supercells they could produce locally significant hail given the moderate to strong CAPE and steep mid-level lapse rates.
Fingers crossed that when I roll over and peak at the radar at 8 AM that the MCS isn’t already messing everything up!
I drew this map around 5 PM and this incorporates snow after that, so a bit of this has already fallen.
My thoughts on tonight haven’t changed much…the synoptic snow this evening is not that heavy or efficient, but may drop up to an inch or two in the Cleveland area and east side. Lake enhancement ramps up starting 9 or 10 PM and is most intense about 11 PM – 6 AM east of Cleveland, tapering slightly faster in the Cleveland area and secondary Snowbelt (and lingering past sunrise in NW PA). I still think the duration of the lake enhanced snow and improving ratios will allow for 4-6″ in the hills of the secondary Snowbelt by morning (when including the ongoing synoptic snow) and 4-7″ for most of the higher terrain east of Cleveland, with 6″+ likely in northern Geauga County and most of interior NW PA. Hi-res models continue to have 0.4-0.5″ of QPF as snow in the higher terrain by morning which supports the potential for 6″+ in the most favored hills with improving ratios overnight. Winds ramp up overnight as the ratios improve so it could be a pretty wintry scene by morning. It will likely only be 2 or 3″ tonight closer to the lake and in Cleveland proper, dropping off towards Sandusky where most of their snow was from the synoptic snow today with lighter snow from here on out. With a strong NW flow 2″+ totals tonight likely make it down to Mansfield, Akron and near Youngstown.
There’s likely a bit of a lull Thursday morning, though lake effect conditions become passable in the afternoon. Bands are likely disorganized Thursday afternoon but may drop a Coating to 2″ as they go, and may start southwest of Cleveland and in the secondary Snowbelt before shifting into the primary Snowbelt towards evening. Thursday evening through Friday morning is interesting as a surface trough and vort max move through slowly, adding lift. The edge of the better synoptic moisture just grazes NE Ohio, though with a long fetch and some Lake Michigan moisture should be sufficient. Although instability and moisture both are marginal to moderate, they are well focused in the DGZ Thursday night into Friday morning, so ratios will be good. In terms of band placement, NAM guidance has a band well southwest of Cleveland Thursday evening but the RGEM lifts it into the primary Snowbelt quicker…given the westerly flow, I heavily favor the RGEM, with a few W-E oriented bands likely from northeast Cuyahoga, Geauga and Lake through NW PA and SW NY. Given the good ratios, long fetch, and synoptic lift for a pretty long period of time I expect these bands to drop 3-6 or 4-8″ type amounts Thursday evening through Friday morning on parts of the primary NE Ohio Snowbelt into NW PA and SW NY. There will be little south of Cleveland, though some snow showers and the synoptic lift could produce a local coating to 1″ in the rest of NE Ohio in this window (though nothing organized outside of the primary Snowbelt).
There’s likely a relative lull later Friday morning into the afternoon, before a fairly deep lobe of synoptic moisture and embedded shortwave drop across Lake Erie Friday night into Saturday morning. As winds slowly shift from W to NW over the lake convergence will increase over the Snowbelt and there will be some upslope, with snow spreading south into the Cleveland area and eventually perhaps the secondary Snowbelt south of Cleveland again. Ratios again look high with a decent amount of moisture and some synoptic lift, so I think another few to locally several inches can occur Friday night into Saturday, most in the higher terrain east of Cleveland into NW PA and SW NY, but also with some snow for the Cleveland area and secondary Snowbelt.
Given the multiple windows for snow and ratios, I think a widespread 10″+ is likely for the hills in the primary NE Ohio Snowbelt into NW PA, with 15-20″ where bands are especially persistent. The Cleveland area and lakeshore will see less, but occasional periods of snow will gradually add up some. The secondary Snowbelt I’m debating if I went a bit too high, but if they get a few inches tonight, a little bit Thursday afternoon, and a couple inches or so Friday night into Saturday can squeak out the low end. Most of the snow west of Lorain and south of Medina, Akron and Youngstown will fall tonight, though they may see some snow showers at times Thursday through Saturday that can add additional light amounts.
Suppose I should post this before it starts snowing…
With the 850mb low track still near or just west of Sandusky into western Lake Erie, expect the heaviest synoptic snows to fall in NW Ohio. There, a long duration light snow from tonight (Tuesday night) through Wednesday evening, along with a period of moderate snow Wednesday afternoon when some modest fgen and deform ramp up will likely lead to a widespread 4 to locally 7″. The rain/snow line will slowly progress southeast during the day, with the west side changing over by early-mid afternoon, and Cleveland and the Snowbelt late-afternoon or early evening.
In north-central and NE Ohio there will be less synoptic snow as the weak deform band quickly swings through, thinking the synoptic snow is 1-2 or 3″ in Cleveland/Akron and 2-4″ on the west side…generally, the synoptic snow will be heavier closer to the lake as the best lift passes to the north. I do like the lake enhanced setup…temperatures get cold enough for lake enhancement from west to east between 8-10 PM and are quite optimal for several hours overnight with a NW flow backing towards WNW, with 700mb moisture not stripping away Cleveland points east until close to sunrise Thursday. Usually when the lake induced instability gets above the -10C level (which occurs late-evening) there will be moderate to heavy orographic lift snow in these setups in the terrain downwind of the lake until the 700mb moisture strips away. This gives a prolonged window from late evening through around sunrise Thursday of good lake enhanced snow east of Cleveland (tapers off slightly quicker south of Cleveland).
My general rules of thumb in these lake enhanced scenarios (when it’s cold enough, which it will be by late evening) are to take the highest QPF and assume at least a 15:1 ratio. Some hi-res models have up to half an inch of QPF as lake enhanced snow through Thursday morning, which yields potentially over half a foot. Another rule is that once it gets cold enough that rates in the higher terrain will be generally 0.5-1″ per hour until the 700mb moisture strips away…given the duration (on top of the light synoptic snow in the early-mid evening) this also suggests over half a foot of lake enhancement in the prone spots.
I’d like to see the hi-res stuff just a little juicier for widespread 6″+ totals, but think anyone in the hills south or east of Cleveland gets 4-7″, with 6″+ being likely in the highest terrain of Geauga (and also NW PA) through Thursday morning. With wind gusts of 25-35 mph and an increasingly fluffy snow overnight, there will be blowing and drifting snow and it will be quite wintry by Thursday morning. There will be much less lake enhanced snow near the lakeshore and I’d expect the immediate lakeshore to come in on the low ends of the ranges across the board…worried about the western lakeshore missing the better synoptic snow to the west and better lake enhanced snow to to the east, so we’ll see if those 4″ totals can be met in northern Erie County. Otherwise, I think the terrain does fine.
In terms of the additional lake effect, conditions quickly become favorable for traditional lake effect Thursday morning and afternoon, though winds go westerly (maybe even slightly S of W at times) by late Thursday through the day Friday. The lake effect will likely be moderate and accumulate efficiently Thursday afternoon into Friday morning with a long fetch and little shear, good lift and moisture in the DGZ, good synoptic support from a vort max rotating around the closed low to our northeast, and the edge of the synoptic moisture grazing Lake Erie…but, I think the heaviest snows in this period are focused near or just north of the 322 corridor in the northern Snowbelt into NW PA with a more westerly flow. Given the duration of this setup and likely high ratios, where bands persist could see another 6-10″ of snow Thursday afternoon through Friday morning.
There may be a lull Friday afternoon as we briefly get into some synoptic-scale sinking motion, but conditions become favorable again Friday night into Saturday morning. A lobe of deep moisture and a good vort max move through, with winds slowly shifting from W to NW. I think this is more of a lake enhanced setup with weaker instability but abundant moisture and some lift, but good snow growth given the temperature profiles. Winds will favor broad convergence over the Cleveland area and Snowbelt which will help focus snow, and orographic lift will help too. Given the decent duration and likely good ratios, this likely supports another 4-8″ type accumulation from eastern Cuyahoga County into southern Lake, most of Geauga, inland Ashtabula and the higher terrain in NW PA Friday night into Saturday morning. Some accumulations likely get into the secondary Snowbelt south of Cleveland by Saturday morning as winds go more NW.
The lake effect likely winds down on Saturday as ridging builds in…time permitting I’ll post a map for just the lake effect Thursday afternoon-Saturday on Wednesday.
The winter of continued failure continues as any promising signals constantly get muted as we try to pull them inside of day 10.
In terms of snow from here on out, I think the Plains, Midwest, Great Lakes and perhaps northern NY into central/northern New England do just fine. I think the Ohio Valley and most of the Mid Atlantic struggle, with places like southern New England more on the bubble but probably not great from here on out. There are a couple of colder windows between now and mid-March, but also a couple of very warm windows in the eastern U.S…
February opens very mild. I am not opposed to 7-10 days where the cold actually presses far enough south to drop Upstate NY and New England to somewhat colder than average, and the northern Mid-Atlantic and Ohio Valley to near average somewhere around the second week of February, but am worried about a prolonged and significant warm-up after the middle of the month along the East Coast, with what may be winter’s last attempt at much of anything then possibly arriving late-February or early March. I agree with many on February likely coming in warmer than normal overall across the Ohio Valley and Mid-Atlantic, closer to normal in New England (but leaning warm in southern New England), and perhaps on the cold side over the central U.S. into the Upper Midwest and Great Lakes.
The second week of February interests me a bit as there will be a prolonged +East Asian Mountain Torque through the first week or so of the month, extending the Pacific jet through about mid-month. This will coincide with when the stratospheric PV is temporarily disrupted, which may increase the odds of a +EAMT actually dropping the EPO substantially. The EPS has trended towards a more -EPO over the last couple of days. The issues is tropical forcing; does it cooperate, or does it support a continued -PNA? Where the Kelvin Wave interacts with the low frequency forcing and amplifies in the Pacific may be key. Amplifying near the Dateline as the CFS has may support a brief rise in the PNA near or just before mid-February; amplifying much farther west as the EPS has would likely maintain and even intensify the -PNA. At the moment I’d lean towards the EPS, but not with 100% certainty. The prospect of a fairly significant -EPO does increase the risk of cold into the U.S. with a legitimate tap of Arctic air, though it will likely dive into the western and eventually central U.S. If the PV gets displaced towards Hudson Bay it may depress heights enough over New England to allow cold to seep south, but we’d need a rise in the PNA to really dislodge bigger cold into the Mid-Atlantic, which seems less than likely.
I am worried that a lot of warm signals occur at the same time just after mid-February after some confluence of colder signals (but not unanimous with tropical forcing question-marks) around the second week of the month. How quickly warmth returns may be influenced by the PV’s location, if the PNA does rise and push the PV to eastern Canada, it may be sneakily slow to warm across the northern tier (though the southern Mid-Atlantic and Southeast are different). It’s currently well-agreed-upon that the stratospheric PV quickly rebounds after the first week of February and is again quite strong around mid-month. This while the EAMT is projected to go negative by the EPS and CFS, while any Pacific tropical forcing from the Kelvin Wave likely dies down. This all suggests a retracted Pacific jet and very negative PNA developing after mid-February along with a +AO and +NAO. Yuck.
How the MJO plays out may modulate the duration and intensity of this warm-up. As Mike Ventrice pointed out on Twitter, there is signal for a strong MJO starting in mid-February. While the mean VP anomaly plots quickly wash out to the lower-frequency forcing in the Indian Ocean and central Pacific on the EPS weeklies from Monday, many individual members have a very strong MJO developing over the Indian Ocean somewhere between February 10th and the end of the month and propagating east. This is initially a very warm signal if it passes through the Maritimes at a high amplitude, which many members show. If this occurs around or just past the middle of February when other signals argue for warmth, it could get very ugly. Could we get 80 degrees into PA or NJ in February for the second time in three years? Maybe that’s too aggressive, but man a lot is pointing to that warm-up.
As much as I hate to seem like I’m wishcasting, a strong MJO would eventually significantly weaken the strat PV in later February or early March and as it worked east into the Pacific would eventually teleconnect to cold. That exact evolution is uncertain but would bring what would likely be the last chance of legitimate wintry weather either at the end of February or beginning of March, perhaps lasting up to a few weeks if stuff breaks right. Maybe the end of the season as we start shifting towards spring brings the best chance of a shake-up to actually happen.
At the end of the day this winter has been a train wreck both as a snow weenie and as someone attempting to forecast weeks out in advance, and I wish I had caught certain trends earlier. A borderline record strong +IOD followed by a stratospheric PV that’s been flirting with daily records at time this month are delivering strong and tough to stop results. As a snow weenie I hope it snows more, and I’m not ready for winter to be over…but a large part of me wants to launch this winter into the sun. Certainly some lessens to be learned.
The pattern may not be record cold with a KU threat every 4 days, but there’s some interesting potential over the next two weeks, with 4 things to keep an eye on after this weekend’s sloppy system.
Threat 1: Tuesday/Wednesday of next week
I ultimately don’t think this one amounts to much except for maybe the Carolinas/southeastern VA. A fairly potent sub-tropical jet shortwave slides off the Southeast Coast while a lobe of the PV gets displaced into southeastern Canada. I don’t think the two can phase and as is, the trough axis looks too far east for anything to come up the coast without a phase. But, it’s not horribly far off and if this was farther out I’d say to watch it closer. As is there have been some hints that the southern piece may bring some snow to the Southeast near the coast, and that the northern piece may bring snow showers or squalls to parts of the Northeast, even without any sort of phase.
Threat 2: Somewhere around the 24h-26th
This one is fairly complex, but does have a low probability yet fairly high ceiling.
A robust sub-tropical jet shortwave will eject out of the southwest around the 23rd-24th, with what can be called bootleg (but persistent) blocking centered near Hudson Bay.
This is a robust piece of energy and some risk for an initial cutter does exist. With the blocking and surface ridging over the top, there would likely be an eventual transfer to the coast after any cutter. If there isn’t an initial cutter, the shortwave looks poised to go negatively tilted as it approaches the East Coast, which could favor a robust low developing offshore and moving up the coast. There would be some needle threading involved for the coastal plain, but the lack of a SE ridge with a trough sitting off the coast ahead of this shortwave and blocking over Hudson Bay does increase the odds of this occurring…again, if there isn’t a cutter. An initial cutter would warm our mid-levels quite a bit along the East Coast as the hour 240 QPF/850mb temp mean image shows…though, if there isn’t a cutter, or if there is a cutter but the Miller B takes over quickly enough, it is a plenty workable airmass.
How exactly this storm plays out will depend on the pattern in front of it over the Atlantic, the location/intensity of the Hudson Bay block, and what state the shortwave ejects into the Plains in. Anything from a strong cutter with little snow or just some front end snow in the east, to a cutter with useful Miller B development, to no cutter and a much higher risk of a coastal, to perhaps a southern slider across the Mid-Atlantic seem to be in play based on the pattern and individual ensemble members. Given how moisture-laden this system appears to be, if there is a coastal storm the potential exists for a significant snowfall somewhere. Given the cutter risk and somewhat marginal airmass, at first glance I think this narrowly favors the interior Northeast for snow over the coast, but this is by no means impossible to pull off along the coast either. If there is an initial low that comes out of the Plains and tracks towards the Ohio Valley or Great Lakes, some snow would certainly be possible in the Midwest and Great Lakes.
I am curious to see how quickly ridging may try amplifying over western Canada between the 24th and 26th…that has the look of something that may amp more than the ensemble mean suggests at this distance…if that occurred there could be more robust northern stream influence with this system which would increase snow potential.
Threats 3 & 4: January 28 – February 1
The ensembles suggest two more waves ejecting out of the southwest the last few days of January, with the polar vortex drifting towards the Davis Strait and allowing colder air to funnel into the central and eastern U.S. With wave after wave moving off the East Coast through this period and potentially developing into a robust storm, the ensemble mean has a persistent 50/50 low feature. With signs of more of a polar connection, a continued active sub-tropical jet, and a combination of an EPO cold press and 50/50 low ahead of any possible system keeping the baroclinic zone to the south, this period is very interesting for the East Coast. However, it is way too early to determine which waves may amp, cut, be suppressed, etc, and it will take quite a while to sort out these wave spacing issues.
The EPO and NAO will be worth watching over the next week or two…I definitely can see a brief EPO tank during week 2, as there will likely be cyclonic wave breaks over the northern Pacific starting in about a week. The NAO may dip if any of these storms deepens enough to get a cyclonic wave break over the NW Atlantic, though right now only a small number of ensemble members do that at some point. Either occurring would make things more interesting and I do think we see the EPO try to go more negative than the ensemble means suggest right now during week 2.
Overall, the sub-tropical jet will give us plenty of opportunities over the coming weeks for snow across parts of the Midwest, Great Lakes, Ohio Valley, Northeast, Mid-Atlantic, and maybe even Southeast. With still some uncertainty in the pattern and wave spacing issues that will take a lot of time to work out, along with some marginality to the amount of cold especially initially, it’s way too early to guess which wave may develop into a storm and produce snow as there’s a wide envelope of possible solution with every wave ejecting out starting around January 25th. But, there’s definitely potential.
The very mild pattern that has developed to end December will slowly transition to a colder pattern through the first half of January…however, a positive AO and EPO, with a lobe of the polar vortex settling over Alaska to end December and start January, will slow the transition back to a colder pattern. Any cold through the middle third of January is likely driven by improvements to the Pacific pattern, with a likely lack of a -AO or -NAO. The Pacific pattern may become quite conducive to significant cold shots from late January into February, with the AO and NAO also likely trending more negative starting in late January. Basically, winter will gradually return and then worsen through January if I’m correct, though don’t rush the transition in early-mid January.
Very detailed, technical discussion:
The fall and first few weeks of meteorological winter have been interesting…there have been periods of active convection in the western/central Pacific, resulting in relatively higher AAM orbits, some shots to the stratospheric PV, and a generally amplified ridge near the West Coast and into Alaska. There have also been periods of very little convection in the western/central Pacific and lower AAM when Indian Ocean forcing has dominated, during which we’ve generally seen the stratospheric PV strengthen with the Pacific pattern turning less conducive for cold weather in the central/eastern U.S.
The NAO has been variable and overall somewhat positive, though a period of –NAO in late November contributed to a Miller B off the northern Mid-Atlantic coast to start December that brought wintry weather from parts of PA and NJ into New England, and while an ongoing –NAO is temporarily staving off a significant Pacific-side-induced warmup. The NAO was staunchly positive for much of early-mid December. The AO has also been quite variable, with at least some of that variability tied to the evolution of the stratospheric PV.
We’ll start by looking at the analyzed 200mb velocity potential anomalies…the strong positive Indian Ocean Dipole is a constant feature, with uplift over the western Pacific much more variable. There was a distinct lack of Pacific forcing from mid-September through much of October, generally more forcing for most of November (though even then, it waxed and waned a bit), and then generally less during the first half of December.
I’ve marked the periods of relatively inactive/active western Pacific convection on a plot of the overall global AAM anomaly over the last 120 days…note how generally, lower AAM has occurred when convection is quieter over the western Pacific, and higher AAM occurred when convection was more active over the Pacific. There are other factors influencing momentum, such as mountain and frictional torques, and positive East Asian mountain torque events occurred in early November, and again in late November/early December, helping to add momentum. Mountain torques are currently negative.
The generally more amplified Pacific pattern that has occurred during periods of more active western Pacific convection and higher momentum has featured a more +PNA/-EPO, and has also favored disruptions to the stratospheric polar vortex. The opposite has been true during periods of less western Pacific forcing/lower momentum, and this general evolution is clear when looking at the evolution of the annular mode/zonal wind anomalies with height since late October:
On both charts, warm colors indicate a stronger polar vortex/annular mode/Arctic Oscillation. Note how during October and the first half of November, following a prolonged, deep period of low momentum and little western Pacific forcing, the strat PV intensified quicker than average. However, the period of increased Pacific forcing and higher momentum in November resulted in an amplified/blocky tropospheric pattern that favored disruptions to the stratospheric PV. These disruptions weakened the stratospheric PV in late November and early December, with that weakened PV/stratospheric -AO downwelling into the troposphere in mid-December and contributing to the recent shot of colder weather in the Northeast U.S. It should be noted that any vortex disruptions/warmings were considered minor, but it’s cool to see the troposphere -> stratosphere -> troposphere interaction on the above graphics, and tie it back to other processes (tropical forcing/AAM).
The recent period of a lack of Pacific forcing and lower AAM has again resulted in a strengthening vortex, with no sign of significant disruption into the start of the New Year.
Based on all of this, it’s possible that both camps in the little Twitter/forum disagreement about the AAM/stratosphere have merits to their argument. There have been periods of a lack of Pacific forcing and low AAM that have resulted in a stronger strat PV…there have also been periods of more notable western Pacific forcing, relatively higher momentum, a more amplified tropospheric pattern and resultant disruptions to the stratospheric vortex. At this point, I’d say the former has outweighed the latter over the course of the last few months, though both have occurred at times.
We’re getting our annual Christmas “torch” this week, mostly driven by a very poor pattern on the Pacific side…the PNA is negative with a strongly positive EPO as a large tropospheric polar vortex (TPV) anomaly sinks into Alaska. There is enough of a –NAO to induce confluence just east of New England, which keeps the warmest surface temperature anomalies west of the East Coast. Regardless, this isn’t a good pattern for snow for anyone in the central or eastern U.S.
As mentioned above, Indian Ocean forcing has largely dominated this month as a coherent MJO constructively interfered with the uplift over the western Indian Ocean that has been in place all fall due to the strong +IOD. This registered as a prolonged phase 2 RMM, moving into phase 3 after mid-month. The lagged phase 2 RMM composites for DJF show a propensity for much lower heights from AK down the west coast, as we’re seeing now, especially at lags 1-4. Each lag corresponds to about 5 days:
The phase 3 composites show a similar pattern, except moved up roughly 1 lag:
These lagged composites do show a strongly +NAO and AO, which does not match what we’re currently seeing. So, it’s likely that a combination of the Indian Ocean dominated forcing and low momentum has resulted in our very poor Pacific pattern, while the lingering effects of the late-November/early-December minor stratospheric warming along with other, smaller-scale features (such as cyclonic wave breaking over the NW Atlantic) are still influencing the pattern over the high-latitudes and Atlantic. The ultimate result is the torch isn’t quite as torchy for the East Coast, but both factors are worth consideration moving forward.
Moving forward, the easiest ways to dislodge the TPV anomaly over AK and overall unfavorable Pacific pattern are to bring a return of forcing to the western Pacific and to increase the AAM. Both start happening soon, but I’m not sure how much I want to rush this pattern change given 1) how long the phase 2-3 lags take to become more favorable 2) the strength of the TPV anomaly over AK and 3) how the lower AAM state with dominant western IO forcing has been the “base state” (though as November/early December showed, are not as overwhelming as they were earlier in the fall).
I’ll try to tackle the convection/tropical forcing portion of this equation first:
As the longer-running 200mb VP anomaly time-longitude plot above showed, we have recently seen a notable uptick in convection/forcing over the western Pacific. Another area of enhanced lift is approaching the central Indian Ocean, though is not quite as convectively active currently.
Multiple sources suggest that the activity over the Pacific right now is associated with higher-frequency forcing, while the activity over the Indian Ocean is associated with the slower-moving and more coherent MJO.
Carl Schreck’s 200mb chi anomaly plots suggest that the MJO is still over the central Indian Ocean, and that the current uptick in western Pacific convection is as a result of an Equatorial Rossby wave and perhaps a convectively coupled Kelvin Wave juxtaposed over the lower-frequency uplift over the western Pacific. These plots suggest the MJO emerging over the western Pacific during the second week of January, when the MJO itself may then constructively interfere with western Pacific warm pool uplift.
Mike Ventrice’s analysis plots also suggest the MJO is still somewhere over the Indian Ocean, with the bulk of the forcing over the Pacific a result of higher frequency processes. Along with the forecast plot two images up, the EPS, via multiple different plots, suggests the MJO emerging over the western Pacific during the first half of January:
Mike Ventrice’s EPS plots clearly show a coherent MJO progressing east, slowly, across the eastern Indian Ocean and Maritimes over the next two weeks. As this occurs it will destructively interfere with the subsidence from the +IOD, so it will not be that convectively active.
The Weathermodels VP anomaly charts also show a more coherent, slowly-propagating area of uplift emerging east of the subsidence in early January, and moving towards the Pacific, jiving with both the CFS forecasts above and Mike Ventrice’s plot.
The multiple sources for lift and convection are likely causing the strange RMM behavior on the models over the next two weeks…there’s the shorter term western Pacific forcing from higher-frequency processes (that are resulting in more convection than the actual MJO over the Indian Ocean), with the MJO then emerging in the western Pacific likely during week 3, around or just after the end of these plots. That would likely result in a more coherent RMM propagation in early-mid January.
Based on generally weaker convergence over the western Indian Ocean, along with strengthening convergence over the central Pacific, it seems that the +IOD is slowly losing its dominance while Pacific forcing is gradually starting to take hold. Given the waters over the western Pacific being warmer than average and the lower stratosphere remaining colder than normal over the tropics for the foreseeable future, it seems distinctly likely that once the MJO reaches the western Pacific during the first half of January that it becomes quite convectively active and propagates coherently through phases 5-8. While some affects from the +IOD will remain, I don’t think they’ll be able to dominate as much as they did in the fall. This has been my feeling for a while, though the general progression is a week or so slower than my thoughts in late November, which will likely slow the pattern’s evolution somewhat in January compared to my prior thoughts. As I mentioned in my last very long post, MJO propagation through the western and central Pacific tends to lead to blocking down the road at increasingly short lags from phases 4-8…for perspective, here’s phase 6’s lagged composites, where we may be somewhere around January 5-10th:
So, tropical forcing should lend itself to a more favorable pattern for high-latitude (in particular, Atlantic) blocking during the second half of January, though the current convection over the Pacific is from higher-frequency forcing and may not have as significant of an impact in early January.
In terms of AAM/mountain/frictional torques…using the CFS for time/simplicity’s sake, though it’s not completely different from the EPS over the next two weeks…
There will be a very brief, weak +East Asian Mountain Torque during week 1 (so brief it doesn’t really show up in the mean forecast for the entire week on the CFS), a –Rocky Mountain Torque, and probably a modest increase in Frictional Torque due to the somewhat lower pressures just west of the Dateline than what we’ve seen. Overall, this will (and already is starting to) result in a very brief, modest increase in AAM…but not enough to really shake things up.
The CFS and EPS both have a much more substantial +EAMT during the first week to 10 days of January. This would add a more significant amount of momentum.
During week 3 the EPS and CFS suggest the +EAMT weakens (though does not turn as negative as it has recently been), though lowering pressures over the western Pacific likely result in some increase in FT. The CFS suggests a +RMT in week 3 as cold air descends east of the Rockies, though the EPS is more persistent with the +EPO and likely would maintain a –RMT in week 3.
The CFS and EPS weeklies both hint at a potentially more significant +EAMT in late January while FT remains positive.
In general, it seems like AAM will increase modestly this week but likely remain slightly negative through the end of the year…however, I think between an increase in FT and a strong +EAMT during the first week of January that we will see the AAM go modestly positive in early January. With FT increasing as lower frequency forcing returns to the Pacific and no signs of a prolonged –EAMT through January, along with some potential for RMT to increase if we develop a –EPO, the AAM will likely be neutral to modestly positive through much of January, with some potential for a more bonafide positive orbit the last week of January into early February.
As alluded to above, the stratospheric PV is intensifying after the displacement event earlier this month. The GEFS and EPS suggest a pattern the last couple days of December that may yield some disruption in early January, with hints of a stronger Aleutian low for a brief time along with a Scandinavian ridge. The Euro does show the first hints of some minor warming in early January. However, the pattern thereafter at the end of the EPS does not look conducive to further warming. The CFS, above, shows some modest weakening to the strat PV in early January, but has incredible spread thereafter, with a few members showing a more significant weakening in mid-late January while a larger portion intensify the vortex further. The anticipated propensity for more Pacific forcing by mid-January, and likely somewhat higher AAM state, may argue against the vortex continuing to intensify through mid-January, but it’s difficult to anticipate a significant warming event anytime soon.
Trying to sum it up…
The tendency this fall/early winter has been for a +PNA/-EPO to occur when there’s forcing over the western or central Pacific along with a relatively higher (near neutral or weakly positive) AAM…however, Indian Ocean forcing has been more dominant at times, resulting in multiple prolonged periods of limited/no Pacific forcing and lower AAM. One such period during most of December has resulted in our current +EPO/-PNA, lower AAM pattern along with a strengthening stratospheric PV.
A strong +EAMT in early January will attempt to amplify ridging towards Alaska…however, the current high-frequency forcing in the Pacific will dwindle by early January while the MJO remains over the eastern Indian Ocean. Along with the strong TPV anomaly currently over Alaska being tough to displace, this all will likely slow the flip to a much colder pattern in the CONUS. The AO should trend positive in early January beneath the intensifying strat PV, and the NAO also looks to trend positive too. So, we will need to rely on the Pacific side for cold. I do think we see a cool down over Canada in early January as the cold air with the PV over AK begins spilling southeast, but with a +AO and EPO (at least initially) it will likely struggle to surge south into the CONUS. We may see most of the CONUS average near or above normal for the first half of January with a north-south gradient, with places like the northern Rockies and upper Midwest likely averaging normal or somewhat colder and the STJ perhaps keeping the southern tier from completely torching. It’s not the warmest pattern ever, but with a +AO and EPO to start January it will be tough to see any large-scale negative anomalies over the CONUS for the first 10-15 days of January. Given the location of the PV and overall teleconnections to start January, I think there’s more risk to bust warmer than colder the first 10-15 days of the month…though for now I’m not forecasting a torch by any stretch.
As forcing returns more permanently to the western/central Pacific by the middle of January, I think we can more effectively improve the PNA and EPO and turn colder in most of the CONUS by late January. Increasing frictional torque (and perhaps Rocky Mountain torque) should make up for a likely downturn in East Asian MT around mid-January, keeping AAM from returning to negative values that dominated much of December. If East Asian torque increases more substantially again in late January as the CFS and to some extent the EPS weeklies suggest, the AAM may turn more significantly positive. If this occurs quickly enough, it may combine with any PV disruptions that occur in early-mid January for a more noteworthy weakening of the strat PV in late January. This could combine, along with the MJO progression I expect through early-mid January, to drop the AO and NAO late January into February.
In general, the pattern we’ve been in for the last few months with a tendency for amped ridging into the EPO region whenever we see an increase in Pacific forcing and AAM can be good for the central and eastern U.S., but we have gotten away from that recently and it likely won’t return immediately in early January. If this pattern continued through the winter, a 13-14 or 14-15 type look where we see PNA/EPO induced cold with less NAO help is doable. Assuming Indian Ocean forcing doesn’t overwhelm again for any extended periods and the strat PV doesn’t become too strong, that is a possibility. We have seen a tendency for cyclonic wave breaks and a –NAO over the NW Atlantic at times, and have to assume that without a major shakeup that tendency could return in later January/February when the tropical forcing becomes more conducive, and when the strat PV hopefully weakens a bit.
What I’m saying is for the central and eastern CONUS, we don’t need a significant strat warming event for a decent second half of winter in the central/eastern U.S. (the UK may be a bit different), as long as the vortex doesn’t remain uninterrupted well into January. A SSW would likely shake-up the pattern if it occurred, perhaps bringing a temporary +AO and warmup when it occurred followed by an increased risk for a –AO and –NAO. This currently seems less likely to occur, but given signs that the tropical forcing and AAM may become more supportive of disrupting the strat PV during the month of January, it’s something that can’t be at all ruled out. A SSW would change the evolution of things from late January through March, and I don’t want to speculate on how until one appears more probable.
This is going to be a long post discussing my analogs that I finally got around to updating along with how the next several weeks may play out…and attempting to tie those things together.
We’ll start with the analogs…after weighing a number of variables from tropical and extra-tropical SSTs, the solar cycle, QBO, low-frequency pattern in the tropics this fall and the general hemispheric pattern this fall, the larger set of analogs is:
Before diving in a bit more and analyzing features this month/trying to tie in to this year, I ran only certain sets of the analogs (based on factors such as QBO, low-frequency forcing in the fall, northern hemispheric fall pattern, and the stratosphere) and found some of them yielded a somewhat better representation of the November and likely December pattern than the full set (not that the full set is bad for November, it’s not)…I’ll post a couple of the more intriguing sets…
The closest set in November/perhaps December, likely not coincidentally, is the set where I only take years in the analog set that at least somewhat match this fall’s pattern…here’s those monthly 500mb composites:
Here are the analogs with at least somewhat similar fall low-frequency forcing and QBO to this year:
And to round out, the four analogs that had fairly perturbed stratospheric PV’s in December with at least some QBO similarity to this year:
I toyed with several other things, but I don’t want to post anymore analog 500mb composites right now and you likely don’t want to look at them, so I’ll stop. A few takeaways are:
Various composites do capture this month’s pattern respectably well
Although a weak signal among the entire set, the various “enhancements” definitely strengthen the strong Aleutian low and generally warm North America look in December with a neutral to positive AO, and that look is quickly emerging on the EPS as we head into December after a chilly start. This is a strong signal for a mild December on the analogs for much of the continent.
January shows a strong blocking signal in all composites, with –EPO blocking strongest with a decent –NAO signal too.
February has the strongest –NAO signal in general, and is absolutely brutal for the Midwest and Northeast in terms of temperature anomalies on nearly every composite I did.
March is not signaled to be particularly warm in any of the composites, though the QBO/December stratosphere set is not quite as chilly as some of the other sets.
So, tying everything into the pattern we have now and may see in December…
The start of December will likely be chilly for the central and eastern U.S. as a transitioning NAO, EPO-induced cold, and PNA spike work together to force cold into most of the CONUS. However, the traditional teleconnections above quickly revert to opposite phases that are not supportive for cold as the AO also goes moderately to strongly positive in early December. This suggests that the cold pattern to start December will moderate quickly, likely by the second week of the month. A number of factors, aside from ensemble teleconnections, support this warm-up…
Trying first to look at the GWO, week one does feature a few sources trying to add momentum…a strong East Asian Mountain Torque, some Rocky Mountain Torque, and active western Pacific convection/tropical cyclone activity. This will likely cause the GWO to rise modestly from its current weak negative value:
However, week 2 sees the East Asian Mountain Torque weaken quite a bit while we lose frictional torque as pressures rise over the Equatorial Pacific, with at best weak positive Rocky Mountain torque. We do gain some frictional torque back in week 3 if the models are right in some convection/lower pressure starting to return to the western/central Pacific, though the mountain torques are both progged to be strongly negative. The EPS guidance is similar to the CFS in all of these areas, except it loses the +RMT quicker in week 2:
A general lack of momentum would argue for a less blocky high-latitude pattern, along with a weaker/more retracted Asian-Pacific jet and mid-latitude ridging over the Pacific, likely putting troughing near the West Coast. For reference, here is the December correlation to AAM, with the color table reversed to depict what the correlation is when the AAM is negative:
Some similarities to the pattern showing up later in the EPS runs, with a generally +AO, +NAO, and mid-latitude ridges over the western/central Pacific and across much of the Atlantic.
My general thought is that the sources of momentum that we should see over the next 7-10 days support the generally amplified pattern, and that the +EAMT through week 2 does support the +PNA that the ensembles re-develop during week 2 that would linger into week 3. This added momentum would also support disruptions to the stratospheric PV. However, there will be a clear loss of momentum most of weeks 2-3, which supports a generally +AO and poor Pacific pattern when combined with other factors.
Tropical forcing over the past few months has been dominated by a strong positive Indian Ocean Dipole, with essentially a standing wave over Africa and the western Indian Ocean. The strong, displaced West Pacific warm pool has been somewhat active near and west of the Dateline, with frequent, strong subsidence in between in the vicinity of Indonesia. Note that the AAM was at its lowest during the second half of September and first half of October when convection subsidence dominated much of the Pacific.
After a strong MJO propagation in early-mid November, the MJO has slowed considerably over the western Indian Ocean as it ran into the standing wave/enhanced lift that’s been there all fall due to the strong +IOD. While a phase 2 MJO is not necessarily bad in the short term, it usually turns warm shortly after in December:
Each lag is roughly 5 days…note how with time, after a phase 2 RMM (which we’ve had for the last week or so and will have for another week or so), the pattern trends awfully close to what the EPS is showing developing within 2 weeks; a strong Aleutian low with troughing into the western US, a +AO, and anomalous mid-latitude ridges over the Pacific and Atlantic, and is not too different in some of the lags than the December pattern in a number of the “enhanced” analog packages.
In addition, Paul Roundy’s tool that incorporates the low-frequency forcing shows a similar pattern for a phase 2 MJO to what is likely for a time 10-15 days from now, and is an absolute torch in phase 3 (which is where some guidance tries taking it towards mid-December):
So, between analogs, ensembles, GWO, and tropical forcing, a number of factors argue for a +AO, lack of blocking, and unfavorable Pacific pattern taking hold in December. Although the chilly start is very likely, it doesn’t last long, and the warm signal thereafter is very strong. Note how the EPS, despite having a +PNA, generally is very mild at 2m at the end of its 12z Tuesday run:
Now, despite the very strong (in my opinion) warm signal for a large chunk of December, there are some positive developments from above…
The added momentum from the mountain torques during the next 7-10 days, along with a subsequent strong Aleutian low, are favorable for disruptions to the stratospheric PV (in particular via wave 1 fluxes). The GEFS and ECM both show very strong wave 1 activity ongoing now, and the GEFS suggests another surge towards mid-December (and the EPS pattern may allow for it).
It’s unclear whether or not this will cause a major Sudden Stratospheric Warming or not…while we have recently seen a minor warming, and another at least minor event is likely during the first half of December, only the extended range GEFS has been actually breaking down the polar vortex, while the op Euro, which only goes out to 10 days, has not yet shown this at 10mb. While the vortex will likely be weaker than climo in December, whether or not we pull off a major SSW event or not is still very much up in the air. That said, the PV will be fairly disrupted, and any downwelling for the upcoming disruption during week 2 would occur later in December into early January.
Attempting to tie the tropical forcing, GWO, stratosphere and analogs all back together now, here we go…
The stratospheric warming events will keep the tropical stratosphere anomalously cold for the foreseeable future, which should lead to enhanced tropical forcing. The Western Pacific warm pool is somewhat warmer than normal displaced east towards the central Pacific:
This does argue for continued convection over the western and central Pacific, and the EPS weeklies and CFS both show that resuming mid-late December:
Although the eastern Indian Ocean and western Pacific are vaunted as the so-called “warm phases” of the MJO (and, at T=0 they do correlate to warmth for most of the CONUS), unlike phase 2, their lagged composites get considerably better with time. So really, we do want the MJO to come out of the eastern Indian Ocean and propagate at some point in mid-late December…the improvements at later lags become apparent as early as phase 4, and improve through phase 8 for NDJ…I’ll post phases 4 and 8:
Given the cold stratosphere and general (albeit weaker) lower frequency forcing over the Pacific this fall, it seems likely we see another MJO propagation at some point. Based on the EPS and CFS, that could happen at some point mid-late December. The subsidence over Indonesia would suppress the MJO in phases 3-5, though the MJO would hopefully still progress through those phases and through the western Pacific, as the eventual result of that is a better pattern than what we’re seeing.
The GWO, after what may be a period of moderately negative values in mid-December, will likely climb later in December when/if convection returns to the western and central Pacific. There are also some signs on the EPS weeklies and CFS that the –EAMT will weaken, and perhaps even become positive, in late December/early January. This would also add momentum and support a more amplified Pacific pattern. This would all occur in the general vicinity of when any stratospheric warming events in early December would try to downwell, and when the analogs all strongly suggest blocking becomes more likely in January.
So, while I think evidence is strong for a mild, if not full-blown “warm” period for a good chunk of December, I do think that if we see the MJO cooperate (propagate east by late December), along with at least a minor stratospheric warming event during the first half of December, that the factors would try to align at some point during the first half of January. The type of pattern in late December, should the MJO propagate, may support further stratospheric PV disruptions should any warmings over the next couple of weeks remain minor. The analogs are then very favorable through February for the eastern U.S., and don’t suggest a March torch either.
After all that, the tl;dr version is:
I think, with much more confidence than several days ago, that December winds up mild for much of the U.S., though there will be cold weather with snow potential to start the month, and I don’t want to rule out some improvement to the pattern towards the end of the month
Analogs strongly point to increased blocking in January, and I believe that if the (seemingly likely) upcoming at-least-minor stratospheric warming event does in fact occur, and the MJO is able to propagate east within the next few weeks, that this year would not be an exception
Per the analogs, once it gets blocky in January it can stay quite cold through February and perhaps into March. This is seemingly supported by the QBO becoming more favorable for blocking through the winter, and by us likely becoming (in my opinion) farther removed from the very low AAM/GWO state we saw earlier in the fall.