Isotherm

Frd, there are a couple [major] reasons why the NAO is becoming increasingly negative. One is the z10 reintegration of the vortex and attendant push downward of the -ve circulation anomalies; aka, the downwelling is finally reaching the critical layers of the troposphere. And, secondly, the tropospheric receptivity to high latitude blocking is increasing by way of vicissitudes in the Pacific tropical forcing regime. Reweighting of centrally based warmth, easterly trades increasing in region 1+2, enhanced LL convergence/UL divergence near the dateline, signalling canonical walker cell emergence. Posted this on another board. U850 wind proxy is instructive. It's incontrovertible that the walker-cell orientation and attendant tropical forcing is altering for February, toward a more stable regime which has been largely elusive to date; namely, enhanced low-level convergence and upper level divergence near the more canonical dateline location. The U-wind at z850 is a valuable proxy to ascertain this eastward propagating WLY wind and thus convergence line, serving as a focus for tropical convection. The result of which should be the induction of the canonical Aleutian low / +PNA regime once the contaminating intra-seasonal signal propagates toward late phase 7 / near phase 8 [circa Feb 8th]. The Western troughing is courtesy of the slowly propagating MJO phase 6-7 signal, which has already been covered numerous time. We saw the same situation in late December w/ amplified 6-7 and resultant trough deepening in the West. I think we all realize that this winter has not "cooperated with the time scales delineated in many winter forecasts" but I believe the apposite thought in everyone's mind at this point is as follows: one no longer cares how it has been timed, but will winter ever show up? And the answer is that there's a good chance for 4-5 weeks of winter [which quite frankly, if one thinks about, a large percentage of our good winters have came in about a month's period]. The debate of course is the precise z500 structure going forward. Let me refrain from equivocating: the GEFS is dead wrong with the circling back to phase 6. The ECMWF is closer, but still a bit too much wandering in phase 7. The VPM has been most instructive, and indicates some slowing in phase 7 in early February before phase 8 entrance at the beginning of the second week of February. So the bottom line is that the pattern, a priori, should become progressively improved.

I've been noting a slow, gradual cave by the ECMWF suite toward the GFS for this one re: worsening angular momentum from the TPV. It's now similar to the GFS in that regard. The ECMWF/EPS prior, had the TPV oriented in a much more elongated favorable manner, such that it acted constructively to the sern vort intensification. Now it depicts the TPV as a "ball of vorticity" with little elongation and thus not much aid to the sern vort; it dampens its intensity. It's possible this reverses, but not looking good for the more intense sfc low option right now, unfortunately. Fairly sizable error for medium range ECMWF suite.

No, not yet. The call was for weakly negative DJF NAO, and weakly negative AO as well. December was +0.6 NAO, January will probably average around near neutral, and I'm expecting February will be solidly negative (could average -1 or lower), so that should push the trimonthly average negative. We'll see.

@PivotPoint, I think the west Atlantic is a "result" forced by other mechanisms, rather than a cause in and of itself. It's partially a function of the upstream dilemma delineated above re: Nina-esque forcing, which, as this ameliorates over the coming week(s), should begin to decrease the WAR's deleterious effects. The WAR is also a function of the oscillating, positive NAO. A classic -NAO features low-heights from the East Coast across the ATL to Europe.

Bob, I concur, and would add that one of the primary reasons weak Nino's are felicitous for -NAO is the long-wave structure in and of itself -- which this winter has been devoid of thus far: Aleutian low, Boise, ID ridge axis, downstream trough, which naturally tends to send cyclonic wave breaks northward and force the -NAO.

@psuhoffman - You may have a valid point with respect to non-optimal TPV location, though, intensity wise, the tropospheric vortex has been running weaker than normal this winter. The proxy of which is z150 zonal winds, which indicates upper tropospheric vortex strength: With respect to the MJO, the apposite maps for El Nino are different than the ones you posted above. Phase 7 Jan/Feb is actually quite good, particularly in the high latitudes: Further, VPM diagnostics have been superior to other guidance, though the ECMWF is now in concordance on eastward propagation to phase 7. This will likely continue propagating through low-orbit phase 7 and 8.

@frd - the notion that the SSW messed up the -NAO is erroneous in my view, though it did mess up timing significantly this winter. Here's my response to a poster on 33 which included an answer to your inquiry: The answer lies precisely in my explanations which I've been posting ad nauseam in the teleconnection thread: Nina-esque tropical forcing, uncharacteristic for a weak Nino. Maritime Continent forcing will align the Western US ridge axis too far west -- near the West Coast, rather than Boise, Id, which permits cutters. The NAO domain is not principally why snowfall has been underwhelming. If the NAO domain were solely culpable, then how have we had numerous snowy winters over the past - essentially decade - without a stable negative NAO? The answer is the Pacific ridge axis has been west of ideal. The PNA is simply everything w/ respect to significant snow threats in our area. If the upstream is dismantled, it's very difficult to consolidate energy in the right places downstream. Now, this may finally be changing, that is, more Nino-esque forcing which will properly place the ridge axis near Boise ID, conducive for us. Re the SSW messing up the TPV -- while yes, I've always been of the opinion that this winter would have progressed well without a SSW, and yes also, the SSW did aid in messing up timing big time this winter, the SSW is not the reason for a lack of negative NAO. That's a red herring and completely erroneous in my opinion. I can point to numerous examples in which the upper SPV was located adjacent to Greenland, yet we had robust -NAO in the troposphere. The key is that tropospheric receptivity has been unfavorable to date to support a negative NAO. This, again, if some of us are correct in our diagnosis of the global regime, will change for the remainder of the winter, and a legitimate negative NAO signal should appear. EDIT -- Amy Butler states the NAO is not forecasted to trend negative. That's only based upon the GFS. EPS guidance suggests otherwise. This is due to divergence in tropical forcing handling, not the location of the TPV.

Rapid analysis of the status of tropical forcing this afternoon: I continue to believe that alterations remain positive going forward. Westerly wind burst now progged near the dateline courtesy of the eastward propagating tropical wave, and with the MJO low-orbit, likely to continue into phase 7, then halt motion there, we should see an increase in Nino-esque long-wave pattern, theoretically. Tropically induced FT will increase, torques should remain positive, as should AAM consequently, and negative -AAM deposits will continue to increase in the high latitudes. The VPM index diagnosis has proven more reliable than RMM, as has the CHI Z200 analysis. All of these proxies are suggestive of central Pacific forcing near the end of January onward, w/ increased convergence, FT, etc. z200 -- note subsidence emerging over the Maritime Continent by late January: We recently saw this regarding the MJO : many models hit the putative brick wall, erroneously. A low-orbit signal should progress into phase 7: Note guidance increasingly redolent of the dateline WWB: The low amplitude phase 7 MJO z500 composite for February yields a hemispheric structure not too dissimilar from the EPS and GEFS, with fairly robust -AO and -NAO blocking, and an Aleutian low signal. The latter is what we'll see improve as we move forward in my opinion; namely, the predilection for improvements in the PNA domain. The low height signal currently progged in British Columbia should retrograde quickly to the Aleutians, operating under the assumption the tropical forcing behaves as anticipated, which it should. In closing, while models are not detecting any material snowstorm threat yet for the Northeast coast, I don't see sufficient reason to push the "Cancel winter" button at this time. Re discussion of putative post mortem: it's not the magnitude of the El Nino that was a surprise [weak expected]; it's the amount of time spent, and magnitude of, intraseasonal MJO activity in Nina-esque phases, as well as atypical Nino forcing w/ a weak walker cell. The forcing regime resembled NIno at times (early-mid Dec, southern snowstorm; and early/mid Jan, mid atlantic snowstorm). But the Nino forcing simply has not, to date, maintained for sufficiently long enough to provide us a large enough window within which to receive a snowstorm. With the apposite variables coalescing, we'll see how we progress.

Re: concerns pertaining to the lack of robust negative NAO initiation thus far: 1) Recall my last update, I noted that we would be entering a period of destructive interference post the Mid-Atlantic snowstorm, due largely to poor tropical forcing. I expected this to yield a rainstorm for the Northeast coast for the most recent event, which did occur. 2) The development of constructive interference, whereby the troposphere and stratosphere act harmoniously, was said to return after the 25th, in the last week of January. 3) The NAM has been negative; however, the weakly negative NAO ineffectual to date; why is that the case? 4) There's a misconception that the stratosphere entirely governs the progression of the NAO modality, but that is erroneous... If tropospheric receptivity is such that conditions are unpropitious for -NAO maintenance at the z500 level, it is irrelevant what transpires in the stratosphere If the stratosphere were entirely governing, we wouldn't have years w/ SSW events such as 2011 in which no tropospheric blocking resulted. 5) So, the stratosphere plays a significant role, but until the rossby wave train is oriented such that a proper block can retrogress into Greenland, it won't occur. 6) The etiology of the destructive interference lies principally in the propensity for upper divergence [convective generation] near the Maritime Continent. 7) This has ripple effects. It disrupts the canonical Nino-esque angular momentum cycle, by creating easterly trades in the central Pacific, lowering momentum, and inducing positive deposits in high-latitude regions which activate the polar jet such that the flow becomes too fast 8] Extratropical momentum pulsing attempts to countervail, but until the Maritime Continent convection [which has also been obviating proper SOI response] diminishes, a strongly negative NAO will not initiate This is changing. A weakly negative / transient NAO develops later this week, which could play a salutary role in the resolution of the 27-28th winter storm threat; although, it is largely EPO/PNA driven. Usual caveats apply, but it is certainly a threat to monitor, and the most conducive threat to date. The Maritime Continent divergence will finally subside by the end of January. This is evinced quite ostensibly on the chi z200 propagation plots, and the easterly negative trades will subside as well, permitting emergence of more classical walker cell forcing. It's not necessary the MJO itself, but overall divergence has been antithetical to proper +AAM transport through the sub-tropics and consequently negative deposits poleward. Conclusion: The atmospheric pattern has changed and is much colder. The NAM has reversed, but the spasmodic NAO has permitted rainstorms for the East Coast [expected for the last event]. A real negative NAO will finally emerge as tropospheric receptivity increases dramatically by February 1st. Notes and asides: Snowfall timing is the most difficult parameter of a long range forecast. However we get there - we get there. A forecaster should not be penalized for snows not being "evenly spread throughout the winter." If the final tally at the winter's end is close, then it's a good forecast. So, while there's been no snow to date, that will simply have to be dealt with. That doesn't intimate non-emergence prospectively. Key indicators and atmospheric changes were foreseen, and now we will watch the final dominos this week. I have read some comments here and there across the boards that forecasters promised a historic winter, etc., or a historic winter period; I simply want to make clear that those words never emerged from my end. My winter snowfall forecast from November for NYC-area was 37-47". This is still attainable. Tom

@frd -- sometimes I see people misusing analogs. A significant commonality on one front does not necessarily intimate that the analog's every attribute should be projected upon the current year. Concordantly, this year will continue to be different from 1985 in terms of sensible weather. The reason I broached 1985 was entirely due to the stratospheric similarities, timing, of the tropospheric response (which has verified well). However, 1985, as a lower AAM year, with low-orbit GWO circuits and Nina-esque background, had a feeble STJ, and thus, moisture was generally lacking. Snowstorms originally almost entirely from the northern stream. This year, we will have a heightened AAM state, w/ nino-esque GWO cycling, and split flow structure w/ much more opportunity for moisture laden storms.

Your post a few weeks ago stated the following: "Personally, I believe the SSW event and possible split of the PV is a game changer to most winter forecasts and not for the better." Why didn't you quote your whole forecast? We'll see if your forecast is correct over the coming weeks, but the NAM reversal has already occurred, and it's quite clear to me that a protracted period of blocking is ahead. So, it's far too early to claim victory on your call that the SSW/split event will cause winter forecasts to fail or perform poorly. Further, there's a major snowstorm ongoing currently in the Mid-west, which will carry to the Mid-Atlantic coast as a significant snowstorm. That wouldn't happen, but for, a favorable GWO/MJO circuit through p7-8, and AAM pulse. It is incorrect to assert we've been in "fast flow" the entirety of January thus far. The only piece of your post that makes a bit of sense if that the Pacific has been problematic this winter to date w/ regards to a stable PNA structure, and I noted that in a lengthier post the other day. However, that will be altering later this month as AAM/MJO and the SSW after effects constructively interfere. But, again, I don't recall seeing your winter forecast in November, so it's much easier to see these issues after they've already revealed themselves (i.e., the past few weeks).

Yes - agreed - and, right, Ray [not altering the overall outlook, just explicating why we've been "fighting" to get snow in the Northeast corridor a bit more than anticipated so far].

@psuhoffman, thanks for the summary - you are correct. The polar field has already rapidly improved c/o of the SSW, and we have a nice Pacific favorability window this weekend c/o of the MJO/GWO 7-8 circuit which will benefit you all in the Mid-Atlantic w/ a nice snowstorm. As the AAM starts rising again this week, and MJO re-circulates, before we arrive at the period in which all factors are "working together perfectly", the PNA may be a bit unstable. This makes the 20th threat a bit risky, with unstable PNA and non-retrogressed -NAO, but that will rapidly improve thereafter, which is why I think 22/23rd--> could bring the lower-risk / higher ceiling opportunities. Everything is progressing as expected atmospherically, but as PSU said, that post was more directed toward the NY forum (applicable to BOS too) which has rolled snake eyes in every favorable window thus far. @frd

Yes, @frd -- NAM diminution 1 week from SPV-split on track, as well as nascent -NAO, and more felicitous EPO/PNA domain as a function of propagating tropical forcing; ERW contamination finally decreases w/ MJO coherency uptick into phase 8 over the next several days. Good to see model data reaching concordance on the reality of tropospheric alterations today, and I think it's ostensible that the GEFS had the upper hand w/ respect to the detection phase [this was expected due to MJO disparities]. All in all, the winter outlook is still on the table. And yes, I'm not sure if I posted this here, but below is the NYC data for December 1984 and January 1985. The SPV-split [EQBO] occurred January 2nd, 1985. The temperature reversal really intensified January 8th+. Will be similar this year.

Right, I agree, chi 200 definitely detects the ewd propagation better. We will probably see a compromise but weighted more toward the GEFS propagation, in my view.

I posted this just now on the New England thread. The RMM plots are currently apocryphal/erroneous: "Yes - this is what I was posting a few days ago re: the stratospheric amplification of the MJO signal, hence the propensity for increased warmth/cutters over the Dec 20-Jan 5 period. The MJO is actually in phase 6 right now. RMM plots suffering from rw/kw interference. The ECMWF has been attemping to send the wave into the null phase since p 3, and has been correcting since. The stratospheric driving augments the MJO signal, and there's no physical reason for the ewd propagation to cease. The potential threat period around the 8th-9th will feature a phase 8 MJO by that point."

Yes - this is what I was posting a few days ago re: the stratospheric amplification of the MJO signal, hence the propensity for increased warmth/cutters over the Dec 20-Jan 5 period. The MJO is actually in phase 6 right now. RMM plots suffering from rw/kw interference. The ECMWF has been attemping to send the wave into the null phase since p 3, and has been correcting since. The stratospheric driving augments the MJO signal, and there's no physical reason for the ewd propagation to cease. The potential threat period around the 8th-9th will feature a phase 8 MJO by that point.

I suppose it depends upon one's definition of "better." KU pattern - certainly no by the 7th. However, I do believe the troposphere will be rapidly improving beyond the first week of Jan. The inchoate NAO response occurring this week, with NAM/AO diminution initiating post the first week. The new EPS coincides more closely with the GEFS now re: the NAM decline in the second week of Jan. We'll see.

The reason for the significant bifurcation b/t the GFS and ECMWF based guidance in the D7+ period is almost entirely MJO/tropical forcing related. The ECMWF has been insistent upon killing the wave, while the GEFS propagates this wave eastward. The ECMWF has been correcting more / more coherent each day with respect to the MJO; further the stratospheric amplification suggests this will carry eastward. I cannot envisage a physical reason for a rapid COD entrance. Clearly, both sets of data are intransigently insisting upon their respective solutions, and the inflection point w/ the MJO is right now. We'll know by January 1st what the picture looks like. Note: EPS much less unfavorable than the operational beyond D7.

Yes, that was my point - that December was not a canonical-Nino pattern. It was more Nina-esque. CONUS temperature departures were running cooler than normal until the MJO began to amplify, "out of control" in the warm phases, due in part to stratospheric elicited feedback / convective growth, in my view.

I agree, Scott. Not saying it's the only variable, though I do think it's a significant one. There's quite a bit of contamination currently re: the RMM plots due to rossby / kelvin wave interference. The correct initialization of the MJO as of today is phase 6, in my view, and will be propagating through 7 and 8 by the 4th +/- a couple days. There are plenty of papers on strat-trop coupling and feedbacks, and additionally, on the propensity for enhanced intraseasonal signals in easterly z50 QBO winters. From a physical forcing standpoint, the brewer dobson circulation strengthens significantly prior to SSW events, which cools the tropical stratosphere. As I posted on the prior page, z70 temperatures 25N-25S are approaching record lows, which increases lapse rates, inducing enhancement in tropical divergence and convective potential - i.e., invigoration of the MJO signal. And while, no, strong MJO does not proceed every single SSW event, it is a fairly common occurrence. As said, easterly QBO winters tend to feature the MJO/strat dual pathway more significantly than westerly QBO, and there's no secret the former is a more perturbed background state.

Thank you, Ray, and again, I didn't intend to imply that your call for this month was a lucky one. You've had a great handle on the pattern thus far. Hopefully, this winter will continue in accordance to what we've both forecasted.

Ray -- I don't think it's a coincidence that sensible weather tends to be warmer than normal preceding and during sudden stratospheric warming events in the Eastern US. Historically, this is a fairly strong common denominator. As I said, last year had a similar temporal association w/ the MJO amplification. Certainly, correlation is not tantamount to causation, but in light of the available literature which seems to underscore this dual pathway, and the empirical evidence of MJO amplification concurrent with precursor stratospheric warming events, I think it's tenable to assert that 1) if you're expecting a SSW event, 2) forecast a warm period for the 10-15 days preceding / during the time wherein you expect it to occur. The upside to all of this, is it could potentially render the core of the climatological period more productive this year.

Ok, we agree completely then. I'm mostly talking about the specific reasoning for this late December decline in sensible wx prospects. And agree, most would have a different "flavor" of this month if the early December event ended up farther north.

Correct, IMO that is a large part of the pathway. If we note z30, z70, etc., temperatures right now 25S-25N as a proxy, they're down near record lows, which enhances the convection signal. Do you agree with this line of reasoning?