Glacier Point

the modelling is dopping big hints that the wave 2 actvity will be strong in the next 10-14 days, and the lagged influences of the pattern this week are taking effect further up the atmosphere. Don't forget this will likely be enhanced by the other favourable factors.

GEFS mean height anomaly at day 10. Note the omega signature there in the western Atlantic as the jet flow splits and sets up the ridge development into Greenland. Whilst this is the most agressive of the GFS runs today in developing this block, it is not new and has been there on the ensemble means for the last few days. We are entering a window where a holy trinity are at play which will set the scene for some rarified synoptics...MJO wave entering phase 8...poleward +AAM tendency...seasonal wavelength changes to the pattern dictated by the ridge in the Pacific. All of this at a time when the troposheric vortex is going to take repeated hits from wave 2 activity. The 12z GEFS ends like this: That mean anomaly includes some peturbations which flatten out the Pacific ridge and can be readily discounted. That makes this an understatement of the height anomaly, which is extremely strong at that range. However, to take this back to earth somewhat, it's just one GFS run so we need to the ECM and other global models picking up the lifting out of the vortex from Canada and split flow developing in the Atlantic t192 through t240.

... and flow all the way from the North Pacific across the Arctic. Stunning run, tropospheric vortex obliterated. This starts to get modelled t192 and is now starting to tick through the timelines. Look West !!!!!

GEFS and ECM mean height anomalies at day 10 are in very good agreement w/r/t the longwave pattern across the NH, depicting three anomalous ridges extending into the Arctic. The key message is that the AO is trending negative as we head into the final week of the month. It's a bit of a race at the moment to see whether the ridge to our NE extends over the top or the ridge over Canada migrates eastwards, or even of we get the Atlantic trough disrupting and setting up a ridge over Iceland, but the final evolution here looks to be southern Greenland and Iceland area for the block to lock (in line with upstream ridge placement).

.. wave 2 activity showing up around the 13th November. When you consider that the precursor to this (lagged about 20 days), we were looking at twin anomalous ridges over the North Pacific and North Atlantic. The output for the next 2 weeks is loaded with wave 2 activity further down the line. In particular, anomalous ridges over the Aleutians and E. Europe / Western Russia. Add to that a little potential for wave 1 activity as upper lows spin across the North Pacific sector. Bottom line, the stuff coming down the tracks at the stratospheric vortex will likely be sustained and favouring splits.

A couple of considerations to put along current modelling. The last 30 days NH circulation shows a particular +ve height anomaly across the Northern Pacific extending from the Aleutians to Kamchatka. In part, the near record advance in Eurasian snowcover can be seen to be influencing this. 250hPa winds over eastern Asia are displaced southward as the cold block is encountered (this is well documented by Cohen). The Asian jet is strengthened and associated with an upper low as the troughing on the poleward side of the jet extends well into the middle latitudes of the Pacific. This leaves a relatively strong eddy to develop in the higher latitudes which is our North Pacific ridge. Even with a +AO phase, this is adopting a high latitude for this feature (one of the key reasons why the stratospheric lead is not enganging with the troposphere). The presence of a strong +ve height anomaly teleconnects to a trough off the eastern US and consequent ridge in the North Atlantic. With no prospect of a reversal in the upstream pattern over Asia, this looks a rock solid modelling feature, set to dominate the NH circulation. The GEFS 12z run tonight has this throughout the run. GEFS for final weak of November has the Aleutian ridge as an extremely strong and poleward displaced. The downstream teleconnector over the North Atlantic is well modelled, and actually gains intensity in the final stages of the output. The SSTA both in the North Atlantic will reinforce this pattern. The key question is therefore the mean state of the AO / NAO which will influence the northern extent of this very blocked pattern. I suggest that with a strong collaboration of MJO wave, poleward +AAMr signal and succession of Wave 2 activity in the next 2 weeks, the stratospheric vortex may be on the bring of collapse 1st week December and ripe for a higher latitide block to take root in December. Whilst not totally against any Scandinavian ridge, I think the wavelengths here more favor a ridge to our W / NW.

interesting analogs for cross polar flow... impact of low sea ice in that list..destination southern Greenland.

Tonight's CPC prognostic discussion "....WEAK POSITIVE HEIGHT ANOMALIES IN NORTHERN ALASKA AS ENSEMBLE MEMBERS TRY TO BUILD A RIDGE NORTHWESTWARD THROUGH CANADA. THIS WILL BE CAREFULLY MONITORED AS IT COULD LEAD TO AN INCREASE IN CROSS-POLAR FLOW LATE IN THE PERIOD, WHICH COULD HAVE IMPORTANT IMPLICATIONS BEYOND THIS PERIOD." Never seen that from them before, and speculation about an event outside of the forecast period.

...another GFS run doing some rapid transitions in mid tropospheric / lower strat profiles over the high latitudes from day 10 onwards. Normally I regard the GFS op with some scepticism at this range, but it's run after run bang down the middle of the ensemble mean anomalies suggesting a developing 4-wave pattern across the NH days 11-15. Note the continual building of the trough off the east coast of the US which will assist in ridge development towards Newfoundland and Greenland.

UKMO model would appear to offer the best guidance in this respect, based upon the empirical periodicity of tropwaves, with the next one 'due' around the end of November. I think the non-tropical element of the global wind oscillation will also lend a hand to the MJO and give it a push around the final week of the month with a strong +AAM signal. Right now there's an interesting westerly wind signal coming in the tropics which will work its way poleward over the next few weeks and successive montain torque peeks (10th, 20th and 30th November) will see large upward spikes in relative angular momentum.

So I'm thinking now that the GFS op has shown something of interest in FI, the question will be asked, what's causing this ? As with all ocean-atmosphere dynamics, there is no one driving force, more a number of converging factors. Here's what I suspect is going on: 1) From the winter thoughts presentation you'll hopefully get some feel as to the general support for this type of evolution over a season that the SSTA within the North Pacific and Atlantic are lending. 2) I discussed the possible implications of a large anomalous ridge over Kamchatka / North Pacific during November. A simple change in seasonal wavelength (going from mid to late November to early December) will generally shift the orientation of the blocking ridges which develop downstream into the Greenland area. 3) perhaps most influential, we are likely to see a favourable alignment of a poleward fluxing +AAM signal (westerly winds - substantive increase in relative angular momentum) and MJO entering phase 8, both of which favour high latitude blocking development.

Don't get too hung up on the detail of where ridges and troughs fall at this range. The key message is that during the last week of the month / beginning of December something is potentially going to shift the core of the PV out of its current position, and where this occurs, we would normally expect pressure rise in its place.

First time we've got some ensemble mean support for a GFS op in extended range.. The blocking developing over western Greenland and NE Canada. Note also the development of a trough off the eastern US (something CPC were expecting last night in line with teleconnection to Aleutian Ridge). That comes as the main core of the PV lifts out and shifts to towards Siberia. This would be beautiful timing for the start of winter. Still too far out from a modelling perspective to draw any meaningful conclusions (witness CFS not consistently advertising this for week 3 /4).

Thanks all for the positive feedback so far. I didn't want to spoon feed but I also wanted the presentation to be reasonably technical so it's encouraging that you've been able to get the gist. Just to be clear, I'm not sold on CFS idea of a milder December, and yes, as that starts to come into a nearer range, we can start to see that this upcoming phase is a likely blip and the stratosphere will likely come under some pressure. The disconnect issue I think is largely related to the PDO and its influence on the global weather pattern.

Nick, for any downwelling cold strat influence we need a strong thermal gradient across the tropical and polar stratosphere. At levels below 50 hPa, we don't have this, so any possible signal at 10hPa and 30hpPa is fighting to get through. Add to that, the lag time for downwelling is 21 days. That takes us to end of the month, when as if luck would have it, the tropopsheric teleconnectors are signalling -NAO. It would be logical to assume that the lower stratosphere would also favourably respond given light polar westerlies. If you take last year and this year, you have a classic case study as to why an early season cold vortex event can succeed (La Nina and strong solar activity) and why it finds it very hard to engage to lower levels (ENSO neutral / low solar / second season east QBO).

No suprise to see the longer range ECM signalling a rise in pressure to the North and lowering pressure to the south. This is not related to the stratosphere but the lead up to this around mid month (troposperically) will likely have a significant on the stratosphere during the first week December. A strong build of pressure over Eastern Europe and Western Russia has a teleconnective influence on Wave 2 activity and I'd be very suprised if we didn't see strong wave 2 response in the stratosphere during early December (wave 2 being more responsible for vortex splits). That places us around late December for a stratospheric influenced pattern. [and wouldn't you know it, we have another tropical wave favouring high latitude blocking mid January - good trop / strat / trop/ strat feedbacks]

... and what happens down the line Ed when the NH wavelengths begin to generate Wave 1 and 2 activity if and when we get the lower stratospheric split at the end of the month, particulary in mind of the favourable lower stratospheric profile ? Hot knife and warm butter spring to mind.

Something 'interesting' for cold synopticians definately beginning to sir now in the extend range for final third of the month / start December. We have various teleconnectors (principally a poleward migrating +AAM signal and coupled MJO wave) siganlling high latitide block at this time. Last couple of GFS runs from around mid month onwards starting to hint at split vortex scenario. This includes a split lower stratospheric vortex programmed with lobes centred over Siberia and Canada. Last 3 CFS runs for weeks 3 and 4 have developed blocking structures over Greenland / Iceland locale, and today forecast is for cross polar ridge. http://origin.cpc.ncep.noaa.gov/products/people/sweaver/cfs_fcst/images1/wk3.wk4_20121104.z500.gif (note the Kamchatkan ridge / Greenland ridge wavelength there) I suspect that this is a failry strong signal if both teleconnectors and modelling is consistently picking this.

Following on from earlier posting... This is GFS end of run, so pinch of salt and other caveats etc. although cited because the timing might represent a rapid atmospheric response to wave 2 activity. Lower stratosphere appearing to be undergoing split or attempts at it with Pacific and Eurasian Ridge pushing poleward: this is traceable all the way up to 30 hPa.

Yes Steve - anomalous ridge to our E setting up wavetrain with enhanced wave 2 activity leading to decceleration of upper level westerlies.

Very interesting nuances developing in longer range stuff over the last couple of days. Remember waxing and waning. We have a developing (anomalous) high pressure cell over Siberia. This is Cohen's first response to anomalous snow cover advance in October. The eastern flank of this has begun to pull the blocking structure over Kamchatka back west. We also are seeing developing anomalous ridge over wester Russia and eastern Europe in response to lowering heights over Greenland phasing with a longwave trough digging through the North Atlantic. This has important potential ramifications w/r/t stratospheric temperatures in December. I agree with the sentiments above, the weakness in NWP to place and model troughing in the Atlantic this year. We are likely to see any toughing further south the closer we get, allowing for trough disruption and downstream ridges to get reinforced. We are now effectively early into a winter synoptic, and models are generally pretty weak with the presence of a strong Siberian High pressure cell. That sets us up with an intriguing prospect. Heights building over the far NE and long term signal to restrogress the ridge towards Greenland (waxing). Longer term still sees CFS week 4 advertising Greenland High, and NAEFS also leading to this with the rebuilding of a wavelength lead by Kamchatkan ridge.

Might be worth keeping on Wave 2 forecasts in about 10 days time. Longer range modelling is keen to split the developing high pressure cell over Siberia and develop a strong ridge over western Russia and eastern Europe. That's in the 'sweet spot' for wave 2 direction poleward. Current modelling has this development around 11th November. Add 20 days for stratospheric impact, and that gives us a date of around 1st December for earliest sign of vortex split.

The risk of a west-based -NAO will be greater the deeper we get into the winter as a general rule, and it is something I have specifically mentioned in the forthcoming Winter Preview video. However, the anologues appear to suggest that the seasonal wavelength with a ridge in the central / NW Pacific will set the downstream ridge over Greenland, or even a shade further east towards Iceland, which gives use a little bit of wriggle room, especially when you consider the reinforcement of this that the SSTA signal will give (persistent +ve anomaly over the NW Atlantic).

There is I think a growing signal for something quite notable to develop final week of November / beginning December based upon the existing pattern of hemispheric ridges being reinforced by a favourable MJO wave and doubling up with a favourable GWO signal, allied to the natural wavelength changes as the season progresses. For analogues rolled forward where we get this persistent Kamchatkan / Mid Atlantic ridge: Current MJO modelling (UKMO the pick) looking to circle back a trop wave through Africa (phase 7/8) 2nd half of November into December (favouring Greenland High solution): Large increase in +ve tendency in angular momentum recorded around 30N, likely to propagate poleward in the next 21 days also increasing support for Greenland High solution: We know where ECM long range is heading.. and now CFS week 3 and especially week 4 forecast: Not a lot has to change from the current conditions for this to come about.

Some useful comparisons with last year reveal the differences that the lower stratosphere and troposphere are exhibiting. Zonal wind anomalies 2011 2012 Mean height anomalies 2011 2012 Notice how this year at around 50 hPa and below the polar field is experiencing negative zonal wind anomalies and +ve height anomalies. That is a lot of atmosphere to try and turn around from above. We know to our costs that not all upper events downwell, indeed,are we upwelling a negative wind anomaly from the lower stratosphere ? This would be consistent with the hard time that any cooling signal at 10 and 30 hpa is having right now. I think the longer this goes on, with the stratospheric temperatures being prevented from free fall, the deeper into the boreal winter season we get and the greater the chance of a favourable tropospheric pattern leading to wave breaking. The pattern over the next week or so looks favourably aligned for Wave 1 activity with that deep North Pacific Low, so 20+ days to some form of disturbance to the PV ? I think I mentioned this the other day, but I would almost want to see an upper level +ve zonal wind anomaly attempt to downwell, as it would meet a stiff resisting force with the resultant torque likely to generate some exciting activity around the mid stratosphere. EDIT: 12z GFS shows no appreciable thermal gradient across the NH from pole to equator at 50hPa downwards right out to mid November with the exception of a warmer blob in the Pacific. That has got to be some resistance to any attempts for cooling from above to penetrate the lower stratospheric layers.