December Thoughts

[BrickFielder]

Some conflicting messages which I think are likely to see saw the pattern first one way then another. Firstly from temperature analysis of the stratosphere we can see anumber of waves going up into the straosphere which tend to have a period of positive AO if they are not reflected back down. At the moment despite the easterly QBO they do not look strong enough to be reflected back down to set a more blocked pattern.

What triggers these waves are strong weather episodes and looking at the MJO we had two periods where activity was more pronounced. First in phase 1 (out west) mid october and then from mid november in phase 6 (central pacific) the second of which is rather unexpected due to the fact that la nina conditions should have dampened activity in the central pacific.

What we should expect is that MJO activity will eventually die down and begin again out west in phase 1. The fact that it has stalled in phase 6 means the pattern change changing the AO back negative is likely to be delayed as well.

Looking closely at the stratospheric forecasts we expect lower pressure towards russia and western canada at 100hPa (just above the tropopause). This is likely to cause height rises in the tropopause and higher pressure at the surface. This does suggest ridging into the artic but it looks to be orientated wrongly for cold lovers in the UK. Long term those areas of low pressure at 100hPa are likely to move closer to the pole and shift away from Russia.

Overriding this signal will be jet stream patterns with cold air meeting warm in the western pacific we should expect the pacific jet to blast across the Pacific under cutting the high towards the west of canada. This cuts of the cold feed into the central states and allows the Atlantic jet to cool off a little.

In terms of the UK then I think we will have a period of unsettled weather for the end of November and into the first week of December. This may continue into the second week although I think a weakened jet will let a northerly flow in. This will reset again after about a week as the pacific jet boots up again. I don't think there is much sign untill late on in December of a blocked pattern although an easterly QBO and more than expected MJO activity bode quite well for a blocked period later on. I shall be watching for the first signs of a warming of the stratosphere a fairly good indicator of a blocked pattern to come.

[Glacier Point]

Interesting thoughts there Brickfielder.

Wave analysis would support a roughly monthly peak in the AO cycle. However, this would appear to taking a slight deviation from the expected frequency with a -ve phase looking very likely now as ridging from Alaska and height rises over the Pole take effect:

This not a usual event and the strength of the block is comparable to early December 1983 and 1958 events. Reanalysis of these years suggest the blocking to last and this is realistic prospect given a strong undercutting of the Pacific, possibly allowing the warm air aloft to drift towards NE Canada.

The polar stratosphere is nicely average or slightly above temperature, with lots of anomalously warm air around the 30hPa level.

http://strat-www.met.fu-berlin.de/cgi-bin/...mps&alert=1

These should start to work down into the tropopause mid December.

The pattern at the moment strikes me as a hybrid between Nina and a -AO. November's AO index is likely to come in at -0.5 to -0.8 which in itself is a suggestion for a -ve AO month (0.6 correlation to December-February AO). We also have an anomalously strong sub-tropical jet typical of Nina conditions.

We also have a sudden surge in global wind, attributable to the pattern in the Pacific. Continued easterly QBO and La Nina will dampen this down but the first half of the month will be zonal in nature I think. Analogues for negative AO and La Nina put up the following guidance which I think will be close to the mark:

Note the -NAO projections there but generally close to average temperature conditions with a return of the mid Atlantic High second half of the month opening the door for a number of northerly attacks, potentially one with a bit more of an assist from a proper Greenland High.

More updates in the winter forecast.

[johnholmes]

both go along with what I would term my 'gut' feeling about the charts at the moment. Its finely balanced to me just looking at ordinary synoptics on the surface and the upper air out to T+240. Beyond that I suspect the models are unable to show what is really more than likely.

As ever time will tell.

[BLAST FROM THE PAST]

both go along with what I would term my 'gut' feeling about the charts at the moment. Its finely balanced to me just looking at ordinary synoptics on the surface and the upper air out to T+240. Beyond that I suspect the models are unable to show what is really more than likely.

As ever time will tell.

Some excellent posts above and demonstrates the difference compared to last year for example. Indeed a pattern that drives LPs directly over the UK and not way to the north IMO is pattern we've been unaccustomed to of recent years and really does allow the developments to go either way mild/cold. It certainly isn't a IB typical even larger teapot set up. Surprises are being thrown up and more to come IMO and I still like the idea of Height rises to our East/NE.

BFTP

[kold weather]

Yeah can't disagree with whats being said. The jet itself is rather typical for a La nina but the northern blocking is pretty intense and given the state of the temps in the stratosphere there is probably going to be a couple more rather large eruptions of high pressure over the next month. The blocking should help to prevent the jet from getting to buckled to the NE and even if it does breifly get more amplfied into a bartlett like set-up its going to get shot away by a split PV racing along to the ESE. This sort of pattern could be pretty good for the north.

Blast, i think its only a matter of time before we see height rises closer to Europe, once the jet eases off I think something will spark, this is a little different from the norm though globally...its not very common to see an omega high in the Pacific in a mod/strong La nina, it would be interesting to see what 1916-17 looked like in that regard.

Edited November 27, 2007 by kold weather

[Terminal Moraine]

I must admit I can never quite get my head around the multitude of interconnections between the various states of the AO, MJO, QB, stratosopheric warming and cooling, the shape of the jet, la nina, el nino, old uncle Tom Cobbley et al and rely heavily on the detailed posts of Glacier Point, Brickfielder, Steve Murr and a few others for guidance as to how this BT junction box can be unscrambled and related to what's happening, or likely to happen, on the surface.

I'm more of the John Holmes school ( 'old school' if you like ) having spent the last 45 years or so digesting the Met' Office Daily Weather Report and Aerological record and, for the last few years, the various charts on the net.

Four and a half decades of studying synoptic charts may or may not count for anything when trying to ascertain what might happen in a couple of months time but I will say that the current synoptics have ' the look' about them to deliver something a bit more akin to real winter than I've seen for some years; there's certainly more of a flavour of the 1960s or late 70s to the current output.

I'm certainly not expecting a 1963 or a 1947 but perhaps a 1966 or a 1968, either of those would cause young hearts to flutter in rapture.

T.M

Edited November 27, 2007 by Terminal Moraine

[johnholmes]

how kind TM, I thought there was only me that had such difficulty.

[Glacier Point]

Suggestions at the moment that the block forming over Scandinavia may not be that fast to move, if anything a trend for a substantial block to form just to the north of the UK with the potential for undercutting by the jet setting up an interesting little run into Christmas.

Levels of global wind have continued to decline rapidly - no coincidence for me that the sudden rise and dramatic halt to zonality mirror these anomalies - and look likely to further drop off as cold SSTAs are shifted westwards in the tropical Pacific.

Easterly winds in the western Pacific have strengthened as a result of a lage anticyclone forming in the Pacific.

These will shift the core of La Nina westwards and nail any potential for tropical convection around the Phillipines where warmer seas are too be found. This will further promote GLAAM to drop off and shift the Global Wind Oscillation into a calm stage 4.

http://www.cdc.noaa.gov/map/images/gcm/gsdm_90d.jpg

http://www.cdc.noaa.gov/map/clim/gwo.htm

This is shown nicely by outgoing longwave radiation (OLR) anomalies with positive anomalies across the tropical Pacific indicative of an absence of tropical convection:

http://www.cdc.noaa.gov/map/images/olr/olr.anom.gif

Some negative anomalies there over the Indian Ocean which may well be teleconnected to the development of a Scandinavian High

This drop off in global winds should futher stall the jet and allow underlying SSTAs to influence the pattern.

For the Atlantic, this means a height anomaly over northern Scandinavia. For the Pacific (influencing the Atlantic) this is for a large blocking high in the Atlantic.

Atlantic:

Pacific:

The polar field is looking increasingly volatile. The AO has skyrocketed positive as a result of surface cooling although looking back over the last week we have seen the development of upward warming waves and a general propensity to warmth within the mid layers:

http://www.newx-forecasts.com/ao_2.html

The stratosphere is starting to look quite cold now, and forecast to carrry on this way which has important implications for 3-4 weeks time possibly leading to an increase in stratospheric winds and more sustained +AO conditions.

http://strat-www.met.fu-berlin.de/cgi-bin/...mps&alert=1

The wildcard here though is the distribution of the upper cold pools. ECM has continually forecast something of a split stratospheric vortex with the coldest air over Siberia-Scandinavia and warmest over the opposite side of the Pole. This is lifting up the troposphere and allowing blocking to develop beneath it.

To get to the point where height rises are observed to our west, we need to loose the upper cold pool extending across the Atlantic south of Greenland. There are signs that some of it is being drawn west towards Alaska and the western States consistent with a La Nina pattern. The remainder has to shift either south-east (undercutting) or north-east (Bartlett).

This is the dilemma at the moment, shown in Ensemble members going either very warm or cold.

With the AO so positive, the Bartlett option would ordinarily be the favoured option but the AO is really bouncing around and the block would seem to be very strong with SSTA support so not such a clear cut call, infact, the way the models look out to t240, the block could well hold sway forcing an undercut.

At the end of this, we end up with a high pressure block, probably located to the west of the UK by the end of the month given the strength of the +AO and stratospheric lead. If the AO shows signs of crashing, then we could be in business for seasonal cold with the high migrating across from Scandinavia to Greenland but the favourite would be for gradual building of the high across the UK from Denmark then westwards.

[BrickFielder]

Yes I would agree with the suggestion that the underlying pattern from the sea surface temperatures across the Atlantic is for a undercutting jet. Angular momentum does look like it will drop away although convection in the Indian ocean I think will enhance the MJO through phase 3 and 4 before it is cut off by la nina.

Where I Thought the Global wind would drop off and stay largely in phase 3/stage 1 I think the MJO will fire up and raise up angular momentum again before it drops away. This I think will change the pattern breaking up the scandinavian high before christmas.

The following pattern could be for cut off lows trundling into France or a Bartlett type scenario acording to the models and I think we should expect a roller coaster ride over the next few weeks untill we get a clearer idea of timings and the strength of the MJO. GFS and ECMWF stratospheric forecasts are quite a way apart with GFS rather quickly getting rid of support for the Scandinavian high, so I am not quite sure how this will play out.

[Glacier Point]

Building on the monthly analysis, I think tropical convection in the Indian Ocean will begin to play an important role in forcing of inter-monthly patterns.

Over the last week, rates of outgoing long wave radiation (OLR) have decreased markedly and started to shift eastwards towards the Indo-Pacific region. This is indicative of an intense monsoon season with large amounts of tropical convection - something which is forecast to continue over the next two weeks at least (although there is some uncertainty to this):

http://www.cdc.noaa.gov/MJO/Forecasts/imag...orbar_large.gif

This promotes westerly wind development over the equator. At the same time, easterly winds have increased over the western Pacific as a result of strengthening La Nina and the easterly QBO. This is likely to promote the development of a large anticyclone to the east of the converging winds and massive Rossby Wave development which will impact on global pressure patterns over the next 30 days.

Reanalysis of similar December monsoons of a similar magnitude supports this analysis:

Sample years:

Westerly winds:

Crucially, levels of gobal wind anomalies are low - due to cold water anomalies over the western Pacific. This is likely to allow the Rossby Waves to transfer rapidly to the polar regions and at a more perpendicular angle due to less deflection by zonal winds. As these disturbances work their way through the polar atmosphere, they will increase the probability of warming of the atmosphere above the polar field - driving the Arctic Oscillation negative towards month's end.

Of the years when we observed a similar monsoon, and filtering for low global winds, we get an average AO for the December - January period of -0.818 (1965/6, 1973/4, 2005/6) so there is reasonable evidence for a downward pressure on the AO. This includes January 1966 which witnessed a massive AO of -3.232. This must be balanced however by a polar stratosphere which is still really quite cold and not conducive to blocking so a moderately negative AO is the expectation.

Reanalysis of these sample years suggests the angle of the jetstream to be tilted NW-SE across the Atlantic and interestingly a dropping off of the Scandinavian High. Adjusted for SSTA, this is supportive of a mid-Atlantic ridge.

Just one thing to watch. If the tropical convection (negative OLR) shifts further eastwards over the western Pacific, the positioning of the Pacific High will be further east and zonal winds will likey stir up once again as global winds are increased.

[BLAST FROM THE PAST]

Hi GP

GFS has been picking up this signal it seems for a couple of days now and the 00z crtainly reflects this. What are your thoughts on the chances of the last paragraph....lowish?

BFTP

[Glacier Point]

SSTAs are warmish in the western Pacific which might promote such activity but against this we have a strong Nina which is likely to counter-act any easterly migration of the monsoon. The reanalysis includes a few Nina years there so we would probably favour tropical convection to be confined to the Indian Ocean.

http://www.bom.gov.au/bmrc/clfor/cfstaff/m....Last40days.gif

... also, MJO wave has hit a wall in phase 3 which is indicative of suppression of convection around Indonesia and futher east suggesting the core area of convection to be west of this point.

[Gavin P]

Perhaps we'll geta northerly January like that of 1955, which was also in a La Nina winter?

[Nick F]

GP, what are your thoughts on the effects of the projected ridge over E Canada progged by the models for the medium range?

Looking at the the 8-10 day mean H500 comparisons there seems to be a rather strong Pac jet developing which will reinforce the ridging over the east coast/NW Atlantic by increasing the H500 warm flow NE-wards aloft ... and troughs crossing the US Plains may eventually end up undercutting this ridge or at least splitting with a block forming over E Canada/NW Canada and result in a west based -ve NAO into early January?

The 8-14 day H500 outlooks do at least show some energy undercutting the Ern Canada ridge - and in the meantime a more NW to SE orientated mean flow across the N Atlantic which suggests a Pm regime rather than Tm regime affecting the UK at the end of the month/first few days of Jan:

[Glacier Point]

Hi Nick,

The Pacific anomalies look rock solid in terms of Nina and tropical forcing from the Indo-Pacific region. As you say, the logical response to this flow pattern is an east ridge. I'm slightly puzzled by the strength of the low anomalies in the Atlantic - there's just so much stacked against it so it may just be the models over doing it.

The problem (if there is one) as I see it at the moment is a lack of blocking over NE Canada which is allowing the jet to roll through preventing any deflection towards western Greenland. This may be related to surface cooling rates still catching up from that very warm start to the winter although the logical migration of the high eastwards should increase the -NAO west.

The AO continues to defy any sort of prediction beyond 3-4 days but the lower latitude blocking looks to be there, witness the recent Scandi block and now one programmed over East Canada. Maybe just a question of waiting for it all to click.

The real concern lies with the polar stratosphere though which is running happily like many of the recent winters well below average. Signs from the latest data plots that these cold anomalies have started to work down to the 200 hPa layer which is not good although there is high volatility. We badly need a warming event in the next 10-20 days I think to counteract this.

What we may have in the next 2-3 weeks is a warming of the mid layers over the Pole as a result of planetary wave disperal from all that convective activity in the Indian Ocean. The way the polar field is right now, Canadian Plains looks the most obvious route for any large ridge to devlop into the Pole.

Edited December 18, 2007 by Glacier Point

[Nick F]

The real concern lies with the polar stratosphere though which is running happily like many of the recent winters well below average. Signs from the latest data plots that these cold anomalies have started to work down to the 200 hPa layer which is not good although there is high volatility. We badly need a warming event in the next 10-20 days I think to counteract this.

What we may have in the next 2-3 weeks is a warming of the mid layers over the Pole as a result of planetary wave disperal from all that convective activity in the Indian Ocean. The way the polar field is right now, Canadian Plains looks the most obvious route for any large ridge to devlop into the Pole.

Thanks for the reply,

yes the reluctance for the AO to develop a sustained -ve phase is a major hinderance for a pattern conducive to more sustained cold for the UK going into early Jan. Though maybe the fact that the PV is programmed to move away from the western hemisphere into Siberia may allow some kind of warming event to push a ridge north across Canada where cold pooling is set to relax, the strengthening Pac Jet and deep troughing on the West coast may help.

I just hope the similarities of the current pattern with Nov/Dec 1970 - with a moderate Nina and -QBO do not lead into a similar vein as Jan/Feb 1971 where the QBO died down to neutral values - but the UK endured a very mild run in those two months.

[Nick F]

Looking at the reanalysis H500 anomalies over the last few months, a striking correlation can be see between +ve height anomalies and areas of warm SSTAs. One area of note over the last few months has been the blocking centred just west of the UK just downstream of a large area of high SSTAs that have persisted through much autumn over the mid-latitude N Atlantic. Also another area is the +ve height anomaly over the Bering Sea/Arctic Sea - this corresponds with high SSTAs due a huge reduction of sea ice this year - these high heights over the Alaskan Siberian side of the pole may well have been responsible for pushing the Polar Vortex over NE Canada/W Greenland:

-17/10 to 16/12

- 17/11 to 16/12

since the begining of the month we've seen the blocking migrate NE to be over Scandinavia, the low heights increase in depth around the Labrador Bay area, +ve anomaly still persisting north of Alaska/Ern Siberia

- Oct and Nov SSTAs

... the SSTA over the Bering/Arctic Seas North of Alaska/Siberia seem to have weakened into Nov due to the rapid refreeze, this suggests that the +ve H500 anomaly here fading and indeed there's evidence of the PV and low heights movng out of N Canada towards eastern Siberia ... this looks to allow height rises to replace the low heights over NE Canada which have dominated this area over the last few months ... another thing that has not occured thus far is the retrograde of the +ve anomalies/ridging that have persisted over the East Atlantic/Nern Europe these last few months - this obviously due to the overiding La Nina signal with low heights over NW Atlantic and the +ve NAO/AO driving a strong jet over this area preventing this.

There are signs of the low heights relaxing perhaps over this area which may allow the chance of a retrograde as we enter the new year ... but equally there is a strong signal that low heights may remain over Greenland driving a strong SW to NE jet which may allow the chance of another block developing to our NE.

Given the possibility of a block developing over Ern Canada/NE US - a split flow may well develop over Ern N American continent with energy going into the Sern arm enhancing the possibility of a -ve NAO, albeit west based initially.

[Glacier Point]

Good analysis Nick which shows just how much SSTA forcing we have had over the NH over the last 4 months or so.

The question of where the PV sets up is a key one and has a significant bearing on the distribution of cold. I would put forward the theory that the location is essentially influenced by summer and autumn skin temperatures over the polar field and the polar stratosphere.

All of this comes in the context of record low levels of summer ice cover and huge surface temperature anomalies but my theory would be that the warmest areas leading into the winter actually tend towards lower heights - especially when the surface warmth is unevenly distributed - possibly related to rates of surface cooling - the warmer areas having a larger net loss.

I looked at years with summer / autumn polar skin anomalies in couplet (http://www.netweather.tv/forum/index.php?s...t=0&start=0). This suggests the location of the winter-time vortex to be east Siberian Sea into the Gulf of Alaska and a secondary one western Siberia.

Obviously the current programmed trend is highly encouraging from the theoretical point of view. Possibly, the early season build up of cold over Greenland and PV there was a response to the system 'catching up'.

In addition to this, we are seeing the core stratospheric vortex (lifted troposhere) starting to migrate across the pole from Siberia towards the Pole (which might be related to surface feedback mechanisms):

[edit: just clocked the trend in the programmed movement of the warm stratosphere over the Pacific - carry that on and it may end up over the Atlantic in 2-3 weeks possibly setting up a southerly displaced jetsream - any thoughts on that one ?]

All of this is likely to translate to the height rises (or a recovery from low values) over the Canadian Arctic for the next month and possibly beyond. Rossby Wave dispersal in the Indo-Pacific region will also start to have a greater impact if we get this movement of the vortex and a greater chance of reflection of upward propagating waves back down through the tropopause in the Siberian sector.

We still await the easterly QBO to do its stuff wrt the stratosphere which remains anomalously cold.

Edited December 20, 2007 by Glacier Point

[BrickFielder]

The stratospheric vortex does tend to migrate from siberia towards the pole around this time and the models are depicting a typical response which I am not entirely happy with.Convection over the indian ocean I think will slip south as it comes up against the twin anti cyclones of la nina. The models seem a little too ready to shift those anti cyclones and even though we have a cooling in the stratosphere I would not expect them to shift untill angular momentum picks up.

This is roughly where I would expect the anti cyclones to be (perhaps a little more west ?).

We can hope that the recent convection will result in a warming in the stratosphere top levels with a resultant blocked pattern in about 4 weeks.

What does interest me is the consistent signal for cut off lows crossing the atlantic.

This will tend to lower the temperature gradient between north and south in the northern atlantic and will eventually put more energy into the southern arm of the jet. A good signal for cold perhaps latter in the winter. More cold looks to be on its way for the central states so we should expect some news stories over the festive period to this affect with some cold ramping as result on our winter discussion threads.

Pressure ensembles show models fairly accurate out to 5 days and pretty dreadfull beyond that.