Winter Outlook

[Glacier Point]

Winter 2007/8

Summary

This Winter has many parallels with that of 2005/6. We have a similar number of variables likely to consign last year's horror story to the distant memory. In many respects, I think we're even better placed than we were in 2005/6 with much less blocking to our east and a polar field which has shown a propensity towards an all important negative Arctic Oscillation.

However, I'm not seeing enough evidence to suggest a really strongly negative Arctic Oscillation and the colder phases that this will bring are likely to be cancelled out by extreme volatility in the patterns resulting in some ridiculously warm phase coinciding with positive phases of the North Atlantic Oscillation. La Nina is already showing up in the overall pressure patterns and I think she will actually send us down a pretty average route.

So, a close to average winter is the call, maybe a shade over given a warm start to the season and mild phases during January and February. In the context of recent Winters, that may not go down too badly. It does leave the door open after all for something from the east….

Method used

Following on from the success of the autumn forecast, I'm adopting the same methodology for the winter forecast, with the additions of the Pacific sea surface and Global winds to reflect greater influence from the Pacific during winter-time conditions and expanded learning on the significance of tropical forcing and the pole.

The factors, and their relative weighting are as follows:

• Leading mode of the Arctic Oscillation – 40%
  
• La Nina – 35%
  
• Atlantic and Pacific Sea Surface Temperatures – 25%
  
• Globally averaged angular momentum (no direct weighting but consideration in terms of impact of other factors)
  

Key factor: Arctic Oscillation (AO)

The more I study long range forecasting, the more I appreciate the importance of the AO on NW European weather patterns. Last winter's horror story was directly attributable to a big positive AO and even the 'nearly winter' of 2005/6 fell short of expectation in my view because of an indeterminate AO signal (recorded index -0.1 which is considered neutral). Last summer was dominated by a negative AO phase, and that says it all.

There are no hard and fast rules to forecasting the AO and forecasting agencies cannot claim reliable successes beyond 7-10 days. This shouldn't stop us from trying however and I believe there are some statistical and analytical tools that give us a 60-70% chance of reasonable prediction:

1) November statistical guidance

Figure 1: Monthly trends in the AO and current conditions.

This November's AO is likely to come in at an index value of between -0.4 and -0.6. In doing so, it will join a long list of negative AO Novembers, most of which went on to record a negative AO for the winter. It also fits the profile of the long term trend in the AO which is downwards for winter months.

The correlation between November and D-J-F AO average value is 0.4. Whilst this might look insignificant, this is by far the strongest monthly lead throughout the year and, when you consider all of the atmospheric 'clutter', such statistical links are of definite interest to the long range forecast.

Moreover, when the November AO was preceded by a negative sign, 86% of winters (sample size 28 years) went on to average negative. When the November AO was less than -0.5, 78% of winters (sample 18 years) also went on to record an average AO of less than -0.5. This is important because it implies a very mild winter is very improbable.

I'm also looking at a particular sequence of AO indices throughout the year (Arctic Spring, Summer and Novembers) leading to an enhanced correlation between the November and winter AO. This sequence is valid this year and improves the correlation to 0.8 making this November's AO a likely strong indicator for the winter, both in terms of sign and magnitude.

The link to this (highly experimental) research is here:

http://www.netweather.tv/forum/index.php?showtopic=40066&st=0&start=0

2) Quasi Biennial Oscillation (QBO)

The QBO is a measure of the upper level zonal winds across the equator which flow either east or west in an approximate 11 month cycle. Since April, we have been in an easterly phase which has come close to breaking the record value, most probably related to the quietness of the sun.

http://www.cdc.noaa.gov/Correlation/qbo.data

These winds also have an important influence on the way the wintertime polar vortex behaves, modulated by the solar signal. Labitzke et al. have updated their original work on this factor and identified a reasonably strong correlation between the QBO and polar stratosphere impacting on the AO.

http://strat-www.met.fu-berlin.de/labitzke/moreqbo/MZ-Labitzke-et-al-2006.pdf

During easterly phase QBOs, there is an increased probability of warming of the stratosphere and a generally slower, less well organised polar vortex which assists a more negative AO to develop. Reanalysis confirms the work of Labitzke et al.

Figure 2: Linear seasonal correlations of the QBO and 30hPa temperature and pressure (reverse the colours for a negative (easterly QBO))

We are following closely five other years where the QBO switched from west to east in the Spring. Interestingly, most of them went on to develop quite rapidly into strong events returning to a westerly phase the following year. You can see here how this year fits in and where it is projected to return to a westerly switch in late Feb / March.

Figure 3: Comparison of similar QBO years.

Reanalysis of these comparable years validates the relationship between QBO and stratospheric conditions.

Figure 4: 30hPa temperature and geopotential height anomaly for analogue years for the QBO.

Why is this important ? Well, the upper atmosphere is vital in predicting the strength of the wintertime polar vortex. A strong upper vortex (anomalously cold air) will help lock down a lower tropospheric vortex retaining a fast westerly flow. A weak upper vortex (anomalously warm) will lead to a weaker and less well organised polar vortex which is much more prone to mid winter warming events and an early break up of the wintertime vortex.

I believe this year we have already seen signs of this volatility in the vortex with several unsuccessful attempts for the vortex to set up shop over Greenland and two distinct tropospheric vortices. As the polar westerlies expand southwards during the winter, this will likely result in a complex ridge developing across the pole maintaining a –ve AO.

3) Stratospheric conditions

A warmer than usual polar stratosphere dampens down any tendency for a stronger, faster polar vortex by cutting off the supply of cold air aloft and reducing the thermal gradient between troposphere and stratosphere making it difficult for any permanent vortex to establish.

Current stratospheric conditions are about average, some warmer spells, some cooler spells.

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

Comparison with last year reveals a very different upper air profile. Note the red (warm anomalies) this year as opposed to last year's purple (cold anomalies) in more or less the same position in the upper atmosphere. This is a strong hint that the stratosphere is no where near as cold as last year.

Figure 5: polar cross section thermal anomalies 2006 vs 2007

This leaves us open to mid winter warmings of the stratosphere. Analysis of current conditions also shows the development of a series of warming waves (+ve height anomaly) penetrating up through atmosphere on a 3-4 week cycle – very different to last year's conditions.

Figure 6: Current profile of the polar atmosphere and wave responses

4) Polar surface temperatures during the Arctic Summer

A lot of attention has been given to the extraordinary warmth of the Arctic this summer where a large part of the East Siberian Sea remained free of sea ice and the North-West Passage was navigable. Reanalysis of years where there was a warm-cold couplet across the Arctic suggests that this has an influence on the probable location of the core of the polar vortex. Somewhat curiously, the analysis suggests that the warm summer anomalies are associated with the polar vortex and cold anomalies related to higher pressure. This may be related to outgoing long-wave radiation, rates of cooling and snow build up in down-wind areas.

More details here:

http://www.netweather.tv/forum/index.php?s=&showtopic=40066&view=findpost&p=1054819

The presence of such a warm anomaly this Summer is an indication for a couplet, possibly enough to suggest the western hemisphere to attract the core of the vortex with more height rises towards Greenland, Svalbard and western Siberia with a westward displaced Siberian High.

This does also indicate an increasingly volatile polar field with dramatic fluxes and some balmy conditions as we get transport of tropical activity poleward shifting the NAO positive for brief but intense periods.

Key factor: El Nino Southern Oscillation (ENSO)

Cold water at depth has persisted in the tropical eastern Pacific. These have upwelled in the last two months, largely through easterly wind flows and decreased tropical convective activity. Anomalously cold water now persists across much of the tropical Pacific to qualify for La Nina conditions.

http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_update/wkxzteq.shtml

Easterly winds remain in force over the region which is likely for La Nina conditions to persist. Current conditions suggest the core of the anomalies to be centred towards the eastern Pacific:

http://www.bom.gov.au/climate/enso/indices.shtml

The QBO influence, brought about through easterly surface wind anomalies and shown by the analogue years, will likely shift the core of these anomalies towards ENSO regions 3.4 and 4 in the central-western Pacific as the core easterly winds are strengthened in these areas whilst the winds towards the eastern Pacific drop off..

Figure 7: Easterly QBO impact on 850hPa winds over the tropical Pacific.

All of these suggest to me a moderate to strong La Nina to stay in place during December and slowly, very slowly decline into February as the QBO falls apart allowing westerly winds stirred up by an anomalously warm Indo-Pacific region. Incidentally, this area will, I think, be a huge confluence of winds supporting a substantial amount of topical convective waves.

Model guidance is reasonably good on the projected La Nina.

Figure 8: Summary of ENSO forecasts.

In isolation, La Nina is likely to force a substantial high pressure anomaly in the mid Atlantic and longwave trough over Europe which will phase with the sub-tropical jet. I believe this pattern is already very much in evidence. Analogues for La Nina actually suggest this to be an influence for average temperatures. This also opens the door for the build up of deep cold over western Siberia as the jet stream is deflected southwards forcing pressure rises over Scandinavia. On the flip side, this could maintain us in a mild NW'ly flow given the current warm state of the North Atlantic.

Figure 9: 500 hPa geopotential height anomaly associated with La Nina.

Key factor: Atlantic and Pacific Sea Surface Temperatures (SSTAs)

This method proved to particularly good effect during the Autumn so I'm using it again and building in a Pacific Basin analysis. The method is based upon a seasonally lagged SSTA analysis, September to November.

Eyeballed matches for the North Atlantic SSTAs (pattern of warm N. Atlantic and indeterminate anomalies beneath) suggest a baroclinic response of slightly increased height anomalies over southern Greenland and negative height anomalies over the central Atlantic akin to a weakly negative North Atlantic Oscillation (NAO). The average NAO for all these composites comes out at -0.09, close to the UKMET forecast.

Figure 10: Current N. Atlantic SSTA, analogue match and projected baroclinic response.

Eyeballed matches for North Pacific Basin SSTAs (pattern of La Nina, warm central-western band, cold eastern sector (negative Pacific Decadal Oscillation) suggest an Aluetian ridge, Alaskan trough, ridge over the SE USA and weak ridge in the North Atlantic.

Figure 11: Current N. Pacific SSTA, analogue match and projected baraclinic response.

The jet stream response to this pattern is for a strong jet over the sub-tropical Atlantic and North Africa and weaker polar jet.

Figure 12: Projected jet pattern according to Pacific SSTA signal.

Key factor: Globally averaged angular momentum (GLAAM)

GLAAM captures level of global winds. This Summer, GLAAM has been at very negative values. The levels for September and October were the two most negative since records began in 1958 indicating a very slow moving pattern and low jetstream activity. This is not surprising given the easterly phase of the QBO and La Nina, both of which are correlated to negative GLAAM and both were very strong.

During November, GLAAM shot up. Research (and just eyeballing the response of outward propagating waves of wind from the equator) suggests a lag effect of rapid changes in GLAAM. These changes may be attributed to increases in sub-tropical winds and the Indo-Pacific SSTAs.

Figure 13: Theory of increasing GLAAM

Figure 14: Latest GLAAM

Latest GLAAM suggests a dropping off the upward curve. It is likely that the combined effects of La Nina - particularly a westward shifting cold core as ENSO Region 4 becomes the coldest which has the most significant correlation with -ve GLAAM – and the easterly QBO will ensure that over the course of the Winter, GLAAM remains at below neutral values. Global winds will, as a result, be less strong and this is likely to weaken the jetstream in the Atlantic.

This is important as low GLAAM will make it more likely that SSTA signals will be of a greater significance, just as they did this Autumn.

FORECAST 2007/8

Prior to the increase in GLAAM, this was looking like a straightforward forecast. However, zonal winds have increased and this looks like dominating the pattern and obscuring the strongest signal for high pressure in the mid Atlantic and forcing any mid-latitude high that bit furth south.

However, as we progress into the Winter, two things will happen:

1) The polar stratosphere will assume a greater influence bringing about negative phases of the AO with increasing frequency;

2) Zonal winds will decrease as the impact of La Nina and QBO reassert in the tropical Pacific.

The predicted AO value of -0.6 is indicative of a weakly negative AO. This teleconnects to average temperatures for the UK – we need to sustain a winter-time AO of less than -1 to get below average. This is however a signal for below average temperatures for Scandinavia and central Europe. If correct, this is not a rampantly mild pattern and I'm suggesting the Greenland vortex will have a really hard time in establishing any great foothold –sure some periods of low heights – but without the upper level support, this will not be a sustained feature.

In fact, which ever way I have cut the analysis – AO, Atlantic SSTA, Pacific SSTA, Nina – it always comes out somewhere near average. I'm going to take the hint and my forecast for the UK Winter is +0.3C, close to average temperature-wise with a maximum 0.3 C deviation from the long term 1971-2000 average but reflecting the anomalously warm waters out to our west. La Nina's NW'lys are still likely to come in a fraction above average because of this.

With dropping GLAAM, the Autumnal pressure anomaly pattern will re-emerge. That's a strong signal from La Nina and the SSTA for more of a mid-latitude high pressure anomaly located just to the west of the UK.

This is a potentially interesting pattern with a –PDO in place over North America which will take the jet towards Greenland allowing for retrogression and transient northerly attacks. These will be of greatest frequency before and immediately after Christmas.

As we progress into January, Pacific Basin SSTAs suggest a weakening of the high pressure anomaly resulting in something of an atmospheric free-for-all with tropical forcing and northern blocking all up for grabs. This is likely to be a split pattern of mild ridge over the UK and more colder incursions from the north and north-east to coincide with deeply negative phases of the AO triggered by stratospheric warming events (we will get 3-4 weeks' notice of these). Dare I say it, perhaps a more traditional January pattern with possibly the best chance in a long time for a cold incursion from the east ?

February is likely to represent more of the same, possibly a bit more of the milder stuff as zonal winds are likely to increase from the western Pacific.

With the increased probability of mid and high latitude blocking precipitation will be close to or below average for the UK.

Key areas where it could go wrong are a resistance for global winds to slacken off, which would result in a weak ridge over the UK and jet to the north. I'm particularly concerned by the potential impacts of that warm western Pacific fuelling plenty of planetary wave activity (could be beneficial though to the –AO). A stratospheric cooling event is also possible, indeed at the time of writing, one is forecast which needs watching in terms of the end of December. I'm also very mindful of those very warm sea surface temperatures in the North Atlantic which could turn an average synoptic into an above average CET return.

This forecast will be finalised once the final CPC data for November's AO is in but here's a summary of forecast indices:

Enjoy !

Edited November 30, 2007 by Glacier Point

[BLAST FROM THE PAST]

Thank you GP. An excellent read and level headed forecast. I tell you what it seems to fit very nicely with RJS thoughts.

regards

BFTP

[Glacier Point]

Many thanks for the hard work and careful thought once again put into compiling this forecast GP. In the nearer term, lets hope that the current warming event puts enough 'in the tank' to provide something of interest for around perhaps the festive season and also enough to help offset the impact of what may follow afterwards.

A very realistic assessment overall for the season and certainly at least better prospects than this time last year

Well done :lol:

Tamara

Thanks All.

Mods, could you suspend the policy for posting into this thread ? I'm happy to take feedback.

[Paul]

Not a problem GP, we'll go with the flow!

[grab my graupel]

Wow Thanks for that G.P. :lol: Superb stuff and I very much appreciate your obvious

hard work.

I must say I might have to read it over and over again to try to digest it, but I get the

General overall picture.

If things go to plan, it sounds like there may be some pleasant suprises for us this winter

and our patience may be rewarded afterall

I hope you don't mind me saving this post as it is an excellent learning tool as well.

Brian

Edited November 30, 2007 by grab my graupels

[Glacier Point]

Thanks Brian,

I meant to post this link which contains all sorts of goodies.....

http://www.weatherlinkhub.com/climate/telec.html#GLAAM

[nick sussex]

Thanks GP for the obvious hard work you put into that forecast. An excellent read , its very interesting regarding the correlation between the AO in November and the following winter. Of course given last winter something closer to average will seem like a great success! I think a very positive pointer as you highlighted is the instability of the PV and its reluctance to set up shop over Greenland. :lol:

[Polar Continental]

Thanks very much GP, for your winter outlook, a most interesting read, have managed to digest a fair bit of that so far. Will still though, be one I will be revisiting for some time to grasp more.

I am looking forward to this winter with more optimism than in recent years, mainly because of the many of the signals of the past 6 months, which you have explained in good detail, and with your previous postings and this have been able to get to grip as why this winter could be more interesting.

Have also found interesting your good explanation as to why the Arctic was so extraordinary warm this summer.

Regards

Paul

[kold weather]

Sounds like a slightly milder version of 1970-1971 to me GP, indeed thats probably the closest match at least in the Pacific right now, not so sure about the Atlantic mind you.

It is also similar to 05-06 in numerous ways but we can't ignore that the La nina is far stronger and i think that'll show in the end.

I certainly don't want to hijack your thread but on average i'd say above average based on the warmth of the SSTA in the Atlantic but not without some cold spells as well as the PV seems to be rather fragmented...alot of the cold mind you may come from "through the backdoor" type set-ups, much like what we saw in November at times...also a lot of NW airflows and I wouldn't rule out an easterly of some sorts when the AO does go very negative even though it probably won't last longer then a week before the old pattern reverts.

Edited November 30, 2007 by kold weather

[johnholmes]

it probably won't last longer then a week before the old pattern reverts.

!!

when was the last time we had a week of easterlies?

a week??

and to GP wow, I totally admire the immense amount of work that went into this analysis and provisional prediction for winter 2007-08.

You have given very clear comments about all the various factors which may go towards affecting our winter. As you admit there are one or two question marks in a couple of areas. credit to you though GP you do not cover these up but state them very clearly.

A first class piece of work.

many thanks

Edited December 1, 2007 by johnholmes

[Glacier Point]

Thanks.

Today's ECM 8-10 day anomaly pattern is a pretty good expression of the overall patterns I'm expecting..

• Aluetian ridge
  
• trough extending Vancouver - SE to Central Plains
  
• ridge over the SE with a slight tendency for cut off lows western Atlantic
  
• weakish ridge of displaced high in the Atlantic
  
• Arctic height anomaly delivering -AO
  
• trough over central Europe, slight tendency for positive height anomaly over N. Scandinavia
  
• maybe the Azores high not as strong as shown here
  

[SMU]

Thanks.

Today's ECM 8-10 day anomaly pattern is a pretty good expression of the overall patterns I'm expecting..

Hey mate-

Excellent stuff- A one to be expected around the current teleconnections & their modalities-

Im kinda thinking though that you may have the NAO overall to negative at -0.6, with the rossby wave propergating more towards scandi & western Russia- with more of a vortex over Greenland-

Therefor I think my leanings would be NAO Neutral arriving in at +0.2 Starting strongly +VE & seasonally corrected downwards later on-

I would punt for your -0.6 to be the mean negative anomaly for the POL -

Best of luck-

Cheers

Steve

Edited December 1, 2007 by Steve Murr

[Glacier Point]

Thanks Steve,

The key thing for the 1st half of the season is a relaxation of the jet, directly related to GLAAM, which in turn is related to the strength of the gradient across the Pacific and Canadian Plains - Hart did some useful work relating GLAAM, ENSO and blocking suggesting a positive GLAAM equating to more blocking and likely tightened gradient as we see now.

This one of my thought processes: QBO and trend for westerly migration of core Nina anomalies; impacting on GLAAM (regions 3. and 4 have a 0.76 and 0.77 correlation with GLAAM during Jan); dropping of GLAAM allowing for a similar pattern to November but with more intense -AO and proper winter type set-up.

[Gavin P]

Excellent piece of work, thanks GP Most of this goes over my head, but it certainly sounds like an interesting winter by modern standards.

Good luck with it.

Edited December 1, 2007 by Gavin P

[J10]

A very well argued post by Glacier Point, I'm not going to pretend I understand all of it, but it certainly seems plausible and time will tell if it's spot on.

[Jack Wales]

Winter weather forecast 2007/08

Good evening all, firstly can I apologise for not uploading this yesterday as anticipated but circumstances prevented me. Before I go any further I would like to state that although I’ve been interested in meteorology for 20 years, this is only the fourth time I’ve attempted a long range forecast and the first time I’ve published one! That’s not to say you cannot rip it apart if you so wish, in fact feel free to do so as I’d welcome all feedback. I merely wish to manage your expectations as this forecast will not contain the same level of expertise as forecasts by Glacier Point, Kold Weather, John Holmes to name but a few.

Finally I would say that I’ve not viewed anybody else’s winter forecast yet (apart from the met office) so here it is guys and girls…

ENSO:

Forgive me for stating the obvious here but I’m going to start off by analysing the current ENSO signal, we are currently in a negative (La Nina) phase and this is how I categorise the MEI numerical values…

>-0.75C Neutral

-0.75C to –1.5C Slight

-1.5C to –2.0C Moderate

< -2.0C Strong

Sept/Oct Reading: -1.117 (Slight)

MEI

Oct/Nov Reading: -1.5 (Estimate)

Current Reading: -1.7 (Estimate) SST

Also the ENSO region 3.4 (though to be the most influential), is currently recording an anomaly of approx –1.9C which is moderate (borderline strong).

ENSO 3.4

The is some debate surrounding possible strengthening of La Nina but the general consensus is that it has peaked or will do so very shortly…

ENSO Forecast

I therefore predict that La Nina will peak at –1.7C and remain in the moderate category.

This does have a strong correlation on our weather, in fact an ENSO signal with this strength often overrides other tele-connections with the pattern resulting in zonal conditions for western Europe. Having said that we are unlikely to see the very mild conditions experienced in winter 2006/07.

The last time we experienced a La Nina of this strength was in 1988/89 (CET D7.5, J6.1, F5.9) or with a combined e’ly QBO 1976/77 (CET D2.0, J2.8, F5.2).

In a nutshell from this signal alone I would expect to see a strong Atlantic jet interspersed with brief height rises over the northern mid Atlantic (classic toppler scenario).

QBO (Quasi-Biennial Oscillation). Calculated at PSD (from the zonal average of the 30mb zonal wind at the equator.

We are currently in a negative QBO phase, strongly negative figures have been recorded

QBO

There is a correlation between a negative QBO and a larger emphasis on blocked weather conditions, although the ENSO signal could be expected to override this the sheer extent of the anomaly is worth further consideration which I have factored into my forecast.

Although the QBO has probably already peaked and should decline sharply to neutral values by the end of winter. The lag effect should ensure the effect of negative QBO are in force throughout the winter months.

It has also been observed that an e'ly qbo can assist in locking-in synoptics i.e. so that given conditions can prevail for longer periods.

AO…

The AO ensembles indicate a switch to AO- conditions shortly, with the trend being towards AO negative conditions. Considering the tentative link between the QBO and he AO I'm certain we will not see the extreme positive figures of last winter and predict a weakly negative AO for DJF.

If you look at our cold winters of the past you will note that a negative AO regularly prevailed (Jan 1963: -3.3).

AO

See ensembles…

Ensembles

Hints at a change in FI for mid December on the 1/12/07 12Z but I suspect GFS are a bit ahead of themselves here although I expect to see this sort of development from Christmas onwards. However the 1/12/07 00z GFS 850 HPA Temp ensembles show no particular trend with equal members option for mild/cold.

Stratospheric Observation

Stratosphere

There is a correlation between a warm stratosphere with colder conditions at the surface although a 4 - 6 week lag is generally required for this to be observed. Therefore we need to look at the stratosphere from mid October to the end of November. You will see that at the end of October thru most of November slight cooling of the stratosphere notably between 1 - 30hpa. This possibly contributes to the recent change to a more zonal pattern (also note the very recent spot of yellow at the surface). The cool anomaly remains until the last week of November so this would favour a more zonal pattern at the surface for December. The pattern does change during the final week of November where finally some warming is recorded. Although this anomaly is relatively slight it does represent a pattern change which could favour blocked conditions towards the end of December.

The stratosphere currently bears a similarity to 2005:

2005

NAO (North Atlantic Oscillation)

The Met Ooffice predict a neutral NAO and although they do not stipulate exactly how they arrive at this, Atlantic SST's in spring time (May) are thought to be the main driver. At this time La Nina had not shown her hand so I predict a weakly positive NAO to the tune of +1.0

December:

NAO

PNA (Pacific North American Pattern)

Owing to the Moderate La Nina this is likely to be predominantly negative this winter which teleconnects with zonal conditions for W Europe. Having said that the ensembles are currently progging the PNA to turn positive during mid December which could help enhance –AO and NAO for late December early January.

PNA

I predict the overall PNA for DJF to be in the region of -1.3.

SST

SST signals in the Atlantic are mixed and favour neither NAO+ or - conditions, the tripole we saw earlier this year has all but disappeared there are few 'cold over warm' anomalies to invigorate the jet further.

SST

Ice extent/Albedo effect

There is a correlation (albeit loose) to cooler winters and ice/snow coverage. During summer 2007 we saw record breaking extent of minimum ice cover in the NH. Although a significant recovery ensued during autumn, ice levels remain low at approx 1m sq km below average.

Summary:

Needless to say La Nina is the spoiler for cold weather lovers this winter. My forecast would have been quite different had ENSO neutral conditions prevailed as expected during the summer.

Having said that I do not expect it to be continuously mild as it was last winter. As previously mentioned I expect N’ly incursions only to be toppled by the jet. However I do expect the jet to take a slightly lower track due to the negative AO although not significantly so, instead the jet will be squeezed more tightly around 50 – 55 deg N resulting in some stormy conditions.

I don’t expect the mild to get all it’s own way and although this winter look likely to be above average in terms of temperature I wold not rule out at least one significant cold spell (to last around 7 days). Although I’m not going to chance my arm and predict when, I expect January to be the month most suitable to blocked conditions and have reflected this in my CET predictions below.

I do believe that winter starts to show it's hand in November, not necessarily in terms of temperature and precipitation but synoptically. Whilst there has been a trend towards more mobile and zonal weather we have seen on two occasions that the Greenland high has tried to form and ridge down to the mid Atlantic high. Although the jet has blasted through on both occasions I believe this synoptic will come off at least once point during the winter.

Finally I expect northern Britain, to experience more frequent spells of polar maritime air which should mean a better year for the Scottish ski resorts. Here are my predictions for this winter

Precipitation

December Above average

January Slightly below average

February Average

Overall Average

Temperature (CET)

December 6.0

January 3.5

February 5.5

Overall Above Average

Thanks for taking the time to read my forecast.

Adrian

Edited December 2, 2007 by Jack Wales

[johnholmes]

a lot of research and many hours work has gone into that as well JW. Thanks for that and sharing.

[Jack Wales]

Winter 2007/8

Enjoy !

GP, I've only just skimmed through you're forecast but what a magnificent piece of work. Thanks for the days/weeks you've obviously put into this as your technical knowledge and understanding are inspirational. This is definitely one to be printed off and mulled over at bedtime with a glass of scotch!

a lot of research and many hours work has gone into that as well JW. Thanks for that and sharing.

Cheers John.

[BrickFielder]

Some excellent forecast details in this thread and I preety much agree. What I would like to do is perhaps try to explain some of the concepts so that perhaps the forecast and its concepts is better understood. First I would like to look at the SST (Sea Surface temperature Anomalies) currently. What is obvious from the charts is La Nina that cold band of water extending out from south America as the trade winds blow east. Just recently we have had a flare up of MJO activity which has pushed eastwards from around the phillipines which was a little unexpected. This reflects increasingly warm seas around that area and has resulted in an increase in angular momentum (more on this later). We can see how prssure systems have formed as a result of La Nina and how it has affected sea temperatures basin wide. Notice the small tongue of cold water just off the U.S. coast which may extend eastwards as winter progresses which increases the risk of a mid Atlantic block latter into the winter.

Back to angular momentum (AAM) and what it means and I like to think of it as a measure of how much and many low pressure systems are spinning up. Typically when it is high you see fast jetstreams which arch northwards. Through the summer it has been low but the recent MJO (madden julian oscillation - A phase of thunderstorm activity in the western pacific which moves eastwards) as significantly and surprisingly increased AAM. The diagram which follows is an attempt to show how increased tropical activity can propigate northwards increasing AAM.

What reduces AAM is friction whether it is trospheric wave breaking over mountains (Mountain torque) or other features. The following diagram is a sort of rough representation of how I think about it. One of things I think you can see is that whether the winds are blowing in a westerly or easterly direction in the stratosphere makes a difference to the friction created. It is important to note that if you have high pressure sitting over the mountains then winds will not be blowing up the mountains to create the wave. In other words AAM will go lower as low pressure systems cross high mountains, and here I am thinking cut off lows crossing the rockies.

Similarly a tropical depression (a group of thunderstorms) with over shooting tops can also create some friction although perhaps not to the same extent.

Friction creates heat and heat results in changes in pressure. Warmings and pressure changes in the stratosphere are reflected back down into our weather to some extent by nature of changing the tropopause height.

I can not promise that I have got everything correct but hopefully I have given you something to think about and perhaps a glimmer of understanding about some of the terms and interactions talked about. So to sumarize GLAAM (Global AAM) is an indication of jet stream strength and low pressure system strength. GWO (Global Wind) balances AAM with torque to give a picture of what we should expect. It starts with sea temperatures and then returns full circle via the stratosphere, pressure systems and winds back again.

GWO explained

GWO current state

Ed Berry Explains

What should become evident is that a period of zonal weather should set things up for a blocked and meridional pattern later on. The GWO chart shows a looping action and as a cut off low bombs into the west coast of the US expect a pattern change as a result mid next week with the GWO looping back towards stage 1. High pressure developing over the artic is likely to help cold build there which could signal a fairly cold spell when the pattern does change.

[Glacier Point]

http://www.cpc.noaa.gov/products/precip/CW...ent.ascii.table

CPC data for November in and AO value was -0.519. This is within the expected range and the forecast therefore remains unchanged.

[Yorkshire Snow]

Thanks Steve,

The key thing for the 1st half of the season is a relaxation of the jet, directly related to GLAAM, which in turn is related to the strength of the gradient across the Pacific and Canadian Plains - Hart did some useful work relating GLAAM, ENSO and blocking suggesting a positive GLAAM equating to more blocking and likely tightened gradient as we see now.

This one of my thought processes: QBO and trend for westerly migration of core Nina anomalies; impacting on GLAAM (regions 3. and 4 have a 0.76 and 0.77 correlation with GLAAM during Jan); dropping of GLAAM allowing for a similar pattern to November but with more intense -AO and proper winter type set-up.

Hi Gp,

Absolutely stunning. Brilliant. Whether the end result is bang-on or not, it is the reasoned and scientific approach to your reasoning. The presentation is spot on.

Its folks your yourself, Steve and others that really fire up the interest in long-term weather forecasting and I thank you for going to the effort, its certainly helping me learn more about the science.

The very best of luck and lets pray for a TEITS Beast to hit us soon !!

Regards

YS

Hi Gp,

Absolutely stunning. Brilliant. Whether the end result is bang-on or not, it is the reasoned and scientific approach to your reasoning. The presentation is spot on.

Its folks your yourself, Steve and others that really fire up the interest in long-term weather forecasting and I thank you for going to the effort, its certainly helping me learn more about the science.

The very best of luck and lets pray for a TEITS Beast to hit us soon !!

Regards

YS

Just to add that the above wasmin relation to your Winter forecast ...... hit the end button too soon !!!

YS