In Depth Model Discussion, Analysis and Summaries

[johncoolj]

Joe B from AccuWeather says that ECMWF NINO forecast is picking up on the collapse of the Nino. He's ramping this up quite a bit. Interesting times ahead.

Yes but Joe B said worse to come over east europe as in germany etc not uk ireland he says cold will not be as bad here on the return of the cold snap

So looks like mild and wet for west europe for the next few weeks and i wouldent say much of a chance for gettin anything like we have had no way .

[Glacier Point]

Heights still building to the north according to ensemble mean guidance as the Atlantic trough migrates eastward and disrupts, indeed, trough disruption is the word of the moment.

Relative angular momentum is continuing to fall as easterly wind anomalies develop across the tropics.

http://www.esrl.noaa...m.sig.90day.gif

Mountain torques are positive but frictional torques (which lead mountain torques) are negative and dropping.

http://www.esrl.noaa...ltauf.90day.gif

http://www.esrl.noaa...ltaum.90day.gif

Tendency in relative angular momentum is falling. As the net impact of these falling torques drives angular momentum down further towards the low angular momentum base state, we get a strong signal for the GWO to be in phases 0-1.

http://www.esrl.noaa...gcm/gwo_40d.gif

The composites for these phases argue for a ridge in the North Atlantic (and over the UK on some composites - so basically high pressure across the mid latitudes west of the Meridian) and cut off lows off the eastern US and Iberia / central southern Europe.

However, the upper atmosphere is casting a long wave trough across the Atlantic with the support of low heights between Iceland and Greenland. Something of a conflict there.

The 8-14 day solution therefore is low confidence, but I would go for a trough disruption ejecting energy SE into Europe (helping to build heights to the north) and cut off low developing in the Atlantic helping to reinforce the westward movement of the ridge over Svalbard / northern Scandinavia. The dip in 850 hPa values around the 25th Jan picked out by the GFS ensembles makes a lot of sense and perhaps we should be looking NE for the cold to come last week Jan.

Edited January 16, 2010 by Glacier Point

[bluearmy]

stewart, the NAEFS shows all the high anomolies dotted about at T240 (west scandi, east canadian and unusually for this season, azores) being sucked into an elongated ridge stretching from s greenland to siberia. we just about get the trough to our east. infact the T240 anomoly doesnt look that far away from the ecm op run at the same timescale. 'animate' the run and you see the trough rolling east across the retrogresing azores ridge as the greenland ridge builds behind it. it does look a very similar sypnotic outcome to the two previous cold spells with the cold uppers encroaching with the trough from the northeast although it may well be that this time there is a longer draw of CAA to tap into. are the teleconnections still at odds with this being a possibly long term cold spell or are we maybe looking at something short but sharp ??

[Glacier Point]

Agree about the evolution last week January with the idea of discontinuous retrogression.

The declining angular momentum is now starting to rule out the prospect of a GWO phase 4 'final destination' and heading more towards (the Nina-like) phases 1-2-3. The ESRL web page is down which precludes reanalysis at this time, but I'm gessing combined with the Atlantic mean trough, we're looking at a SW'ly vector with the trough solution still in play for Scandinavia and ridge close to south of the UK.

Edited January 16, 2010 by Glacier Point

[lorenzo]

GP and Chino - if you be kind enough, please drop past the learners area a couple of questions there on GWO and charts that could do with some clarity.

[bluearmy]

GP, should we be concened that the fi NAEFS is so keen on the greenland ridge that it actually shows an omega block by the end of the month whereas the extended ecm ens arent showing any particular trend at that time.

[BLAST FROM THE PAST]

Posted 12 January 2010 - 23:51

Quote

This LP for Tues Wed will introduce less cold air as it tries to move in. I think a degree of trust has to be given to the models particularly upto T96. It seems that generally upto Thurs we are seeing the general plan BUT it is from the weekend where we may well see changes...and I agree with TEITS here, a SW/NE split will develop. Renewed colder attack [relevant to next few days but not as deep as previous by some margin] from the east as the block re-asserts itself. LPs attacking 16-20 from SW on southerly track. The period I mentioned 14-20 has to be re-defined to 16-21, but that is small adjustment as grand scheme remains. The block all but shunted out of the way come last 5-7 days of the month ie no influence over UK as LP barrels across the centre of the UK and then the more pronounced LP coming in for the end of the month. Feb looking very, very interesting, J Bs update is interesting yet he remains with the 02/03 composite...I disagree inasmuch the deepest cold was somewhat further west than he anticipated [although still a very good call] and so there could be a renewed attack of deep cold as we enter Feb. I am reconsidering the last month of my LRF with RJS as indeed everything looks like being further west hence warm sector west of UK and cold sector more towards NW Europe. Next weekend likely for Feb outlook.

BFTP

Just re-posting post I made yesterday following on from others and seeing that its in keeping. Now the less cold we have had/having is a blip inasmuch that the calls for the cold spell to be over by the weekend and all gone by likes of Francis Wilson are incorrect. The jet is on a southerly tracking longterm holiday [seriously long] and as the sun is awakening it will send out disturbances that will buckle the pattern temporarily. Cold will renew as LPs will start cutting off and sliding SE as the southerly track re-asserts. We are not talking bitter cold but cold from this weekend starting the set up for the renewed pulse from the east. Now taking into account the great in depth posts you have seen and timings on the model thread, big buckle coming and proper end of cold from 25-31 with deep LP barreling over the UK. This will end the current/renewed eastern influence..Kaput. Now a tad earlier to post than expected but this is where I start looking for renewed cold shot. I don't know about the teles GP superbly reads and projects BUT I think they will develop into a very favourable pattern. We must as we enter Feb look to our northern quadrant, Northely and NE'ly winds will set in again with strong HP devloping over the arctic [exactly where I don't know but I suspect a similar development to what brought our last deep cold spell. I think Feb may well now develop off the back of the peak energy period end of Jan into a prolonged and very cold pattern.

BUT I have a nagging and so I'm posting my nagging.

BFTP

Hi Stew

Just wanted to post my musings on RJS and my LRF site. Its very interesting to note your updates and latest one. I personally think late Jan for cold shot is too soon as the 25-31 is a peak energy period and it seems the models are 'currently' picking up on that. I agree that the LPs will get past the3 date line and trough into Scandi and also that IF there is to be a renewed cold shot iot will be similar development as previous one.

I also think we will see a collapse of Nino sometime soon, the signs are there already.

BFTP

Edited January 17, 2010 by BLAST FROM THE PAST

[Glacier Point]

Model guidance is continuing to advertise a window for a cold (brief?) pattern late January into early Feb.

Tropical forcing (organised thunderstorm activity and cloudiness) is now centred just west of the Dateline. Further eastward motion, even just marginal, will begin to favour a -NAO signal, particularly given the SSTA which clearly is likely to favour a weakness over the Azores.

http://cawcr.gov.au/bmrc/clfor/cfstaff/matw/maproom/OLR/m.3d.html

http://weather.unisys.com/surface/sst_anom.html

The potential sharp downwelling of waves associated with the increasing temperatures in the middle and upper stratosphere (as opposed to more sustained long term impacts) are also manifesting themselves in modelling of the polar field with a split or highly displaced vortex being programmed. This remains the most problematic aspect of the medium range modelling.

However, the GWO is tilted towards the low angular momentum base state. There is little to suggest that it will lift out from orbits in phases 0-1-2-3.

http://www.esrl.noaa.gov/psd/map/images/gcm/gwo_40d.gif

Phase 1 composites more or less capture the GWO signal going forward across the 'jump' from January to Febraury w/r/t changing wavelengths.

This signals high pressure over or to the west of the UK and appears to be supported by the majority of model guidance 12z 18/01/10 allowing colder air to be advected westwards.

Under these circumstances, a blend of GWO and MJO composites would seem to be the best solution taking account of the various factors. This would be for the Atlantic trough to disrupt and for tendency for high pressure to the west and north of the UK, and initially to the east before the ridging to the west becomes strong enough to drop a trough into the east and retrogress the ridge further NW.

[Nick F]

Thanks for the analysis GP, 12z ECM 8-10 day mean 500mb height chart certainly heading towards your idea of HP to the W and N of the UK, bearing in mind the ECM has the best verification for the medium range:

Warm SST anomalies over NW and N Atlantic mainly north of 55'N, do you think these are contributory at all to helping more ridging in this area in the future? Or would this be over-ridden by teleconnections upstream?

[Glacier Point]

It certainly will not interfer with the pattern Nick.

If we look at the MJO composites for phase 7 (January) and phase 8 (February) we can see clearly the emphasis long-range modelling is going:

MJO forecasts suggest high amplitude phase 7 to come:

It should be noted though once this tropical forcing signal wanes, the support for the cold pattern likely goes, particularly as the GWO is tilted towards Nina-like phases which favour more mid latitude blocking features and there is something of a battle here going on between tropical and extratropical signals as to where that high pressure will end up.

[BLAST FROM THE PAST]

It should be noted though once this tropical forcing signal wanes, the support for the cold pattern likely goes, particularly as the GWO is tilted towards Nina-like phases which favour more mid latitude blocking features and there is something of a battle here going on between tropical and extratropical signals as to where that high pressure will end up.

Stewart

Wouldn't the jetstream kicking south provide the footstool for the cold pattern to remain. In winters pre 08/09 and this current period / cycle we find ourselves in I woulsd say yes that the cold pattern would quickly collapse. Hence my posts above for the cold to potentially really dig in and be lasting through Feb?

Great analysis and thread by the way.

BFTP

[Lettucing Gutted]

And also a nina-like low GLAAM signal should reinforce a southerly tracking jetstream.

[Glacier Point]

Analysis of the global circulation and ocean-atmospheric coupling seems to suggest something of a conflict between the tropics, which are trying to repeat the mid December evolution to cold, and the extratropics which are less reluctant to embracing a sustained cold pattern renewal - at least for western Europe.

As predicted over two weeks ago, an organised convective signal (thunderstorm activity) has arrived around the Dateline. This is ahead of the MJO wave which is centred around 150 East based on latest visible and infra red wind data:

http://cawcr.gov.au/...m/OLR/m.3d.html

http://cimss.ssec.wi...wavetrkP-1.html

The atmosphere is responding accordingly with a positive mean height anomaly projection in the mid Atlantic and upper low over the Azores. Model guidance for the next 10 days is solidly in agreement with composites for the MJO phase 7/8:

http://raleighwx.eas...Phase7500mb.gif

http://raleighwx.eas...Phase8500mb.gif

The satellite imagery shows an organised ribbon of westerly winds across 30N in the Pacific. This has been a feature of the winter thus far due to poleward eddies of mean zonal wind anomalies transporting energy into the sub-tropical jetstream that remains strong.

Total angular momentum anomalies show a profile which is very similar to the previous 2 cold episodes in mid December through early January with strong positive (westerly) wind anomalies over 30-40N and negative anomalies above these in the mid latitudes (easterlies):

http://www.esrl.noaa...m.sig.90day.gif

Crucially, there are some small but significant differences this time round which are likely to preclude sustained cold and the high latitude blocking necessary.

The trade wind surge which started in late December dumped a large amount of easterly winds into the tropics. Whilst starting to fade, there are still negative wind anomalies holding back total and relative angular momentum. The lower the angular momentum, the greater energy flow into the northern arm of the jet. Whilst mountain torques are positive overall, there is a disconnect between the Asian and N. American mountain torques. The Asian torque is negative (impact of the Siberian High ?) and this is adding easterly wind (scrubbing westerlies) to the tropical atmsophere preventing angular momentum from taking off:

http://www.esrl.noaa...ltaum.90day.gif

Negative frictional torques are also present in the high latitudes (westerly winds stirring) and negative overall, mainly related to the strength of the sub-tropical jet:

http://www.esrl.noaa...ltauf.90day.gif

This confirms that the polar westerlies are increasing, allowing the polar vortex to shift back towards the Pole.

The GWO, which is the key measure of both tropical and extratropical wind flows, is centred in phase 4:

http://www.esrl.noaa...gcm/gwo_40d.gif

Mountain torques are likely to continue positive for a few more days and overall tendency in relative angular momentum is likely to remain positive holding the GWO in phase 4. Composites for this phase into February depict nicely the anticipated deep polar trough over Scandinavia and resultant northerly over the UK:

The MJO is still to reach the Dateline (phase eight) This means that we can expect to see convection remain around the Dateline for at least 10 days giving us a fairly strong signal for a block to our west or north west in the Atlantic.

A couple of other pointers:

- frictional torques are negative and likely to remain so;

- mountain torques are likely to go negative within the 5-10 day timeframe.

This means that the GWO will likely shift from its centre in phase 4 towards phase 0 in early February. Composites for phase 0 in February identify a trend for the ridge to transfer towards western Europe:

The stratosphere over the Polar region has warmed significantly in the last few days and we are seeing the first signs of rapid downwelling occuring. Zonal wind anomalies identify a negative wind anomaly stretching across the tropopause:

http://www.cpc.ncep....JFM_NH_2010.gif

The ECM plots suggest some propagation of the negative zonal winds in the 8 day timeframe:

http://strat-www.met...cast=f192&var=u

This suggests that the blocking structures over the northern hemisphere (North Atlantic) may get some enhancement in the immediate term from this downwelling so there is an argument for the northerly being something more than just a standard toppler. I would support the notion of it lasting beyond 2 days, probably setting a cold period end January into early February allowing for residual surface cold.

Thereafter, the colder temperatures in the middle and lower stratosphere may well help to sustain a tropospheric vortex over Greenland until the lagged downwelling of the warming impact the middle troposphere around 20th Feb onwards. It should also be noted that the polar vortex has become completely displaced and is a good signal for sustained trough over Scandinavia:

http://strat-www.met...0&forecast=f192

Putting this all together, the extended long range discussion revolves around a mid Atlantic ridge retrogrossing north-west late January into early February. I then think it likely for the high to transfer south and east towards the UK and western Europe (eastern Europe remaining cold) during the early to middle part of February associated with falling angular momentum. There is a window I think around mid to late February for a combination of the GWO phase 4 and warming of the stratosphere to reinforce a similar situation to now with the northerly. If angular momentum continues to fall, then the potential impacts of the warming will be limited. However, if angular momentum increases, a more sustained colder pattern and blocking could well emerge.

Overall, the next 2-3 weeks look cold initially but becoming milder and more settled, probably marking the most prolonged spell of milder weather in this cold winter. The main feature will be the persistence of high pressure across a longitude 10W - 30W in our part of the Northern Hemisphere.

Edited January 24, 2010 by Glacier Point

[bluearmy]

thanks for the update. the extended ens are starting to show a trend for blocking to our north (likely centered nw) and the related drop in uppers spreading sw across nw europe. however, the spread off newfoundland in a week is very large and this then spreads across the n atlantic. there are obviously, as you say, conflicting signals. your description of our likely route is pretty well represented by the NAEFS anomolies for the next fortnight. however, they didnt seem keen to keep the northern blocking quiet for long. i guess they can only take account of what the current data shows rather than doing some assumptive extrapolation as you have done.

[Glacier Point]

Over the next few days, keep an eye on relative angular momentum plots for the tropics and the polar region. If they start to show decreasing easterlies in the tropics and increasing easterlies over the high latitudes, then maybe the blocking signals would be justified.

http://www.esrl.noaa.gov/psd/map/images/reanalysis/aam_total/glaam.sig.90day.gif

[lorenzo]

Great summary GP, no mention of the AO and NAO signals though are they still in a state of indecision?

[Glacier Point]

I would go for the AO to be negative over next 10 days, rising positive towards neutral or even positive thereafter, perhaps falling again towards 20th February.

I would favour the NAO to be negative becoming nuetral or slightly negative, mainly due to a weakness in heights towards the Azores due to a strong sub-tropical flow.

[Duane S.]

Hi GP, Oustanding post as per always. Thanks for taking the time to do so.

If I've got this right... Increasing Mountain torques of North America can be nullified by similarly decreasing ones over Asia having the net effect of not increasing AM by as much than if both were on the increase??

[Thundery wintry showers]

Reading over the posts regarding the teleconnections, I gather that we could be in for a February similar to the likes of 1973 and 1999? That's certainly what the current pattern reminds me of, in conjunction with the signals for HP to the south and west and trough over Scandinavia. Both of those Februarys had alternation between mild anticyclonic westerly types and cold northerly types, and came out mild overall but with many areas having close to average snow amounts.

I think something like that, if it comes off, would be a minor victory for cold/snow lovers as there has always been the real possibility of a very mild snowless February.

[damianslaw]

Reading over the posts regarding the teleconnections, I gather that we could be in for a February similar to the likes of 1973 and 1999? That's certainly what the current pattern reminds me of, in conjunction with the signals for HP to the south and west and trough over Scandinavia. Both of those Februarys had alternation between mild anticyclonic westerly types and cold northerly types, and came out mild overall but with many areas having close to average snow amounts.

I think something like that, if it comes off, would be a minor victory for cold/snow lovers as there has always been the real possibility of a very mild snowless February.

I'd be happy to see synoptics similiar to Feb 73 and 99, perhaps bringing a colder feed than those two monts, polar maritime air was frequent with pulses of arctic maritime air at times bringing snow showers to many north western northern and north eastern parts, I'd prefer the polar air to be in greater residency though than tropical maritime air.

[LomondSnowstorm]

Perhaps this is just a bit stupid but I can see similarities with the start of February 2005, with high pressure sitting to our west and troughs to our northeast - http://www.wetterzentrale.de/pics/archive/ra/2005/Rrea00120050201.gif http://www.wetterzentrale.de/pics/archive/ra/2005/Rrea00120050210.gif

http://www.wetterzentrale.de/pics/archive/ra/2005/Rrea00120050219.gif This pattern evolved into a fairly snowy easterly by the end of the month http://www.wetterzentrale.de/pics/archive/ra/2005/Rrea00120050224.gif

Not sure if this is likely but with some talk of more northerly blocking by late month due in part to the MMW (not sure if this is likely to occur either!) this might be a decent analogue. In the end though few people actually compare previous months to other months due in part to how different most months are from each other.

[Duane S.]

Perhaps this is just a bit stupid but I can see similarities with the start of February 2005, with high pressure sitting to our west and troughs to our northeast - http://www.wetterzentrale.de/pics/archive/ra/2005/Rrea00120050201.gif http://www.wetterzentrale.de/pics/archive/ra/2005/Rrea00120050210.gif

http://www.wetterzentrale.de/pics/archive/ra/2005/Rrea00120050219.gif This pattern evolved into a fairly snowy easterly by the end of the month http://www.wetterzentrale.de/pics/archive/ra/2005/Rrea00120050224.gif

Not sure if this is likely but with some talk of more northerly blocking by late month due in part to the MMW (not sure if this is likely to occur either!) this might be a decent analogue. In the end though few people actually compare previous months to other months due in part to how different most months are from each other.

I would say a very good analogue! The main difference (bar the greater energy in this week's Scandi trough) between these two charts is the low pressure under the High in Mid Atlantic (which possibly answers my quesion on the model thread reference why is the NAO not going through the floor in the next few days?)

[Snowyowl9]

I can remember that atlantic high in february 2005 stayed there for sometime giving mild N/NW winds around that high.

If we had an easterly like feb 2005 or even 2006 it would be much colder now to what it was then..

[Thundery wintry showers]

Although there has been much talk of the Arctic being anomalously warm it is nothing like as anomalously warm as it was in January & February 2005 when a high-powered northerly jet kept flooding the pole with sub-tropical air.

Therefore one major difference is that we're unlikely to see a repeat of 13 February 2005, when we got a northerly straight from the pole but there was so little cold pooling up there that temperatures merely fell to around average (maxima of 6 or 7C). By the 19th February the Arctic had cooled off a bit allowing a more potent northerly in (maxima of 3 or 4C), but even so I would expect any northerlies this month to be more potent than any of the February 2005 northerlies.

[LomondSnowstorm]

Although there has been much talk of the Arctic being anomalously warm it is nothing like as anomalously warm as it was in January & February 2005 when a high-powered northerly jet kept flooding the pole with sub-tropical air.

Therefore one major difference is that we're unlikely to see a repeat of 13 February 2005, when we got a northerly straight from the pole but there was so little cold pooling up there that temperatures merely fell to around average (maxima of 6 or 7C). By the 19th February the Arctic had cooled off a bit allowing a more potent northerly in (maxima of 3 or 4C), but even so I would expect any northerlies this month to be more potent than any of the February 2005 northerlies.

Exactly why I think this month is unlikely to be milder than average - in previous years we've needed really good synoptics just to get reasonable surface cold (other than inversion highs) but with cooler SSTS and a cold continent any cold spell is likely to be colder than in recent years.