January thread

[Glacier Point]

As we reach the core of the winter, some extraordinary global factors at play and some tremendous uncertainty, largely related to instability over the polar field but some tentative signs for increased frequency of blocking over Scandinavia and NW Europe with a possible extension to Siberia and later on Greenland.

Key players:-

1) Globally Averaged Angular Momentum

Sea surface temperatures over the Tropical Pacific continue to drop as La Nina matures..

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

and pushes westwards driven on by anomalously strong easterly trade winds. During the middle part of December, these easterly winds intensfied over the western Pacific, driven on by the formation of a large anticyclone consistent with La Nina...

ENSO regions 3, 3.4 and 4 have a high correlation (0.72, 0.76, 0.77 respectively) with global angular momentum in January.....

http://www.cdc.noaa.gov/Correlation/Table/corr.table.jan

The combination of easterly winds reinforcing cold sea temperatures by upwelling and surpression of tropical convection will maintain levels of angular momentum (and zonal winds) at anomalously weak levels. This will allow other factors to influence the pattern.

2) Tropical forcing

Staying in the Indo-Pacific region, the second half of December has been characterised by intense tropical convection denoted by large negative outgoing longwave radiation anomalies. This has aided significant Rossby Wave dispersal, the effects of which are being felt across the Northern Hemisphere - particularly in terms of setting blocking ridges over Scandinavia and disrupting the polar field. These impacts are being enhanced by low levels of angular momentum through less deflection of the waves.

This convective actvity is ongoing and lastest MJO wave analysis confirms this...

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

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

Reanalysis of Decembers which featured strong monsoon (negative OLR anomalies), and filtering for low levels of angular momentum, shows a January pattern predisposed to blocking over the NE quadrant....

3) Arctic Oscillation

December's AO is likely to come in at a very positive index value. This has been related to significant surface cooling (possibly delayed due to the anomalous warmth) which is shown well by the polar atmosphere cross sections....

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

More recently, we have started to see negative height anomalies appearing in the upper atmosphere (driving +AO) and these are consistent with the anomalously cold stratosphere experienced in December. Right now, the polar stratosphere is forecast to cool further - at a time when it should be starting to warm...

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

This should be horrifically bad for the UK and any prospects of cold, but there are some important differences in terms of where this cold is distributed and how the stratospheric zonal winds are structured which may well be off-setting this upper cold anomaly.

By looking at the latest available polar cross section....

we can see the cold air to be more over the Scandinavian locale with warmer pools in the upper atmosphere over eastern Siberia and Gulf of Alaska. We can also see development of distinctive columns of warm and cold air below the tropopause which is reflective of Rossby Wave dispersal.

By looking at stratospheric zonal wind anomalies.........

...we can see no organised wind pattern circling the North Pole - which is in total contrast to last year. This is the thing that is really allowing blocking to develop around the margins of the polar field and the overall instability of the polar vortex and the distribution of upper cold pools with tropospheric blocks beneath them.

Latest ECM stratospheric plots continue to trend the core of the stratospheric vortex (high pressure beneath) towards the Arctic interior and Greenland with a transfer of the polar (tropospheric vortex) towards eastern Siberia and Alaska.....

....which should assist in the disruption of the formation of an organised polar vortex and mitigate for the impacts of the +AO which we are most likely to endure c/o the cold polar stratosphere (remember 23-25 days lag time for warmings to work their way down to the troposphere).

Even though the AO has been largely positive in December, it has shown signs of great instability and blocks have formed consistently between 50 degrees North and 60 degrees North.....

With more tropical forcing, I see no reason why we shouldn't be looking at further blocks develop on the edge of the polar field but not quite due to fastish (albeit eddying) polar westerlies. What may also be interesting here is if you compare OLR for a week or two ago with current anomalies...

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

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

we can see that the amount of OLR from the polar field has dramatically reduced indicative of a normalisation of surface cooling and possibly making the AO less extreme in its fluctuations.

4) Atlantic SSTAs

December's pattern will show a high similarity with the predicted 500 hPa height anomaly derived from analogue SSTAs in the Atlantic - despite the +AO signal.

Predicted...

So far.... (allowing for more recent low pressure to our west and high pressure likely to build NE)

No reason therefore to go against January's predicted pattern....

.....showing continued +ve height anomalies over the eastern third of US and Canada (especially Newfoundland) and extending across to Scandinavia with a -ve height anomaly over Iberia.

Putting these all together, I think the atmosphere will continue to respond to tropical forcing and the SSTA with ridges over the eastern third of N. America, a weak trough in the Atlantic and ridge over NW Europe. This is a pattern consistent with the SSTA, tropical forcing and the underlying +AO pattern. As we progress through the month, there are some signs that the ridge over Canada will become more pronounced with a hint of a negative AO/NAO developing as an expected warming of the stratosphere may take place in early January.

This is a pattern with a strong easterly component. If we get a significant enough ridge extension into the Arctic early in the month, this could well set up a slow, sure-footed progression towards a deep cold pool moving all the way from central Siberia towards western Europe, something that is being picked up by the GFS extended range.

With a cold surface established over Europe, this is not likely to be an anomalously warm month. Analogues suggest something not far from average but these probably mask some large variations and I would put the range anywhere between -1.5 C and +0.5 C.

The one caveat I would put in here is IF we start to see the massive cold anomalies in the stratosphere take effect and drive an organised upper wind circulation, then any hopes of easterly's would collapse and highh pressure will get lodged over the southern half of the UK.

Edited December 28, 2007 by Glacier Point

[Gavin P]

Thanks very much Glacier Point. So January should hopefully be a month thats starts off interesting and gets increasingly exciting as we move through?

[Roger J Smith]

My research profiles have shown a tendency for anomalous warmth in the middle third of the month in analogue situations, with the best cold profiles early and late. The way things are now setting up, I would not be surprised if it's a real up and down ride, the current model discussion shows the uncertainty developing early but there seems likely to be at least some significant cold and snow, then possibly some very mild weather 10th-20th, becoming stormy, then colder again.

Besides the impending snow and cold event, the highlights may include some record warmth later, a windstorm or two in the period 20-23, and a turn back to quite cold weather near the end of the month. The January CET could be almost anything the way this is setting up. Would refer anyone to the daily CET for January 1867 to show just how variable a month can actually be (-10 to +10 in a few days, then back to the deep freeze).

[Glacier Point]

My research profiles have shown a tendency for anomalous warmth in the middle third of the month in analogue situations, with the best cold profiles early and late. The way things are now setting up, I would not be surprised if it's a real up and down ride, the current model discussion shows the uncertainty developing early but there seems likely to be at least some significant cold and snow, then possibly some very mild weather 10th-20th, becoming stormy, then colder again.

Besides the impending snow and cold event, the highlights may include some record warmth later, a windstorm or two in the period 20-23, and a turn back to quite cold weather near the end of the month. The January CET could be almost anything the way this is setting up. Would refer anyone to the daily CET for January 1867 to show just how variable a month can actually be (-10 to +10 in a few days, then back to the deep freeze).

One thing to bear in mind here is that the AO is going through a 3-4 point index reduction:

This is telling us that there will be a huge outflow of cold air and corresponding influx of warmer air. This will I think serve a less mild pattern in the warmer interludes that are inevitable with such an amplified pattern that is being built right now.

GFS vs ECM mean height comparisons for the start of the month bring both models toghether quite nicely. Even though the longevity of the cold pattern might be shorter than desired, anything that happens now has implications further on down the line.

These outputs suggest ridging back to Greenland and a building -AO.

These outputs are a good (and encouraging) match on reanalysis based on analogues for strong December tropical convection in the Indian Pacific:

The inference here is that the block will not go that far east - we might get caught in a cool zonal pattern for a time - but there will be an overwhelming case for the ridge over Scandnavia to rebuild.

[Gavin P]

I have a hunch the Siberian High will be a major player this January. That means we'll alternate between cold easterly and mild southerly spells. From around January 18th, though, I do think the SH will really take control and the final third of the month could be the coldest period since the mid 90's!

Edited December 28, 2007 by Gavin P

[BLAST FROM THE PAST]

My research profiles have shown a tendency for anomalous warmth in the middle third of the month in analogue situations, with the best cold profiles early and late. The way things are now setting up, I would not be surprised if it's a real up and down ride, the current model discussion shows the uncertainty developing early but there seems likely to be at least some significant cold and snow, then possibly some very mild weather 10th-20th, becoming stormy, then colder again.

Besides the impending snow and cold event, the highlights may include some record warmth later, a windstorm or two in the period 20-23, and a turn back to quite cold weather near the end of the month. The January CET could be almost anything the way this is setting up. Would refer anyone to the daily CET for January 1867 to show just how variable a month can actually be (-10 to +10 in a few days, then back to the deep freeze).

yes a breakdown is likely and could be of the Jan 79 scale but with an overall cold winner. Also when the cold hits us around 2/3 I don't think its a 2 day affair..indeed Roger I think a week like you suggest is more the outcome.

regards

BFTP

[kold weather]

Without wanting to be totally untechnical I think an easy weay to sum up this month is a battle between the Siberian High and the poler vortex located close to Greenland.

When the jet is blowing strong then I think overall the jet will win out at least for the first 15-20 days however at the same time like yesterday there are going to be times when the jt eases off just a bit and thats going to be the time when your going to see cut-off lows as the high tries to force its way westwards again...

I think however by the 20-25th the high will actually fully win out and send the jet stream to the south, tohugh there will be very likely still a SE slant to the jet eventually we would get a pure easterly esp if the high gets to extent towards Greenland.

Will be interesting to see how it all pans out. If the PV stays a little too dominant then a Jan CEt in the 5-5.5C mark seems reasonable...if the high ends up dominating from about the 15-20th then clearly we have a chance to be between 2-3C, something inbetween would argue probably 3-4C IMO.

[Glacier Point]

Happy New Year all.

We are currently seeing a westerly burst of winds over the western hemisphere associated with tropical convection which, is an unexpected development, but one which could well be the catalyst for the type of evolutions which might well deliver the easterly goods into the second half of the month.

Tropical convection (shown as red anomalies) in the Indo-Pacific region are leading to westerly winds which are registering in an upward shift in global zonal winds (although not yet of the order of magnitude as previously). A similar episode took place during late November and lead us orbiting briefly through westerly wind propagation in the Global Dynamic Model:

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

Last time this pretty quickly connected to an increase in winds over the Atlantic and I think a similar response will follow, which is already beginning to show up in the modelling. However, as before, SSTAs in the Pacific and Atlantic are not predisposed towards endless zonality and this unsettled spell probably will take us no further than the halfway point of the month. One important difference though is that we have seen two significant downward shifts in the AO which have expelled large amounts of colder air equatorwards which will ensure an averge winter zonal flow pattern rather than the much milder ones we have experienced over the last few years:

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

Staying with the AO, the trend towards a dropping off in positive outgoing long-wave radiation anomalies over the polar field has continued. This is signifying that the field has undergone pretty much all the anomalous cooling associated with a delayed freezing over and, as a result, we are now much more likely to see the AO trending neutral negative rather than very positive - despite the polar stratosphere being very cold during December.

Compare previous seasonal trend in OLR with current anomalies and we can see the dropping off of positive anomalies across the North Pole indicative of more normalised cooling at the surface:

There is some slightly better news regarding the polar stratosphere with ECM forecasting an upturn in 10hPa and 30hPa values towards neutral values.

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

This could be signficant if the trend is continued and we might start to entertain thoughts of proper northern blocking in a month's time if this continues. However, we probably have 23-28 days worth of the legacy of the colder stratosphere to deal with making any phases of northern blocking transitory for the time being.

Looking at current and projected H500 anomalies, we continue to see the evidence of a large block to our NE over the Barents Sea and northern Russia. This is entirely consistent with the anologues for strong tropical convection in the Indo-Pacific and week 2 NCEP / PSED forecasts:

The combination of an upward spike in zonal winds (with dropping off to follow day 7-10) and influence of Rossby Wave dispersal from tropical convection and formation of a large Pacific anticyclone will set up the conditions and open up the window for another easterly attack during the second half of the month perhaps allowing the retrogression of the Siberian High and development of a Scandnavina High due to undercutting lows. The end of the month is perhaps more favoured using the stages of the Global Wind Model as we have a greater chance of cut off lows triggering easterlies in stage 4 of the model - currently stage 2.

Interestingly, the main body of the polar vortex looks ideally placed towards eastern Siberia to facilitate this and we have had two easterly flows (the second albeit brief) already this winter despite an unfavourable polar stratosphere.

Edited January 5, 2008 by Glacier Point

[bluearmy]

welcome back GP = we need some help mate

[Gavin P]

Thanks Glacier Point. :lol: All is not lost yet, then?

When would you expect to see the block to the east starting to fight back?

[kold weather]

I'm not GP but i don't think the block will fight back from the east, rather the PV will shift back westwards. At that point we need to look for a cut-off low to develop and head SE and its wake high pressure would form. Its how the huge block to our NE formed this week though that ended up just a little too far east for the UK in general for us to last too long in a SE airflow.

[BrickFielder]

I don't see tropical convection being strong enough to change the mature la nina pattern. There is no evidence of stratospheric warming at this point so we should not expect blocking to occur as a result of this.

What we do have is some very cold conditions building in siberia, a jetstream which is a good way south such that much of the UK is on the north of the jet stream and 850hPa temepratures across the north atlantic which barely get above 10C even off the coast of Africa.

Short term I would guess the jet stream will continue to move south and this raises the possibility of potent lows crossing the UK further south raising the threat of very strong winds across more southern parts of the UK. Into January and the wet atlantic weather coming in from the west is likely to turn cooler and ever more snowy.

If I was looking for pattern change then I would be looking at the Sea surface temperatures across the northern atlantic. As winds continue to blow westwards so cooler water will be lifted off the coast of the US similar to la nina but the opposite way. This will give a tendency for high pressure to develop mid atlantic, eventually we will get a blocked pattern which in turn I think will trigger a stratospheric warming bringing a blocked pattern and colder air out from siberia sometime later in february.

In short I don't see a very mild pattern no matter what pattern develops and SST's could well be key.

Watch for a pattern change across the US as this may be crucial with a high pressure shift across Canada which may change the outlook.

[WINTRY WALES]

I don't see tropical convection being strong enough to change the mature la nina pattern. There is no evidence of stratospheric warming at this point so we should not expect blocking to occur as a result of this.

What we do have is some very cold conditions building in siberia, a jetstream which is a good way south such that much of the UK is on the north of the jet stream and 850hPa temepratures across the north atlantic which barely get above 10C even off the coast of Africa.

Short term I would guess the jet stream will continue to move south and this raises the possibility of potent lows crossing the UK further south raising the threat of very strong winds across more southern parts of the UK. Into January and the wet atlantic weather coming in from the west is likely to turn cooler and ever more snowy.

If I was looking for pattern change then I would be looking at the Sea surface temperatures across the northern atlantic. As winds continue to blow westwards so cooler water will be lifted off the coast of the US similar to la nina but the opposite way. This will give a tendency for high pressure to develop mid atlantic, eventually we will get a blocked pattern which in turn I think will trigger a stratospheric warming bringing a blocked pattern and colder air out from siberia sometime later in february.

In short I don't see a very mild pattern no matter what pattern develops and SST's could well be key.

Watch for a pattern change across the US as this may be crucial with a high pressure shift across Canada which may change the outlook.

I must say Brickfielder, your forecasts and observations are one of the few on here that comes with the highest regard, and what a possibly exiting few weeks we may have in store then

[BrickFielder]

It definately looks like a pattern change is coming up for the US. Most noticeably high pressure for the gulf of Alaska but preceeded by a blocked pattern. The change over I think will likely give a cold spell for the plains and eastern seaboard of the US. No immediate change for us but warm air going across the pole will likely change the pattern in our area.

[Jack Wales]

Another factor that would indicate a pattern change towards the end of the month into February is the six week anomalously cold NH Stratosphere apparently coming to an end...

NH Stratosphere anom

Up until very recently there were only small variances between ensemble members, even into FI but look at the 12/1/08 00z 850hpa for Cardiff...

GFS Ensembles

There is now considerable disparity between members undoubtedly due to the changes referred to by BF over the US and the track of the subsequent LP off the eastern seaboard.

Edited January 12, 2008 by Jack Wales

[Glacier Point]

Boy or boy, have I got this month wrong. I have no problem fingering a sudden pulse of westerly winds originating over the western Pacific early in the month and, to top that, the pattern change mid month that we were looking for, but only a change to a really strong +AO. The latter is hardly surprising given the anomalously cold stratospheric temperatures experienced during December.

Moving to the outlook last 10 days January into the first week of February, I expect to see a slow movement of the pattern, essentially becoming drier and trending towards a cooling as high pressure migrates first over the UK and then slightly west then north west.

Tropical convection in the Indo-Pacific looks to be restricted to a narrow area along the equator from Indonesia towards Australia whilst easterly surface wind anomalies remain in place consistent with a mature and strong La Nina where negative surface temperature anomalies have increased over the western Pacific. Outgoing long wave radiation anomalies identify the areas of tropical convection (orange - negative anomalies) in the Indo-Pacific:

This is likely to lead to moderate to low levels of zonal wind propagation given the strength of the positive OLR and cold SSTAs forming in the western Pacific. The MJO has largely stalled in phase 7 and is some way from phases 3-5 where it may serve to pep up the zonal wind anomaly - I estimate it will hit phase 3 in another 15-18 days with lag of 10-12 days for any winds to increase over the Atlantic - all of which makes a context for a more quieter phase of global winds.

The dominating feature of our weather pattern has been the strongly +ve AO. ECM is currently forecasting a sharp rise in stratospheric temperatures however and the models will have to grapple with lag times for this to permeate downwards (likey duration 21-23 days) and more immediate impact on the polar vortex.

Notice in this sequence how the ECM has, over time, shifted the day 8 forecast for 10 hPa height anomalies in an anticlockwise motion with the high pressure migrating towards North America and the low pressure centre moving more towards Scandinavia.

and most recently...

At the same time, we see falling OLR anomalies which might just be hinting at a falling AO, or less positive AO. Latest GFS AO Ensembles show this drop, which is important because even though the index does not go very negative, it does fall from +4 to 0 in a short space of time which is indicative of a large body of cold becoming drained from the Polar Field.

Drawing this together, I think there is an increased tendency for the pattern to subtly shift once again. This time, for high pressure to form further north of its current position in Biscay towards the UK and then for the jetstream to become increasingly amplified with pressure rising in the western - central Atlantic and then bing shifted towards the eastern Atlantic in line with SSTA forcing.

CPC analogues rolled forward (base -4 days ago) to 10 and 15 days depict this type of evolution:

NCEP and PSD Ensemble week 2 forecasts also go along with a theme of rising pressure over the UK and eastern US:

.. whilst the operational ECM and GFS are going for at least a transitory disruption to the polar vortex before the impact of any stratospheric warming takes place.

The one concern I have here is that the models may be a tad pre-emptive in breaking down the polar vortex although this is really a question of lag times and stratospheric zonal winds are likely to drop out markedly over the next 10 days or so.

Edited January 20, 2008 by Glacier Point

[BrickFielder]

One thing to note about stratospheric warmings is that they do not all reach down to the tropopause and they can normally be categorized as minor or major warmings. Typically a minor warming will appear out of the blue at the top levels of the stratosphere but will will fail to propagate downwards. Major warmings are normally preceeded by smaller minor ones and propagate downwards over a period of about 3 to 4 weeks.

Major warmings are categorised by a complete reversal of the winds at the top of the stratosphere like below.

What really happens is the stratospheric vortex gets disturbed over the pole so that it either breaks in two or meanders south of 60 degrees north. What is unusual about the current warming is that the vortex has been disturbed when it is so strong. This shows up as a 38 degree temperature rise at the top of the stratosphere within a 48 hour period. This is undoubtably a record and shows what an extraordinary event is occuring.

My take would be that this is an extreme minor warming which will take about a week to propagate downwards and will not be that long lasting. This might well be similar to the warming in early 1999.

Who is to say further warming or oscillation in the stratospheric vortex will not occur so we need to keep an eye on the stratosphere for clues to how long and when cold periods might occur.

One thing I might be looking for is for a scandinavian high to develop if the atlantic high topples over us based on the fact that the lifting (Or should that be depression I am never quite sure) of the tropopause which helped create the high will have shifted east.

Edited January 25, 2008 by BrickFielder

[chionomaniac]

Whilst awaiting for another update from GP or Brickfielder it appears that the record warming event in the Stratosphere appears to be reversing.

The forecast is for a rapid cool down

I guess we will have to wait to see if this warmth will filter down to the tropopause and disturb the polar vortex in the troposphere which could then cause cold to flow out of the polar region hopefully in our direction.

Looking at the wind anomaly at the top layers in the stratosphere it appears that there has indeed been a strong reversal of the wind direction and strength.

I wonder if this could therefore be classified as a major warming event.

The AO is still strongly positive but there are signs at the top of the stratosphere that things are changing

The AO forecast suggests that a reduction to less positive values is underway, however the full effects of the stratospheric warmings are just out of forecast range.

I guess it will be interesting to see how/whether the stratospheric warming will have a knock on effect on us in about two weeks but I for sure will have a keen eye out.

c

[BrickFielder]

The strong stratospheric vortex is being pushed back over the pole meaning the very strong but minor warming is over. It did just about reach down to affect the troposphere but we should not expect much from it.

Looking at the two forecasts for the top of the stratosphere it looks like the vortex will be pushed away from the pole again with temperatures rising there. Another strong wave is forecast so we shall see whether this slows the vortex enough for a major warming.

[BrickFielder]

I we be watching the stratosphere closely after Thursdays deep low as I would not be surprised to see a warming shortly after.