Feb thoughts...

[Glacier Point]

January has been dominated by a rampantly positive +AO pattern, driven on by a strongly cool polar stratosphere. La Nina has strengthend further, and tropical convection has helped the inter-monthly variability. Will February follow the same pattern, or will the old adage - what the weather does in December, the atmosphere will remember - remain true ?

Surface temperatures across the eastern - central and western equatorial Pacific remain well below normal. These cooler waters extend to some depth and easterly trade winds remain in place. This is very likely to persist La Nina conditions across the Pacific with the event now taking on very strong magnitude. The latest Australian Gov. data has yet to be updated at the time of writing but CPC data suggest a move towards -2 SD anomaly for ENSO Region 3.4 indicative of a very strong La Nina.

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

http://iri.columbia.edu/climate/ENSO/curre.../technical.html

Analogues for strong La Ninas in February suggest the following...

.... a deeply +AO type pattern with strong polar vortex and high pressure to our south - no change from January there then.

What's interesting about this Nina is that it has also seen strong tropical convection in gthe western Pacific and Indian Ocean which has helped force a strong inter-seasonal variation. For the UK and western Europe, this has meant an alternating anticyclonic-cyclonic type pattern.

Shifts in the pattern can be traced back to the zonal wind anomaly moving up and down in three distinct peaks and troughs. This is associated with anomalously strong bursts of westerly winds from the Indo-Pacific propagating poleward and being enhanced by the Nina pattern.

The current wind anomaly has peaked and will start to head back down as a result of a normalising of outgoing long wave radiation in the tropics....

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

.... and removal of westerly wind anomalies through mountain torque, an event picked up recently which increases the likelihood of stratospheric warming in the next 14-21 days.....

Perhaps of most interest here is the negative (easterly) wind anomaly extending outwards from the tropics (shaded blue). This is likely to result in strong mid latitude highs developing week 2/3.

Current SSTA plots suggest that rates of OLR will not be as great as previously in January - after all, convection tends to reduce sea surface temperature. The exception here is the region between the Philipines and Japan in the western Pacific (MJO phase 5/6) which is likely to pep up convection rates when the next MJO wave reaches the area - likely mid month. This will stir up +ve zonal wind anomalies once more leading to another zonal phase week 3-4.

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

Mentioning the MJO, the variability of the winter pattern is explained exceptionally well by the combination of an active MJO and strong La Nina. Analogues for these have a strong resemblance with the reanalysis plots for each phase of the MJO. If the MJO is to be active once again, the MJO will become a highly useful forecasting tool.

This time round, the MJO is indeterminant around phase 2/3, probably a result of the neutral SSTA in the Indian Ocean. However, I do expect it to move on and become stronger as it approaches phase 4/5 (remember those warm SSTAs and confluence of the easterly flow off La Nina pattern). This gives us an MJO orbiting through phase 4-5-6-7-8 for February with most influence likely to come from phases 4-5-6.

Phases 4, 5 and 6 look like this (analogue and reanalysis for this year so far)......

As you can see, lots of signals for high pressure there around the eastern Atlantic / NW Europe and Atlantic trough.

The one wildcard here is the AO.

The recent stratospheric warming has, up to now, not shown signs of moving down through the atmosphere. Current forecasts suggest a second warming event which ties in with my thinking wrt mountain torque events. This leaves us with the warming of the very upper layer but 30hPa layer that refuses to warm as of now.

30mb zonal winds remain quite strong....

so the +AO pattern and well defined polar vortex will be difficult to break down but I suspect there will be a gradual shift over time with warmer anomalies permeating downwards but only slowly with more neutral AO conditions from week 3 onwards. With pressure on the polar vortex coming from waming upper levels, we could well see some extraordinary behaviour of the polar vortex during the month.

Current long range forecast tools give high confidence to week 2 position of high pressure building at mid latitudes:

Putting this all together:

• zonal week 1
  
• tendency for mid latitude week 2 - 3 with some potential for retrogression of the high further NW allowing a brief NE'ly regime
  
• zonal wind increase but short lived week 3
  
• possibility for arctic high and colder mobile pattern week 4 if the AO shows signs of going -ve or strongly milder zonal pattern if not - great uncertainty here
  
• overall pattern something of a blend between December and January's pattern with greater emphasis on the simlarities with December perhaps given the balance of high pressure.
  

Edited January 31, 2008 by Glacier Point

[mezzacyclone]

tendency for mid latitude week 2 - 3 with some potential for retrogression of the high further NW allowing a brief NE'ly regime

As per the strong signals for 12th/13th Feb here GP?

[John Cox]

Thanks GP,

I have read in detail your predictions for Feb,

Basically, if I read you correctly, it may ge a good winter month after week 1 with HP dominating with a possibility of cold incusions from north.

What I like about your posts GP, is how difficult to predict the forecast for Feb, but you give the possible technical options.

Thank you!!!

Edited January 31, 2008 by John Cox

[BLAST FROM THE PAST]

Thanks GP

As ever in depth and informative. My thoughts of Feb have similar make up to yours BUT as I mentioned in Feb CET thread i think week 3 could bring very mild if not warm conditions to us with HP being dominant in a 'warm' position. Indeed it looks like a kick in the teeth again but cruelly early signals are in my method that March will be decidedly cold in comparison to the norm and a late winter again?

BFTP

[Glacier Point]

Ooohh, are we about to be heading for a sustained -NAO block ?

So far, the medium and long range forecasts have given solid backing to a mid latitude high centred E-Atlantic / UK at mid month. This is consistent with the poleward propagation of easterly zonal wind anomalies and movement of the MJO from phase 4 towards a stronger pulse in phases 5/6.

What happens next will be crucial in determining whether we head towards a flattening out of the pattern with the high forced eastwards and south, or whether we see true undercutting reinforcing a -NAO pattern for the last 10 days of the month.

The analysis for the later end of the month is premised on tropical convection increasing in the western Pacific lifting up zonal wind anomalies and corresponding with the movement of the MJO through phases 6-8. This year, phases 7 and 8 have teleconnected to falling heights in the Atlantic:

We are not approaching this yet given latest extended range outputs which still (correctly IMO) deal with mid latitde high pressure. However, this is the likely next move 'after the next move' and it is this evolution which brings us to the point where the block will get undercut or shoved eastwards.

Key to this will be the state of the polar vortex and the AO. Current projections are suggesting a splitting of the polar vortex allowing a +ve height anomaly over the Pole consistent with a neutral / -AO:

This -AO is absolutely vital. Without it, the polar vortex will reorganise and drive the block back east as the polar jet is cranked up by a tightening thermal gradient. With a -AO, the thermal gradient across the mid latitudes is decreased with cold air being replaced by warmer air at the pole.

It's still a long way off, so keep an eye on how the AO is projected over the next few days to give a big clue on the movement of the high in the longer term. Recent form suggests no -AO although not such as clear cut as any comparable period over the last 30-40 days.

[Glacier Point]

As anticipated, the third MJO of the winter has moved into phase 5 at medium amplitude. This, allied to strong easterly wind zonal anomalies at mid latitudes, has lead to the establishment of a strong blocking anticyclone over the UK. Phases 5 and 6 MJO have an influence for high pressure over the NE Atlantic sector. The movement of the wave is, based on previous form of the first two MJO events and current tropical convection, likely to orbit through phases 7-8 before the month's end although the timing and amplitude are at this stage uncertain.

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

Phases 7-8 teleconnect with a breakdown of the blocking anticyclone with phase 8 noted for a signficant reversal in the PNA with a ridge extending into the Arctic from the NW Pacific. Long range models are starting to pick up on this development.

Against this, westerly zonal anomalies are being removed from the northern hemisphere due to the mature La Nina, low level negative outgoing radiation anomalies to the north of the equator (strong in the southern hemisphere) and sinking easterly zonal wind anomalies at mid latitudes. All of this translates to falling zonal wind anomalies and greater tendency towards high pressure at mid latitudes.

http://www.cdc.noaa.gov/map/images/aam/glaam.gif

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

The recent (1st) stratospheric warming event, albeit brief, has had an infleunce on the tropospheric vortex with a recent split developing over the pole and shift of the main body towards Asia. The vortex is forecast to reform towards Greenland as the projected ridge over Alaska and the Pacific NW builds which links nicely the tropical and polar sides of the equation.

What is worth noting is how the AO has responded to warming events. The first warming event, and its associated height rises in the stratosphere, have been associated with a surging +AO. This is forecast to happen again with the GFS Ensembles suggesting the second warming event to be preceded by a surging +AO (also consistent with a reforming polar vortex). What is likely to follow however is a drop in the AO towards the end of Feb and first week of March coinciding with the MJO in phase 8.

http://www.cpc.ncep.noaa.gov/products/prec...hgt.ao.cdas.gif

http://www.cpc.ncep.noaa.gov/products/prec...ex/ao.sprd2.gif

This second stratsopheric event (or maybe 2nd and 3rd) is much more likely to impact on the general pressure pattern. Notice how the recent warming is forecast to drive up 30hPa temperatures much more than the first event:

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

http://www.cpc.ncep.noaa.gov/products/stra...JFM_NH_2008.gif

Putting this together, it seems likely that pressure will remain high for the next 7-10 days, followed by a brief Atlantic pattern as the polar vortex organises and becomes shifted towards Greenland and Scandinavia and a strong ridge develops over Alaska. Thereafter, the door is left open for a the ridge over Alaska to extend into the Arctic towards Greenland resulting in a much colder pattern last day or so in February into March. The main synoptic feature to be alert for in the extended range is the initial development of the ridge across the NW Pacific coast.

Edited February 12, 2008 by Glacier Point

[damianslaw]

The synoptical evolution you are projecting suggest a northerly at the tail end of the month - this is a pressure pattern that has been a regular feature of recent years, at this stage i think there is a good chance of it coming off again this year, any return of the atlantic i see as being rather short and i think we will see mid atlantic ridging before the month is out

[BLAST FROM THE PAST]

An excellent post again GP. I agree and the forecast set up is as I have been posting over the last 2 weeks that this warmth or early Spring is a false dawn. 'For the time of year' some impressive cold may strike the UK as we enter March but moreso as I've also hinted at is the chance of a sustained cold spell. The pattern that has been in place with widespread cold affecting other parts of the world barr W Europe is going to change and allow a cold early spring to develop. No armageddon, but decently below average. Type of set up? I think like GP says a spell of mobility, then a northerly as the PV moves east but then more importantly true Height rises to our N [Greenland Favoured] and winds predominantly from the E and a sinking jet. My CET in March will be a fair bit below average.

BFTP

Edited February 13, 2008 by BLAST FROM THE PAST

[kold weather]

Problem I have is that a strong Greenland high in strong La Ninas are about as rare as a 2C below average month nowdays...

I had a fear earlier in the winter that we would get a strong la nina we'd be in trouble. As it happens December was about what you would normally expect in a strong la nina, high pressure to the east with a CEt close to average. However Jan came in a good deal above due to high SSt's to the west. Feb at the moment looks being close to the S.La Nina as well.

I've got to be perfecty honest since 1950 in winters that have been classed as strong I don't know one that had a sustained -ve NAO signal, thats not to say we can't get northerlies as we saw in the first few days of Feb but note how fast it toppled...same with the high pressure to our north right now. Unless the Arctic dips very negative and the whole jet becomes completely amplified like back in December then a true greenland high lasting more then 48hrs won't be easy!

However what I would watch for is a Scandinavian high. If the jet does ease off and he AO dips I'd watch there for a high pressre to build much like December if it does then we may set up for a below average March, like March 1976.

However saying all that there is enough variation to suggest that other factors may overwhelm the LN factor if strong enough.

Edited February 14, 2008 by kold weather

[Glacier Point]

Some fundamental changes in the polar stratosphere now taking place which will impact on the overall pattern into late Feb / 1st week March.

30hPa as well as 10hPa layers showing substantial warming (30hPa layer didn't warm during the 1st episode):

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

30hPa heat fluxes showing upward tendency likely to signal further warming down the line:

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

Note also the falling zonal wind anomalies at 10 and 30 hPa - forecast to reach the lowest levels since November and entering -ve territory. The zonal mean sections showing up a big negative wind anomaly reaching down to about 100 hPa:

http://strat-www.met.fu-berlin.de/cgi-bin/...=f192&var=u

Wave 1 temperature shows the core of the warming and also the extent:

http://strat-www.met.fu-berlin.de/cgi-bin/...192&var=ta1

... and height rises so far...

http://strat-www.met.fu-berlin.de/cgi-bin/...192&var=ha1

We should start to see these positive height and temperatures start to show up in polar cross sections in the stratosphere in due course, but initial reaction from the atmosphere looks to be happening with the AO surging upwards as a precursor response to the warming.

http://www.cpc.ncep.noaa.gov/products/prec...ex/ao.sprd2.gif

As the warming works its way down through the tropopause, the AO will drop sharply downwards, particularly given the -ve zonal winds developing in the stratosphere.

Edited February 14, 2008 by Glacier Point

[BrickFielder]

I am not a great fan of stratospheric warmings causing blocking about one month in advance. To me the time it takes for a warming to reach down to the troposphere depends on the strength of the stratospheric vortex, the strength of the warming and the direction of the QBO. The following diagram shows my current thoughts on when a blocked pattern may occur. I tend to think of the stratospheric vortex as like a spinning whirlwind which can wobble off course and even split in two at times.

Like GP I am monitoring the wave activity to see when the next warming will take place and I think the following chart shows how that one is likely as wave 1 picks up.

Stratospheric vorticity diagrams can give strong hints about where high and low pressure will be. If we compare the current stratospheric vorticity chart with a northern hemisphere 500hPa heights chart we can see some notable similarities (see below). We can also see some notable differences like the high pressure towards siberia.

One thing that happens every year is that the stratospheric vortex migrates from the pole towards siberia and russia during late winter into spring.

The result is a strongly stratospheric driven weather pattern across Europe , Russia and Asia with a more SST driven pattern across the Pacific and US. This to me means without a significant stratospheric warming la nina weather patterns are likely to resume. This will give a deep trough over the central US which will speed up the jetstream over the north Atlantic. Without height rises over greenland this will most likely just open up the storm track towards Iceland although storms may well have to navigate round a stronger mid atlantic high.

Perhaps a chance of a northerly as a long wave trough sets up into Europe, but beyond that we should expect blocking patterns to develop as the stratospheric vortex weakens and starts to be affected by warming down to lower levels. Most noticeable this year is how strong the stratospheric vortex has been and it may be late breaking up as it was last year perhaps a sign for the season ahead.

IRI Seasonal forecasts ahead

[BLAST FROM THE PAST]

Again thanks BF and interesting read. Only thing I'll pick up on is I take the IRI seasonal with massive reservation...I personally rate them pretty low. Your imput though is of good benefit

BFTP

[BrickFielder]

A quick look at the CFS charts gives a very variable outlook for this year.

March looks cool with a mid atlantic high and a trough into the continent.

April sees a reversal with a bit of a spanish plume look with the prospect of warm perhaps thundery air up from the south.

May looks a bit north south split or possibly bartlett like.

Deep lows crossing the south of the UK. wet and windy comes to mind.

July and we see more typical atlantic weather.

Further out August looked warm while september looks stormy. What interests me is not any accuracy but the variability from month to month.

[Glacier Point]

With the month nearly at an end, time to wrap this one up and look at the initial forecast verification.

February 2008 will be characterised by a blocked upper air pattern over the eastern Atlantic and NW Europe:

This is similar to strong La Nina composites although this is not for me a comprehensive explantion for the overall pattern.

Note the differences in the two patterns, particularly the more meridional flows in 2008 and cut off lows to our SW compared to the flat Nina analogues.

By far the best explanation for me is a combination of MJO variability within the context of a strong Nina and, crucially, +AO regime.

MJO wave #3 passed through phases 2-8, nearly completing a full orbit in the month. The strongest phases were 2 through 5 and analogues this winter for the previous phases 2-5 go a long way to explaining the dry conditions with a characteristic mid latitude block.

Phase 2:

Phase 3:

Phase 4:

Phase 5:

The original forecast majored on this key driver (although the MJO wave amplitude was slightly off for the early and later stages) and was therefore rewarded with a pleasing degree of accuracy.

At the end of the month, we are seeing some indications of disturbances to the polar vortex although not the anticipated drop in the AO just yet, although latest ensembles suggest a fall off in the next few days....

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

With another MJO wave possible, the predictive value of this tool will be employed once again in March, although there is the added complication of the polar vortex and stratospheric jet being interrupted by a whole succession of stratospheric warmings, with the potential for more warmings to occur with strong turbulence and Rossby Wave dispersal across east Asia having take place in the last 7-10 days.