JH and a look into the period from T+144, usually

[johnholmes]

Update on Monday 18 February

By T+240 GFS and ECMWF have a rather different set of charts. The main 500mb trough is quite different and thus much of the ideas on surface features, pressue value and position are thus different. They do have the main Atlantic surface high in about the same place and intensity and also have a suggestion of a 500mb ridge over the ne of N America with a surface high underneath it. The GFS version causes the flow to veer much more north westerly behind the low they both take the low towards the UK, ECMWF some 10 degrees faster, so whether it is to be believed in its subsequent charts is hard to tell.

It has a pretty unrealistic looking very deep low way off the south west approaches by 1 March. Nevertheless it does seem to have what is probably the right idea in taking 500mb contour heights down with some suggestion of perhaps a cut off low towards the end of the run. This might allow the Azores high to link north with pressure rises in the Greenland area, although that are far from certain at this stage.

This does seem to be a bit at variance with what the 500mb anomaly chart from NOAA shows for the back end of the month but it is rather hard to see this very clearly.

Once again it’s a wait and see game.

[johnholmes]

Update on Tuesday 19 February

first a comment regarding the last para last evening;

the latest NOAA chart did fit with the possible set up, it was late arriving and I had only got the earlier one, the later one was similar to the Met O Fax chart with human intervention, the one I referred to was model generated only.

to this evening.

All 3 models show a good deal of agreement at T+144 with a deepish low in mid Atlantic under a broad 500mb trough ahead of what will be an extending 500mb trough developing in the east of N America.

By T+240 GFS and ECMWF are rather different although they both show a 500mb trough extending to the south of the UK with a surface low somewhere to the north west of the UK. Further west and east they are rather different.

NOAA progs for 500mb anomaly's show -ve areas extending further east across the Atlantic with the +ve anomaly being pushed almost off the chart to the east. This with a further =ve anomaly developing off the east of USA does lend support to a more Pm type of airmass slowly developing over the period, post T+168, admittedly not that clearly with GFS. The ensembles show quite wide divergence but do lend some support to the overall idea of a PM airstream by the end of the month but not a direct north west or northerly flow.

So interesting watching continues with no real definite sign. The info from GP was that a pattern change is likely end month-early March so we just wait and watch.

[PB22]

Thanks John, excellent reading as usual.

[johnholmes]

tks mate

sometimes its not far from what happens, just about to post about that in the model thread, re outlook from 8/9 Feb for this past weekend

[Greyowl]

Thanks John

Your updates are like waiting for the next instalment of a book.

[snowray]

Thanks John

Your updates are like waiting for the next instalment of a book.

Indeed, great stuff John, I await tonights installment with I feel could get even more interesting.

[johnholmes]

Update on Wednesday evening 21 February

The T+168 charts from GFS and ECMWF are not the same at sea level but the general 500mb pattern from eastern USA is fairly similar, which is the main item this far out.

They also both show the low coming out of the Newfoundland area at T+168. Obviously this is the same on the N American GFS issue. This is something that NOAA comment on this morning. They seem quite happy with the two models evolution of this with a marked 500mb trough over the east by T+168(Wednesday 27 February) and an upper ridge in the far west.

Their comment-‘FORECAST CONFIDENCE AT MID-LARGER SCALES STAY AT ABOVE NORMAL

LEVELS INTO NEXT MON-WED EVEN WITH A SIGNIFICANT TRANSITION TO A

POSITIVE PNA PATTERN WITH BUILDING OF A HIGHLY AMPLIFIED MID-UPPER

LEVEL NOAM WEST COAST RIDGE AND AN INCREASINGLY PHASED

EAST-CENTRAL NOAM TROF.

Their 500mb +/- chart for 28 February to 1 March is here

This obviously confirms their text comments and is what GFS is showing post T+168 at 12z. That is for an increasingly PM type air mass affecting the east of the Atlantic and western Europe along with the UK and a tendency also for the flow to veer with pressure rises over the eastern Atlantic and falls in the area north of the UK.

I’m new to this game but IF the two +ve areas west of N America become one then along with the extending trough in the east it would tend to develop a similar pattern over the Atlantic due to the length of the, what are called, Rossby waves. Interesting and something I’ll watch to see if my idea has any truth in it!

Just how this will pan out by end February/early March is unclear. Thanks to the state of La Nina it seems unlikely that, no matter how much GFS may try the prospect of anything more than a transitory ridge in the Greenland area seems small. What may happen, and has before this winter, is that the Azores high may ridge across the UK into Scandinavia, behind one of the short wave surface lows, with a block reforming in that area. Again whether it forms in an area conducive to deep cold air getting to the UK is open to a lot of doubt this far out.

What does seem increasingly likely is that much of the Uk will lie on the north side of the main Polar Front with sub zero 850mb temperatures, occasionally lower in the north and occasionally lower near frontal zones for the southern half.

Well worth watching if you are a cold/snow lover in my view, or if you just like watching to see how well the models appear to cope this far ahead. As usual when it gets to T+00, say a week Saturday 1 March, I’ll do a comparison of this check

[johnholmes]

Update Thursday evening

The two main models by T+168 have a fairly similar 500mb flow with ridge and main trough about the same in position and values. The surface high and the low coming out of eastern USA are also fairly similar. They are at odds with the low near Iceland so what effect that has on the subsequent runs I'm not sure.

It certainly ends up with them having rather different patterns by T+240. The 500mb flow being quite different and as a result so is the surface pattern in that area. ECMWF has a well extended trough around 10-15 east with a deepish low associated with it. GFS shows almost a cut off 500mb trough centred around the meridian and down to Spain.

Beyond that, bearing in mind this difference 7 days ahead then GFS seems to have a reasonable solution with a 500mb trough, for much of the time, until late in the period over/west of the UK and the upper ridge, which NOAA are confident about further west and a deep trough over eastern parts of the USA.

So the trend to a colder and more unsettled period is still there although there is no sign of any prolonged cold but that may be the model(GFS) having a problem at that time frame.

NOAA 500mb prog for +/- 500mb heights for today covering Feb 27 – Mar 03 is shown below.

It seems to show to me, at any rate, that the 500mb flow is likely to veer over that period with a 500mb ridge off/over north east N America with a subsequent downstream trough tending to dig further south and thus cause the flow to veer more north westerly. Just how far this will develop or how long, is not clear yet. Unless height rises start to show over Greenland I would suspect not for more than about 72 hours. As to what happens further down the line then that is pure conjecture for me, so I'll not even attempt it.

Whether there will be a polar northerly at the differing time frames shown by the two models is still some way off and I suspect they will slowly become one, which one though I don't know!

So to sum up, turning colder and more unsettled with the usual idea of wind/rain with snow for higher parts, some frost when to rear of any low.

Edited February 21, 2008 by johnholmes

[johnholmes]

Update for Friday evening:-22 February

It seems a reasonable start point to show the NOAA take on likely events out to the end of the month.

VALID 12Z TUE FEB 26 2008 - 12Z FRI FEB 29 2008

ALL MODELS AND ENSEMBLES ARE IN GOOD AGREEMENT ON THE TRANSITION

OF THE CURRENT SPLIT FLOW PATTERN INTO BUILDING EPAC HTS AND

RIDGING WITH CORRESPONDING DOWNSTREAM HT FALLS LEADING TO THE

DEVELOPMENT OF A POSITIVE PNA PATTERN OVER CONUS. THIS WILL BRING

A RETURN OF MORE SEASONAL NORMAL TEMPS TO WRN CONUS AND A PERIOD

OF WELL BELOW NORMAL TEMPS FROM THE PLAINS TO THE EAST COAST. BY

THE 8-10 DAY PERIOD MODELS AND ENSEMBLES BREAK DOWN THE PNA TO

MORE PROGRESSIVE SEMIZONAL PAC FLOW WITH THE POTENTIAL FOR HIGH

AMPLITUDE SYSTEMS COMING THRU THE FLOW AS CLIMATOLOGICAL SEASONAL

TRANSITION BEGINS.

This is borne out by their run of 500mb +/- charts, the latest below

This continues the idea of a veering upper flow over much of the Atlantic with a subsequent extension in the area over/just west of the UK of the 500mb trough, later extending further south east into the near continent.

If we look at the NW Extra jet stream charts for the world you can see that, for a time, the flow does buckle with a flow from west of north at times but not consistently.

Again this fits in with the NOAA idea but it also shows that the prospect of any prolonged arctic cold is, for the moment, not likely. This will be shown again a little further on in this evenings update, with different charts.

http://www.netweather.tv/secure/cgi-bin/pr...43a738a71480989

(you will need to be able access NW Extra to see this chart)

Turning to what I’ve shown at an earlier date.

GFS on 17/02 for 04/03, along with comments I made

GFS 12z today for the same time to see if its anything like, it will be a total surprise to me as I’ve not yet looked!

The answer is pretty clear, nope not a lot, thus in 5 days a ‘possible’ northerly has once again disappeared.

The chart below for 2 days previous is nearer to it but I am far from clear as to if it is the same lows shown near Newfoundland or over/east of the UK on both charts?

If it is one wonders if the attempted link between the Atlantic high and the attempt at rising pressure in the Greenland area will connect this time. On today’s’ 12z run it does not.

The comments from GP and to an extent those from NOAA do suggest that at some time in the early March period it SHOULD occur.

I have to say at the moment a true north or north west polar outbreak, over several days, with deep polar air is not occurring in the time frame out to T+384.

Summary:- in general terms

Over the past several days GFS has shifted from a high cell trying to form over the far north of the Atlantic(Greenland area), to one to the north east, or no major high cell north of the country and a cold mobile west occasionally north west flow behind depressions as they run across mostly north of the UK. Occasionally (here is where the burst of polar air comes from) across northernmost parts into the North Sea/Denmark/southern Norway areas. This seems to me still the most likely scenario. That does not mean that a brief deep polar outbreak will not occur, indeed even more than one. However to get a spell lasting several days of this type, such as 2005 and to a lesser extent 2006, by 4 March, seems some way from occurring at the moment.

[johnholmes]

Update Saturday 23 February, again for the 29-02-03-01 period, cantering on Saturday 1 March.

Just a brief update this evening. (for relevant charts commented on please read back over the last few days, or see the ouput for 12z today)

T+384 some time ago showed what GFS suggested for then=just prior to that it was showing height rises across the whole of the northern part of the chart. On Sunday 17/03 the chart for 12z Tuesday 04/03 showed much promise with a deep low to the east of the UK with signs of the Atlantic high merging with some of those northern pressure rises but with a low moving out from Newfoundland. The 12z chart for today typifies this problem with the low now shown running between Greenland and Iceland.

I’m no expert about La Nina but if the comments are correct about its effect on preventing height rises in the Greenland area then it would go a long way to explaining how GFS and ECMWF over the past few days have tried time and again to do just that without any apparent success.

It’s a couple of days since GP posted his observations of what is happening in the atmosphere above the Tropopause and its probable effects lower down. So far it seems to have been over ridden by the effects of La Nina. Nor is there any consistent sign of any real build of pressure from the Atlantic high into the ‘classic’ surface high over Scandinavia, the possible alternative.

Yes its going to get colder than it is now with a generally unsettled, at times quite windy PM flow, becoming at times a Tm flow as depressions run relatively quickly across the Atlantic. Behind some of them short bursts of quite deep and quite cold polar air but mainly 24 hour duration, although GFS is showing one from about T+288-336 currently. IF one of these did somehow allow the attempted height rises in the far north to connect behind a low with the high over the Atlantic then we MIGHT be into a different set up. The odds on this happening seem to me to be rather long at the moment.

The 500mb trough extension, created by events over North America is usually extending out into Europe somewhere rather than a deep extension occurring near the meridian. NOAA seem happy with the expected evolution over their area and this SHOULD help the flow to buckle in the longer term, say T+288 onwards but we shall just have to wait and see.

[johnholmes]

sorry no update this evening.

The new forecast I issued this evening, see here for Extra users

deleted as not sure if it should be avilable to all - Paul to answer perhaps?took rather longer than I expected, following on from a hard day in the garden, late dinner etc etc!

I'll try and do an update on data collected this evening and issue sometime tomorrow morning

Edited February 24, 2008 by johnholmes

[johnholmes]

here with the update from Sunday as promised

late update for Sunday done mid am Monday 24 February using Sunday 12z output

Starting with the NOAA forecast discussion on Sunday morning.

noaa summary sun am

VALID 12Z THU FEB 28 2008 - 12Z SUN MAR 02 2008

MODELS CONTINUE TO HAVE A SIMILAR HANDLE OF THE GENERAL FLOW

PATTERN...BUT DIFFER ON THE DETAILS. THE 00Z ECMWF HAS THE

SUPPORT TIMINGWISE OF THE NCEP AND ECMWF ENSEMBLE MEANS WITH

VARIOUS SYSTEMS...BUT IS ALONG THE NORTHERN PERIPHERY OF THE

GUIDANCE WITH A SYSTEM MOVING ALONG THE US/CANADIAN BORDER EARLY

NEXT WEEKEND. THE 00Z GFS IS AN OUTLIER WITH A COUPLE

SYSTEMS...ONE NEAR THE WESTERN CANADIAN COAST AROUND FRIDAY AND A

SECOND APPROACHING FROM THE PACIFIC NEXT WEEKEND...BUT HAS THE

SUPPORT OF THE ENSEMBLE MEANS WITH A CLOSED LOW NEAR NORTHWEST

MEXICO FRIDAY MORNING.

NOAA seem happy with the models output for their ‘neck of the woods’ out to 02/03

GFS shows a fairly mobile and breezy westerly with temporary north westerlies behind the lows but generally on the average to rather less than average temperatures for late Feb-early March. The NOAA extended 500mb +/- 500mb chart shows even more of a +ve anomaly mid Atlantic) suggesting even more of a north west flow so perhaps the ideas/comments from GP are still holding. I certainly have not the faintest idea at that range what the weather will be.

There are quite large differences between GFS and ECMWF at shorter time frames, taking the T+240;

GFS has a rather different take on the main 500mb flow, basically a touch north of west across the Atlantic into a deepish extension over Europe with a surface high over and south of the country and a deepish low near northern Iceland.

ECMWF has two surface highs, one concentric in shape over south west England and the other in the far west of the Atlantic, with an odd looking dip in the 500mb flow between the two. It also has an even deeper plunge of cold air over Europe with the low well north east of Iceland and less deep.

By T+316 it has another burst of polar air behind a deep low over southern Scandinavia with a 500mb ridge showing east of Newfoundland thus creating a more north west pattern to the 500mb flow.

By T+384 this has become a deepish extension of the 500mb trough to the south west of the UK with the ridge still evident off eastern America and a pair of lows at the surface from the 500mb trough up north over the country and across Scandinavia.

Again no sign of any major polar outbreak, no Scandinavian high and no real height rises to the north or north west.

The summary is no sign of any major polar outbreak from any source for anything more than a 24-36 hour period behind travelling low pressure systems.

[Glacier Point]

The more I follow longer range modelling, the more I appreciate the value the models place in tropical forcing.

At the moment, the MJO signal (as an indicator of global wind anomalies) has become indeterminant due to SSTAs in the Indian Ocean and lack of organised tropical convection.

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

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

There is a much more organsied body of convection around the Indo-Pacific region, but the general circulatory pattern is not responding to it just yet - maybe when convective waves develop more coherently towards MJO Phase 4.

This is likely to make the longer term outlooks liable to greater fluctuation and I have to say I'm pretty sceptical of the GFS right now into March week 1 / 2 when I'm expecting some pretty unusual synoptics to be thrown up.

As perhaps anticipated, the GFS (and ECM) was possibly too aggressive in the breakdown of the +AO modality, shown here by the AO Ensembles which still drop but in incremental stages rather than a massive plummet:

http://www.cpc.noaa.gov/products/precip/CW...ex/ao.sprd2.gif

The end of the ensemble run range must for me show signiifcant drop into -ve territory, hence my scepticism at some recent outputs suggesting a return to +AO. It may just be an issue of timing here.

On the plus (forecasting) side, if there is a trend emerging amoungst all the variability, it is for below average conditions to return from 27th Feb as suggested by the GFS Ensemble Mean for quite some time, with now the ECM (12z) joining in...

http://www.knmi.nl/exp/pluim/vijftiendaagse/index.html

[johnholmes]

thanks for that input GP, GFS and ECMWF do seem to want t edge the temperature levels dow and NOAA are also quite firm in their belief of the 500mb flow across the Atlantic showing a more north of west flow into the second week in March.

more model watching!

[johnholmes]

sorry this was not posted yesterday evening, but not back in until midnight

Update Monday evening 25 February

Well we are already into the period when, a week or so ago, we were looking at what appeared to be the first signs of the flow over the Atlantic at 500mb becoming more north west rather than westerly. I suppose it depends on your point of view but to me its not changed as much as the various models suggested it would. Granted the main change was predicted to be the end of February into early March and that can still happen. Indeed there are indications at times with GFS that it will happen. We also have the posts by GP which continue to point to events above the Tropopause working down into the lower atmosphere by then and also working to do that. Possibly overcoming what seems to be a grip on things doing that, so far, by La Nina.

NOAA on the one hand seem less sure about events over N America by early March due to conflicting and varying outputs from the major models. Their last 500mb +/- chart into this period still suggested a veer in the Atlantic flow with colder air gradually working further south and east.

The outputs from GFS and ECMWF in the time frame of T+240 show some similarities but not what I think shows close agreement.

By early March, say T+300 GFS has settled for a large and deep low between Iceland and Scotland with a veering 500mb flow to its rear as a result of events over America. It trundles this feature so that eventually by T+348 the UK is in a northerly flow to its rear, finally ending at T+384 with an attempt, behind this now filling low for a link from the Atlantic high to something well to our north east.

Will it happen, will the flow become northerly with pressure rising somewhere to our north?

I'm no ??? so I have to say I honestly don't know. It is still in the balance for a deep cold plunge and one to last for several days. I don't think it's a nil chance though, less than 50:50 in my view for more than a 48 hour spell but we shall see.

Edited February 26, 2008 by johnholmes

[johnholmes]

Update on Tuesday evening 25 February

NOAA suggest in their am synoptic review for Saturday-Tuesday (01-04 March) that a large 500mb trough will settle to cover much of North America. This will obviously have repercussions down the line for our area assuming its correct. To me it looks very likely that it is a good summation of the main Rossby waves upstream into the Pacific and onwards to Western Europe.

We seem likely to get a short spell of cold arctic air between Sunday and Tuesday before this major development begins to impact on the area of the UK/eastern Atlantic and Western Europe.

By about T+216 this change is quite evident as the main 500mb trough/ridge set up develops with a large 500mb trough becoming slow moving around 20 west. Initially it will extend south east to be into the Iberian Peninsula but will then retrogress to finish by about T+324 at around 40 west. Once there, with the pattern between it and the next major upstream trough (as suggested by NOAA) to be around 90 west or so, thus 60 degrees spread. This is an accepted slow moving wave pattern in Rossby theory and has been shown to last several days with only minor changes in the main positions.

Assuming this is correct then the pattern change is under way and likely to settle as suggested above. Further it may well last for between 3 and 7 days or more.

This leaves the UK in the eastern side of this deep trough with minor surface disturbances likely to be created by the very large temperature differences over relatively short distances both near the surface and aloft. These will run quickly across the country from wsw to ene giving short spells of TM air before cold PM air floods back.

ECMWF and GFS by T+240 are not that similar especially at msl.

NOAA comments on various issues, NAO, AO etc suggest that there is not much sign of the needed blocking occurring and they also believe La Nina will, ‘remain strong or moderate through March and into Spring’.

So my summation is:-

a touch more wnw flow to the 500mb flow, and as above the major change is on the way but that the deep cold air will be around 30-40 west at 50 north not over the UK in the time scale of end of February out to about the end of the first week of March. No idea beyond that to be honest.

By all means disagree with the above, knock holes in it, I'm still learning about trying to predict this far ahead. But please no dismissive one liners, your reasons for not agreeing may help us all to understand trying to predict with some accuracy at this range.

I'll do a check on Saturday 1 March to see how close the initial guidance was.

[J10]

Certainly looks as if there will be a period of Polar Maritime air for next week. I think it is better to have prospects of cold which of course may be downgraded then endless zonality.

There is at least a certain amount of synoptic agreement at the models at T+96 and T+120, if GFS were to verify then a chance of some wintry conditions for at least some parts of the UK, however looking quite decent for some snowfall for both the Scottish Mountains and the Alps.

I have attached the these model comparison as this may well show if that match up well to what happens. In particular I will be watching out to see if the High pressure manages to push as far north as suggested and West as suggested, at least temporarily allowing the Northerly winds to take hold, or if the high ends up further East blocking off any Northerly incursions as has often happened so far this winter.

[damianslaw]

Increasing confidence that much of scotland and hilly regions of northern england will see some snow cover come next Monday, the heaviest and frequent bouts of snow will come courtesy of any troughs that may embed themselves in the polar flow.

Where such troughs occur it will be down to the intensity of the precipitation to give any real chance of low level snow cover from cumbria/north pennines southwards.

Lapse rates will be high so certainly places above 200m for the northern half of the country look like having a fairly high chance of seeing snow cover.

Every chance at this stage that the colder spell of weather for sunday-tuesday will be upgraded and prolonged.

[johnholmes]

Update on Wednesday evening 27 February

First my apologies for this being even more rambling and indecisive than normally! But the situation is pretty complex and leaves me rather baffled as to what may happen.

Looking at the 12z from all 3 major models and they are pretty similar both at 500mb(two troughs one ridge-all about the same place and depth) and the main surface features, values and positions.

At T+240 they are not that similar with the main 500mb trough to the south west of the Uk with GFS and over and south south east with ECMWF. The main low is similar, both have it over south west Eire and about the same value. A suggestion of a 500mb ridge upstream but the surface low running behind it is rather different in position and value on them both.

Taking GFS out over the next 100 hours plus and it suggests the low off Newfoundland coming east and settling as it starts to fill somewhere west to south west of Iceland as another low comes out behind it. The 500mb trough flattens out to the west of the UK and then another reforms east of America.

So no sign out to 14 March of any build to the north west, north or north east. No sign on this 12z run of any deep cold at all, a suggestion of more TM air for southern areas at times as the flow remains west to south west.

Looking at other sources for the same time scale.

NOAA today out to 05/03

TELECONNECTIONS WITH A NEGATIVE ANOMALY OVER NORTH-CENTRAL CANADA

AND A POSITIVE ANOMALY BOUNCING AROUND 40N 130W FAVOR LARGE-SCALE

TROUGHING ACROSS THE PLAINS/MIDWEST AND RIDGING IN THE WESTERN

ATLANTIC.

This has subsequently had added;

ECMWF and GFS develop the ridge in the central Atlantic area from about D 7 onwards. This is borne out by the longer term 500mb anomaly chart issued by NOAA today

If it happens it would lend support to the idea that the 500mb flow, in spite of GFS not really showing it on the 12z as having any major showing could still occur. Perhaps it has not yet locked on to the change being predicted by other methods. The link between what is happening in the Stratosphere and its effect extending the Troposphere is another factor.

Once again, I’m left with not really any definite feeling of what is going to happen in what is termed FI land, certainly not beyond T+240.

It may not be a very scientific comment but I’m left with the feeling that GFS is as much in the dark as we all are. The changes predicted outside the direct remit of the GFS model I don’t think are able to be modelled very well into the basic T+00 to T+384 set up. I suspect that the overall suggestion that I’ve been peddling for over a week now that the 500mb will veer more north of west with decidedly colder air affecting more of the Uk is probably going to occur. There could be a cold spell, (by Met Office definition=some 3-4 degrees below the expected average) occurring for anything from 2-6 days say, sometime during the first half of March.

[johnholmes]

Update on Thursday 28 February

ECMWF and GFS 8-10 day mean 500mb charts for the northern hemisphere, both have 4 main troughs and a secondary trough in the Atlantic. What is crucial to any veered flow for the area over and west of the UK is the actual positions. The GFS is the one more likely to do this. Which is right, who knows! Both are about as likely to be correct.

NOAA are happy enough with the idea of two ridges over each of their coastal areas, one Pacific one Atlantic. So the basics seem okay its just the exact potion and orientation of them that will decide what the weather is over the UK. One will give a rather cold west to west north west flow the other will give a colder north west flow. The second one would also give the greater chance of a deep cold northerly developing and for it to persist for several days before the upper trough either relaxed or moved further east. Again who knows which will occur?

GFS and ECMWF 12z runs today beyond T+168 are rather like chalk and cheese.

The GFS, as you would probably expect, is much like the NOAA idea 05-09 March with a large +ve 500mb anomaly showing up in the Greenland/Iceland area. Since I’ve been tracking this data it’s the first time I’ve seen that, copied below, it also almost has the anomaly linked to the Atlantic high. Not shown here but its next period 7-13 March has it even more pronounced.

Summary

It will turn colder, in stages, from this weekend, and probably stay with temperatures for the whole country in the range below normal to rather cold. A possibility of cold for some. Look at the Met O description, cold means 4-5C below what would normally be expected at this time of year. Their web site will show you what you local area is.

We will have another look tomorrow evening.

[J10]

So in this comparison

(2 Mar)

A clear distinction between the GFS and ENS Models which go for pressure rises over Greenland to the ECM/UKMO models which is having nothing of it. However a period of PM air across the UK seems certain looknig at these models.

(3 Mar)

This feeds in later down the line with the GFS going for a better cold spell than either UKMO and in particular the ECM.

goes for stronger High pressure over Greenland than the ECM

( 4 Mar)

Again the GFS going for the strongest block over Greenland, with no real high Pressure at all forecast by the ECM.

After this stage a major disagreement with the GFS reloading the Northerly as the LP pushes SE, but the ECM allows winds to come in from the west.

[johnholmes]

Update on Friday evening 29 February

A longer and more detailed look this evening, especially at how good or bad the guidance was when I did the first post on this on Sunday 17 February

GFS comments

The general trend it predicted, that is of a change to a more north of west Polar Maritime flow has occurred(about to occur I should say to be correct) at the end of Feb-early March. Remember this was done 12 days ago.

That said there is no general indication of a spell (say 4-7 days) of polar air affecting the UK on any of the GFS charts. That is apart from the 12z 'The Ice Age cometh' from yesterday which had polar air from one source or another over the country right out to T+384.

ECM comments

Through the period I've been doing updates, yes it supports the trend, but without the emphasis that GFS has shown. Hence my e mail to NOAA to see if its charts are used in the 500mb assessments for the anomalies.

Other

A post on the same evening from GP said this

'So putting this together, I think we are still progressing along the lines as set out in the February thoughts thread. The presence of an Alaskan ridge is consistent across operational GFS and ECM modelling, which may lead to a split vortex and height rises over Greenland as the AO tanks -ve into March.

Weather-wise, perhaps more of a signal for high pressure to persist into the next 2-3 weeks with only partial Atlantic breakdowns. Winter may not have done with us just yet, for all its disappointments, into the 1st week March with emphasis on a northerly or NE'ly type wind during this time.

the link to this is here

http://www.netweather.tv/forum/index.php?s...4416&st=119

page 8 and post 124

NOAA

The latest two 500mb anomaly charts take us out to mid March. They continue to show the 500mb flow should be north of west for here, possibly as far round as north west later on in that period.

I have sent an e mail to them asking what model(s) they use to arrive at these charts. Is it a mix of the models/ensembles or is it solely GFS. Hopefully they will reply and then I know how much weighting to give to these products.

All along they have been consistent in suggesting anomalies which would give a west north west, possibly even a north west, 500mb flow over the near Atlantic and UK.

They believe that La Nina will remain strong or at least moderate through March.

Their charts, in spite of the 500mb anomaly charts, do not really suggest any major northern blocking.

Reading their technical discussion for their own area (USA) they weight their confidence as 3 out of 5 for days out to 10 and 4 out of 5 out to day 14. This may at first seem odd to us but even if the upper high over Greenland is not there their weather will be largely governed by their confidence in the predicted flow from the eastern Pacific through North America.

There are two possible variations out to mid March in my view.

1) The upper ridge remains over Greenland or the Atlantic surface high is sufficiently far north to give a cold or rather cold flow north of west.

2) If the above does not hold(and the recent post from GP, link here,

http://www.netweather.tv/forum/index.php?a...sult_type=posts

suggests that it will, but if it does not then a milder westerly or even south west flow would be in place by mid month.

That would be after a mostly below average to cold spell for the 7 days or so from about Monday of next week.

Cold using Met O terminology is 4 to 5C below the expected temperatures for early March

Overall I would sum up the guidance on Sunday 17 February as good. (always assuming the talked of colder weather does arrive as most centres are predicting!)

Edited February 29, 2008 by johnholmes

[Glacier Point]

Models seem to be converging on the idea of an anticylone centred in the mid Atlantic and cut off low- the next pattern to be alert to and very much Nina driven.

http://www.meteo.psu.edu/~gadomski/ECMWF_12z/hgtcomp.html

I'm still very sceptical of current week 2 projections for March. There is no way in my book that this is a 'non propagator':

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

with temperature anomalies extending downwards to the 200 hPa layer already. Add another two weeks onto this and we are very likely to see blocking at high latitudes.

Mid Atlantic ridge + fragmented polar vortex = Greenland High. I wish it were that simple, but this is where I'm looking week 2, for potential at least.

Edited February 29, 2008 by Glacier Point

[bluearmy]

Hence my e mail to NOAA to see if its charts are used in the 500mb assessments for the anomalies.

John, the prognostic discussion is clear as to what model input has gone into the chart - here is today's 6-10 day:

TODAYS OFFICIAL 500-HPA BLEND CONSISTS OF 15 PERCENT OF TODAY'S OPERATIONAL ECMWF CENTERED ON DAY 7...15 PERCENT OF YESTERDAY'S OPERATIONAL ECMWF CENTERED ON DAY 6...20 PERCENT OF TODAY'S 0Z GFS ENSEMBLE MEAN CENTERED ON DAY 8...20 PERCENT OF TODAY'S CANADIAN ENSEMBLE MEAN CENTERED ON DAY 8...AND 30 PERCENT OF TODAY'S EUROPEAN ENSEMBLE MEAN CENTERED ON DAY 7. MODEL(S) OF THE DAY: 12Z ECMWF ENSEMBLE MEAN. FORECAST CONFIDENCE FOR THE 6-10 DAY PERIOD IS BELOW AVERAGE... 2 ON A SCALE OF 1 TO 5 DUE TO UNCERTAINTY REGARDING THE EVOLUTION OF A CUT-OFF LOW IN THE SOUTHWEST CONUS AND POOR AGREEMENT AMONG THE TEMPERATURE TOOLS.

the ecm is not used in the 8-14 day chart.

[johnholmes]

tks GP and BA

I'm exchanging e mails at the moment with them