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

[johnholmes]

Update on Tuesday 25 th March 2008, finally issued on Wednesday morning

For the first time in the relatively short time I've been doing this there a lot of 'scatter' in the advice. Much of it seems to be accumulating to suggest that the earlier predicted extension, for most of the period, of cold rather than mild, is now turning the other way. There are still indications of it staying on the colder side. So a difficult decision at the end, but that is often the case in forecasting whatever the time scale. Weigh up the evidence carefully and objectively and then decide what the forecast will be.

In terms of the models, just one of the tools used, GFS and ECMWF, both have shown much more oscillation and changes over the past few days than is usual. NOAA were driven, twice, over the past 2 days to make unusual comments as they battled to try and get a consensus from the major models.

Comments like' flip flop', and 'the models have not disappointed concerning the limited predictability part of the argument'. This just for their own area let alone the rest of the northern hemisphere!

Their ensemble 500mb charts for the period 24/03-04/04 were described thus 'Days 8-14. For this forecast period, the ensemble spread is quite high, with a mean forecast of mostly zonal flow across the CONUS.'

The comment for the issue today for the period 28/03-8/04 was, 'A large spread among the ensemble members continues.', again that was really commenting on what they are showing for the N American area. As you can see below this applies all the way across the Atlantic, especially near the more eastern end of the chart, our area of main interest.

Regarding La Nina it is seen to be 'moderate across the tropical Pacific Ocean and they say 'La Nina will continue through the northern hemisphere spring'

On the MJO they say the mjo expected to weaken and that seems to suggest that the flow will become more zonal in early April.

AO conditions are expected to show a fall by early April (the suggested fall is like some before and I suspect it will not be as large as the chart suggests).

The NAO shows some fall then a split with either a smallish rise or fall.

The last post from GP gave this (remember his links will not show what he was trying to demonstrate, they will update each time a new run comes out)

By and large (allowing for the inevitable inter-run variability), the operational GFS continues to advertise a theme I'm keen on backing.

Tropical convection in the Indo-Pacific has resulted in disturbances to the hemispheric flow pattern which is very likely to result in an eddy in the jet stream which in turn is likely to result in height rises over Scandinavia. We can see the impact of this Rossby Wave Dispersal by the setting up of a trough in the western-central Atlantic:

http://www.wetterzentrale.de/pics/Rtavn1441.html

This should not be seen as the precursor to a warm pattern change IMO. Much more likely, we get a rise in pressure over France then the UK then Scandinavia as the wave disperses and sets up a sub-Arctic ridge.

Tropical convection remains very strong in the Indian Ocean SW of India.

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

This has continued for some time now and is likely to lead to a large downstream anticyclone in the western Pacific sending further disturbances around the NH, key message for the UK probably being a pulling back west of the sub-Arctic ridge towards Iceland and southern Greenland. Also advertised by GFS 12z:

http://www.wetterzentrale.de/pics/Rtavn3601.html

So what of the two major models from T+240 to way out T+384 with GFS? AS some may know I've just started doing a long check on the output each day by GFS, when possible, logging each run and looking for the first signs of any solid idea of the long term. The first check I am doing has 12z 31/03/08 as the check time/date. Its taken it almost to day 11 before any really recognisable signal started to show. The pattern you see below

This is now only 168 hours away. About 3 days prior to that it did seem to suggest that high pressure would become anchored near/over the UK but has changed that position several times since. Obviously its position is fairly crucial to the temperature being above or below normal for much of the country. Currently its showing that the 500mb trough will move east allowing less mild air, after a milder, if fairly windy for some, weekend, to move in. The ECMWF now shows a similar evolution and indeed out to T+240(Friday 04/04/08) they now both have gone for a block somewhere north of the country(not identical by any means) but keeping a colder or at least less mild type of weather over the UK.

So that leaves a fair amount of spread from between a rather zonal, albeit not especially mild type of Atlantic flow, or a more blocked pattern with surface high pressure north of 50N?

Trying to weigh everything up is the hardest so far in this short spell of trying to give guidance on the most likely weather pattern for the UK for 10-20 days or so in advance.

In the relatively short term, say 5-7 days (weekend into Wednesday), the weather pattern seems fairly well set. A disturbed spell with the northern portion of the country still not really out of the current cold air whilst the south does have temporary visits into more Tropical Maritime type air, albeit more of a modified Polar type; windy for some with the lows crossing the UK at times. By the end of the weekend and into next week then a return to a more Polar Maritime also seems the most likely option followed by a build of pressure to the north west of the country as a surface high develops to give a coldish flow from the north east. Into the second week of April and the high will probably be eroded from the north due to the upper flow creating low pressure way north thence across the north of Iceland and into Scandinavia. Beyond that and it is quite possible that the surface high will rebuild north of 50N and to our west with mostly Polar Maritime type air over the country for much of the time.

Like I've just commented this is the most difficult one I've had to call and if I gave it % marks it would be 60 to 40 in favour of cold over a less cold and more mobile flow.

I'll monitor it as usual.

Edited March 26, 2008 by johnholmes

[johnholmes]

update will be done on Saturday

[BLAST FROM THE PAST]

update will be done on Saturday

Thanks John. ECM and GFS still favour the northern blocking of type theme.

BFTP

[johnholmes]

it looks that way overall but once again some of the evidence suggests it may not be rock solid and way out in FI there seem to be hints it might not last all that long. Anyway I'll ponder over the evidence this pm and 'jump' one way or the other, probably still for cold rather than mild

[johnholmes]

Update on Saturday 29 March 2008

I expressed some reservations about the last update weighting it 60-40 in favour of cold rather than mild into the second week of April.

Over the past few days the various inputs keep the uncertainty over the outcome to, say, 14 April, still pretty finely balanced.

To take just one comment, see below, from NOAA this morning, as they try to give a coherent view of likely developments for north America out to about next weekend

VALID 12Z WED APR 02 2008 - 12Z SAT APR 05 2008

CONFIDENCE IN THE OVERALL FCST FROM THE PACIFIC ACROSS NORTH

AMERICA IS WELL BELOW AVERAGE. MODELS/ENSEMBLES GENERALLY SHOW

SOME DEGREE OF ERN PACIFIC MEAN TROFFING OVER THE COURSE OF THE

MEDIUM RANGE PERIOD BUT CONTINUITY/AGREEMENT OVER DETAILS IS

UNUSUALLY POOR. THRU DAY 5/THU THERE IS ALREADY A WIDE SPREAD OF

POSSIBLE SOLNS

They are obviously not very confident in any of the models. So trying to give good guidance for a week further down the line is even more difficult. Still lets have a go and watch to see how events turn out for the period 5-12 April.

Looking first at the nearer time scale and the GFS/ECMWF 500mb ensemble charts. T+168-240, the period Saturday 5 April to Tuesday 8 April, see the latest chart from this morning below

Over the past few days, much as their operational models have shown, they have differences in the basic flow pattern at 500mb.

Today ECMWF favours a much more pronounced ridge over Greenland into Iceland than GFS shows. It takes the line of a ridge much nearer and less pronounced with the major trough that NOAA talk about over the eastern states.

Picking up on this and looking at the NOAA charts for 500mb flow pattern from the end of March out to about 14 April and the two charts, one below the other, show their ideas today, along with their comments about how the ensembles are behaving in those time periods.

for 4-8 April

and for 6-12 April

both go for +ve anomalies to the north west of the country.

Below is the NOAA 500mb ensemble set of charts for the latter part of the period along with their comments through the whole period on spread etc.

Days 3-5.

Following several model runs of poor agreement among ensemble members, much better agreement exists in today’s model run. Split flow evolves from the eastern Pacific into western North America. A shortwave trough rapidly tracks across the central and eastern conus, while a southern stream trough amplifies into the southwest conus at the end of the period.

Days 6-10.

Spread among the ensemble remains low indicating a southern stream trough progressing from the southern Plains towards the East coast during the early part of the period. During the latter half of the period, a moderately amplified pattern develops with a trough near the west coast, a ridge over the west-central conus, and a trough over the eastern conus.

Days 8-14.

Spread increases among ensemble members which is typical for this time period. The mean pattern favours a shift of the dominant storm track northward. If this occurs, then above normal precipitation can be expected across the northern tier of the conus with dry, warm conditions across the southern tier of the conus.

For around 11 April, no real sign of any marked ridge to the west of the UK although with the wide spread NOAA mention some members do suggest that but the mean is pretty zonal.

GFS and ECMWF outputs from T+168 to T+240 and they have shown quite large differences over the past few days. ECMWF being the keenest on having some kind of northern block through most of the runs.

La Nina has no change

AO shows a drop and very few suggesting any rise late on

NAO is a bit more variable but generally its more down than up

PNA shows a rise

Looking at these sets of data what can we draw from them?

I would punt for cold rather than mild as a basic comment out to the end of this lrf – Monday 14 April, and that from about T+192 for the daytime temperatures to fall below normal. They then stay that way out to about the 12th a fortnight today when they show signs of recovering to above for most. At times the values by day have to be rated ‘cold’ and there is obviously potential for falling snow through a similar time scale to the ‘below’ average day time temperatures quoted above.

Putting %’s to it I would say 70-75% for cold or below normal and 25% at best for it being overall normal or above. That is through the majority of the period from T+168 out to T+384.

Obviously not everyone will agree so please post and give your reasons if you feel this comment is wrong.

There will be no update until next Friday 4 April 2008.

[PB22]

Thanks John, excellent as always and its nice to read a balanced summary of the longer term prospects.

[johnholmes]

cheers Paul, its the fairy story after the factual chapter you give mate!

very interesting doing it though, I might even be learning something.

[mcweather]

Many thanks for all your hard work yet again John Its is now Monday and the models seem to have hardened up on the cold out come during the last 48 hours so a spot on call from yourself I would say yet again. At the moment I think that this spell may well have the potential to be snowy on a more widespread basis than he Easter blast. I saw just a couple of short lived but potent snow showers from that down here in balmy coastal Dorset which was all I expected really. But this time what chance another April 81? We'll see.

[johnholmes]

this will be updated late this evening into around 18-20 April.

Obviously not talking about the next 3-7 days, plenty about that on other threads.

Suffice to say that the lrf did quite well predicting this spell some 2 weeks ago.

Not blowing my own trumpet its thanks to various charts and outputs from NOAA and not least the regular inputs over the past 2 months from GP.

[johnholmes]

Update on Friday 4 April 2008 for the period from the weekend of 12-13 April out to around 20th April

Just a note to those new to some of the terms used below, this link should help

http://www.netweather.tv/forum/index.php?showtopic=46150

it it possible as Martin has handed over this exhaustive and time consuming task to someone else that the link above may not work. try this one if it does not, failing that go to the NOAA site and search through their full explanations

http://weatherfaqs.org.uk/

http://www.hpc.ncep.noaa.gov/

or this one

http://www.cpc.ncep.noaa.gov/products/pred...10day/500mb.php

Not as detailed as usual as I've not been able to look at the daily output from all the sources. From what I've seen its still nowhere near more than an even bet that it will turn milder overall in the period in question.

Let us look at what has been shown over the past couple of days.

First from NOAA

La Nina they say

30-DAY OUTLOOK DISCUSSION FOR APRIL 2008

MODERATE LA NINA CONDITIONS CONTINUE THROUGHOUT THE TROPICAL PACIFIC AND WILL CONTINUE TO FORM THE UNDERLYING BASIS FOR THIS MONTHLY FORECAST.

The MJO is mjo expected to remain weak next 2 weeks

The AO=a general fall then a rise, less definitive than the fall it has to be said.

NAO=also shows a fall with something of a rise.

The GFS/ECMWF 500mb +/- charts for last evening showed this below. ECMWF on the left and GFS on the right, and valid over the period 10-14 April. They are, as might be expected quite different from the last charts I saw (last Sunday). Both have large –ve anomalies over/south west of the UK. This, along with other anomalies showing on both charts would suggest rather more of a westerly not north west flow at 500mb.

If we take a look at the NOAA ensemble charts and comments, again from last evening they show this.

GFS ENSEMBLES FOR DAYS 3 THROUGH 14

Valid Apr 05 - 16, 2008

Days 3-5.

Ensemble spread is relatively large at the start of the period reflective of the difficulties in timing individual systems in zonal flow. Some consensus exists on a trough moving from the Pacific Northwest to the Northern Plains as a ridge moves from the Gulf of Alaska to the Northern Rockies. In the southern stream the members agree fairly well that a broad low amplitude trough will dominate most of the CONUS as a ridge builds from the Pacific west of California to the interior Southwest. A closed-off trough northeast of Hawaii will weaken and open up as it drifts slowly eastward.

Days 6-10.

Spread remains large during the period across much of the forecast domain. Best member consensus indicates a broad trough across most of the CONUS and zonal flow across the Gulf of Alaska.

Days 8-14.

Spread becomes very large over the forecast domain during this period.

Looking at their first chart in the period day 3-5 shows this

To me this suggests they are happy with what is being shown by the GFS model in our area for the period up to Monday at least.

If we then jump to their latter period, around 14 April, ie: a week on Monday we can see that the north of west flow is certainly not the mean although there are several members which still suggest that. Note also the NOAA comment, 'Spread becomes very large over the forecast domain during this period.' which is for their main area of concern the USA. However, it does impinge on our area of concern. From it we can see that the north of west flow has largely disappeared from most of the members.

Turning to their 500mb anomaly charts from this evening; the first for the period 10-14 April. This shows a more westerly flow with a large –ve anomaly just west of the UK but with some indication of slight blocking between Greenland and Svalbard along with more major blocking north west of Greenland.

Secondly their chart for the period 12-18 April, which shows a westerly flow again with a much smaller –ve and +ve anomaly areas for pretty much the same areas as the previous time scale.

Finally, we had another first class post from GP with his usual clear explanation and links showing his view on the longish term, see below

http://www.netweather.tv/forum/index.php?s...ic=46808&hl

This is his summation of the post above, but do read his full post with the various links.

Putting some 'weather detail' on these thoughts.....

The impacts of northern blocking will make the first 10 days feel pretty raw with some snow potential. As the month progresses, northern blocking will stay in evidence but a shifting of low pressure to our west will tend to draw up slightly milder air but still close to average temperatures for a traditional April with emphasis on falling pressure to our west and over the UK during the month with above average rainfall totals so feeling particularly unsettled, cold and wet. The last week of the month holds some promise for a welcome rise in temperatures as the AO might trend back towards positive but generally a total contrast to last year's rampant warmth.

Finally, not a forecast but perhaps a cautionary notes to raise our awareness. It's 10 years since the big Easter floods that affected much of the Midlands. The overall pattern suggests the potential for the same volatile mix of polar and tropical air within a slow moving and unsettled theme. We should therefore just be on the lookout for extreme weather events within the next 4 weeks with the Midlands, Yorkshire and western fringes of the UK.

The final sentence is a rather sombre assessment for some areas. However I do find my self, after totting up all the data from various sources agreeing almost completely with the assessment by GP. Cold and northerly becoming dominated by low pressure and that tending to settle to our west or even south west but with blocking in evidence quite a lot of the time to our north. Towards the end then perhaps, no more than that at the moment, perhaps, signs of more genuinely mild air by this period end.

Thus to sum up. I'll do in % figures again. This time splitting the country into two approximate halves, north and south.

I would rate the probability of colder rather than milder temperatures over the period in question, taking the north first as 60% colder with 40% milder. So no long term cold but no long term mild either. One might say the average mix for mid to late April I suppose.

For the southern half of the UK then the reverse with 60% of the time in milder air and 40% of the time seeing some colder incursions.

I would hazard a guess at the cold incursions, both for the north and south being from a west of north rather than east of north direction.

Another update is due on Tuesday evening for a further 4 days ahead. I am checking the T+00 for various points in earlier issues.

Edited April 4, 2008 by johnholmes

[johnholmes]

Update on Tuesday evening 8 April 2008-04-08

This covers the period from T+168 today (15/04) to T+384(24/04), perhaps a little further!

Just taking a quick look at what the 3 main models show at T+144, the furthest the UK Met model is available to us. All 3 show a fairly similar pattern with low pressure east and west of the UK and highish surface pressure to the north of the country. So in that time scale there is little probability of any real warmth. A mix of sunshine and showers, some frost in places under clear skies, still snow for the Scottish ski areas, at least above about 1200-1500ft.

As to further ahead well the 2 models remaining, GFS and ECMWF by T+240, the furthest main charts are shown for ECMWF. Not unexpectedly at this range they do doffer. GFS favours surface high pressure to our east and south west with still some hint of it in northern latitudes, with the actual T+240 chart suggesting much of the UK in a COL situation between the two main high pressure zones. At this range its not much use trying to use a chart to predict what the localised surface weather might be. GFS would tend to suggest some warmer air moving north with some rain especially for western and northern areas. ECMWF has low pressure to the north west much more a feature but with a southerly flow. It still has a large 500mb trough covering the country with surface high pressure around its periphery, to the north east, the far west and way north. So quite a different probable weather pattern would be likely to evolve from those two models.

Taking a look at the 500mb anomaly charts from GFS and ECMWF for today and we can see how they differ in this respect

ECMWF on the left with GFS on the right, and they well illustrate the differences I have just outlined above. The two charts cover the period T+168 out to T+240 so from 15/04-18/04.

Turning to outputs from NOAA and their latest 500mb ensemble charts along with their overall comments

this is the period ending 19 April

a pretty wide spread in the eastern Atlantic, the mean showing only a slight tendency for a flow north of west but the ensemble members, as you might expect, look about evenly balanced either side of that mean flow.

If we now look at their version of the 500mb anomaly charts, and again these go further out than the ECMWF/GFS versions.

this is for the period 14-18 April, fairly similar time period to the ECMWF/GFS charts above

and this one out 16-22 April

The second one has about the smallest anomalies I've noticed in the past 2-3 months.

Other information available is again mostly NOAA originated:-

NO=a fair amount of split towards the end period as one would expect but still showing mostly zero or below

update for tue 08/04

NAO=showing a rise through the period on offer and some spread as time goes on.

Their take on La Nina is that its likely to continue, albeit not as strong as earlier this year, probably for the next 3 months. beyond that and it also shows a divergence of view as we go into the mid summer period.

GP made the following comment in response to a post on 6th April: - from GP on 6th

Disagree: May. Both La Nina and tropical forcing from the Indo-Pacific trend warm at this time and quite settled. Add to that, stratospheric conditions are starting to cool signalling a change in the stratospheric jet over the next 3-4 weeks.

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

http://www.cpc.ncep.noaa.gov/products/stra...AMJ_NH_2008.gif

As all on here know I have not a lot of understanding of this branch of meteorology so I'll leave it to GP to explain if he spots this!

To me, I may be wrong (!) it seems to suggest that there are indications of some changes impacting on our part of the world as we go further towards the end of April.

Once again, time to put finger in air and decide which way to jump.

To me the indications have been growing, that, in the time scale given at the start of this post, that is the period from T+168 today (15/04) to T+384(24/04), perhaps a little further!, say from the end of the third week in April then we may begin to see some genuinely warmer type weather as the various factors begin to allow a more south westerly flow to develop over western Europe including the UK. That is unlikely to show for some time yet and blocking in the nearer time scale, to the north of the country, will be the main factor. When the change begins towards the month end just how that will turn out, mobile from a west to south west, or more southerly is open to either possibility at the moment. I certainly do not have either enough understanding of the long term 'drivers' or a confidence in them, to give any decision.

As to the shorter term of the lrf, then its still an average, possibly less than average type of temperature regime, rather than a milder outlook for much of the time for the next 7-14 days in my view. Certainly some short spells of less cool conditions at times but the blocking to the north is likely to prevent any really mild air surging very far north, at least other than on a very temporary basis.

I think the charts are n the right place, if not please post to tell me, its been a long evening getting this ready

Edited April 8, 2008 by johnholmes

[johnholmes]

only just noticed the post by GP on another thread

tks for that, I'll have a read, hope we are still singing from the same hymn sheet?

[jam]

Thanks John from this part of Sheffield,your efforts are greatly appreciated.

Starting to look for signs of fair weather for the start of our cricket season down at Baslow so this thread will be read with interest

[johnholmes]

lovely spot to play cricket, hope the summer allows it!

[azores92]

lovely spot to play cricket, hope the summer allows it!

hi john, can you explain, or give a good link, asto what the effects are on the british climate, in the MJO cycle? thanks.

[johnholmes]

one I would have to look up, but I would imagine that GP may be able to help or Brick.

If they are it would be good if they or you could post that idea into here?

thanks

not sure if this might help

http://envam1.env.uea.ac.uk/mjo.html

I've done more reading on various items re MJO, La Nina, El Nino etc than the days of being on my first forecast course many many years ago.

At the moment I must admit to being a bit confused by it all but light is slowly dawning in the old few gray brain cells that remain!

[azores92]

one I would have to look up, but I would imagine that GP may be able to help or Brick.

If they are it would be good if they or you could post that idea into here?

thanks

not sure if this might help

http://envam1.env.uea.ac.uk/mjo.html

I've done more reading on various items re MJO, La Nina, El Nino etc than the days of being on my first forecast course many many years ago.

At the moment I must admit to being a bit confused by it all but light is slowly dawning in the old few gray brain cells that remain!

many thanks, it clears things up slightly.

[johnholmes]

Update issued Friday morning 11 April for period from Friday 18th out to end of the month?

For what is likely to happen prior to the start of this lrf then please go to the daily output By Paul B here

http://www.netweather.tv/forum/index.php?showtopic=46977

For a realistic look at the probable weather in that period.

Into the longer term and there are certainly conflicting signals. Both major models have made considerable changes at the end of the ECMWF time scale over the past 3-4 days. The 500mb ECMWF and GFS 500mb anomaly charts have also shown differences and also between the models and continue to do so. Long range indications suggest other drivers MAY be starting to change but some still indicate that the colder air has still not accepted its supposed to be spring just yet!

So, all in all a mixed set of signals but its now time to have a go at the prediction out to the month end.

As usual I’ll go through the various items and then give my idea on what is the most likely outcome.

First, comments recently with links from GP and I’ll just give his quote summing up his view.

With no suggestion of a strong area of organised tropical convection to shift ridges, I would suggest that we will follow a pattern dominated by La Nina and northern blocking. See the above post for the forecast detail for this although this does not mean cold throughout as lows attempt to undercut from the SW.

His link can be found here

http://www.netweather.tv/forum/index.php?s...ic=46808&hl=

NOAA in their weekly update on ENSO suggest that La Nina will continue for the next 3 months, although in the link they do show that it has declined somewhat.

http://www.netweather.tv/forum/index.php?s...ic=46808&hl=

The AO shows an increase towards zero with close agreement but with a fall then expected with a rather wider scatter.

The NAO also shows a rise with a rather less definite idea later but with a tendency in most of the runs to decrease again.

The links to these are below

http://www.cpc.ncep.noaa.gov/products/prec...o.shtml#current

Turning to the NOAA 500mb charts (from yesterday), they make the following comment in the 8-14 day outlook (remember its really for their major area of interest, the North American continent)

Days 8-14.

THERE IS RELATIVELY GOOD MODEL CONSENSUS SHOWING WEAK TROUGHING OVER THE NEW ENGLAND AREA AND WEAK RIDGING NORTH OF THE NORTHERN PLAINS, ALTHOUGH THERE ARE TIMING DIFFERENCES AMONGST THE MODEL MEMBERS. IN THE SOUTHERN STREAM, THE ENSEMBLE MEAN FAVORS A TROUGH SOUTH OF THE SOUTHEAST AND GENTLE RIDGE OVER THE SOUTHEASTERN CONUS.

This is valid for around 20-21 April. Some divergence over our area of interest as you would expect this far out but not much suggestion of any flow from north of west in my view. A few yes but nothing like the predictions of a few weeks ago for the last cold spell we had in this country.

Their anomaly charts for a similar period show this (again from last evening 10 April)

Chart 1 is for 16-20 April

And chart 2 is for the period 18-24 April

To me neither suggest any major type of blocking to the north west nor really to the north east. Thus any cold air is kept way north over the polar regions. Sure its still there but not flooding down as it has done at times over the past few weeks.

Looking then at the two major models that go(ECMWF) out to 10 days ahead T+240 and as I commented in the first paragraph both have shown changes at that time scale over the past few days. ECMWF more than GFS to my eye. The predicted cold flow from it has now disappeared. Both have a major 500mb trough over the eastern Atlantic which up to T+240 will prevent any marked warming of the air over the UK. However by T+240(21 April) they are quite different with cold air flowing down the western side of the surface lows on ECMWF being the main feature. With GFS it’s the southerly flow ahead of these lows which is most obvious. This is where one has to decide which of these two models is showing the most likely outcome another week down the line. Not easy as both look reasonable charts at T+240. Both have that large pool of fairly cold 500mb air to the west and over much of the Atlantic. In terms of consistency over the past 3 days then GFS has it I feel so I’m more prepared to follow that line than the probable ECMWF(we have no certainty as to what that might be either!), supported by the comments from NOAA out to about T+180 that GFS, for once, is looking a better guidance than ECMWF. Also the various charts do suggest little sign of surface pressure building west of the UK let alone north west. If it rises anywhere it seems likely to be to the north east. The evidence suggests that with the considerable warm air advection likely to occur around T+240 ahead of the surface lows, along with the jet pattern which also seems to suggest the surface high over Scandinavia BEFORE any Atlantic high gets in to the act. If this does happen, 60:40 I would say, then a much milder interval seems likely for the country, all of it, from about 23 April with daytime temperatures over the south of the country likely to exceed 20C in a number of places. Having said that I am still not completely convinced, as my first paragraph suggests, that cold air is now something we can forget about.

So summing all that up, I would say, in the time scale in the heading, turning much milder with a southerly flow by 23 April BUT probably another cold shot around the end of the month.

I’ll update this on Tuesday 15 April

[Greyowl]

Great summary of what maybe coming in the next few weeks John.

Thank you

[Glacier Point]

If I could tag this on John:

There is very little tropical convection at the moment, with the exception of a well organised and strong band over the Atlantic. This means two things:

1) La Nina forcing will be the major influence.

2) Tropical convection in the Atlantic teleconnects (I think) to a strong sub-tropical jet and weak polar jet flow.

Both signal falling pressure in the N. Atlantic across to the UK. Today's ECM is a better example of this evolution.

In the longer range, there may be some early indications of the AO returning to a neutral phase and posssibly +ve into May. This rising index is a signal for cold air to retreat northwards. Certainly the polar stratosphere is continuing to cool:

http://ds.data.jma.go.jp/tcc/tcc/products/...e/pole30_n.html

Underlying this, there is a -ve NAO signal due to cold air and SSTAs over the tropical Atlantic and atmospheric coupling from the upper level zonal flow.

What we could be looking at last week April into May is a transition to a -NAO / +AO which teleconnects to some very warm and humid conditions with trough formation in the Atlantic if not mid latitude high - more or less as per initial thoughts for the month ahead.

Edited April 11, 2008 by Glacier Point

[johnholmes]

tks a lot GP, please do post in here with your thoughts and links, they are great to read.

The more I read the more I think I understand but I'm very far from feeling comfortable with the data.

Give me a forecast to do for 3-7 days ahead no problem, years of experience and practice kick in but this is all very new, also very interesting, must go and do some more reading!

for GP

have you any link for initial reading, additional to what you have already suggested, re the 'link' between the polar warming at 30mb and subsequent Troposphere cooling please?

thanks

Edited April 11, 2008 by johnholmes

[Glacier Point]

for GP

have you any link for initial reading, additional to what you have already suggested, re the 'link' between the polar warming at 30mb and subsequent Troposphere cooling please?

thanks

http://www.weatheranswer.com/public/NAO_ROSSBY_IMPACT.pdf

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

http://ams.allenpress.com/perlserv/?reques...IT%3E2.0.CO%3B2

http://www.math.ust.hk/~maklchan/papers_pdf/mayr_jastp03.pdf

http://www.appmath.columbia.edu/ssws/index.php

Edited April 11, 2008 by Glacier Point

[johnholmes]

tks again GP

a lot of reading there and a lot of new stuff to me so another very sharp learning curve required, tks though

[johnholmes]

Update Tuesday evening

This will be just a short one this evening.

To me little has been seen that really alters my view on the last lrf that went from 18th April out to the month end.

There is still a fair amount of spread between the various outputs as to just what may happen.

This is what my final sentence gave last Friday morning

So summing all that up, I would say, in the time scale in the heading, turning much milder with a southerly flow by 23 April BUT probably another cold shot around the end of the month.

On balance I still favour temperatures recovering to more usual April values through next week as a surface high settles to the east or possibly south east of the UK with a largish low off to the west pulling in fairly mild Tropical Maritime air for the whole country. For a few days, late next week and perhaps into the start of the following weekend, afternoon values over 21C do seem likely for places in the southern half of England and into the Midlands. Parts of northern England, along with some of Wales, may see values near or just above 21C.

Thereafter into the start of May and it still looks like most signals suggest that high pressure will take over again north of 50N, probably to the north west it seems at the moment. This will lead to an obvious lowering of those almost summer like afternoon values. Beyond the start of May is way too far ahead for me to even pretend I have any real idea.

I hope to do a fuller update late on Thursday evening of this week.

[mcweather]

Many thanks John. I have very much enjoyed your honest and accurate looks at the possibilities in the longer range as this is my specific area of interest.

I looks forward to next winter when I,m sure you we get the chance to forecast some Siberian blasts.