Model Output Discussion 26/08/2012

[Gibby]

Good evening. Here is tonight’s look at the 12zs of the big three.

All models show a Low pressure area and attendant front cross the UK in the next 24 hours. A band of thundery rain moves across in association with the front followed by WSW winds and thundery showers later in the day. Further showers are possible overnight and at first on Thursday before a drier and brighter afternoon develops as a ridge of High pressure pushes in from the West. Thursday Night will see rather cool conditions under clear skies and a decreasing NW breeze as the ridge advances to be situated over Southern Britain on Friday and Saturday. Through these two days there would be dry and bright conditions with broken cloud while the North sees a warm front cross during Friday delivering rain and drizzle to these parts continuing on Saturday.

GFS then shows 5-6 days of reasonable weather as a belt of High pressure lies from the Atlantic east across Southern Britain. Some fine and warm conditions would occur with some overnight mist and valley fog patches. Further North a Westerly flow does remain with cloudier skies and a little rain in the far North. In FI tonight the weather gradually becomes more unsettled as Low pressure moves in from the NW bringing outbreaks of rain, cooler and sometimes windy weather to all areas by the end of the run.

The GFS Ensembles show uppers above the long term average from the warm up at the weekend to the end of the run for Southern and Northern locations. Rainfall amounts are much less a feature than we’ve seen on any ensembles of late indicating a High pressure dominance. The operational shows an undulating pattern in the North as the Lows in FI approach and pass while in the South it does show the weather cooling much quicker than the average of the members suggest.

The Jet Stream Forecast shows the flow ridging strongly North in a few days time taking up a position well North of Scotland as we go through next week.

UKMO shows High pressure down to the South and Southwest at the end of the weekend though a weak front crossing SE through Sunday could produce some very light rain for a while before pressure builds strongly once more. It looks like next week from the end of the run chart could bring warm and dry weather for many with sunny periods for many by day and mist and fog patches at night.

ECM too shows a weak trough moving SE over England on Sunday with a band of cloud and very light rain in places. By Monday most places are dry again with sunny spells especially in the South with a light West or West North-Westerly breeze blowing over most areas. Thereafter the run is more disappointing than the others as it pushes High pressure away to the SW pulling Low pressure down from the NW with a gradual decline in temperatures and bringing the risk of rain South-eastwards across the UK late in the run.

In Summary the models continue to jostle around in the exact positioning of the High pressure over the weekend and next week. The much favoured position now is down to the SW with Westerly winds predominating over the UK throughout. The far North will see only marginal improvements early next week while the South sees some respectable if not remarkable conditions for quite some time. ECM continues to show a slower evolution to the really good conditions tonight with Low pressure allowed to make a temporary incursion to many areas for a while midweek away from the SW.

[johnholmes]

Ah time to highlight my ignorance. Whats is a 98L?? A 98 low??

A tropical disturbance moving west out of Africa - where pretty much all the storms/hurricanes begin life just north of the equator.

go to the NASA hurricane site for lots of information.

here is the link

http://www.nhc.noaa.gov/

Edited August 28, 2012 by johnholmes

[bluearmy]

short comment on how poor the T96/T84/t72 faxes have been re the depth of the cool on thursday night. yesterday's T96 had the 546 thickness in s norway. this mornings T84 dropped it into the north sea and now the T72 takes it into holland. something was wrong somewhere at such a short range ????

[Alex]

Actually two subtley different ones I'd say...

June 1 - July 19 H5 anomaly. Note the position of the mean trough directly over the UK.

July 20 - present H5 anomaly. Note the position of the mean trough to the south and west of the UK.

That's a big difference in the mean trough position for the UK, the one that turns around a significantly below average temperature regime to one that is around or just above average. The earlier period was characterised by a strong gradient in the atmospheric profile fuelled by sharp downwelling signal in the Arctic during the spring and early summer - this is no longer present and we're likely to see more of the charts we're seeing now rather than those for the first half of the summer, but still with the mean trough to our west drawing up mild SW'lys.

Definately the second half of summer has been alot better for the southeast and August for me has been a typical summer month in Essex which means we avoid most of the rain and we get some hot spells coupled with more average spells. Not one day has so far been below 22c in Chelmsford from my recordings, that's in line with July 2010 just not as dry!

[mushymanrob]

The 500mb profiles shown by GP (Stewart) indicate clearly the differences to the upper air pattern over the period he shows. Because, I suspect, the weather has generally been unsettled over the meteoroloogical summer there are some posters who seem not convinced that we have had at least one major pattern change. The charts shown by Stewart really do show just how different our upper air pattern has been.

As to what may happen beyond 2 weeks or so down the line-that is not my area of expertise so I will leave that to others.

ay john, i accept there was a pattern change, but as gp stated, it was subtle, and replaced one unsettled regime with another, the net effect for me here in central uk was.... no change!

this mornings runs appear to be more reluctant to bring in the azh, leaving it centred to our southwest and leaving us in a westerly/northwesterly airflow. so my cloud concerns posted yesterday do look more likely now IF this mornings suggested ops become reality. however, its still predominantly dry...ill settle for that!

[bluearmy]

The inability of the AZH to ridge across us with any solidity is something that has characterised the summer. However, with persistent high heights around Greenland, that's not a surprise. Plenty of ex TD's showing in the extended output coming into the n atlantic so I think the longer range ens output will continue to swing about a bit.

[Gibby]

Good morning. Here’s my interpretation of the 00zs today.

All models show an Atlantic depression and associated trough moving East through the UK today. A band of thundery rain will move West to East across Britain followed by sunny intervals and scattered heavy showers. This weather continues tonight and for a while tomorrow before a further trough swings SE through tomorrow followed by NNW winds and drier and brighter conditions with a strong rise of pressure in the second half of the day. Subsequently a ridge topples over the UK on Friday and persists near the South over the weekend. In the North a warm front moves East in the Westerly flow bringing rain and drizzle with it to these parts while further South a bright day on Saturday is followed by a zone of cloud and light rain on Sunday as a weak trough moves down over England.

GFS then opens next week with High pressure straddled across Southern Britain through the first few days while the Westerly flow over Scotland and Northern Ireland continue to bring the risk of rain here at times. Under the High pressure in the South things will be pleasantly warm and dry with some sunny intervals at times. Later in the week GFS continues to keep High pressure in charge with dry and bright weather extending to the North too late in the week before through FI High pressure is pulled back out into the Atlantic allowing Low pressure near Iceland to erode a ridge over the South and bring a return to unsettled and much cooler air to all areas by the end of the run.

The GFS Ensembles show the first week of September with higher than average uppers with little rain under the influence of High pressure. From the 7^th the trend is for cooler uppers to gradually take over with signs that the incidence of rain is on the rise again as we head towards the end of the run showing the breakdown illustrated by this morning’s operational has some support. The operational is shown to be a cool outlier for a while around the 10^th following the passage of a cold front.

The Jet Stream Forecast shows the flow is scheduled to move North over the eastern Atlantic over the next few days. It then spends most of next week blowing east to the North of Scotland gradually slipping further South later next week.

UKMO shows High pressure centred in mid Atlantic to the SW of Britain at midnight on Tuesday with a ridge East over the English Channel. A Westerly flow is blowing over all of Britain. With Low pressure out to the NW its only a matter of time that after Tuesday a front crosses the UK with rain moving West to East late in the day Tuesday or Wednesday.

ECM is similar but delays the trough midweek a little. Nevertheless, the influence of High pressure subsides all too quickly as we move through next week with troughs moving East over Britain delivering rain to the north at first before reaching all areas by Thursday. The end of the run shows a deep Autumnal Low just North of Scotland with troughs swing East over the UK in very strong winds leading us all into a very cool NW flow with strong winds and squally showers to end the run.

In Summary the models continue to decline the strength of the High pressure’s influence as we move deeper into next week. The North now looks like there is just a window of better weather midweek if GFS’s evolution evolves. The Euro’s look poor this morning with the ridge near Southern Britain quickly collapsing away as we move deeper through the week with ECM setting up a major Atlantic storm to batter the North in particular late next week. Unfortunately the main reason for this decline in conditions is due once more to rising heights over Greenland gradually nudging the Jet Stream back southwards again later next week.

Edited August 29, 2012 by Gibby

[Frosty.]

The GFS 00z is showing a heatwave next week in southern parts of the uk, especially the southeast quarter with temps up to 26c which is hot for early september and pretty warm further north and west for the time of year at 22-24c. The anticyclone across the south looks like struggling to extend north due to a persistent band of low pressure near iceland continuing to push east which means scotland, n.ireland and northern england could remain in a broad run of moist westerly winds next week with a lot of cloud and some patchy rain (mainly for western scotland) although to the east of high ground it could become brighter and warmer at times, the high does eventually extend further north but only briefly before the main body of the high is pulled west into the atlantic and then a deepening depression sweeps southeast towards the uk bringing a much cooler unsettled spell but not before the south has had at least a week of very warm and sunny weather with light winds.

Edited August 29, 2012 by Frosty039

[Gibby]

The problem is that we never really lose a feed from off the Atlantic with the High held too far South. With such a fetch cloud cover will always be prevalent at times even in the SE developing in the mornings and spreading out under an inversion. So warm uppers are not alone in ascertaining surface temperatures with cloud factors playing a dominant role in maximums achieved though I concede the potential is there if the cloud breaks enough.

Edited August 29, 2012 by Gibby

[johnholmes]

The ec-gfs version of the anomaly charts this morning has certainly backed away from the upper ridge idea. ECMWF has a trough back where it had a ridge only 24 hours ago. GFS has it without any sign of the upper closed high over southern Europe it has had previously.

The NOAA version has shown differences for a couple of days now with the 6-10 day version still showing a +ve area into western UK but the 8-14 day has not really shown this. It has kept a more w’ly type of flow over the last couple of days.

Looking at these recent outputs then a flow of air from the Atlantic does seem more likely rather than one from south of west. Given the changes and differences in the anomaly charts then the dry settled spell may be fairly transitory and a NW-SE difference is perhaps being set up with a more unsettled spell likely to be taking over before the end of the first week of September.

The hurricane season does often cause serious problems in trying to predict more than a few days ahead, with the end of meteorological summer and the start of autumn seeming particularly prone to this.

[Frosty.]

The problem is that we never really lose a feed from off the Atlantic with the High held too far South. With such a fetch cloud cover will always be prevalent at times even in the SE developing in the mornings and spreading out under an inversion. So warm uppers are not alone in ascertaining surface temperatures with cloud factors playing a dominant role in maximums achieved though I concede the potential is there if the cloud breaks enough.

Agreed, I think there will be some disappointment now that the high is unlikely to move east of the uk and enable a continental flow which could have pushed the temps up to 30c next week in the south but as things stand, the gfs 00z is still projecting a very warm spell for the south/southeast with BBQ type weather and temps close to 80F, at least southern england will be closest to the high with light winds and more chance of long sunny periods even in the worst case scenario, a raw deal for the north though in comparison.

[Frosty.]

If this verified there would be a lot more disappointment, so next weeks weather in the south at least still looks very good if temps can get to between 75-80f, not to be sniffed at in early autumn.

[Gavin Hannah]

The 00z run in FI going for ridging in the north atlantci/greenland which looks like bringing in some colder air from the north. A stronger signal compared to the 18Z run. But as has been noted over the last few days, the Azore high is getting pushed around so I expect this could easily change. That large storm moving east appears to move into greenland but will be interesting to see if it heads more west than north and straight into the UK.

Edited August 29, 2012 by IBringTheHammer

[summer blizzard]

Ah time to highlight my ignorance. Whats is a 98L?? A 98 low??

A Tropical Wave (they get a designation if they have the potential to become Tropical Depressions and then Hurricanes), quite common in the models during peak season which is why i would'nt back a 10 day forecast during peak season (August-October).

The one in question may develop and re-curve down the line.

We have a hurricane sub-forum you can visit for more information.

[Gavin Hannah]

A Tropical Wave (they get a designation if they have the potential to become Tropical Depressions and then Hurricanes), quite common in the models during peak season which is why i would'nt back a 10 day forecast during peak season (August-October).

The one in question may develop and re-curve down the line.

We have a hurricane sub-forum you can visit for more information.

Thanks for the info.

[Scorcher]

Agreed, I think there will be some disappointment now that the high is unlikely to move east of the uk and enable a continental flow which could have pushed the temps up to 30c next week in the south but as things stand, the gfs 00z is still projecting a very warm spell for the south/southeast with BBQ type weather and temps close to 80F, at least southern england will be closest to the high with light winds and more chance of long sunny periods even in the worst case scenario, a raw deal for the north though in comparison.

Thing is GFS looks a lot better than ECM for those looking for warm, settled weather. I think the jury's still out on what will happen next week given the models are being affected by the tropical storm/hurricane activity. The GFS 00z has the high further north and east than the ECM 00z and it would probably be considerably warmer for most with far less cloud. GFS has stuck to its guns over this for several days now while the ECM has got progressively worse for those wanting settled, warm conditions. At T+120 the UKMO seems to be closer to GFS with high pressure further north, but it then retreats south at T+144.

[Frosty.]

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Thing is GFS looks a lot better than ECM for those looking for warm, settled weather. I think the jury&amp;amp;amp;#39;s still out on what will happen next week given the models are being affected by the tropical storm/hurricane activity. The GFS 00z has the high further north and east than the ECM 00z and it would probably be considerably warmer for most with far less cloud. GFS has stuck to its guns over this for several days now while the ECM has got progressively worse for those wanting settled, warm conditions. At T+120 the UKMO seems to be closer to GFS with high pressure further north, but it then retreats south at T+144.<br />

Yes the ecm was showing the better outlook for next week only a day or two ago so that's a worry, it will be a shame if the very warm anticyclonic spell fails to develop but it may just be a model wobble.

Edited August 29, 2012 by Frosty039

[Thundery Shower]

It certainly looks like a breakdown from thursday next week onwards. Mon - Wed 24 -26 then by thursday evening cooler and fresher weather returning.

[Frosty.]

Just for a blink or miss it time next week, the Gfs 06z has all of mainland britain under high pressure but the outlook beyond next week looks like turning cooler and very autumnal with northerly winds for a time and a risk of ground frosts. The anticyclonic weather this weekend and next week is mainly concentrated from central wales and across the midlands to east anglia and southwards, there is some very warm and settled weather shown on the 6z, next tuesday looks hot in the southeast quarter with temps as high as 26c and the rest of next week also looks warm but nearer 20-22c, that also applies to the coming weekend with southern britain under the influence of high pressure with temps in the low 70's, but progressively cooler and more unsettled further north with quite a lot of rain and strong winds across parts of scotland and maybe n.ireland between fri/sun as lows continue to track east to the north of mainland scotland, at least tomorrow looks dry and fairly sunny in northern britain as a ridge pushes east but wet and windy up north by friday and at times through the weekend.

[Gavin Hannah]

I wouldnt be surprised if this high pressure disappears completely...

[Weather Boy]

I wouldnt be surprised if this high pressure disappears completely...

Erm, that's pretty unlikely seeing as it is due from about +T48!

Edit: although to be fair given your location (which is somewhat vague) I suppose it's not totally out of the question, although your sentence suggests disappearing for the whole country, which I think is definitely not going to happen.

Edited August 29, 2012 by Weather Boy

[Gavin Hannah]

I was referring the post above. The chart for Wed 5/09.

My bad, I should have wrote, "that high pressure" lol.

Edited August 29, 2012 by IBringTheHammer

[ANYWEATHER]

I wouldnt be surprised if this high pressure disappears completely...

Indeed,! Looks as though all models have frayed away for any significant pressure build next week and away from the East and Southeast of England the general theme of unsettled conditions looks the format. The weekends a long way off in terms of detail but looking at the current model output, a cold front will be affecting the northern half of the uk on saturday and then the southern half of the uk on sunday. Anyway, in terms of a downgrade , high pressure now, unfortunately looks very reluctant to be the dominating feature for most of the Uk.

[Summer Sun]

Plenty of high pressure on the 12z GFS tonight we could be looking at a decent first half to september, certainly for the south, the north could be prone to rain at times

Edited August 29, 2012 by Summer Sun

[shedhead]

As I said several days ago this was never going to be a smooth, linear transition to fine, dry and warm weather from Scapa Flow to Scilly, with the main HP always likely to change position and intensity as the runs unfold. 10-14+ days of wall to wall sunshine was never on the cards, but a good deal of fine, dry and warm weather is still likely as we move through the first week of Sept, with the models likely to pick up a consistantly more setttled pattern once again during the next day or so.

Edited August 29, 2012 by shedhead