Model Output Discussion 26/08/2012

[Frosty.]

Thankfully the days are still long enough to bring some decent heat as the chart you posted shows

Yes that's true.

It's good to see solid model agreement for the anticyclonic spell next week, I think the models have handled it very well in recent days. We look like having a similar day to saturday on wednesday with some torrential downpours/storms but things then quietening down quite dramatically beyond wednesday.

[shedhead]

I may have taken a little while, but as expected all the models have now locked onto the theme of a substantial rise of pressure and the eventual development of a protracted fine, warm/very warm spell from the south. Subtle changes in the position and intensity of the high will mean the detail is still likely to remain elusive for a time, but the Paralympics certainly look set to do far better than the Olympics themselves, despite the lateness of the season.

[Summer Sun]

Lovely 12z run from GFS looking like 7 or 8 days of high pressure the perfect start to September

Edited August 27, 2012 by Summer Sun

[damianslaw]

Significant changes on the way, the models are all suggesting the azores high will be ridging NE later in the week sending the jetstream and its associated low pressure systems away to the NW.. just in time for the start of autumn and the start of the school term in England and Wales. How long the settled spell will last remains to be seen, but the signals suggest such conditions could well last for at least a week and quite possibly longer.. but we can't be certain at this stage just how long.

So summer 2012 looks like one bookended by summery conditions just before it started and just after it finished.. i.e. late May and possibly early Sept delivering the best conditions nationwide.. and nationwide is the key word, whilst central,east,south and se england has seen some lengthy decent summery weather in the second half of the summer, much of the NW quarter esp N Ireland, SW Scotland and NW England hasn't seen any such spell this summer, but the whole country joined in with superb weather in late May.

[Summer Sun]

UKMO continues to show the pressure rise very nicely

[shedhead]

Lovely 12z run from GFS looking like 7 or 8 days of high pressure the perfect start to September

Personally I'd add another 10 days to those numbers Gavin, but I'd expect that would attract the same kind of critisism that my call for a lengthy fine, warm spell to start September did. So let's go with the 7-8 for now and look forward to enjoying them, they've been a very long time coming for 70% of the UK.

Edited August 27, 2012 by shedhead

[Summer Sun]

Personally I'd add another 10 days to those numbers Gavin, but I'd expect that would attract the same kind of critisism that my call for a lengthy fine, warm spell to start September did. So let's go with the 7-8 for now and look forward to enjoying them, they've been a very long time coming for 70% of the UK.

I've played safe with 7 or 8 as beyond that looks uncertain atm hints of it turning unsettled but who knows

[Gibby]

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

All models are in fair agreement on the synoptic pattern from now until the weekend. The current front bringing wind and rain to Northern and Western areas moves on eastwards over the next 12hrs clearing the East coast by dawn. Tomorrow is then shown to deliver all areas a SW flow with sunny periods and scattered showers these mostly in the North and West with very few in the SE. It will be less windy and warmer tomorrow. Wednesday sees an active and complex weather system drift slowly East over Britain later in the day. Heavy and potentially thundery outbreaks of rain starting the day in the West will move further East later. A partial clearance to sunny intervals and heavy showers could reach the West later. Thursday sees another showery day with less in the way of showers later in the North and West as a ridge approaches from the West. By Friday it will be the turn of the South to see better conditions as a ridge transfers to Southern Britain. Further North a warm front crosses East bringing cloud and rain to these parts through the day. Saturday sees High pressure becoming established close to Southern Britain with a Westerly flow over Britain. Still cloudy weather likely for Northern and Eastern areas with some rain too in the North while Southern areas see the best of the dry and brighter conditions.

GFS then shows High pressure centred near Southern Britain for a good week from Sunday with a lot of fine and bright weather before a gradual transition to less settled conditions slowly develops later in FI with a much more distinct Autumn feel to things by then. During the fine period the daytimes will be warm with some good sunny spells but nights may be chilly in lowland areas where winds stay light with some morning mist and fog patches possible. Also Northernmost areas will be slow to see much improvement until midweek when it joins the rest of the UK in some decent conditions for a while.

The GFS Ensembles show the warm up clearly as we reach the new mont with near +10C uppers over the UK for a week before slowly falling to levels just above the long term mean by the end of the run. With only patchy rain shown from the start of the new month the most striking feature of tonight’s Ensembles is the agreement shown between the members right up to the 7^th now for Southern locations especially.

The Jet Stream Forecast shows the flow currently located over Southern Britain and Northern France buckling North through the Eastern Atlantic by the weekend and then locating in a much more usual position between Scotland and Iceland next week.

UKMO shows High pressure centred near SW England at midday on Sunday with a dry, bright and warm day for England and Wales with Scotland and Northern Ireland continuing to see more cloud with some rain in the far North in the moist and humid Westerly flow.

ECM has High pressure centred much further SW than GFS or UKMO on Sunday with a tenuous ridge over the extreme South. A westerly breeze elsewhere brings a lot of cloud towards the North with some rain too in the NW. Further South fair weather would prevail with a few sunny breaks but a lot of cloud too. By Monday and Tuesday High pressure holds on but more as a ridge over Southern Britain as opposed to a full blown High. The weather would be OK with some warm sunny spells in the South but cloud amounts will still be quite large over the UK and the North would remain close enough to fronts to be at the risk of rain. The run ends on Wednesday and Thursday with a weak front crossing East on Wednesday bringing a risk of some rain before High pressure reassembles itself over the top of Britain returning dry and settled conditions to all.

In Summary the models are not in total agreement in what form next weeks’ fine spell takes place with GFS pulling High pressure gradually East through the week while ECM tonight shows High pressure hardly making it to our shores before receding back SW by the same time point. It still looks pretty good but the fundamental final position of the High will determine how warm and sunny it will be and how long or short the spell will last. A retracting High to the SW tonight has allowed a weak front to come into the mix from ECM for next Wednesday but the model then quickly redeems itself for the close of the run with dry and settled conditions returning. I think we may see plenty of jockeying about in the position of that high until things become clearer after the next few days unsettled weather is out of the way.

[steelermark]

Personally I'd add another 10 days to those numbers Gavin, but I'd expect that would attract the same kind of critisism that my call for a lengthy fine, warm spell to start September did. So let's go with the 7-8 for now and look forward to enjoying them, they've been a very long time coming for 70% of the UK.

And so it should. I believe this is the model discussion and the models don't go out the extra 10 days your talking about. I very rarely see you substantiate anything on here...so would you like to show us why you "expect a 2-3 week spell"??

[Methuselah]

Aye, but we all get a bit carried away from time to time. Back to the models?

[Killerbeast]

Tomorrow a new weather system is expected over Great Britain. In terms of its dynamics it can be said that it will be in its last stages of development, with its axes almost vertical which gives us a barotropic atmosphere. Anyway, the cold occlusion will cause widespread precipitations and almost no change in temperatures. A relative shift towards more sunny spells will come with the ridge from southwest. The temperatures at 850 hPa will rise to 10 - 14 degrees.

[mushymanrob]

so high pressure is pretty much nailed as taking control from the weekend onwards qand for the forseeable future after that. good! but initially at least im not so sure that itll be sunny for all. with a pretty long draw off the atlantic i expect cloud will be an issue, especially in western and northern areas.

[Gibby]

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

All models show a Low pressure out to the NW and a SW flow over the UK. A further Low will move slowly East across the UK later on Wednesday with a trough of Low pressure swinging East with it over the UK. By later Thursday a strong ridge of High pressure moves in from the West and topples SE to Southern areas by Friday. From Friday to Sunday a belt of High pressure lies West to East just South of the UK with a Westerly flow over all areas. The weather for the rest of the week will see scattered showers today before more general and potentially thundery rain moves East tomorrow. Brighter and more settled conditions move in on Thursday and Friday before a warm front in the Westerly flow brings rain at times too Northern areas over the weekend while the South stays dry and becomes warmer if rather cloudy.

GFS then moves into next week with High pressure dominating things with a centre near Southern Britain gradually transposing into a belt from mid Atlantic across the UK to Germany. A lot of dry weather is shown though the far North will continue to see cloudier periods with occasional rain as troughs brush by to the North. Some warm sunny spells would occur for many Southern locations in what should be a dry week. In FI pressure remains High in mid Atlantic with the ridge to Britain weakening with time allowing a cool off in temperatures and the eventual return of Low pressure swinging down from the NW bringing rain and wind in late on.

The GFS Ensembles this morning show a warm up at the turn of the month for all areas with uppers near +10C or above in the South for the first week of September before cooling off at the end. The operational was a warm outlier for the South in its later stages with rain a scarce commodity here. Further North after the initial warm up there is a huge amount of spread between the members in the 1^st week of September with a quicker cool off to normal uppers than locations in the South.

The Jet Stream Forecast showsthe current position of the Jet near Southern Britain on the move soon as it buckles North through the Atlantic to a position flowing NE through the Atlantic towards Iceland by Friday and diving South over Britain. By next week it is flowing well North of Britain between Iceland and Scotland.

UKMO has a High pressure cell centred in the SW approaches at midnight Monday with a cloudy Westerly flow over Northern areas with some rain in places. Further South the proximity of the High will maintain dry conditions with a lot of cloud tumbling down from the NW at times but with some clear or sunny intervals too, especially in the SW.

ECM today is once more not as good in its evolution with High pressure to the SW pulling back further west as we move into the midweek period. After a cloudy start to next week in Northern areas with some rain in the far North a drier interlude occurs here by Tuesday when it joins the South in fair conditions with a mix of broken cloud and some sunny intervals. By midweek pressure begins to fall as the High cell drifts West into the Atlantic with a cool and potential return to showers as a slack NW flow develops. As last night though this is a temporary phase as High pressure rebuilds more strongly encompassing all of the UK by the end of the week delivering dry and bright conditions with some chilly nights by then.

In Summary there is little overall change in the models general pattern from that of last evening’s 12zs. The deeply unsettled conditions of recent times are coming to an end by this weekend with High pressure domination shown by all models from thereon. However, it will not all be plain sailing warm and sunny weather as the location of the High has been shifted South and SW persistently over recent runs to the extent that a Westerly flow will bring a lot of cloud and weak fronts across the North with some rain for a while and the cloud could well spill across Southern areas at times as it moves around the Northern flank of the High. Further out ECM still shows room for a trough to move SE on Wednesday giving a brief risk of showers though this is the catalyst for a reload of High pressure in a much more favourable position late on to keep all places dry and increasingly sunny if a little cool.

[bluearmy]

Presumably some posters will now appreciate why it's important to pay attn to ens output rather than ops at more than a week out as we see the core of the high and heights failing to get to our east first half next week. Beyond and that would currently seem unlikely for the time being although I suspect that eventually those higherheights have to transfer east as a breakdown comes in.

[Frosty.]

The remains of Ex Hurricane Isaac have some impact on the Gfs 00z in FI , as isaac becomes absorbed into the main area of low pressure in the iceland region and just adds fuel to that system, the result is eventually for a shock cool plunge to sweep southeast across the uk later in FI. it would be a shock to the system following the benign settled and warm weather across the south of the BI next week. For northern britain, the anticyclonic spell being shown on the gfs 00z is not looking as good as it did yesterday with more of a moist sw'ly flow as the main core of the high is centred rather further south and eventually the main body of the high pulls well back into the atlantic which helps to tilt the jet nw/se and eventually leads to the much cooler and unsettled spell. The south of britain can still look forward to a warm and settled spell lasting at least a week but this op run is not that good for the north at least.

[AWD]

According to Matt Hugo:

Latest EC 32 day signalling a clear trend towards high pressure up to approx 9th of Sep, after that low pressure/unsettled conditions return

Which is backed up by the NAEFS charts:

Obviously still a long way off, but if something similiar to the above does happen, it doesn't necessarily mean a return to cool, endless wet & windy weather. If the trough stays out west in the Atlantic, then something similiar to August would prevail, in that whilst turning more unsettled, it would still remian rather warm. My bet is on it remaining rather warm generally, but turning less settled after the opening 10 days or so of Sept, with more convective activity rather than frontal rainfall.

So a fine, settled spell ( especially so for southern areas, northern areas plagued by cloud/light drizzle a little more ) to open September before pressure falls and it turns less settled in Mid Sept, with temps still remaining average - above average!

[Milhouse]

The ECM mean ends with high pressure still in control and upper temperatures above average for all.

Edited August 28, 2012 by Milhouse

[Bobby]

According to Matt Hugo:

Latest EC 32 day signalling a clear trend towards high pressure up to approx 9th of Sep, after that low pressure/unsettled conditions return

Which is backed up by the NAEFS charts:

Very likely I'd say, that's been the pattern more or less since April hasn't it - a few days a month where high pressure ridges in before the Alantic and rain moves back in. The NAEFS chart setup looks like a copy and paste from most days since early April, high pressure over Greenland and troughing limpeted near us. The pattern that doesn't want to shift.

Edited August 28, 2012 by Barb-

[Glacier Point]

Very likely I'd say, that's been the pattern more or less since April hasn't it

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.

Edited August 28, 2012 by Glacier Point

[SMU]

I would tend to agree with Stewart although with one caveat-

with increased wavelengths of mid lattitude highs developing towards the pole - due to the reduced snow cover & arctic ice cover I think that the 500 MB anomaly for Sept + OCt looks a lot like the second one above-

However because the polar cell will be expanding erratically south through a meridional jet as opposed to gently south across a sinuous jet I would say the the mean trough & the quasi stationary Euro high with be pushed maybe 5-10 degrees south - especially as we head to Oct -

The net effect of this will be less of an influence from the South & more unsettled influence from the North & NW-

I expect Iceland to be below average as well- I think we could see some early frosts in the extremem North & the polar air escapes South rather that South east- / east from greenland....

S

[johnholmes]

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.

[Summer Sun]

Still on course for 7 decent days from the weekend, ex hurricane isaac is looking like tracking north of the UK, at this stage it should have no or very litle effect on the UK though this could change over the next week or so

[Gavin Hannah]

Most of the ensembles show high pressure in to the south-south east out in FI. The exact location of this ridge wanders all over the Atlantic and on top of the uk however.

Edited August 28, 2012 by IBringTheHammer

[summer blizzard]

Still on course for 7 decent days from the weekend, ex hurricane isaac is looking like tracking north of the UK, at this stage it should have no or very litle effect on the UK though this could change over the next week or so

Thats not Issac and at a guess i suspect it is 98L (not even a TD yet).

[Gavin Hannah]

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