Model Output Discussion - September Onwards

[chionomaniac]

For the first time in a long time it seems that we are seeing an MJO forecast that sees an increase in tropical convection to significant levels that will drive us towards phases 7 and 8. Both the GFS and UKMET agree on the general pattern.

The outcome of this is likely to drive any upper trough that is residing in the Atlantic eastwards towards Scandinavia. And indeed that is what is projected by the GFS and ECM. As Steve says if retrogression of the ridge is great enough then some frosts are likely. I love this time of year when one minute we will be looking towards record October temperatures one minute and frosty mornings the next.

[Gibby]

Good evening folks. Here's my balanced, non biased look at the 12zs tonight.

GFS tonight shows a warm week to come but we have 24 hours to see out first when some areas will actually see some rain and windy weather too in the NW. Thereafter GFS shows a large Anticyclone over Germany with warm Southerly winds extending North through the British Isles. Temperatures would become progressively warmer and warmer with some very high temperatures each afternoon late in the week, the highest temperatures in the SE and East of England. Over the weekend high pressure transfers to a position near NW Britain cutting off the warm southerly flow. Instead winds will become variable or from a Northerly point with temperatures falling steadily day by day and night by night with the risk of fog increasing. By the end of the run High pressure remains in control having moved from the UK back into Europe with another centre to the west allowing a surge of cold air to approach Northern and Eastern Britain by the end of the run. So temperatues look like they would become a little above normal by day but some chilly nights.

UKMO is very similar in showing an increasingly warm then very warm week with most areas sharing in some of this by the end of the week. The east and southeast will see the most sunshine and highest temperatures but nowhere will be immune to some very warm days. By the end of the run things still look very warm with the high pressure hanging on just to the east.

ECM tonight also simplistically shows a prolonged very warm spell on the way. With the exception of tomorrow when Southern areas could see some sharp showers from an old trough currently in the NW the rest of the week will be dry and increasingly sunny for all. Temperatures by the end of the week will be in the range 22C over Southern Scotland to 28C in the SE with a light Southerly drift most days. Nights will also remain unusally mild with limited fog patches. As we move towards the end of the run and into October some warm air still holds on for many but it looks like cooler air will be running down North sea coastal areas by 240hrs.

In Summary there could well be some temperature records toppling late this week as all models show considerable warmth and sunshine later. With High pressure over Germany Southerly winds will become dominant with only Northern Ireland and Western Scotland seeing some spoiling cloud at times. The longevity looks like it could well last into the early days of October before a cool down of sorts is hinted at.

[Geordiesnow]

Good charts this morning for warm/very warm weather next week. If this had happened in mid summer, would 31-32c be the most likely maximums in favoured spots in the SE.

I'll be shocked if that did happen, sun is weaker at this time of year and the days are much shorter but temps of 26-28C seems more realistic to me.

The models has changed slightly as I feared in some ways with the Atlantic trying so hard to push the high away and too me, there is a risk that Northern Ireland and Western Scotland in particular could see quite a cloudy week if the Atlantic fronts advance too far Eastwards. As I said a few days ago, the further South and East you are the more room for error you got. The models seem to trend on pushing the high westwards in the later part of the run so there is hope for folk in Western Scotland and Northern Ireland.

As per ever, the hype of the high retrogressing towards Greenland seems to ripe up whenever the models hints at this but normally shortwaves tend to stop this from happening and we get a half hearted attempted NW'ly flow with mild air mixed in and high pressure is never too far away, don't be surprised if this happens again.

[Weather Boy]

From Summer Sun to Winter Cold in less than a week!! Gfs 06z run is a stonking run for the heat and cold lovers! You couldnt make it up! What it does go to show ,is that despite this weeks very warm weather ,the cold season in now not too far away and is now building very cold air in and around the Artic...

True, but of course the real difference is that the first chance is only four days away and therefore highly likely to happen (or very similar) and the second is FI and therefore possible rather than probable.

[J10]

I'll be shocked if that did happen, sun is weaker at this time of year and the days are much shorter but temps of 26-28C seems more realistic to me.

The models has changed slightly as I feared in some ways with the Atlantic trying so hard to push the high away and too me, there is a risk that Northern Ireland and Western Scotland in particular could see quite a cloudy week if the Atlantic fronts advance too far Eastwards. As I said a few days ago, the further South and East you are the more room for error you got. The models seem to trend on pushing the high westwards in the later part of the run so there is hope for folk in Western Scotland and Northern Ireland.

As per ever, the hype of the high retrogressing towards Greenland seems to ripe up whenever the models hints at this but normally shortwaves tend to stop this from happening and we get a half hearted attempted NW'ly flow with mild air mixed in and high pressure is never too far away, don't be surprised if this happens again.

Either I worded it badly, or you have misread, probably the former, I meant could 31-32c be possible if this happened in mid summer.

It certainly won't happen next week

[Frosty.]

I like the trend of the long range models out to mid october with a hint that the floodgates might open to a much colder phase with widespread frosts and even a risk of wintry showers on northern hills, there is a feeling that high pressure may pull back into mid atlantic and allow a Northerly but the next 7 days or so look hot in the south and at least warm everywhere else with rain becoming restricted to the far northwest of the uk and almost unbroken sunshine for many areas, especially further south and east..signs then that the heat will disappear the following week with more chance of ground frost, mist and fog before possibly turning much colder and very unsettled towards mid october..fingers crossed.

[Geordiesnow]

Either I worded it badly, or you have misread, probably the former, I meant could 31-32c be possible if this happened in mid summer.

It certainly won't happen next week

Nope it was the latter, as per always!

If this set up occured in summer we could be looking at the high 30's even low 40's with uppers of 20C(even 25C before being "diluted") making inroads but in summer we don't get such large blocking highs and they tend to be more flabby. It must be very rare to get a Euro high of around 1030MB in the middle of summer?

As it is, I think its perfect summer weather to be honest, very warm but sun is weaker at this time of year so it should be enjoyable. It will be quite humid though but a shock to the system for those joe public folk i'm sure!

[Grimsby Snow Lover]

I like the trend of the long range models out to mid october with a hint that the floodgates might open to a much colder phase with widespread frosts and even a risk of wintry showers on northern hills, there is a feeling that high pressure may pull back into mid atlantic and allow a Northerly but the next 7 days or so look hot in the south and at least warm everywhere else with rain becoming restricted to the far northwest of the uk and almost unbroken sunshine for many areas, especially further south and east..signs then that the heat will disappear the following week with more chance of ground frost, mist and fog before possibly turning much colder and very unsettled towards mid october..fingers crossed.

I do like the sound of that, warm even hot weather is great in SUMMER, not Autumn. I do feel for those areas of the British Isles where it has rained for pretty much the whole summer, but here in the east the ground is still very dry with streams drying up. I for one prefer a more seasonal feel and with talk of the high possibly shifting into the mid Atlantic after next weekend then perhaps the first wide spread frosts could finally occur, lovely.

[Frosty.]

I do like the sound of that, warm even hot weather is great in SUMMER, not Autumn. I do feel for those areas of the British Isles where it has rained for pretty much the whole summer, but here in the east the ground is still very dry with streams drying up. I for one prefer a more seasonal feel and with talk of the high possibly shifting into the mid Atlantic after next weekend then perhaps the first wide spread frosts could finally occur, lovely.

Hopefully we will see a cold front pushing south after next weekend followed by more high pressure but with a cooler airmass over the uk with frost beginning to feature more in the forecast but any rain on the cold front will probably be light and patchy as pressure still looks like being high which will squeeze the life out of the front but then it could turn more interesting from mid month with the jet possibly tilting nw / se with depressions sweeping down from the north west and arctic air behind the fronts..long way off though but there are at least some early hints.

[johnholmes]

'If this set up occured in summer we could be looking at the high 30's even low 40's '

oh come on let's have some thought about what we put in our posts, how many times has the temperature ever exceeded 100F in this country, 100F=38C ?

42C=108F(low 40's)!!

Edited September 25, 2011 by johnholmes

[SP1986]

Even in August 2003 when Gravesend hit 38.5C, the 850hPa temperature were +20C in the southeast. The thicknesses were close to 580. This setup doesn't come close... 850's of +15C and thicknesses of 560-570, even in high summer that would at best translate to 32 or 33C in the southeast.

[Liam J]

'If this set up occured in summer we could be looking at the high 30's even low 40's '

oh come on let's have some thought about what we put in our posts, how many times has the temperature ever exceeded 100F in this country, 100F=38C ?

42C=108F(low 40's)!!

Only once and it set a new record high temp for the UK with 850 temps of 20c into the far SE - chart below and very rare to see this so close and touching the UK.

I'm not sure why some are thinking that these synoptics would produce temps of 35-40c had they occured a few months ago, 850's of 10-15c with the correct thickness would likely peak around 30-33c.

Edited September 25, 2011 by Liam J

[damianslaw]

Tempted not to look at the models for a few days - whilst high pressure is very pleasing for those wanting dry settled weather- it doesn't make for very interesting weather model watching. Expect a very slow migration of heights to our west later next week, possibly setting us up for a much cooler spell of weather after next weekend. In the meantime - its all about the cloud levels, and warmth with very little else to talk about. BBC forecasters may as well get the same script ready from tuesday onwards for the rest of the week ...

[davehsug]

So you don't find the potential for mid-high 20s temps for late September, followed by the potential for a cool-down to low teens-high single digits interesting? I think the models atre pretty gripping stuff at the mo.

[summer blizzard]

'If this set up occured in summer we could be looking at the high 30's even low 40's '

oh come on let's have some thought about what we put in our posts, how many times has the temperature ever exceeded 100F in this country, 100F=38C ?

42C=108F(low 40's)!!

As i mentioned in the seasonal section, you are correct because while uppers are on average 5C higher in July (so they may get close to 20C) and lapse rates are also higher (13C-15C), we would still only get 33C-35C because unlike 2003 we have previously had a rather cool and damp period while the heat built in August 2003 for around 10 days, this spell looks to last just under a week (wednesday-saturday).

In regards to the models, ECWMF12z has the breakdown complete at day 10 while the GFS18z has it at day 9 so it looks like the trend is being firmed up.

[ZONE 51]

500hpa - GFS 18z

+12 to +186

cooling down by early october

temperatures next 7days(afternoons)

Edited September 26, 2011 by ElectricSnowStorm

[reef]

GFS 00z is certainly interesting again with a proper change-around of the sort we more commonly see in Spring. We'd go from maxima in the mid-twenties on Sunday to single figure maxima, air frosts and wintry showers in the north the following Saturday. Its all FI of course so has little chance of happening, but from a weather enthusiasts point of view it would certainly be welcome after the long periods of homogeneous weather we've experienced so far in 2011.

+15 uppers touching the south coast:

http://www.wetterzen...s/Rtavn1442.png

to -7C touching Scotland within a week:

http://www.wetterzen...s/Rtavn3002.png

The most likely outcome I suspect is a weak north-westerly as the GFS is well known for these monster-northerlies in FI. I cant help but secretly hope we get something along these lines, but theres a few days of warm and hopefully sunny weather to come yet, which should be very pleasant indeed.

[Aaron]

Wow.. that second chart is amazing. If only it was a bit closer..

[mushymanrob]

i think the record temp might have been under threat IF these forthcoming charts had been around several weeks ago in summer... unfortunately there doesnt appear to be a thundery breakdown progged by any run, which is a shame, a late thunderstorm off france would have been welcome!

[Gibby]

Good morning. Here's a look at the 00zs of GFS, UKMO and ECM.

GFS today shows the weather settling down over the next 24 hours in response to High pressure to the east of the UK. Warm Southerly winds will move North across the UK in the next 48 hours bringing progressively warmer winds up across all areas. The Southeast and East of England will see the highest temperatures around Friday/Saturday with 27C likely around the London area and East Anglia but pleasantly into the Low 20s in most areas. Over the weekend the highest temperatures leak away from the North as the Southerly flow dissipates and Westerly winds take over from the North with High pressure retreating South of the UK. In FI things become very much colder as winds strengthen and turn Northerly with cool days and cold nights, possibly meaning the first frosts of the winter.

UKMO also shows a warm week to come with the highest temperatures likely towards the SE while the rest of the UK see comfortably warm conditions too but more cloud than GFS as the winds are shown to continue a SW component for Scotland and Northern Ireland. By Sunday the Southerly flow is also shown to weaken with calm conditions leading to the more likely incidence of overnight fog.

The Fax Charts also show a warm setup with Southerly winds wafting North over the UK though troughs never far away from NW areas with some rain in the far NW.

ECM today follows the same theme with a cut off Southerly flow by Sunday with the same risk of overnight fog shown by UKMO. The hottest weather through the week will be in the SE with comfortably warm conditions elsewhere too. Towards the end of the run High pressure sinks away South with an increasing Westerly then NW flow heading south over Britain with temperatures reverting to nearer normal values by then with the warmest of conditions towards the SW by then.

It looks this morning that a very warm week is still on the way. Things look a little less likely for any record breaking temperatures on a National scale but the SE may approach such values on Friday and Saturday. The far NW misses out once more keeping mild SW winds, cloud and a little rain at times. After the weekend all models show something of a collapse in temperatures from the North as deep Low pressure throws cold fronts SE over Britain in 10 days or so although rainfall from these is likely to be light and sporadic.

Edited September 26, 2011 by Gibby

[Frosty.]

'If this set up occured in summer we could be looking at the high 30's even low 40's '

oh come on let's have some thought about what we put in our posts, how many times has the temperature ever exceeded 100F in this country, 100F=38C ?

42C=108F(low 40's)!!

August 1990 rings a bell as being hot, 32c 90f in yorkshire and mayber it was nearer the mid 90's?

This morning's Ecm 00z keeps us on track for a very warm week, hot in the southeast with temps of 26c by midweek and 27-28c by friday with a hot weekend to follow but then turning cooler everywhere but maybe warming up again in the south for a day or two before the real onset of more seasonal temps and a fair chance of the first cold plunge into the north in around 8-10 days time with snow for the scottish mountains and widespread frosts for a time..so the next few weeks look like producing some exciting weather.

[Aaron]

'If this set up occured in summer we could be looking at the high 30's even low 40's '

oh come on let's have some thought about what we put in our posts, how many times has the temperature ever exceeded 100F in this country, 100F=38C ?

42C=108F(low 40's)!!

Very unlikely but certainly possible, just like -30C is possible, and would have been reached in the UK in 1995 or any other times -27c was reached, for example, if it weren't for cloud..

Edited September 26, 2011 by Aaron

[chionomaniac]

So you don't find the potential for mid-high 20s temps for late September, followed by the potential for a cool-down to low teens-high single digits interesting? I think the models atre pretty gripping stuff at the mo.

I agree with you here Dave. For the second time this Autumn my ears are pricked up by the potential pattern in the offing. There are a number of potential reasons why. Firstly we have the Atlantic trough sucking up the warm uppers from southern regions. With the potential to see mid to upper twenties this week we have a noteworthy pattern.

Secondly we see that the trough is forecast to be sectioned leaving an Azores cut off low. For those who do not know what a cut off low is it is worth demonstrating with some charts of the jet stream.

The first chart we see below shows the jet stream with a big loop - this is known as a long wave trough as the upper air pressure and heights are lower inside the loop then the equivalent latitude just East or West.

As we progress a stronger pulse of the jet stream by passes the loop cutting it off from the main flow completely. At this point the residual eddy is like a spinning top that will slowly spin itself out. The cut off low is born and is the equivalent to the jet stream as an ox bow lake is to a river.

On the 500 hPa charts this appears as the low around the Azores seen below

The significance of this cut off low is that it will interfere with heights building in the short term around the Azores. When we add this to the possible increase of tropical convection activity leading to a progression of the MJO to a higher amplitude phase 7, we will witness that the long wave trough will try and establish itself over Scandinavia. With the cut off low preventing heights rising over the Azores then some retrogression of the existing ridge will be forced northwards. But will the jet stream flow encourage linkage with Greenland allowing a sustained northerly blast or will we see another solution?

Intriguing.

[BLAST FROM THE PAST]

C

I think that the retrogression will not be that high and hold but get toppled out eventually.

BFTP

[johnholmes]

I agree with you here Dave. For the second time this Autumn my ears are pricked up by the potential pattern in the offing. There are a number of potential reasons why. Firstly we have the Atlantic trough sucking up the warm uppers from southern regions. With the potential to see mid to upper twenties this week we have a noteworthy pattern.

Secondly we see that the trough is forecast to be sectioned leaving an Azores cut off low. For those who do not know what a cut off low is it is worth demonstrating with some charts of the jet stream.

The first chart we see below shows the jet stream with a big loop - this is known as a long wave trough as the upper air pressure and heights are lower inside the loop then the equivalent latitude just East or West.

As we progress a stronger pulse of the jet stream by passes the loop cutting it off from the main flow completely. At this point the residual eddy is like a spinning top that will slowly spin itself out. The cut off low is born and is the equivalent to the jet stream as an ox bow lake is to a river.

On the 500 hPa charts this appears as the low around the Azores seen below

The significance of this cut off low is that it will interfere with heights building in the short term around the Azores. When we add this to the possible increase of tropical convection activity leading to a progression of the MJO to a higher amplitude phase 7, we will witness that the long wave trough will try and establish itself over Scandinavia. With the cut off low preventing heights rising over the Azores then some retrogression of the existing ridge will be forced northwards. But will the jet stream flow encourage linkage with Greenland allowing a sustained northerly blast or will we see another solution?

Intriguing.

nice post laying out the possibilities and I like the explanation about cut off lows and what conditions they can create.

thanks ch