Model Output Discussion 18z 30/09/15

[Cloud 10]

ECM op run looks too flat this evening in the mid-term,and is out of step with other op runs and ens. means.

 

[knocker]

The transition to the flatter pattern on tonight's GEFS anomalies.

http://www.tropicaltidbits.com/

 

[bobbydog]

If you're expecting cold and snow in the month of november you was setting yourself up for disappointment.

'were' not "was"...

[Man With Beard]

With the start of November, we see like clockwork a stepping up of the polar vortex. Much more organised by D6-D10. Chances of early November being below average look unlikely now unless the Azores High has enough proximity to the UK to keep things stagnant, or to allow a very light easterly/northern influence. With eastern heights starting to give up, the chances of Atlantic storms tracking ever further south increase to. An early taste of winter looks quite far off

[ANYWEATHER]

Evening All .hope you had a good weekend and enjoyed the extra hour in bed! Unfortunately , the weather is giving the models some huge headaches ! The Atlantic is in control, a little bit in slow motion the further east you are ! If we go to the end of the week some wet and windy weather to the west . Forget the rest....

[Vorticity0123]

As autumn is well underway, the days start to get shorter and leaves are quickly changing colours. So far this autumn has seen quite a bit of blocking over mainland Europe, with the ‘traffic jam’ often starting at the UK itself. As of writing, we are once again heading towards a period of European blocking. However, this time troughing is residing much closer to the UK itself. How will this situation evolve, and why is the uncertainty in the expected surface weather greater compared to average in a situation like this? You will find the answers below!

 

The building blocks towards yet another block

 

For the analysis here I will make use of GFS forecast maps from Wetterzentrale.de.

 

If we look at the current weather map, we can find a few hints that we are yet again going to enter a situation of blocked weather. If you are confused about the term blocking, a good guide what explains what blocked weather means is: http://www.theweatherprediction.com/blocking/

 

GFS surface level pressure (white lines) and 500 hPa heights (colours) 12Z run analysis. Courtesy: Wetterzentrale.de.

 

One notable feature is a 500 hPa trough (blue colours, indicated by the blue line) which is extending to the south towards the west of Ireland. At the surface, a rather deep low pressure area (central pressure of lower than 975 hPa) is present at that location. This low pressure will be a primary actor of the weather in the UK over the next few days.

 

On the other side, high pressure persists over central Europe, and also at 500 hPa height a weak signature of a ridge can be seen (orange colours pointing northward, indicated by the red line). This weak ridge will be the key to yet another block, as we will see below.

 

East-west split over the UK

 

To visualize the buildup of high pressure, we will look at the GFS forecast for Tuesday 27 October:

 

GFS surface level pressure (white lines) and 500 hPa heights (colours) 12Z run T+48. Courtesy: Wetterzentrale.de.

 

The weak ridge of high pressure over central Europe mentioned above has developed in a full-fledged 500 hPa-block oriented northwest-southeast towards Iceland (orange/yellow colours pointing northwestward, indicated by the red line). To the southwest of this high pressure area, the aforementioned trough at 500 hPa is still there (green colours, indicated by the blue line). This is also called a high-over-low situation.

 

For the surface, the aforementioned high-over-low situation results in an east-west split over the UK itself. This is not a northeast-southwest split as one would suggest by looking at the 500 hPa heights. Why is this the case? The high pressure area to the east of the UK is north-south oriented rather than northwest-southeast close to the UK. As a result, the expected weather pattern resembles more of an east-west split.

 

New ‘energy’ from the south

 

Just as the blockade at 500 hPa tends to move eastward away from the UK, a new ridge starts building from the south, as envisaged below:

 

GFS surface level pressure (white lines) and 500 hPa heights (colours) 12Z run T+120 Courtesy: Wetterzentrale.de.

 

Once again low pressure and associated 500 hPa trough reside to the west of the UK (indicated by the blue line). Also, the high pressure area which was previously pointing towards Iceland has weakened and moved further northeastward (indicated by the red line).

 

More notably, a new impulse of high pressure is becoming established from Western Europe. This is visible by higher heights (red colours) pointing northwards towards the Benelux (denoted by the red arrow).

 

1040 hPa high pressure area in GFS ensembles à High confidence?

 

One thing that struck me was the high level of confidence that is associated with the blocked pattern. Comparing the GFS and ECMWF ensembles for 6 days out makes things clearer:

 

http://www.wetterzentrale.de/topkarten/fsenseur.html GFS ensembles

 

http://www.wetterzentrale.de/topkarten/fsecmeur.html ECMWF ensembles

 

Both model ensembles show a strong high pressure area and associated 500 hPa ridge settling near Eastern Europe. In fact, the GFS ensembles show an area of over 1040 hPa at the surface near Belarus. For an ensemble, this is a really high pressure for such a long lead time. This implies that confidence is very high that high pressure will dominate the scene there.

 

The only notable difference is that the ECMWF is somewhat quicker in pushing the ridge away to the east.

 

The surface is the key – or not?

 

Even though there appears to be high certainty that the UK will be located between troughing to its immediate west and strong ridging towards its far east, the weather the UK will experience at the surface at day 6 is much more difficult to pinpoint. For example, compare the runs of the GFS, ECMWF and the UKMO for 6 days out:

 

http://www.wetterzentrale.de/pics/Rtavn1441.gif GFS

 

http://www.wetterzentrale.de/pics/Recm1441.gif ECMWF

 

http://www.wetterzentrale.de/pics/Rukm1441.gif UKMO

 

The operational runs of all models look to be reasonably consistent with ridging to the east and troughing to the west. Therefore, the confidence in the upper (500 hPa) pattern is rather high.

 

However, this does not mean that confidence is also high about the pattern at the surface. The ECMWF has its surface low pressure area much more to the north (to the northwest of Scotland) compared to the GFS and UKMO (to the west of Ireland). The resulting weather would therefore also be quite different.

 

The ensemble temperature forecast for London shows this spread nicely

 

Ensemble temperature forecast for London based on the ECMWF 00Z run. Courtesy: weathercast.co.uk.

 

The square area denotes the area discussed above for Saturday 31 October. Note that there is a large spread between the different ensemble members with respect to the expected temperature forecast in London for the 31st of October. In fact, the temperature is expected to lie somewhere between 10 and 15 degrees Celsius.

 

So, why is the surface forecast for a few days out so uncertain? One could come up with two main reasons.

1. The UK is located at the border of two upper air systems, with a ridge to the west and a trough to the east. Any slight deviation of the weather pattern to the east or to the west with respect to the upper air pattern has got major consequences for the weather experienced at the UK.
2. Even if the upper air pattern is exactly consistent, with such a feeble situation the exact positioning of high and especially low pressure areas at the surface is very uncertain.

A take home message: despite the fact that the surface pattern is ultimately the pattern that determines our weather, it is still important to look at the 500 hPa pattern. This is because the 500 hPa pattern is not influenced by factors like friction by land surfaces, and this pattern is thereby much easier to predict in the future. That is also why 500 hPa charts always show much more consistency in, say, 10 days as compared to surface charts. As a result, the 500 hPa charts are a very good guide to get a rough guide of the weather to expect in long range.

 

Conclusion

 

Once again a blocking ridge of high pressure is forecast to set up shop to the east of the UK like we have seen more frequently this autumn. While the upper air pattern seems to be quite set in stone for the next few days, the details at the surface yet have to be resolved. Therefore, even if we are stuck with a blocked pattern, there is plenty of interest to be found .

 

Sources:

http://www.wetterzentrale.de/

www.weathercast.co.uk/services/ensemble-forecast.html

Edited October 25, 2015 by Vorticity0123

[knocker]

Like the GEFs the ecm anomalies are removing the block and establishing a flatter zonal pattern, around T192, with suggestions of ridging from the south west. This continues into the ext period. The NOAA 8-14 also moving in that direction.

Edited October 25, 2015 by knocker

[Newberryone]

Like the GEFs the ecm anomalies are removing the block and establishing a flatter zonal pattern, around T192, with suggestions of ridging from the south west. This continues into the ext period. The NOAA 8-14 also moving in that direction.

Ecm 12z op pretty much singing from the same hymn sheet as those anomalies with regards to that ridging from the south west. To my untrained eye, perhaps just a transient affair as hinted by Ecm Day 10 chart though as the most likely outcome from that chart would suggest a mini pm incursion.

Edited October 25, 2015 by Newberryone

[Gael_Force]

'were' not "was"...

Since it is not in the past, don't you mean 'are'? ...... <pedantry aside>

[knocker]

Since it is not in the past, don't you mean 'are'? ...... <pedantry aside>

 

I can see a discussion on the Oxford apostrophe looming.

[bobbydog]

Since it is not in the past, don't you mean 'are'? ...... <pedantry aside>

He implied that the poster himself was expecting cold (present tense) but was actually referring to the post itself, which having already been posted, transitioned his statement to the past tense. Therefore "were" not "was" or "are" was the correct word to use in this case.

I can be just as pedantic...

[bluearmy]

I don't remember a group called 'were not was'?

The ops are definitely looking flatter as per the ens but potentially still fairly blocked - just not as amplified. If upstream is a tad less flat come verification then presumably the flatter blocked becomes amplified blocked ?

There are many different options showing across the NH our side in the 8/14 day timeframe - a lot of spread.

Statistically, we are due a more mobile period but that may be pushed back yet again.

[Snowmad79]

From a coldie point of view. As we head into November we need to start seeing this HP anomoly over europe removed and replaced with troughing through the UK / Scandi and into europe. Euro highs are only going to keep the Jet Steam running over the UK on a SW to NE trajectory or just north of the UK prolonger mild and/or wet weather into winter.

 

With a PV already looking stronger at this time of year in comparison to 2010 for example and the lack of HP over the pole, it will just make it even more difficult to get High Latitude blocking rather than what is currently modelled and what I personally would class as mid lat blocking (where the core of HP is centred over Europe).

 

 

Much rather we start seeing the following for our region:

 

 

 

 

 

Not the best charts to be posting but the Jet Stream as it is now with strong HP over Europe is still too far north for my liking. Current modelling shows the troughing just to west of the UK feeding in Wet and blustery weather from the SW before drying out again due to the stagnant Euro high exerting more influence again as it sends the jet SW > NE over the UK.

 

Edit: Sorry added more but the editor cut some out and  will not let me post anymore charts due to object error

Edited October 26, 2015 by Snowmad79

[Ali1977]

ECM at day 10 looks like a change towards colder conditions is heading our way, PV has shifted towards Siberia and hights starting to build to our NW. The GFS and ens are not particularly going with that however. Only time will tell but I do think some charts pointing to another blocked pattern will start appearing on the GFS by the end of this week -for mid Nov.

[IDO]

As many have highlighted the Azore High is still proving stubborn and that will continue to wax and wane with any lower heights that creep down from the NW, so a ridge/ trough mobile pattern in our region is likely as it does appear that in the next 7-10 days the MLB remains over Europe before the ops tend to retrogress it towards the UK.

 

GEM at D6 and then D10 is the fastest:   

 

GFS takes longer. The GEFS at D16 offer minimal interest, we seem to be entering a phase where the late Summer/Autumn blocking is mixing with the inevitable but slowly firing up of the PV, with the removal of any HLB now complete in our region, and we are just waiting on how the MLB is handled. The D16 mean sums up the GEFS:

 

Last night's GEM mean at D16:

 

Not getting any hints from the MJO, it appears to be just travelling through the phases and may not be very instructive as to medium term guidance:

 

 

So a holding pattern as the PV flexes its muscles against the MLB in our region. You would think that a more Atlantic driven pattern would eventually break through but if it does it is looking to be a slower than expected process, and of course it may continue to hang around (hopefully).

 

 

[knocker]

A broad overview of th gfs and ecm runs out to T240. Both have a fairly active Atlantic and the HP to the east is generally too far east to create an effective block so systems sneak into the UK . By T240 the ecm has nudged the HP to the east south connecting to the Azores HP whilst the gfs delays doing similar for a day or two. This scenario is supported by this morning GEFS anomalies.

http://www.tropicaltidbits.com/

Edited October 26, 2015 by knocker

[Summer Sun]

For the start of November ECM shows the high moving over more parts of the UK with any rain baring systems becoming increasingly restricted to the far north and west

 

 

Temperatures should generally be on the mild side for the time of year

[knocker]

This morning's ecm anomaly supports the ops with the flattening out of the pattern reducing the effectiveness of the eastern block and promoting ridging from the SW. In the ext period the block to the east is no more and certainly for a brief the UK comes under the influence of a high cell originating to the south west.

 

[london and south east kent]

Hi all looks like the big freeze is on its way get your snow boots ready...

[Met4Cast]

Hi all looks like the big freeze is on its way get your snow boots ready...

 

That's some chart! It's just a shame we have a weak PV lobe sitting to the South-West of Greenland preventing high pressure from amplifying, instead it just sortof sits over the UK deflecting the cold to our East. 

[carinthian]

Took a break from this site for a few days whilst making a visit to Sussex. Back today in the sunny Austrian Alps. Well done to UKMO to getting it right again for this weeks forecast of mild conditions returning with good Fax chart progs and the longer post 144hours charts for which we cannot access here.

 C

[sunnijim]

That's some chart! It's just a shame we have a weak PV lobe sitting to the South-West of Greenland preventing high pressure from amplifying, instead it just sortof sits over the UK deflecting the cold to our East.

In the abcense of any real 'material' to play with for coldies it's worth a comment.

If anything like that chart came off it could be seen as a 'building block' for cold longer down the line,get some decent cold in place to our East first then pray that other building blocks start to go the UK's way further along and transport it to our shores!

[nick sussex]

The ECM seems to be in default some interest at day 10 mode. The last 4 runs have tried to remove the high over Europe and edge troughing se however this hadn't moved any nearer.

I'd be wary of any removal of that high until it's shown to start moving forward in the outputs and the ECM might be trying to find some amplification which won't verify.

Cant complain too much though as the weather has been stunning down here for the time of year. I'd rather have this then a mobile wet mild pattern.

[Summer Sun]

Looking at the 00z pressure ensembles the first 11 days at least of November look fairly settled whilst we do have some scatter very few runs go below 1000mb and the majority are no lower than 1010mb

 

 

The Aberdeen ENS looks a bit more unsettled this to me suggests high pressure close to the south and low pressure systems crossing to our north

 

[phil nw.]

The 12z GFS and UKMO showing the UK and indeed much of W.Europe on the warm side of the blocking high to the east in the coming days.It looks like some some warm air will be drawn north as we lie between the stalling Atlantic trough and the soon to be Euro high as it sinks south from Scandinavia.

 

 

a hemisphere view at t144 from the UKMO

 

Signs of the high not giving way and building back towards us later driving the jet and the more unsettled weather away to the north.

Edited October 26, 2015 by phil nw.