Model Output Discussion 18/11/2012 12Z onwards

[chionomaniac]

Good afternoon all.

I hope you are all having a very pleasant Sunday afternoon.

Well, with the end of November rapidly approaching we are all wondering what the new season will bring. We have a multitude of signals suggecting that northern blocking is about to occur - but none of these have shown themselves in the reliable timeframe. So to recap and to help start this thread off, let's go through them all.

The GFS and ECM have jumped repeatedly in their outputs recently - often this lack of consistency is indicative of a change of pattern - and certainly there are clues that this could occur.

Currently, we are in a pattern that holds a trough to our Northwest for the next week or so.

There are suggestions that this trough will hold position and lead to height rises over Scandinavia which could give rise to an easterly - but it looks as though the height rises in Scandinavia will be slowly pushed by the trough to our west moving westwards.

There is numerous support for this occur in the 8+ day timeframe.

Here are the comparison H500 anomaly charts for the 8-10 day comparing the ECM and GFS.

Both are pushing troughing towards our SE - never a bad place for this to be!

Other H500 anomaly charts go further with the suggestion that height rises will replace the lower heights to our NW in the longer timeframe. They are all pointing in the same direction.

So is there anything else that can add credence to this scenario occurring?

The ECM and GFS MJO forecasts are both heading towards phase 1 towards the end of the month.

http://www.cpc.ncep....clivar_wh.shtml

This is a good phase for support of GH blocking - in fact it is uncannily accurate when comparing to the anomaly charts above!

Now to the strat. The cooling seen in the autumn has been a fly in the ointment for those wishing to see a -ve AO winter. However there are signs that things are on the turn. During the last week wave 2 activity has put pressure on the vortex creating a monkey nut shape to the vortex. I suspect that this is likely to reoccur towards the end of the month and numerous forecasts right up into the middle strat have appeared. Yet again this has appeared on the latest GFS 06Z run - with the only difference being that the split is stronger and earlier!

30 hPa chart at T+ 12 days

The postion of the split vortex here is more conducive to NWern height rises and cross polar flows. Furthermore, these type of blocking patterns seen recently are precursers to stratospheric warming events, so that I wouldn't be surprised to see a stratospheric warming occuring after - into early December - how significant this will be is difficult to say currently but it will be useful down the line.

So, with a lot of pointers suggesting blocking towards the NW of the UK we are likely to see a cold pattern emerge from the start of December. In fact I can see it repeating and building towards mid December as the vortex takes repeated hits. If these hits can damage the stratospheric vortex and lead to warming then it willl be the perfect start to winter.

Besides the upper stratosphere, I haven't seen such pointers to a cold set up since the start of 2010 -and we all know what happened then.

Let's hope a repeat is on the cards.

Please keep on topic and be relevent as always

c

[JP1972]

I know this post will be deleted but hopefully before it is someone from the NW team might just consider my request ...

When posting charts etc can the knowledgable among you also describe what conditions said charts might bring to which areas please? Obviously I'm not about a localised forecast but something along the lines of "If this chart materialises this might well bring rain/sleet/snow/showers to the NE, E, SW" etc?

If this isn't possible then fine, but just thought it would add to the educational value (for me anyway) of visting this thread and the site in general.

Sorry and thanks for reading (and deleting!).

Mark.

Edited November 18, 2012 by JP1972

[weather eater]

Great post Chiono not sure about your monkey nuts though, better keep those to yourself.

Edited November 18, 2012 by weather eater

[chionomaniac]

I think the 6z Ensembles suite say it all really , that's the mildest set I've seen for a while now.

I don't think that the GFS ensembles will pick up cold 850's just yet, EML. We won't get to see that until the pressure has risen to our NW. And the ensembles to look at there are the Icelandic slp ones - and lo behold there is a very definite pressure trend there - upwards. The cold UK 850's will follow on from them later.

The extended ECM ensembles have certainly been trending colder in the last week - and not because winter is getting closer either!

[TonyH]

A model guided look at the nearer time frame for Wales and the Midlands:

http://forum.netweather.tv/blog/309/entry-4503-weather-guide-monday-19th-to-sunday-25th-november-2012-cf-west-wales-and-central-midlands/

[bluearmy]

http://www.meteo24.fr/fr/home/meteo-france-et-europe/passion-meteo/ensemble-de-previsions.html?eps=london

Once we get past midday, we can use London rathr than de bilt.

Getting colder with the less cold scatter dropping out. Ed - are you surprised how far up into the strat the split is continuing to show on gfs and are the ncep height forecasts on a different scale to the ECM ones as they fail to show the split at 100hpa.

[chionomaniac]

http://www.meteo24.f...html?eps=london

Once we get past midday, we can use London rathr than de bilt.

Getting colder with the less cold scatter dropping out. Ed - are you surprised how far up into the strat the split is continuing to show on gfs and are the ncep height forecasts on a different scale to the ECM ones as they fail to show the split at 100hpa.

Yes, and yet the ECM 32 dayer has been very bullish about northern height rises. Remember, most splits on the GFS have occurred 10+ days so no surprise to see that the ECM charts don't show the split yet.

Also, we are seeing a Canadian positive MT which normally is followed by southward momentum transport - right in the right place for downstream height rises.

[Zakos]

The CFS has performed terribly lately, initially forecasting a very cold November pattern, only to backtrack slowly to a milder scenario.

The model is now doing the same, but in reverse this time.

The first chart is the CFS December forecast a few weeks ago; a very mild pattern. The second chart shows the latest forecast. Its not an exceptionally cold pattern, but I have watched the CFS as it updates every 2 days, and noted how every update increases the cold potential for December every time recently.

This is just to demonstrate how the model cannot really be trusted for monthly forecasts, getting them terribly wrong on both occasions.

However, as bad as the model may be performing lately, it has nevertheless picked up on on a signal for northern blocking in December.

As each day goes by, the prospects for a cold and blocked first half of December are increasing. It is too far out for any real detail, but I do think the cold is most likely to come from the North, and the block situated over Greenland eventually. As I mentioned yesterday, I dont see a potent easterly on the cards just yet.

It has been said quite rightly by some that the cold seemed to always be in FI. this was true, however it is now starting to encroach on the reliable timerframe. What we can definitely see in the reliable timerframe is a general pressure rise over the Arctic and also a weakening of the polar vortex.

The real cold may not reach the UK in the reliable timerframe, but I firmly believe all of the building blocks are there in the reliable.

My advice is not to get your hopes up just yet, but It has to be said, as has been by many on here, that things are now looking pretty good for cold weather fans for the first half of December (at least) One thing I will say definitively is there is very little sign of mild and wet pattern. The outlook is definitely blocked, while it may not deliver sufficient cold/snow to the UK, there is very little sign of an Atlantic onslaught from any model IMO

[Frosty.]

Great post Chiono not sure about your monkey nuts though, better keep those to yourself.

I echo what weather eater said, superb post from chio (Ed) It does look like we are in for a cold spell after this week, slowly gathering momentum over the following weeks, the ensembles are gaining support for colder options, NWern height rises and cross polar flows just sounds so good to me, sounds like arctic outbreaks are on the way in december. Looking forward to strat warming as it will be the literal icing on the cake.

super post from fergieweather, fantastic to have a met office guy keeping us informed.

Edited November 18, 2012 by Frosty039

[Thundery wintry showers]

I feel that Chinomaniac's comments about increased northern blocking are starting to be reflected by the model outputs. The ECMWF operational and ensemble mean both hint at increased northern blocking towards day 10, with the main focus of blocking initially around the eastern North Atlantic sector, and the GFS 06Z operational run agrees. Although the critical period is outside of the reliable timeframe, it is unusual to get such strong agreement between GFS and ECMWF (and including the ECMWF ensembles) at that sort of range. The UKMO run doesn't go far enough but the Atlantic systems certainly look like slowing down towards T+144. Generally such setups entail colder-than-average weather, but whether it ends up snowy or not depends on where the blocking ends up.

Before that, we have a week of traditional mild and moist November weather to come up, with cloudy and wet weather for most on Monday and Tuesday, while there will be a "window" of dry and sunny weather spreading from the west on Wednesday and retreating eastwards on Thursday. Temperatures won't be excessively high though- perhaps a couple of degrees up on the seasonal average generally.

[cold snap]

gfs 12z rolling out .never been this buzzing.will i be at the end of the run.eyes down

[Paul]

I know this post will be deleted but hopefully before it is someone from the NW team might just consider my request ...

When posting charts etc can the knowledgable among you also describe what conditions said charts might bring to which areas please? Obviously I'm not about a localised forecast but something along the lines of "If this chart materialises this might well bring rain/sleet/snow/showers to the NE, E, SW" etc?

If this isn't possible then fine, but just thought it would add to the educational value (for me anyway) of visting this thread and the site in general.

Sorry and thanks for reading (and deleting!).

Mark.

I think where possible people do try to elaborate on charts etc, but when talking about the medium range and beyond, it's all about trends really so discussion on what a particular chart may bring on the ground is then starting to get the realms of being too specific. The difference in positioning of 50-100 miles or so of a system or high pressure cell can make massive changes in terms of weather across the UK, which means that even around 24 hours at times it's difficult to talk about precise locations of heavy rain, highest winds, snow etc etc, so at 168 hours+ (which a lot of the discussion is centred around currently) it's pretty much impossible.

That said, I'd definitely recommend you take a look at the winter setups guide written by TWS as that'll give hopefully you a better grasp on the particular scenarios being discussed and what they may bring:

http://forum.netweat...-winter-setups/

[Geordiesnow]

I must be looking at different charts because my first impressions are any hints of blocking have gone and the models just look like a typical November/December pattern without the PV setting up across the Arctic.

The outlook may turn cooler again towards next weekend and the Atlantic may slow down somewhat but I still can't for the life of me see much potential for a cold set up to form. It all seems rather half hearted at the moment, certainly no strong signal of a colder trend imo.

[Cheshire Freeze]

gfs 12z rolling out .never been this buzzing.will i be at the end of the run.eyes down

I'm expecting a bit of a rollercoaster this week. Plenty of time for things to go pear shaped; and with that I'll keep my feet firmly on the ground.

It's perfectly plausible we'll end up with the outcome of HP further south than some would wish for (including myself). We need to see some scintillating ECM runs.

Edited November 18, 2012 by CreweCold

[chionomaniac]

I enjoy comparing runs - just for the fun of it mind you JH!

The 12Z GFS run already has a larger amplitude trough over the UK by T+114 - and a better angle

This is better for heights to grow over Scandi which could act as a precurser to later height rises to our NW.

[bluearmy]

some of you clearly dont play poker. ian has posted that even the conservative (with a small c) people in exeter are now expecting a meridional pattern rather than a flat one and the word retrogression is used. relax and enjoy the ride. its so much more fun if you do.

[chionomaniac]

The surface low around Svalbard, I suspect is going to act as a trigger development (yellow arrows) and will transfer from one hemispheric lobe to the other lobe of the polar vortex, taking with it a large slice of PV energy (green arrow)

I suspect that this transfer of energy needs to occur long term to allow NW heights to rise.

Edited November 18, 2012 by chionomaniac

[nick sussex]

Great UKMO at 144hrs with phasing of those two lows off the eastern USA combining into an amplified trough, if that went further we'd see pressure rising quickly to the nw, as for the GFS 12hrs so far underwhelming!

[cold snap]

my god this is dragging its heels so slow this evolution past 100 shortwaves barely moving.

[chionomaniac]

The surface low around Svalbard, I suspect is going to act as a trigger development (yellow arrows) and will transfer from one hemispheric lobe to the other lobe of the polar vortex, taking with it a large slice of PV energy (green arrow)

I suspect that this transfer of energy needs to occur long term to allow NW heights to rise.

In fact I should have used the hemispheric view of the UKMO to show this perfectly!

[beng]

The surface low around Svalbard, I suspect is going to act as a trigger development (yellow arrows) and will transfer from one hemispheric lobe to the other lobe of the polar vortex, taking with it a large slice of PV energy (green arrow)

I suspect that this transfer of energy needs to occur long term to allow NW heights to rise.

Out of interest, what would be your view on the perfect split - 50/50? I was looking back at 2010, and we ended up with more of the PV on the Euroasian side - which ultimately I think may have led to the block retrogressing too much - at least from a Nimby South Easterner's perspective. The other side of the coin is that having enough PV on the Eurasian side was important in terms of allowing deep cold to come Westwards - so it's obviously a fine balancing act.

[cold snap]

my god this is dragging its heels so slow this evolution past 100 shortwaves barely moving.

thats its gfs just go slow want you.not feeling this one bit.

Still to much energy over the top .snails pace energy.

[snowking]

Somehow still manages to produce a cross polar flow on the GFS by 186, with some weak height rises out of greenland ridging south, despite a very messy picture:

gfsnh-0-186.png

[cold snap]

well it gets to some sort of easterly at the lower resolution part,but what goes on before that anything could happen.perhaps nick could talk about it before i fall asleep

[chionomaniac]

Somehow still manages to produce a cross polar flow on the GFS by 186, with some weak height rises out of greenland ridging south, despite a very messy picture:

gfsnh-0-186.png

The run that shows a clean transfer of energy between the two lobes will show a far cleaner build up of heights to our NW. I suspect the evolution of the UKMO would be thus if it were to go out further.

Still - all good up to that point.

Edited November 18, 2012 by chionomaniac