Model Output Discussions 12z 01/09/2016

[Frosty.]

The GEFS 6z mean shows our high gradually drawn north-westwards from around  T+ 144 /168 hours onwards and at the same time, a trough slipping down into Scandinavia which enables polar / arctic air to dig down across the uk turning our weather increasingly unsettled and colder..this looks very similar to the 00z mean so no downgrade. The mean doesn't tell you how potent the cold shot will be but it gives you a general idea of where our weather will be coming from in just over a weeks time and it looks like it will be from the north with high pressure building north in the atlantic and a robust looking scandi trough. 

Edited October 23, 2016 by Frosty.

[Tamara]

 The daily updating SOI index has fallen into -ve territory for the first time since the end of Spring. As of today, being below the +7 to -7 ENSO neutral band, it "technically" suggests a Nino forcing reading across the Pacific.

https://www.longpaddock.qld.gov.au/seasonalclimateoutlook/southernoscillationindex/

Of course the index fluctuates daily, and much more can be read into the rolling monthly figures - but nevertheless this further underpins the point I made the other day that weak Nina status is not a given at all for the remainder of 2016

Which neatly leads me on to ask why is there an Aleutian Low currently being modelled? 

Well, +ve torque tendency in the extra tropics triggers a rise in angular momentum tendency that causes an enhancement of the East Asian jet and this teleconnects to a fall of pressure the other side of the Himalayas across the North Pacific which in turn gives rise to a Wave 1 poleward eddy to further perturb the early season polar vortex.

But an Aleutian Low/+PNA pattern is commonly typical of an El Nino signature in the Pacific - not one of the attempted La Nina base state advertised through the summer into autumn. The weakly +ve trend of the index has prevailed through October in conjunction with the significant -AO and looks set to remain into November

 

 

I'm thinking that with the vortex so weak, further weakness programmed, and with the Atlantic and Pacific effectively de-coupled in terms of pattern, we might get the best of both worlds heading into winter. What do I mean by the two ocean patterns being de-coupled and the best of both worlds ?  

First of all the ENSO signal is weak and (in deliberately over simplistic terms) that helps the ocean and atmosphere "do their own thing". 

 While there is, at least at present, an El Nino signal in the Pacific pattern, on the other hand the advertised amplified Atlantic Ridge in the modelling to quite conceivably bring the first early season cold, is a commonly seen aspect of a downstream La Nina pattern in the autumn.  

La Nina in the Pacific causes a retraction of the East Asian jet under falling torques - and this is manifested as a defined split flow in the central Pacific and a high pressure anomaly across the North Pacific.

If La Nina conditions establish properly under definitively sustained low angular momentum conditions (that create the retrogressed Pacific pattern) the downstream effect is to, ultimately, pull the Azores ridge westwards in the Atlantic.

The fact that, at this time, the Pacific is showing an opposite pattern upstream to the characteristic pattern has increasing ramifications ahead into winter IF it is  recyclable, and/or is a signal of an adjustment of regime away from overall Nina forcing.  The post the other day discussed those potential prospects

Ultimately, the changes in seasonal wavelengths from autumn into winter proper mean that the upstream Pacific pattern will force itself more and more downstream - and happening at the same time as the annular mode becomes more and more coupled.. The latter I mentioned the other day.

That is why I see importance that we continue to see low pressure across the Pacific to maintain a favourable teleconnectic profile to the stratosphere in terms of trop> strat disturbances. Also with an associated relatively higher angular momentum state, there is chance of seeing sufficient westerly wind momentum across the Pacific to enable favourable tropical convection (Phases 7/8) to force a -NAO in tandem with weakness and instability across the stratospheric polar field.

The other long term benefactor of course under this regime ( if it is sustained)  is the enablement of a strong Siberian High to take advantage of good snow cover in a way that last winters far too strong ENSO and QBO conditions were (famously) not able to.

That is why indicators such as the SOI (in conjunction with monitoring angular momentum, torque and GWO orbit progressions) are obvious key indicators to monitor carefully for trends heading forward. Its why I keep posting on this particular topic. Its not to be exclusive of the overall picture at all, I just think that taking into account all the background factors this year, any bias that the atmosphere shows within a neutral ENSO regime could prove (one) crucial factor m moving forward

"If "La Nina forcing resists the current "destructive" phase, angular momentum falls back and the pattern upstream reverts to Pacific anticyclones (and a SE US ridge) it would be the case that a favourable unstable stratospheric profile that might help a decent end to autumn or even start to winter ultimately leads to the amplified Atlantic ridging  flattering  to deceive. Seasonal wave lengths, especially in the event of any vortex intensification that may occur later, ultimately indicate increasing energy being sent into the northern branch of the jet from across the US and cold air advection sent eastwards into Europe as milder air flows across the top of the high.

In that sense, if the early season stratospheric weakness is to pay dividends, then it will require a teleconnectic pattern framework ideal to perpetuate vortex perturbment our side of the NH - and not once again across the Pacific side... After a promising start.

Maybe September (monthly SOI average of +13) could be the base Nina "low point" perhaps to watch with the ENSO tide. Inevitably in all ENSO cycles, weak or strong, the momentum ebbs and flows according to significant recurrent intra seasonal phenomena like MJO tropical convection activity.   

But should this be the case, then there is reasonable cause to start thinking that the early season feedbacks across Eurasia with snow cover in tandem with a persistence of the Aleutian Low will start to have positive effects in terms of wave activity that in tandem with increased westerly momentum across the Pacific increasing the chances of tropical forcing an a flip in the NAO substantively in conjunction with stratospheric weakness.

But here she is getting ahead of herself...time to do some baking and housework

 

 

 

Edited October 23, 2016 by Tamara

[blizzard81]

3 hours ago, Frosty. said:

A note of caution for coldies from the Gem 00z, not enough retrogression means the high stays over the uk before sinking south and allowing the atlantic over the top.

Never rated the GEM. It is wrong far, far more often than it is right at that timescale so I wouldn't worry too much. I don't even bother looking at it these days.

[Frosty.]

3 minutes ago, blizzard81 said:

Never rated the GEM. It is wrong far, far more often than it is right at that timescale so I wouldn't worry too much. I don't even bother looking at it these days.

I certainly hope the Gem is wrong, we know we can get snow in late october, even in London as we did in 2008. 

[blizzard81]

4 minutes ago, Frosty. said:

I certainly hope the Gem is wrong, we know we can get snow in late october, even in London as we did in 2008. 

Indeed I remember that early cold shot very well. 

[Frosty.]

As you would expect with the mean looking like this, there are an increasing number of chilly looking GEFS 6z perturbations in various timeframes in early november..could be a cold bonfire night this year!

[Ali1977]

If we are going to Profit from at E/NE flow later in Nov and Dec we need a cold Northern Europe and Scandinavia, that is definitely one of the benefits from the forecast cool down early Nov - Scandinavia looks like its heading for an early "big freeze" starting late next week.

[bryan629]

19 hours ago, Steve Murr said:

Yes

a steady overall drop around 1 point...

Spread even less with todays update and much more tightly packed... plus dropping to -4 values too .. Mr Vortex taking it on the chin for been a bad boy last winter 

 

Edited October 23, 2016 by bryan629

[lorenzo]

GFS loves Halloween then... eyes to Greenland. Been a few years since we have seen even a hint of this type of chart, let alone in the NH anoms overall.

Strat impacts are marked ( given unzipped/splits) modelled across nearly all layers... that Aleutian Low is the story of the run. Think of Butler precursor region for assault on the vortex.

Critically different and triggering so many questions for evolution into winter, the propensity for the atmosphere to either 'lock' or 'repeat' with respect to blocking regimes is a point of interest, has the long lasting Scandi High imprint given way to something toward Greenland. Are we seeing a temporary phase shift in the base regime of the NAO.. Brilliant 4 days ahead as the diluted vortex profile decides on it's route to recovery...

[knocker]

In the meantime could be quite a wet and windy day for Scotland on Thursday before the ducking and diving starts at the beginning of next week.

Interesting to note that the energy emanating from Canada tending to disrupt the block south of Greenland on the above chart is in one way and another continued during the run. It's all quite messy and considering the time frame and just this evening's run not worth bothering about at the moment but eyes will have to be peeled looking for indications of where we are going with this.

Edited October 23, 2016 by knocker

[SMU]

2 hours ago, bryan629 said:

Spread even less with todays update and much more tightly packed... plus dropping to -4 values too .. Mr Vortex taking it on the chin for been a bad boy last winter 

 

Cheers yes I saw them in the gym-

The execution of the 12z & its total coverage of the pole in high pressure with 3 main areas of low pressure around mid lattitudes means that for November 1st which coincides with the optimum low point of the next phase of the negative AO means that the 1st day reading could be -5 AO

i will whizz through & check the records...

[Barry12]

Good agreement at just day 6 - the changes are now creeping into the reliable timeframe.

All of the big 3 models agree on the ridging towards Greenland, and a very wounded vortex.

[Ali1977]

Frosts for sure up North, even by the end of Oct. -4/-5 could be poss in Scotland i would think.

[Frosty.]

The Gem 12z is now going for retrogression of the high with a scandi trough which is a big upgrade compared to its 00z which showed very limited retrograde with the high remaining over the uk before sinking south allowing the atlantic to flow over the top..this run is much more in tune with the current trend..to cold.

[nick sussex]

After annoying everyone for the last few months over in the politics thread I've decided to move back to less annoying territory in the model discussion. I've come out of my hibernation a bit early this year due to the shocking appearance of a possible Greenland high.

This rare sight is the Olympic Gold Medal of synoptics for coldies, we're a bit early in the season to expect too much from this but its more the difference with the NH pattern compared to last winter which is good to see.

I wouldn't raise expectations for anything too wintry if the GH does verify but some frostier conditions look likely.

Of more interest is how long this northern blocking can last for because if it can hold then as we get into November things could get more interesting.

[knocker]

The GEFS 6-10 anomaly illustrates the upper pattern very well with the retrogression of the European high pressure NW eliminating the Greenland low and connecting to the Russian ridge. Not forgetting the strong Aleutian low and continuing WAA into the Arctic. The evolution from this position this evening is to mid Atlantic ridging with a Scandinavian trough which would indicate a cool NW upper flow after a brief colder interlude with winds in the northerly quadrant during the transition period. None of this is of course a done deal

 

[Matthew.]

The ECM keeps the warmer upper-airs for Friday-Sunday with HP in charge. What happens next is another story?

[bluearmy]

Looking at the gets anomolys I begin to wonder if the Euro anomoly isn't modelled quite right then we could see a west based -NAO develop at some point once the greeny ridging develops. the ECM 12z op brings this early on at day 8. Much easier to cope with these kind of model evolutions when the stakes are so low! 

[lorenzo]

Just for posterity - let's be clear this is the ECM at 192..  

Aleutian low is outrageous

the next days modelling will possibly map winter 16-17 - superb time to be dialled in..

[knocker]

16 minutes ago, Matthew Wilson said:

The ECM keeps the warmer upper-airs for Friday-Sunday with HP in charge. What happens next is another story?

 

 

 

How about the energy sneaking out of Canada running south of the Greenland HP having more of an influence. First indicated  by the GFS as mentioned above.

Edited October 23, 2016 by knocker

[Frosty.]

The GEFS 12z mean still going with the high heading north-westwards towards Iceland / greenland with lowering heights to the NE which introduces a relatively much colder Nly / NNEly airflow down across the uk.

[ITSY]

ECM goes down a different route - the energy coming across the Atlantic diverts the Northerly outbreak westward and we get left in higher uppers. Just a reminder for how finely balanced things can be for us, even with GHs and HLBs. 

[phil nw.]

Greenland high's can be real teasers as has been pointed out in previous posts.Too far west such as shown in the later ECM frames and we are on the south side of cyclonic activity and in the milder westerlies.

Not really a big deal at this early stage and liable to change in future runs but something to be aware of if new to chart watching.Yes the Aleutian vortex is already quite extensive and seems to be the major feature which is continuing to drive warm air north over the Canadian/Greenland Arctic weakening the vortex on the Atlantic side. 

Edited October 23, 2016 by phil nw.

[Frosty.]

I agree Phil, it's not that big of a deal at this time of year whether we tap into the early cold or not but the charts are interestingly poised and it could go either way..I also agree with BA.

Edited October 23, 2016 by Frosty.

[knocker]

As I noted above re the ecm