Model output discussion - 5th December 2016 - Into Winter we go

[Spikey M]

Well, at last we have some excitement. Snow, wind, rain and sun all on the table. Mad as a bag of badgers, but I like it.

[chris55]

On closer inspection ECM looks a little odd? What the hell happens to the developing, and what looks like should be, an intense trough spinning up at 144 and heading our way, it just dissipates in the  space of 24 hours? Perhaps it does just disrupt against the high, hard to believe, but i guess if the jet is forcing it south east into the Azores high then its possible. 

[LRD]

1 hour ago, IDO said:

No. It was based on looking at the ensembles that are tending by D10 to suggest the mini ridge/trough combo is in a good cluster and as GEM and ECM ops are in that ball park I drew the conclusion that the GFS was overplaying the Atlantic zonal flow (a regular occurrence):

here & here

 

Well you mentioned Christmas eve and Christmas day which isn't day 10

Anyway ECM shows interest at day 10 so let's hope this is a new trend that builds momentum and ends up verifying

Edited December 18, 2016 by LRD

[booferking]

1 minute ago, chris55 said:

On closer inspection ECM looks a little odd? What the hell happens to the developing, and what looks like should be, an intense trough spinning up at 144 and heading our way, it just dissipates in the  space of 24 hours? Perhaps it does just disrupt against the high, hard to believe, but i guess if the jet is forcing it south east into the Azores high then its possible. 

Its joins the low on the east side of Iceland these lows pretty fast moving because of the powerfull jet,

[Dennis]

wait and see how it works - exiting times

[Dennis]

Just now, Dennis said:

wait and see how it works - exiting times ; i try to write some tomorrow

 

[Spikey M]

1 minute ago, Dennis said:

wait and see how it works - exiting times

What does this mean? It's over my head.

[SMU]

2 hours ago, Nouska said:

There is a very cold pool of upper air modelled to traverse central UK on Saturday - cold enough to support polar low development but I don't know if all other parameters would be favourable.

If current modelling looks to verify, please make sure you factor in the possibility of power outages when planning the festivities and feasting for next weekend.

 

 

Hi thats getting close to the magical sub >-40c@ 500HPA however the angle of flow is to westerly-

You need a flow off the greenland shelf ( or occasionally just west of norway ) as well as a steep gradient to get a decent polar low-

Best

S

[SMU]

12 minutes ago, Spikey M said:

What does this mean? It's over my head.

Its the vertical wave eddie flux chart- showing a significant increase @ 10HPA as we end the year, the fall out from this will be a sharp reduction of the stratospheric zonal wind @ 10 HPA into Jan - propergating down into week 2 - So thats where the landing zone is ATM for anyone to be considering blocking to be a feature once again....

[Nouska]

14 minutes ago, Steve Murr said:

Hi thats getting close to the magical sub >-40c@ 500HPA however the angle of flow is to westerly-

You need a flow off the greenland shelf ( or occasionally just west of norway ) as well as a steep gradient to get a decent polar low-

Best

S

Thanks Steve.

The in between on ECM - low well north of the GFS positioning but at least it shows some (in the far north) will wake up to fresh snow on Christmas day - even if it disappears by the time the turkey is on the table.

  

 

Edited December 18, 2016 by Nouska

[Interitus]

That Cohen tweet is three days old and if as appears is based on the GFS operational, the long range strat forecasts are variable - as he acknowledged in a follow-up tweet (to a member of this forum). The latest GFS is showing pretty much zilch weakening to the end of the run.

His latest tweet on the matter is not as upbeat, and rather states the obvious!

 

[Summer Sun]

ECM ens

Stormy Christmas eve

Still windy and unsettled Christmas day 

Gradually settling down post Christmas

Edited December 18, 2016 by Summer Sun

[bluearmy]

Not often you see the eps reverse for day 10 !

Upper trough to upper ridge !   Seems the 00z run was out of step.

.

 

[Scandinavian High.]

10 minutes ago, Summer Sun said:

ECM ens

Stormy Christmas eve

Still windy and unsettled Christmas day 

Gradually settling down post Christmas

High pressure pushes north  slightly .

[knocker]

2 hours ago, Nouska said:

There is a very cold pool of upper air modelled to traverse central UK on Saturday - cold enough to support polar low development but I don't know if all other parameters would be favourable.

If current modelling looks to verify, please make sure you factor in the possibility of power outages when planning the festivities and feasting for next weekend.

 

 

The dynamics of Polar Lows are complicated and they are not easy to spot A quick summation

When arctic-origin air in winter flows southward (northward in the southern hemisphere) across (relatively) warmer seas, strong surface heating acts both to enhance the degree of instability, and trigger vigorous moist convective towers. This is sufficient alone to give rise to heavy, wintry showers/cumulonimbus clusters etc., but often marked troughing, or even a closed circulation in the isobaric flow is found; the resultant low-level convergence/positive vorticity enhancement, plus the localised concentration of the latent heat energy released, enhances development within the system, and an intense (but synoptically small) area of rain, hail, sleet or snow & squally winds can result - a polar low (or polar depression or polar meso-cyclone in some texts).

The dynamics of such systems are not fully understood, and it is only with the (recent) arrival of very high-resolution satellite imagery & sensors in a wide variety of spectral bands that the detail within such systems can be studied. Even so, for operational meteorologists, careful monitoring of all available data is required; Geostationary satellite imagery has a rather course resolution at high latitudes, and the visible channels are of little use in the winter season. Polar orbiter passes (which give much higher resolution imagery) may not be frequent enough to maintain a continuous watch on developments.

Numerical models also have difficulty with such events; they are born in data-sparse regions, and most schemes 'paramaterize' convection i.e. models don't explicitly forecast each individual convective event, but rather indicate the degree of instability expected, its areal extent etc., and thus have problems going one step further and turning an area of disorganised (model) convection into an organised self-sustaining polar low/trough, where upper troughs are not the primary forcing mechanism. The one remaining Norwegian weather ship, and a handful of research and fishing vessels may be the only clues to developments taking place in, for example, the Norwegian Sea.

Polar Lows can develop, and move (in the prevailing flow) with surprising speed, and lead to considerable dislocation of normal life in regions directly affected. Preferred locations for genesis are to the west of large, slow-moving occluded depressions - i.e. those with a pre- existing rear-flank arctic flow. It may be that the geography of the regions in question play a significant part in genesis of polar lows - Dave Wheeler, who I am grateful to for checking much of the above, suggests that vortices shed by high-arctic island groups (e.g. Svalbaard) are enhanced by the land mass of Scandinavia (Norway) to the east and Greenland and its ice shelf to the west.

http://weatherfaqs.org.uk/node/95

[ArHu3]

3 hours ago, Nouska said:

There is a very cold pool of upper air modelled to traverse central UK on Saturday - cold enough to support polar low development but I don't know if all other parameters would be favourable.

If current modelling looks to verify, please make sure you factor in the possibility of power outages when planning the festivities and feasting for next weekend.

 

 

But polar lows come from the north, not west

[ANYWEATHER]

Christmas day.....Will it be peaceful or noisy? Ecm and Gfs differ, but Gfs  really kicks in its output when it sniffs something Atlantic, and it usually wins!!!  Personally I think between  Christmas and New year a very stormy period......

[johncam]

For our learned posters, Do we really want the euro high back again! 

[Newberryone]

19 minutes ago, abbie123 said:

High pressure pushes north  slightly .

Can't say there's too much to be enthused about with a continuation of those euro heights 

[Newberryone]

6 minutes ago, johncam said:

For our learned posters, Do we really want the euro high back again! 

No, but unfortunately that's what the latest Ecm Ens seem to be suggesting as we head towards the New Year. 

[ArHu3]

Ecmwf seems very certain until at least Tuesday 27th, there is virtually no scatter in the plume's wind direction and temperature, all very narrow 

[AGAL]

4 hours ago, Paul_1978 said:

It's "FI"

I suppose you can't blame some for not realising that a long, long time ago the outer reaches of the GFS was named 'Fantasy Island' and not a technical term like F1; unless off course Lewis Hamilton is forecasting now

[knocker]

Well hey ho said Reilly. where do we go with tonight's anomalies. The ecm has gone off on one or else it's spotted something nobody else has.

Upstream they are still quite similar except the ecm has reorientated the Greenland lobe and retrogressed the Atlantic trough which promotes quite strong ridging just to the east of  the UK. Ergo the eastern Atlantic switches from zonal to quite amplifies in a shake of Sidney's tail. What has instigated this is a tad above my pay grade. It does explains the differences in the det outputs earlier but at the moment the ecm has to be taken with a large dose of Saxo I feel although it undoubtedly would be the better outlook.

Not sure there is a lot of point looking further ahead but out of interest the differences do continue. NOAA and the GEFS have the trough to the NW with a zonal westerly whereas the EPS continues to be more amplified and the UK under the influence of the Azores HP apart from the far north,

Edited December 18, 2016 by knocker

[Singularity]

1 hour ago, chris55 said:

On closer inspection ECM looks a little odd? What the hell happens to the developing, and what looks like should be, an intense trough spinning up at 144 and heading our way, it just dissipates in the  space of 24 hours? Perhaps it does just disrupt against the high, hard to believe, but i guess if the jet is forcing it south east into the Azores high then its possible. 

The low in the first chart actually had two centres of action, hence its stretched out. Common where steep thermal gradients occur as instability exists along an unusually long stretch. Models often over-simplify subsequent development, resulting in the two (or sometimes more) lows merging into one powerful system when the reality sees them staying separate.

The 12z ECM merges the leading one with the trough N of the UK while the trailing one is still out to the west and in an environment that causes it to fizzle out.

[Captain Shortwave]

ECM ens

      

There is a suggestion that pressure will build back northwards after a more unsettled spell around Christmas so potential storminess could be limited to just a couple of days.

      

The GEFs however are not really showing this with the pressure gradient gradually relaxing though that could be more to do with an increasing spread of solutions rather than anything else. It will be interesting to see if the ECM is onto something, early day whether we could see a significant rise in pressure towards the UK and whether it could move into a favourable position for cold. The GFS/GEM suites are less convincing on that regard with a generally westerly pattern persisting.