Model Output Discussion 29th December - Into mid-Winter.

[stodge]

Afternoon all

Well, a fascinating and for many nerve-wracking spell of model watching. I confess I lean more to the COLD camp than the SNOW camp and there's plenty for me to like in some of next week's charts.

I liked this morning's ECM 00Z OP as it shifted the wind flow more to an ESE'ly direction which is excellent for COLD fans as it reduces to a minimum the fetch over the relatively warm North Sea. Cold air crossing a snow-covered Europe hasn't much time to warm before it reaches southern Britain so cold all the way though often cloudy and grey.

For a bit of vague amusement (and to keep the mods happy):

The last three 00Z ECM OP outputs for the early hours of NEXT Thursday. This is far from being a done deal but the signs are there and promising no question.

We need the lobe of the vortex that will crash through Scandinavia on Friday and Saturday to exit cleanly to allow the rapid pressure build behind it over the frigid land mass. That's essentially what happened in January 1987 but we don't have the uniquely cold pool of air to tap into we had then and nor do we have quite have the orientation of the HP right either.

Let's not forget that dramatic cold spell was very short-lived - we need the PV to continue to weaken and move away from Greenland/Davis Strait to give both the HP itself a chance and offer a hint (as on today's chart) of the holy grail of retrogression. Some interesting comments and observations from the real experts on here offer a "hope" we may get somewhere but that's all it is at this very early stage.

I know many on here want Snow and plenty of it - the middle chart of the three I've pasted shows the "ideal" situation which might still happen - but I don't yet see from this synoptic evolution big snowfall. Cold, yes, and with luck, some sunshine.

We are at the very tentative early stages of this - we need to get tomorrow, Friday and early Saturday out of the way which may well deliver for some and particularly those with altitude I would guess.

I think by Monday we will have a much clearer idea of how the HP (if it happens) will set up. The strength of the northern arm of the jet remains a huge concern but in this instance (and one of the reasons I like today's ECM 00Z OP output) the second storm is lifting the PV out of Greenland and over to Siberia.

 

This is the third pasted chart above from the NH overview. The next two charts have two encouraging developments for coldies - one, the movement of the PV and two, the likely second undercutting LP:

The 240 chart (with all the usual caveats) is a superb chart for cold fans - with the PV heading for Siberia and a second LP threatening to undercut from the Azores, the possibility of retrogression (though not the probability or the certainty) is clear.

[Nick L]

Please please please keep general snow-related posts that have nothing to do with the models in here:

Or your relevant regional thread. Any further posts will be removed so please just consider others when posting, some are ruining this thread for those who want to discuss the models.

[marksiwnc]

The Arctic Oscillation (AO) is currently positive and is predicted to first trend positive week one and then trend negative back to neutral week two.

The weakly positive AO is reflective of mostly negative pressure/geopotential height anomalies in the central Arctic and mixed pressure/geopotential height anomalies in the mid-latitudes. With mixed heights over Greenland and Iceland but positive heights across the mid-latitudes of the North Atlantic, the North Atlantic Oscillation (NAO) is currently positive.  With heights predicted to rise slowly near Greenland the next two weeks, the NAO is also predicted to slowly trend negative. 

With strong oceanic ridging/blocking cold temperatures have become widespread across the Northern Hemisphere (NH).

In general, Eurasian temperatures are predicted to follow the AO trends first turning milder and then colder again.

Much of North America is in a clear trend to milder weather as a strong polar low is predicted over Alaska that will result in a mild, southwesterly flow of air downstream over much of North America.  However the mild trend will be briefly interrupted by an Arctic outbreak at the end of this week.

With near normal poleward heat transport, the stratospheric polar vortex (PV) has remained relatively strong.  However the weather models are predicting the PV to become more disturbed over the coming weeks.  A more highly perturbed PV could have an important influence on surface weather but for now that remains highly uncertain.

I have been anticipating a stratospheric PV disruption beginning in the latter half of January that will have an impact on the weather; however, as far as I can tell the stratosphere and troposphere remain mostly uncoupled.  Therefore weather patterns are more transitory and that will likely continue unless there is a significant disruption to the stratospheric PV.

Impacts

It is my opinion that the stratospheric and tropospheric PVs were coupled in the fall and through much of December.  In December they were coupled in such a way that resulted in cold temperatures in western North America and western Asia.  Later in December they were coupled that the PV was strong in both the stratosphere and troposphere that resulted in an overall milder pattern across the NH.  But since the end of December the stratosphere and troposphere seemed to have gone their separate ways.  The polar cap geopotential height anomalies (PCHs) warmed in the troposphere characterized by blocking in the northern latitudes of the two ocean basins that resulted in colder temperatures for both continents.  In contrast the PCHs have stayed cold in the stratosphere and for now the Global Forecast System model (GFS) is predicting that the two remain mostly uncoupled.

In the absence of coupling between the stratosphere and the troposphere the weather is likely to remain transitory.  North American weather is sensitive to variability in the atmospheric circulation in the North Pacific.  So far in January there has been strong blocking/positive geopotential height anomalies near Alaska resulting in cold temperatures downstream in Canada and the United States (US).  However the pattern is predicted to reverse with a deep polar low centered over Alaska that will result in a southwesterly flow of mild, maritime air from the North Pacific across much of Canada and the US.  My expectations are that this pattern will be transitory as well with a cooling trend towards the end of January.

Across the larger Eurasian continent, the weather has been more stable with cold temperatures across Siberia and warmer temperatures across South Asia.  The cold air from Siberia has been transported to the southwest into southwest Asia and Southeastern Europe by blocking in the Barents-Kara seas.  Closer to the blocking/positive geopotential height anomalies across Northern Europe including Scandinavia and the United Kingdom (UK), temperatures have been milder.  Variability in the pattern is expected but the overall pattern looks to persist in my opinion.

My expectations have been for the stratospheric PV to become more highly perturbed the second half of January.  There are signs that the stratospheric PV will become more perturbed in the coming weeks but I feel that it remains highly uncertain how much and what will be the impact on the weather.  I don’t see an ideal pattern to excite strong upward Wave Activity Flux (WAFz)/poleward heat transport in the next two weeks.  Until the stratosphere and troposphere become coupled, in either direction with strong or weakly coupled PVs, my ability to anticipate multi-week changes in the weather is limited.

 

Brief update by judah 

Edited January 11, 2017 by marksiwnc

[Nouska]

I'm somewhat limited in weather nowcasting abilities but assume this is the AROME model depicting a squall line on the front edge and snowfall on the back edge.

  

Great satellite image of the system coming in - should be able to double click to get very fine detail if opened in a new tab.

Edited January 11, 2017 by Nouska

[nick sussex]

So far today after the 00hrs runs the global 06hrs outputs which have come out show a bit more amplitude upstream. The latest NAM 12hrs and hi Res Nam for the USA also show a trend to that aswell compared to their 06hrs outputs. This is certainly what we want to see because we need every last ounce we can squeeze out upstream because this will have us in a better position re the ridge to the west and how vertical it can set up before the we see this toppling towards Scandi.

I really hope this trend is picked up tonight by the big 3.

[Ali1977]

2 minutes ago, nick sussex said:

So far today after the 00hrs runs the global 06hrs outputs which have come out show a bit more amplitude upstream. The latest NAM 12hrs and hi Res Nam for the USA also show a trend to that aswell compared to their 06hrs outputs. This is certainly what we want to see because we need every last ounce we can squeeze out upstream because this will have us in a better position re the ridge to the west and how vertical it can set up before the we see this toppling towards Scandi.

I really hope this trend is picked up tonight by the big 3.

Another tweak north and we could really get an Easterly (snowy one) out of this, only 200 miles would have a big impact which isn't too far in the grand scheme of things.  The models have slowly come more alligned, lets hope a shift north for the block from all over the next 4 hours. Hopefully this will also limit a warm up early next week, it would be nice if any snow does fall that some places hold onto it, that would help get some really low temps next week during the night.

[Methuselah]

Right then, folks - here it comes, the model we've all been waiting for - the nearly man of models, Steve Murr's best friend, spender of too much time spent going for the yellow or the green; maybe, this time, it'll be all about the blue and the pink...It's - the GFS 12Z!

[Snowman31]

17 minutes ago, nick sussex said:

So far today after the 00hrs runs the global 06hrs outputs which have come out show a bit more amplitude upstream. The latest NAM 12hrs and hi Res Nam for the USA also show a trend to that aswell compared to their 06hrs outputs. This is certainly what we want to see because we need every last ounce we can squeeze out upstream because this will have us in a better position re the ridge to the west and how vertical it can set up before the we see this toppling towards Scandi.

I really hope this trend is picked up tonight by the big 3.

Hi Nick, what time should we be looking for for these changes, or are we just looking for overall amplification?

[radiohead]

12Z GFS

[beng]

I know the higher resolution view won't look quite as good - but still this looks nice. If only it would stall like this for a few extra hours.

Edited January 11, 2017 by beng

[Ali1977]

Very early but it looks better for the cold to hang on longer with everything a little furthe west, giving a better Northerly at 48

[weirpig]

well  only the gfs  but it seems to start with atleast snow mainly from south wales  up towards Lincolnshire   the rest rain.  will prob change  turns to snow later in the south east

Edited January 11, 2017 by weirpig

[Captain Shortwave]

Well the output this morning pretty much puts us right on the tightrope so to speak, the line between a dry continental feed and an easterly which could bring snow. I wouldn't want to jump either way just yet. ECM/GFS day 6

   

You can see the high building north east right through the UK with cold air heading south west and potentially backing more west with time as the high strengthens its grip. We are very close to a very cold pattern developing, though the disappointing scenario is a cold and dry one.

So lets see what the 12z suite has to offer, also the risk of snow tomorrow, Friday and Saturday needs to be looked at as well.

[SMU]

Afternoon All

so as we wind up for the 12s we have seen the models begin to allign towards the final outcome for day 5-7, at this range the models are fast resolving the longwave pattern & sketching the detail on the shortwave pattern.

Whats very notable is the change in detail of our northerly pattern from the day 6&7 arena that had 'generally' a straight NW to now which is still the same northerly, however as soon as we hit day 5/T120 - the models startsd resolving the kinks & troughs in the flow & now we have arrived at T24 we see significant areas of PPN showing up- just to underin this heres the ECM 5 days ago (144) tomorrow night then just 3 days later (72) for tomorrow -night

the same can be said about the poss rain/snow event tomorrow-

So the possible (snowy) Easterly now hangs in the balance now as we transition from longwave resolution to shortwave one-

From an objective point of view most of us if not all have keenly followed every run & hopefully have noted the changes they have made in the longwave pattern- 

The ECM seemingly has been the best in terms of overall evolution but it must be said the UKMO that took the slightly middle ground may be making a gallop down the outside with the GFS always playing catch up- however the ECM has seemingly got one element wrong in which it has followed the UKMO / GFS blend & thats being over-amplified & lifting that azores shortwave out to much, up until today every GFS run lifted it out & flattened it over the UK, where as the ECM lifted it up past iceland NB below

ECM V GFS

Both models have backtracked today, the GFS 06z is more amplified but takes the low SE still ( after elevating it more ) ECM also does the same - note the energy here north of Scotland-

So the ECM whilst being best overall has moved toward the GFS in one crucial element

Scores on the doors then-

ECM 8

UKMO 8.5 as it always modelled the azores low coming SE bar one or 2 runs

GFS 6

Ita not over though because the azores low can be our friend in the form of the part of a trigger low *IF* it tracks on more of a horse shoe shape ( so ejects out of the azores, loops up close to iceland then tracks SE across the UK slicing the HP- whats important & a usual failing for the UK is the jet is usually flattening off behing however its still firing NE so the high pressure can still build NE over the top.

This is key in the longwave medium & longterm solution as despite having a bit of greyness in the transition all roads seem to lead to rome - at least a continental flow type rome - some are snowy some arent - but nearly all are very cold at the surface.

Now as we roll into the 12s the shortwave pattern starts to develop which is good because the shortwaves when they get resolved will enhance the cold air advection across the continent...

look for 3 key developments this eve

* azores low cutting SE across the UK at a sharper angle

* 2 / 3 shortwaves over europe in the 120-144 arena enhancing westward flow of the cold ( shortwave resolution )

* lower heighs to the east & south of the Uk at a similar timeline as above.....

The 168-240 timestamps look dominated by easterlies & significantly cold dewpoints

look at the debilt clustering around 90 degrees

Anywhere around here for the cut point across the UK is fine

S

** EDIT**

day 3 jet profile showing a better startpoint for that horseshoe shape over the 06z - also elevated more towards southern greenland-

 

 

Edited January 11, 2017 by Steve Murr

[warrenb]

I think we might be in for a good one,, better angle of jet, more gap in the northern express.

[Ali1977]

This looks a bit better than the 06Z, the PV and jet stream are slightly further North which should allow the WAA/High to build in slightly North also, the slight warm up before continental flow (>0c 850s) was 48 hours in the SE, I think it''ll be less this run - good trend.

[Singularity]

  

Well, somethings going on here that's for sure.

If we look at week 1 of the CFS forecast (top right) we can see some activity picked out as being associated with the MJO (black outline) in the subtropical Atlantic - but this seems unusually far north. By week 2 it looks as if the entire Atlantic ITCZ has received an adrenaline boost .

Note, however, the activity crossing the C-E Pacific at the same time, while Indonesian activity continues in-situ. This sort of multiple-activity situation can't be accurately depicted by the H-W diagrams shown above. The week 2 Atlantic activity is however so extensive as to project strongly into the H-W diagram despite ongoing activity elsewhere.

 

So in a sense we have forcing both toward this:

- which supports the easterly but only if you factor in the vortex displacement to the other side of the hemisphere forced by other mechanisms.

...and - to a lesser extent perhaps - this:

 

...the combination of which fits nicely with recent NWP model output and trends. If we're going to have a battle between two different areas of forcing, one between phase 8 and phase 1 forcing is a good one. 

The Indonesian activity supports more of a Euro ridge, but is predicted to be more of a referee than a competitor - hopefully it will stay that way, with no interfering cyclones in that region. Well, unless one moves quickly NE from there and triggers rapid amplification downstream, but that's a wildcard to say the least!

 

p.s. great post @Steve Murr, I'm watching that shortwave with great intensity and willing it to dive as sharply down into NW Europe as possible - ideally to set up shop over N. Germany, but that's being greedy 

[Singularity]

 

Shallow low/wave just E of Scotland is around 40 miles west on the 12z of where it was on the 06z at this point. A good trend.

Consequentially we see the ridge being split already with a more distinct circulation over Scandinavia that allows more of an undercut on its SE flank by the jet.

Edit:



Okay, this is close to the ECM 00z in terms of the shallow low movement so in theory the progression should be along similar lines for next week.

Main difference is the low is shallower than ECM had it.

Edited January 11, 2017 by Singularity

[radiohead]

UKMO a bit less amplified than the GFS at T96. Not a major difference at this point though.

 

[SMU]

As above cut point good plus look at the jet - it has a faster jet streak going south through Scandinavia @120

This means better lift ,deeper lows & more CAA...

@Radiohead - UKMO SHARPER!!

Edited January 11, 2017 by Steve Murr

[Ali1977]

As I mentioned, the above freezing 850s only leave the SE for 18 hours now, it was 48 on the 06z.  

[Severe Siberian icy blast]

5 minutes ago, radiohead said:

UKMO a bit less amplified than the GFS at T96. Not a major difference at this point though.

 

The U.K.M.O is much sharper than the gfs, could that be a snow event for the east maybe? Great hights into scandi there aswel.

[Ali1977]

3 minutes ago, Ali1977 said:

As I mentioned, the above freezing 850s only leave the SE for 18 hours now, it was 48 on the 06z.  

Also look at the 850s in Europe, much more -8c or colder to tap into should we get an Esterly - which is looking good.  Great afternoon/ evening of model watching to come.

Edited January 11, 2017 by Ali1977

[Alekos]

UKMO looks not interested in easterly, northern jet goes over top ?!

[Singularity]

  

Basically a small shift west but the sharper dive has also improved the trough over Italy.

Remarkable consistency elsewhere given which model we're looking at 

 

- and yes, UKMO dives that low sooner and further west. Good going given how far east the 00z was with it.

A good match for GFS with the SE right on the ridge axis so little or no flow - could be pretty cold at night if the skies are clear.