Model output discussion - 1st December onwards

[Mildcarlisle]

And nothing forecasted the Christmas evening snowfall in Galashiels.The uppers were well in order as sleet was falling into +3.5c. I will take this likes. 

[Allseasons-Si]

Negatively tilted trough @ 198hrs only meens one thing,the outcome,well you guessed it

 

[radiohead]

Perhaps the cold is just too far south to tap into on this run?

[Cheshire Freeze]

Inevitable from this point...

 

[bluearmy]

This run seems to have stalled. Not sure where it goes by day 10.  The renewed jet could head ne or re enforce an effective undercut. 

[Allseasons-Si]

1 minute ago, radiohead said:

Perhaps the cold is just too far south to tap into on this run?

Nope,better this run

18z @ 210

12z @ 216hrs

[swilliam]

Just now, bluearmy said:

This run seems to have stalled. Not sure where it goes by day 10.  The renewed jet could head ne or re enforce an effective undercut. 

Jet profile looks good

 

[shaky]

Main thing about this run is its an improvement on the 12z!!stronger scandi high and more of an undercut!!couldnt ask for any better really!!

[Cloud 10]

3 minutes ago, Allseasons-si said:

Negatively tilted trough @ 198hrs only meens one thing,the outcome,well you guessed it

 

 

Yes,the jet deflected away SE.

 

[Vorticity0123]

 

On 22-12-2015 at 9:16 PM, Vorticity0123 said:

A setup to winter weather?

With such a setup forecast, a high pressure area at the surface near Scandinavia is a distinct possibility. Does that also mean that winter weather is on the way? The answer is most likely not. But why?

For the reason why it is useful to look at the ECMWF and GFS for 10 days out:

ECMWF (left) and GFS (right) 500 hPa heights (colours) and surface pressure (black contours) 12Z T+240. Source: Wetterzentrale.

The plots given above are for 2.5 days further out than the stratospheric charts of the ECMWF presented above.

In both cases we see the polar vortex nicely represented at 500 hPa as well by the purple colours (very low heights, so intense troughing). Both charts also show something of a Scandinavian/European high pressure area. But both charts are definitely not suggesting anything wintry. These charts would result in a continuation of south-westerlies instead. Why is that the case?

The ‘block’ is in both the ECMWF and GFS located too far to the east to be of much influence on the weather in Western Europe. It only tends to hold up approaching low pressure systems from entering Europe, instead sending them northward towards Iceland. The troughing over the Central Atlantic, on the other hand, is able to exert its influence over Western Europe. As we are going to be located in the upper branch of the troughing, the consequence is again mild southwesterlies.

 

Some interesting developments have been going on in terms of weather models during the past day or so. While chances on wintry weather are still not remarkably high (especially given the ensemble forecast of the ECMWF presented by many), chances have definitely increased since about 3 days back.

At that time I wrote about the (lack of) chances on wintry weather  for the extended timeframe (about 10 days from then). However, now the odds have definitely turned somewhat. Have the models changed dramati\cally during the past few days? Or are the changes hidden in details? Time for a small recap!

The big picture - still about the same?

Here we compare the charts above (from 3 days ago) with the charts of today (EC12 and GFS18) for the same time frame, Friday the first day of 2016.

 

ECMWF (left) 12Z and GFS (right) 18Z 500 hPa heights (colours) and surface pressure (black contours) for Friday 1 January, Source: Wetterzentrale.

The main 'building blocks' of both charts look quite the same. Deep troughing (purple colours) is forecast to be present near Greenland and further southward, whereas a strong ridge of high pressure (yellow and orange colours) extends all the way up to Scandinavia. One could say that the ECMWF run of 3 days ago does deviate significantly with a much less pronounced ridge, but still to some extent ridging is present over central Europe.

It is all in the details

So if the big picture looks roughly the same, the difference has to be in the small things. The main factor appears to be the extent ridging is able to extend its westward influence. Whereas about 3 days ago every run kept the high pressure area over Scandinavia too far east and south to establish a cold easterly feed towards the UK, today some runs are suggesting that the influence of the ridge is able to extend far enough westward to bring real cold to the UK.

It has to be said that there are definitely some more small differences (like changes in positioning of centers of high and low pressure, undercutting or not etc.), but the difference noted above appears to be the most crucial one to me to explain why some runs are suggesting wintry weather over the UK and others not.

So... could we have seen this coming?

To answer the question, it is good to take a look at the broad picture. With a ridge forecast over Scandinavia one always should keep in mind that there is a slight, but significant chance on an easterly flow reaching the UK. Although none of the models back then forecasted an easterly, critically put one could argue that a very small correction of the whole pattern westward could turn the tides considerably. Therefore, even though the models suggested otherwise, I think it was possible to realize that there would be at least a slight risk on wintry weather.

Back to reality

Even though the trend has been for chances on cold weather to increase, the chances are still marginal at best. Furthermore, it is worth remembring that any shift to the left or right will have large implications on our weather. It could be the difference between a cold easterly and a mild southwesterly. Nevertheless, we are bound to see an interesting period of weather watching!

 

Edited December 25, 2015 by Vorticity0123

[bluearmy]

Mexican stand off between the Atlantic and the block.  The block not aligned well enough to allow any cold to advect west.  No effective undercutting to help in any case. 

[radiohead]

Hard to see a route for the cold from that setup now. Cold is placed too far southeast. Not that it matters, it's way out in FI and will look different again on the 0Z!

[cold snap]

Strange situation but plausible 

The Trigger low that's required to bring in the cold easterlies didn't transpire nor did it push the block away it just fizzled out and went south 

Now according to this a run we don't have a trigger low .

No Mans land is where we end up

[January Snowstorm]

I wouldn't worry too much about FI.

As GP said a few days back let the trend be your friend!

[Mucka]

I don't mind the 18z so much in that like many other runs it suggests we could have more than one bite at the cherry.

 

Scandi high take 2?

without taking each run on face value it may be the case of hoping we can keep some strong anomalies to our E/NE until conditions upstream are more favourable for undercut.

Edited December 25, 2015 by Mucka

[tight isobar]

18z just goes to highlight previous, 

The slightest tweaks in alignment of developing hight rises and forthcoming coming cold becomes "forthcoming" mild inflow. (Again)

However' there's certainly reasons for optimism. ..

Lots to be resolved and some certain twists and turns ahead. 

[Allseasons-Si]

And we are back to were 90hrs started at in the latest frames(similar)

at 90

[Carlrg]

1 minute ago, Allseasons-si said:

And we are back to were 90hrs started at in the latest frames(similar)

at 90

Not quite!

 

[Allseasons-Si]

23 minutes ago, Carlrg said:

Not quite!

 

We had a trough disruption in it's higher res from 192 onwards,that's as far as i would push it,although that could not happen but nice viewing anway

Edited December 25, 2015 by Allseasons-si

[Mucka]

ECM London ensembles

 

[492]

Changes are occurring as early as T +72 with repeated westward corrections near the UK with fronts held to our west as warm air surges north aiding the building of a significant ridge to our east. Watch near time to see if the trend continues tomorrow.

[Allseasons-Si]

6 minutes ago, Mucka said:

ECM London ensembles

 

Do you have the link to that site.i seem to have lost it,due to re-format.

Edited December 25, 2015 by Allseasons-si

[Jason M]

GEFS rolling out and they are looking much better at 174. Be surprised if there are not some lovely runs in amongst them by the time we get to day 10.

[Mucka]

12 minutes ago, Allseasons-si said:

Do you have the link to that site.i seem to have lost it,due to re-format.

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

 

Or you get it from the Metociel site by clicking on ECM and then London (Londres) link under the pressure charts

Edited December 25, 2015 by Mucka

[Allseasons-Si]

7 minutes ago, Jason M said:

GEFS rolling out and they are looking much better at 174. Be surprised if there are not some lovely runs in amongst them by the time we get to day 10.

Yes and the control disrupting the trough at 180hrs

Edited December 25, 2015 by Allseasons-si