Model Output Discussion - Into Winter

[Notty]

For model discussion, any timescale is fair game.

For a sense of what may actually happen, I would say 96/120 hours away is the outer limit.

As for the ECM, many of us remember it leading us up the garden path until about 48 hours away if I recall correctly.

[Catacol]

34 minutes ago, bluearmy said:

Ec46 as expected on ssw

the heights through Jan looking pretty good 

tomorrows met office update will be really interesting as will show how glosea sees the downwelling from the warming irrespective of it not being a technical ssw 

Disappointing, but tomorrow is another day and things may shift again. Signal for significant weakening still there.

[minus10]

Nice dip towards latter stage of 12z eps and clustering below the mean..

[MATTWOLVES 3]

Plenty going on beyond day 10 with the ecm ens. Perhaps we won't see a full on pv split just yet,but with positive background drivers right now its probably not gonna be needed at this stage. Keep in mind if a favourable pattern emerges over the coming couple of weeks and a full on pv split were to occur it would play out 2 ways...either to reinforce the cold pattern or to completely squander it! So as long as the pv remains weak and displaced,with all the other forcing coming into play we would remain in a very encouraging position....this would also serve to further weaken the strat and perhaps bring about a fall a little further down the line.

All in all I'm very happy with the current direction of travel....on top of very favourable met updates,we really can't complain.

 

Edited December 27, 2023 by MATTWOLVES 3

[Mucka]

So ECM ensembles pretty much with the Op, surprising to me given ECM has the low NE of Scotland at 120 and other models have it NW of Scotland.

That is quite a big difference at 120h on how the Arctic/Scandi ridge backs up the pattern on other output.

Here is ECM 120h mean V GFS and UKMO output

 

So it will be interesting to see if UKMO and GFS become more progressive tomorrow or of ECM continues to correct the pattern W

My guess would be ECM will correct W and will show an Atlantic ridge day 8 to 10, we will see tomorrow.

 

[Ali1977]

5 minutes ago, minus10 said:

Nice dip towards latter stage of 12z eps and clustering below the mean..

That’s extremely good to see , the mean is skewed by some outlandish mild ens members - clearly it’s going v cold

[bluearmy]

5 minutes ago, Catacol said:

Disappointing, but tomorrow is another day and things may shift again. Signal for significant weakening still there.

I think we have to wait for the next glosea run (assuming Exeter don’t change their tone on the second half of jan  tomorrow)  to see how the deceleration is expected to play out re downwelling waves 

just because we don’t see a tech ssw, it may not matter too much 

[WYorksWeather]

Think we need to go back to first principles a bit when watching the model output. Firstly, the background chance of a prolonged cold spell with a significant amount of snow in any given winter month is generally quite low, possibly even very low for some of the least favoured areas further south. Hence, background signals and tendencies for an increased chance need to be taken in that context, by both cold ra+mpers and cold pessimists (or mild lovers).

An increased background chance of a cold spell as the Met and others are suggesting for the second week of January onwards still doesn't mean it's guaranteed, so don't count your chickens if you're looking forward to it. On the flipside, even if it doesn't happen, it emphatically does not mean that the background signals are wrong, just that the odds didn't fall in our favour despite the increased chance.

The next point is that the ensemble forecasts are nearly always going to be more useful than any given deterministic operational run, unless you just like looking at pretty charts (which is an acceptable way to look at the models, I'm not policing what people post here!). The other point is that there are so many models, runs, and charts, that you'll nearly always be able to find a 'game over for Winter' chart or a '1 in 100-year snow bomb' chart.

With all that in mind, going to have a look at a few things.  Going to cover background drivers first.

SSW chances

Here's the ECM mean zonal wind chart for today.

And here's the last week.

 

The notable thing is that after getting closer and closer to predicting a major SSW, followed by the vortex remaining seriously disorganised afterwards, we finally saw the mean showing a reversal yesterday. The vortex does become very weak, as has been suggested for some time, but the mean is now in the 5-10 m/s range. The pattern afterwards has also shifted, with the vortex recovering fairly quickly to just below its climatological mean by February. Of course, these charts will continue to change, but the odds of a major SSW in the next two weeks have certainly diminished, but there may be some effects even if we don't actually get a reversal.

Sea surface temperatures

As others have said, well worth mentioning that sea surface temperatures will continue falling in the weeks ahead, despite the fact that we're now past the shortest day. There is a huge amount of inertia in this system. Of note is that most of the waters to our east will drop from the 8-10C range to the 6-8C range over the next two weeks or so.

  

In anomaly terms, not a huge amount of change except a cold spot emerging to the south-west of Iceland. And of course, the sea ice advance is visible on both charts. Still positive that we've lost some of the crazy anomalies around our immediate coasts. I would continue to note, however, that as we have seen at times recently, any south-westerly influence is likely to be notably mild, even compared to what we would normally expect from that wind direction.

GEFS and ECM ensembles

So, with all that in mind, onto the NWP output. Both ECM and GFS show something quite promising by the time we get to the second week of January. 

Promising for the north as well.

Here's my overall take - background factors aren't all that favourable for the next week or so - up to that point you're probably on a hiding to nothing chasing cold away from the most favoured spots (note the large amounts of snow for some parts of Scotland today, as an example!).

But, as we enter the second week of January and go beyond, the chance of cold does increase relative to average, and is starting to be picked up at longer range in the ensembles. All of this is regardless of what the individual OP runs decide to do - as we can see there are some very mild runs in here, alongside some very cold ones, so best not to spend too much time on an emotional rollercoaster. There will always be a mild outlier chart or a cold outlier chart each day.

Edited December 27, 2023 by Norrance

[tight isobar]

7 minutes ago, minus10 said:

Nice dip towards latter stage of 12z eps and clustering below the mean..

now as- again that’s  a correct direct for cold intro… the members are paying their bills… and mingling nicely

Edited December 27, 2023 by tight isobar

[Jacob]

ECM ensemble member 46 would mean 1ft of snow for the SE!

[feb1991blizzard]

30 minutes ago, tight isobar said:

Mid southern England horizontals.. we’ll take that… going forward…@12z-EC

Certainly nothing wrong with ext eps graph, these for the Midlands so no cherry picking.

[Liam Burge]

Anyone up for round 2?

[bluearmy]

9 minutes ago, Jacob said:

ECM ensemble member 46 would mean 1ft of snow for the SE!

it’s a 2% chance then - sadly not close to a foot either 

[swfc]

17 minutes ago, WYorksWeather said:

Think we need to go back to first principles a bit when watching the model output. Firstly, the background chance of a prolonged cold spell with a significant amount of snow in any given winter month is generally quite low, possibly even very low for some of the least favoured areas further south. Hence, background signals and tendencies for an increased chance need to be taken in that context, by both cold ra+mpers and cold pessimists (or mild lovers).

An increased background chance of a cold spell as the Met and others are suggesting for the second week of January onwards still doesn't mean it's guaranteed, so don't count your chickens if you're looking forward to it. On the flipside, even if it doesn't happen, it emphatically does not mean that the background signals are wrong, just that the odds didn't fall in our favour despite the increased chance.

The next point is that the ensemble forecasts are nearly always going to be more useful than any given deterministic operational run, unless you just like looking at pretty charts (which is an acceptable way to look at the models, I'm not policing what people post here!). The other point is that there are so many models, runs, and charts, that you'll nearly always be able to find a 'game over for Winter' chart or a '1 in 100-year snow bomb' chart.

With all that in mind, going to have a look at a few things.  Going to cover background drivers first.

SSW chances

Here's the ECM mean zonal wind chart for today.

And here's the last week.

 

The notable thing is that after getting closer and closer to predicting a major SSW, followed by the vortex remaining seriously disorganised afterwards, we finally saw the mean showing a reversal yesterday. The vortex does become very weak, as has been suggested for some time, but the mean is now in the 5-10 m/s range. The pattern afterwards has also shifted, with the vortex recovering fairly quickly to just below its climatological mean by February. Of course, these charts will continue to change, but the odds of a major SSW in the next two weeks have certainly diminished, but there may be some effects even if we don't actually get a reversal.

Sea surface temperatures

As others have said, well worth mentioning that sea surface temperatures will continue falling in the weeks ahead, despite the fact that we're now past the shortest day. There is a huge amount of inertia in this system. Of note is that most of the waters to our east will drop from the 8-10C range to the 6-8C range over the next two weeks or so.

  

In anomaly terms, not a huge amount of change except a cold spot emerging to the south-west of Iceland. And of course, the sea ice advance is visible on both charts. Still positive that we've lost some of the crazy anomalies around our immediate coasts. I would continue to note, however, that as we have seen at times recently, any south-westerly influence is likely to be notably mild, even compared to what we would normally expect from that wind direction.

GEFS and ECM ensembles

So, with all that in mind, onto the NWP output. Both ECM and GFS show something quite promising by the time we get to the second week of January. 

Promising for the north as well.

Here's my overall take - background factors aren't all that favourable for the next week or so - up to that point you're probably on a hiding to nothing chasing cold away from the most favoured spots (note the large amounts of snow for some parts of Scotland today, as an example!).

But, as we enter the second week of January and go beyond, the chance of cold does increase relative to average, and is starting to be picked up at longer range in the ensembles. All of this is regardless of what the individual OP runs decide to do - as we can see there are some very mild runs in here, alongside some very cold ones, so best not to spend too much time on an emotional rollercoaster. There will always be a mild outlier chart or a cold outlier chart each day.

Post of the day. Cracking

[Met4Cast]

Perhaps worth baring in mind that you don’t need a technical SSW (reversal) to see downwelling & impacts. 

The minor SSW in 1991 led to that February cold spell, for example. 

Technical SSW of not, we’re going to have a very weak vortex in the middle of winter with other variables falling into place. Not the worst position to find ourselves in.. 

[Kasim Awan]

2 minutes ago, Met4Cast said:

Perhaps worth baring in mind that you don’t need a technical SSW (reversal) to see downwelling & impacts. 

The minor SSW in 1991 led to that February cold spell, for example. 

Technical SSW of not, we’re going to have a very weak vortex in the middle of winter with other variables falling into place. Not the worst position to find ourselves in.. 

In my opinion there are factors working both for and against optimal tropospheric connection of the SSW. It has been gloated consistently by the literature that a starting negative NAO / easterly rossby wave activity significantly increases frictional coupling into the troposphere. Analyzing the NWP and it appears that the SSW will probably coincide with development of a North Pacific high / Arctic high which does raise interest for optimal coupling. I've already discussed the negatives working against this action. Does make you wonder, these contradictory factors could result in an air mass battle/ snow event potential mid month.

[Cambrian]

You pays your money and takes your choice, but thankfully all the main models essentially offer consistently good value at day 10 on the 12z EPS, GEPS and GEFS…

  

…courtesy of Iceland / Greenland / polar heights, and a Scandinavia / Europe trough, ultimately leading to variations on the single emerging theme of European cold. 

All wondrously impressive stuff for just a couple of days after Christmas, which I’m far from having finished celebrating  Have a great evening. 

[Polar Maritime]

Let's keep emotions aside and talk about what the models are showing.

[The 500]

Sometimes humour helps ! 

 Good question though . What is a Polar Vortex Warning…… if we all want snow …. Surely it’s good news and not a warning !,,

 

 

 

 

[Daniel*]

Typically when we see a SSW quite often it is preceded by +NAO/+AO, even following as strong westerlies are flushed down to the trop. The fact zonal flows are already weaker, helped by Canadian warming et al, and these indices are coming on board before a minor/major SSW is clearly a good thing. And it would be sensible to assume this would help to sustain -NAO/-AO for foreseeable. I don’t think that’s wishcasting just common sense clearly the probability of a colder than normal January is somewhat higher than normal on recent years with also potential for cold to establish on longer timescales. 

Edited December 27, 2023 by Daniel*

[BleakMidwinter]

1 hour ago, Mike Poole said:

There’s an awful lot going on at the moment, at different timescales and in different places.  Here’s how I see things:

There is a high probability of a significant cold spell starting early in the new year.  There is also a reasonable chance that it may last (with one or two wobbles) into the spring.  But we have some hurdles to get over first.

So how have we got here and what’s in our favour?  Take a step back from the UK for a moment, and this winter has one stand out feature, a weak polar vortex, in the troposphere that is one that can’t find its usual home, in the stratosphere, that is one that has already suffered a Canadian warming, is now under attack, and on the ropes.  We have an easterly QBO, and we have El Niño, which favours cold in the second half of winter.  We add to that (finally) a more amplified MJO in Phase 1, and rising AAM to put the atmosphere in tune with that El Niño.  

There is a timetable as to how this is likely to unfold:

• Forget the next 9 days, there is nothing to see here!
• About 10 days from now, model runs indicate that high latitude blocking will take hold in the European sector.  How exactly that happens depends on luck - where a ridge punches through, we’ve seen all the outcomes on the various op runs recently.  Provided that happens, could be Greenland (more likely, think GFS 12z), could be Scandi, possibly with retrogression (less likely, think yesterdays pub run).
• At about the same time, a warming event in the stratosphere is taking place, it may or may not be a technical SSW, but it will ensure a weak vortex for the foreseeable.  The models are backing off from a split SSW that looked to me a couple of days ago like it could lock in a -NAO until spring; this backing off may mean it happens later - perhaps when we need reinforcements, I’ll come back to that - or it could mean it is less powerful.  There’s also the possibility that a less than convincing split leaves a remnant of the strat vortex near Greenland where we don’t want it.  None of that is likely to markedly impact the next 3 weeks though, for good or bad.
• The AAM surge may continue, the MJO will move away from favourable phases in time, this might be the optimal time to benefit from a SSW, we can’t be choosers in this, but what I am highlighting here is the possibility that things might work constructively if we can get this winter evolution off the ground in the first place.

Finally, I leave you with the ECM extended clusters, at day 15 you have a choice of cold, cold, cold or cold:

Oh jings, thank you SO MUCH for this! 

This kind of clear, reasoned, structured explanation is the reason I keep reading this discussion even though around 75% of the posts are almost meaningless to me even after 12 years on the forum... 

Thankyou. It's appreciated! 

[Paul_1978]

24 minutes ago, The 500 said:

Sometimes humour helps ! 

 Good question though . What is a Polar Vortex Warning…… if we all want snow …. Surely it’s good news and not a warning !,,

 

 

 

 

First of all, what is the source of this? 
 

[Bartlett High]

Just now, Paul_1978 said:

First of all, what is the source of this? 
 

Daily Express 

[CryoraptorA303]

1 minute ago, Bartlett High said:

Daily Express 

You might as well be making it up then. In fact, it'd probably be closer to the truth if you did just make it up

[Bricriu]

1 hour ago, Met4Cast said:

Perhaps worth baring in mind that you don’t need a technical SSW (reversal) to see downwelling & impacts. 

The minor SSW in 1991 led to that February cold spell, for example. 

Technical SSW of not, we’re going to have a very weak vortex in the middle of winter with other variables falling into place. Not the worst position to find ourselves in.. 

I think also December 2000. A minor warming could still lead to a major one later on in any case. At least with a minor warming  there is potential  for furthering warming  going into the strat 

8 minutes ago, Bartlett High said:

Daily Express 

Whatever happens we don't want to see you for the next two months.