Winter Approaching: Forecast and Model Discussion Commentary

[WYorksWeather]

 Mike Poole I think my view on this is more in terms of longevity. A few days ago it looked like there was more of a possibility of the mid-Atlantic ridge getting all the way up to Greenland around that period 8th-10th December, and then staying there long enough for a more notable cold spell. At that timeframe, it does look like a bit of a downgrade, with more of a tendency to topple the high into the UK after 2-3 days. You can clearly see this on the morning 00z for instance, from T+168 to T+240.

 

However, as you say, the models are now showing a potential second round of amplification to our west in the extended range, so further chances for something more notable may come along further along.

Overall, probably an increase in certainty about a colder than average spell, but I would say a decrease in the possibility of anything notable around that timeframe. The later timeframes beyond the 8th-10th, which we are only just now starting to think about, are still a long way off. Very much the way I was thinking about the 8th-10th a few days ago, we need more model runs before we start thinking about the third week of December in any great detail.

 

[Cambrian]

It’s possible to gather a skewed view of the output if we focus on one timeline, forget that the ops can be a bit of a lottery at any time past days 5-7, even in less tricky hemispheric and strat-trop setups, and when the MJO outputs are ambivalent. 

A quick look at 0z GEPS at day 5 indicates a NAO+ profile, long fetch westerly or southwesterly upper flow from the southern United States, around the base of that impressive eastern seaboard trough, an Azores high ridging northeast and a sustained zonal flow beckoning; 

but a closer look shows wedges of heights pushing up through the mid Atlantic towards the Labrador Sea and Greenland. This is a repeated feature this season, and to a fair deal because of this, I haven’t seen more than fleeting indications of raging zonality at any stage so far this season. The GFS has been more bullish at times, but then we know that it can overplay the PV on the Canadian side. 

The strong, southward-diving trough becomes even cleaner by day 8, further developing the mid Atlantic wedges into a handsome mid Atlantic ridge reaching north to Iceland and a downstream Scandinavian trough. 

 

We have been keeping an eye on this window of opportunity next weekend for the last week or so, and there remains opportunity for a northerly blast for a day or two. I wouldn’t be surprised to see further interesting ops and controls this weekend for this period. A toppler, but potentially a good one, as shown by the impressive mean T850s. 

The more telling development is the southward extension of the European trough. Notice as well though that the Siberian Arctic lobe of the PV is at 495 dam compared to 504 dam on the Canadian side. At that stage, just a tad more Atlantic amplification into northern Greenland and downstream troughing through Svalbard might well break the pattern.

But alas, it’s not quite enough on this bite of the cherry. By day 10, there’s another westward flux of energy in the PV and a drawing of the pivotal low heights back towards Baffin, which draws the eastern seaboard trough north for a while and kills off the northward amplification in the mid Atlantic. Hence the toppler. The standout feature though is the establishment of low heights over the Mediterranean basin. 

And so the fluxing continues through to day 16, the precise balance between the two ends of the PV sausage determining the southward reach of the North American trough on that side and the capacity to drop low heights south through Svalbard into Scandinavia and the Mediterranean on the European side. Yesterday’s 12z EPS was playing with similar at day 15, not so clear on the 0z run, but we’d expect this while the ensembles are grappling with such a wide range of outcomes depending on this flux.

 

It’s a fine balance. We need the strong diving NA trough to drive the amplification. That pinch halfway along the PV sausage is the crux to my eye. If heights break through there, shaping the NA trough to push more of the energy back into Hudson Bay, we could get enough, more durable, amplification just a bit further west along the mid Atlantic / Greenland longitude to establish a cleaner and more sustained trough on the European side in the run-up to Christmas. This might then become sustained by the Eurasian Arctic lobe itself as the North American trough weakens.

A very long way off but good nevertheless to be able to discuss some promising indications, supported by a reasonably feasible evolution.

[blizzard81]

Latest mogreps is an improvement for next weekend compared to the 00z run.

[Scandinavian High.]

are we heading for something like this later in december more blocked patterns emerging there might be to much energy to the north west for the high to build. most probably riging though the  uk into scandinavia. 

[Dennis]

let it snow

amazing block

Edited November 30, 2024 by Dennis

[blizzard81]

Ok it's the very last frame of the latest ECM AI run but I'm seeing signs that week commencing 16th December could be when our next proper cold spell takes place.

[TEITS]

Sorry to be negative but I do not share the positivity from some today.

All I see in the models is an unsettled period with only brief outbreaks of N/NW,ly winds which may bring snow to favoured locations in Scotland, N Ireland, but very little for anyone else. The problem is were are seeing these brief NW,lys outbreaks via the heights in the Atlantic rather than Greenland so the N,ly flow is quickly toppled and replaced by a milder W,ly.

I have also seen E,lys mentioned and a Scandi High. However if you look at the 06Z GFS we may have a NE,ly at  +240 and low pressure across Sardinia but if you skip back to earlier time frames and view the actual air mass associated with the NE,ly then you will be disappointed. If you want a classic E,ly bringing a bitter cold spell then you need an injection of cold air from the arctic into Europe prior to the E,ly developing across the UK. The Jan 87 is a perfect example. 

Hope I am wrong but I see nothing for the next couple of weeks of any interest from a cold perspective. The upcoming weather is actually a very typical UK winter weather pattern with frequent alternating air masses.

[Met4Cast]

 TEITS

Yep - We’re looking beyond the scope of current model outputs for something more interesting. 

Wouldn’t be surprised to see some interesting output in the 18th-27th period. 

[Legend.Ice]

Fortunately the consistency of ECM ( ENS ) for the next two weeks is very low, or should I not be grateful for this ? I am not sure

[Chesil View]

 TEITS That's a fair post TEITS but thsnext two weeks isn't tie period that the background drivers are suggesting might be ripe for greater amplification. 

It is Interesting to note though that even the fallow period of the next two weeks shows an atmosphere where the MAR tries to ridge north.

In other words the atmosphere seems predisposed to attempting MAR/Greenland link up. Who know what might happen with a little extra help down the line.

[Dennis]

MJO 

[Mike Poole]

 Chesil View Absolutely this.  There is no proper cold showing within the 10 day period.  After that, the signs from the models and ensembles are now promising with the clear MAR signal, clear signal for Euro trough, and clear absence of signal for a rampant vortex +NAO.  There are at that point cold options showing in the model output, but by no means guaranteed.

But throw in more favourable background signals, especially the MJO moving into favourable phases, and that actually bodes quite well for mid month onward.  

[Legend.Ice]

 Dennis

 

It's a complete scandal, even after three days, the two big models can't get along and they have completely different mindsets. 

[damianslaw]

 Dennis Need to see it amplify into the 6 phase, interesting reading that this is often the point in the MJO cycle forecasters/models struggle with. Interesting to note how quick the last cycle moved through 6-7-8-1 compared in comparison to its sluggish expected move through 4 and 5 right now. Perhaps it may move through 5 far quicker than anticipated. There is the 10-15 day lag affect, so even if we are in an amplified phase 6 by the 10th at the earliest, and swift move into 7 through mid month the affects won't be until the very latter part of December, christmas onwards, but a cold high just before christmas could be served.

All just speculation and conjecture of the potential affects of a favourable cold phase of the MJO combined with a cold favourable PV, trop/strat behaviour. 

Edited November 30, 2024 by damianslaw

[Mike Poole]

 damianslaw This is the thing that bothers me.  Once we get into winter, the MJO has form for letting us down at exactly this point in the cycle.  It did last year if I recall correctly.  

As I noted above, the pattern seems well poised to be able to take advantage of the MJO phases 6 and 7 in the second half of the month, so it would be just our luck if once again it decides to shoot into the COD!  

Edited November 30, 2024 by Mike Poole

[Dennis]

Icon12

a try 

 

 

[northwestsnow]

CFS has it nailed ,we hope..

[swfc-ocs]

 northwestsnow can you elaborate .tia 

[northwestsnow]

 swfc

Yes we want to see the line going to phase 6 .( W Pacific )

Other models  have the mjo stalling in phase 5 before falling into the COD (circle in the middle).

One of my go to twitter chaps..

[Mcconnor8]

 northwestsnow Yep the Atlantic Tripole is very often a strong feature during Winters with lots of blocking, so very good to see that forming now.

[Dennis]

im still in for a 7-9 dec start on the block 

EC6z shows a good start on 6th

 

GFS 

 

[blizzard81]

 Dennis Are these an improvement on yesterday's?

[blizzard81]

 northwestsnow Yep I've been expecting the Atlantic tripole this Dec. Met office used to use this indicator for their winter forecasts. The Atlantic ssts in the previous May are a predictor of the forthcoming ssts in December. I remember seeing the tripole in May this year so hence my expectation of seeing it this Dec. Great news!

[Dennis]

 blizzard81

it seems - it  weakens the low in NE USA... but all is so fragile on weak MJO so hope we see better later the month 

Edited November 30, 2024 by Dennis

[Met4Cast]

18 minutes ago, northwestsnow said:

One of my go to twitter chaps..

Came across this account a couple of months ago, shows some fantastic knowledge on some very technical things, only 19 years old too!!