Model output discussion - The run up to Christmas

[Pennine Ten Foot Drifts]

37 minutes ago, ITSY said:

It's apples and oranges I know and I'm not comparing the substance here (there is no comparison), but I've not seen such persistent and tightly packed cold agreement since Dec 2010. It's startling really and quite refreshing! 

Exactly what I was thinking. This will probably evolve into the most notable cold spell for many years - it's pretty much nailed on as it couldn't have come at a worse time for this country (for most outside this forum anyway)..............

[Uncertainty]

50 minutes ago, feb1991blizzard said:

Don't like the stratosphere on the 6z as much as i liked it on previous runs.

 

No Feb neither do I but I think the will it / won’t it split aspect of this warming remains to be seen. The ensembles (which unlike last year share the FV3 core’s extra stratospheric revolution) are still toying between whether or not to initiate that secondary warming. Many don’t, but this is still an evolving situation beyond the expected Siberian warming in the days leading up to New Year.
The good news is that the ones that have the 2nd warming over the last few days have the split in what would hopefully be a great position: over the N Atlantic.
 

This is the trend I would love to see develop over the next few days.

Onto the trop and I think the GFS needs some credit here. The EPS and GEPS have been 2-3 days behind in firmly spotting this enhanced Atlantic amplification. Though in defence of the EPS this signal has been there throughout including on the 46, there’s just been a few more wobbles than the GFS suite. Overall I think this pattern has been quite well signposted, here’s the EC46 anomaly from the 3rd of December:

Tonight’s run will be very informative, not only for the short term pressure distribution but for the ECM’s take on stratospheric developments. If a)it is seeing an SSW and b)it downwells the -NAM to the trop then this should be evident in the latter stages of its output. 
 

Even if, as I still suspect, the split is delayed vs  recent GFS predictions the continued presence of the Siberian cold/high heights will continue to pile pressure on the SPV. 
 

[DavidS]

I’m loving the way the 500 anomalies evolve on the GFS 6z.  Below some of the snapshots from day 8. 

[Beanz]

Looking good for some seasonal temps and weather, I'm quite impressed how consistent GFS has been over the last few days.  

I'm happy with that, can't say I'll be jumping up at the curtains in the morning at the promise of snow, or sitting watching lampposts just yet though.  As much as I would like that, it will be bitter disappointment for anyone looking for a dumping of the white stuff over the Christmas period.  Its just not in the models, so I'm quite confused about the excitement for it right now...  

 

[SLEETY]

24 minutes ago, West is Best said:

I've been keeping off here because I haven't wanted unintentionally to annoy people.

However, in the interests of balance I have to post this as it has been showing up for days and days. If anyone tells you that the really cold scenarios after Boxing Day are 'nailed on' then there are three significant reasons why I don't yet believe it.

1. We always get this and they always downgrade. Without exception.

2. The upstream blocking still doesn't look sufficiently robust to me. Whether it's the mid-Atlantic High holding in place or retrogressing into Greenland, neither remains for very long and that's, frankly, a recipe for trouble.

3. Most significantly the ensembles continue to show a significant number of members bucking the trend. This from the 6z where, once again, the operational is running below the mean:

 

I'm sorry to appear like a party pooper and I hope I'm wrong.

Plus it's still over a week away, lots chan change still in that timeframe, INCLUDING, UPGRADES, so could get manic in here soon.! 

[Zak M]

   

[Methuselah]

I think it's fair to say that, at this time (and with the data we have at our disposal) snowfall from the upcoming cold snap will be marginal -- most, if not all, of the projected parameters look like being somewhat 'iffy'. And, going by experience, it sometimes only needs one iffy parameter to cast doubt...? But it's still the best scenario we've seen -- potential-wise -- since the BFTE...?

Then again, on the other side of the coin, synoptics like these have the potential for becoming severe:

    

Even assuming no snowcover, any persistent freezing fog could keep daytime temps below freezing, over wide areas...?

Edited December 21, 2020 by General Cluster

[frosty ground]

29 minutes ago, West is Best said:

1. We always get this and they always downgrade. Without exception.

So how do you explain actual cold spells that happen?

[cheshire snow]

29 minutes ago, Kasim Awan said:

GFS 06Z evolution is sensible. Has a polar low diving SSE at D7 creating room for further amplification, I expect the SE Greenland / Iceland area to have higher heights forecast by upconing output for D8-9. This could well yield a surface easterly into the first week of January.

Please correct me if wrong but I was always under the impression that polar lows are not forecast they are features that appear at very short notice.

C.S

[frosty ground]

Just now, cheshire snow said:

Please correct me if wrong but I was always under the impression that polar lows are not forecast they are features that appear at very short notice.

C.S

Very much this.

Polar low - Wikipedia

EN.M.WIKIPEDIA.ORG

 

[Bricriu]

4 hours ago, bluearmy said:

Looks like The Russian ridge holds the sceuro trough in place. The trend for that ridge to strengthen remains this morning on the eps .....as mused last evening, whilst the Atlantic ridge continues to ebb and flow, we see systems dropping in on a mainly nw/se axis into the trough.  How that manifests itself dictates whether wet or white .... 

and do we then see the Atlantic ridge extend across to its Russian cousin ?  I always look for a cold pool somewhere in Russia at times like this when there is a chance we could see a Siberian ridge push west with med low heights likely ....should be nothing more than a note in the back of your mind but there is one at day 10 on the ec op. It likely won’t be there in twelve hours but it might come back .....

This is interesting. If i'm not mistaken there was a gfs run last week that showed the Atlantic ridge  linking up to the Russian high after Christmas? If that were to occur, i wonder what would then happen if we had A SSW with that scenario. I suppose no one can answer that question.

[Ols500]

4 minutes ago, frosty ground said:

So how do you explain actual cold spells that happen?

Severe cold downgraded to actual cold. I get what he means though, that nice -5 850 on the GEFS until the foreseable future will not happen, as we get closer milder blips will occure it always does. 

[Drifter]

4 minutes ago, cheshire snow said:

Please correct me if wrong but I was always under the impression that polar lows are not forecast they are features that appear at very short notice.

C.S

You are absolutely correct, no idea why he keeps  calling this a polar low. 
It’s just a low coming down from the north. 

[Bricriu]

4 hours ago, Nick F said:

Synoptically looking great on the 00z runs this morning, the T850s don't look much for a screaming northerly next week, however, those worried by the poor looking T850s, the air thicknesses and surface dew points modelled don't look quite so bad for snow, but this far out are subject to change. 

I prefer to use the thickness and dew points to look for the potential for snow, I know it's futile this far off, but  850-1000 hPa thickness of 1290-1300 gpdm and 500-1000 hPa thickness below 524 gpdm and a dew point around 0C the upper limit for snow to fall, just about meet that across many parts Tuesday onwards next week as the flow turns northerly on GFS op. 

Longer term, looking at the extended  00z EPS 500 hPa mean, a signal for the trop vortex over Canadian arctic to retrograde toward Alaskan arctic - this may open up the possibility for high latitude blocking while maintaining low heights over mainland Europe as we head into the New Year. So looking good.

With this background state of blocking and the GFS still signalling for a major weakening of the SPV and perhaps SSW in January, it's looking very interesting for coldies for the rest of winter, should everything fall in place ...

If the cards do fall in our favour( i.e we get a prolonged and snowy cold spell), some people will have questions about the Glosea5 model and its outlook for this winter.

[kold weather]

The 06z GFS suite has trended just a touch colder between 27-30th period at 850hpa.

I'd keep a close eye on this time period, very real chance of a decent snow set-up developing, though one that would be rather on the marginal side.

Its tentitive, but things are looking really interesting for that period mentioned above and whilst whats happening further down the line may get even more so, this former period I get the feeling is sneaking up on people a little...

[frosty ground]

1 minute ago, Ols500 said:

Severe cold downgraded to actual cold. I get what he means though, that nice -5 850 on the GEFS until the foreseable future will not happen, as we get closer milder blips will occure it always does. 

Except it’s just not true the part where he says without exception is simply wrong and when that’s your first point it’s a bit a stretch to read the rest.

The current outlook has been well advertised by the ensembles it will be cold with the chance of snow, snow chances will vary as small scale features appear in the flow, these features tend to bring in mild sectors and make snow chances less favourable, usually to the west,

there is no sever cold in the forecast yet and the longevity of this spell is unknown a SSW could leave us wet and windy and remove the current pattern that is more favourable to snow than most normal winters 

 

 

[Jason M]

Yep, I read somewhere there are 10 parameters for snow. The four I tend to use are 850 temps, 528 Dam line, Theta values (10 or less) and dew points. Get these four ducks lined up and the 2m temperature generally becomes a non issue.

Under a day 2 maritime flow I'm looking for 850s of -7c (day 2 to mix out any milder air) and yes I get the arguments that it can snow with -1C uppers given the right situation. I don't know what the dewpoints would be for the next 10 days but under a maritime airmass they must be marginal. Theta values on a lot of the charts are around 14 which feels a bit high. So, maybe reliant on evaporative cooling which generally equates to temporary covering or slush rather than winter wonderland

The 528 dam line is the one that I'm seeing ticked the most on the charts. 

[Frosty_wsm]

1 hour ago, 78/79 said:

Get the right set up, and we can get hammered here.  Seen it a few times, Feb 78 being notable.

Yep, I've seen so much talk recently about the upper air temps not being great & then I think about the awesome snowfall event of Dec '81 with over a foot of level snow here & I'm at sea level just a few miles from the Bristol channel & looking back at the archive charts the upper air temps were only - 2/-3 at the time so you never know. Either way it's great seeing this thread buzzing & hopefully we all get lucky & see some snow. 

[iowpompeylee]

53 minutes ago, Uncertainy said:

No Feb neither do I but I think the will it / won’t it split aspect of this warming remains to be seen. The ensembles (which unlike last year share the FV3 core’s extra stratospheric revolution) are still toying between whether or not to initiate that secondary warming. Many don’t, but this is still an evolving situation beyond the expected Siberian warming in the days leading up to New Year.
The good news is that the ones that have the 2nd warming over the last few days have the split in what would hopefully be a great position: over the N Atlantic.
 

This is the trend I would love to see develop over the next few days.

Onto the trop and I think the GFS needs some credit here. The EPS and GEPS have been 2-3 days behind in firmly spotting this enhanced Atlantic amplification. Though in defence of the EPS this signal has been there throughout including on the 46, there’s just been a few more wobbles than the GFS suite. Overall I think this pattern has been quite well signposted, here’s the EC46 anomaly from the 3rd of December:

Tonight’s run will be very informative, not only for the short term pressure distribution but for the ECM’s take on stratospheric developments. If a)it is seeing an SSW and b)it downwells the -NAM to the trop then this should be evident in the latter stages of its output. 
 

Even if, as I still suspect, the split is delayed vs  recent GFS predictions the continued presence of the Siberian cold/high heights will continue to pile pressure on the SPV. 
 

I see Admiral Akbar, we all want to see charts like Admiral Akbar 

Edited December 21, 2020 by iowpompeylee

[Stu_London]

In terms of upgrades/downgrades for 850temps, I've seen on a lot of occasions, slight upgrades in the just out semi-reliable time frame - this is what we are seeing over the last couple of runs for the period 27th-30th and there is still a little bit of scope for improvement. 

Often when we get what I would call the reliable timeframe (96hrs and under) there is a very small backtrack. Happened in Feb/March 18 which started as -15C ish - nearly made it to -20C at about five days and ended up being -18s and -19s in some places. That spell wasn't an issue as there was nothing marginal to worry about.

This one is a more of a problem, so some expectation management is not a bad thing. However, in here, expectation management for some is about as feasible as trying to get your cat to understand the concept of Norway.

 

 

[Jason M]

4 minutes ago, iowpompeylee said:

I see Admiral Akbar, we all want to see charts like Admiral Akbar 

I'd imagine Admiral Akbar would study the current charts for snow potential and then conclude 'its a trap' 

[SLEETY]

Check out you tube and Shetland Isles polar low, these rare beasts can deliver bucket loads of snow , if the setup being forecast is correct then it wouldn't be surprising to see polar lows developing in the Northerly flow, from end of week, onwards. 

[Kasim Awan]

40 minutes ago, cheshire snow said:

Please correct me if wrong but I was always under the impression that polar lows are not forecast they are features that appear at very short notice.

C.S

It's the trough energy associated with the larger scale low responsible for the polar lows not them directly. They don't always appear at very short notice, though they do have a short duration.

The GFS is quite good at modelling troughs embedded in 500mb temps below -35C, 528dam or lower in an unstable northerly flow with low heights. The 6Z does appear to form a polar low. 

[Kasim Awan]

3 minutes ago, Kasim Awan said:

It's the trough energy associated with the larger scale low responsible for the polar lows not them directly. They don't always appear at very short notice, though they do have a short duration.

The GFS is quite good at modelling troughs embedded in 500mb temps below -35C, 528dam or lower in an unstable northerly flow with low heights. The 6Z does appear to form a polar low. 

The pattern is rife for producing "polar lows". The point I'm making is, the GFS can predict them, plus additional deeping is possible under a nowcast situation. 

Edited December 21, 2020 by Kasim Awan

[Paul]

52 minutes ago, cheshire snow said:

Please correct me if wrong but I was always under the impression that polar lows are not forecast they are features that appear at very short notice.

C.S

A bit more on polar lows here, if anyone wants to learn about them