Model output discussion - The run up to Christmas

[Jason M]

15 minutes ago, LRD said:

There were obviously other factors favouring snow in Dec 1981 but the 850s very rarely got below -5, -6 during the whole month if at all.

In Jan 1982 they did

Rain to snow event:

I'm sure parts of England experienced lows of around -20 in this scenario:

Don't over rely on the accuracy of those charts. Back then the best selling home computer was the ZX81 which had a 1K (yes, 1K) memory !!! And yes, I had one   There is more computing power in your TV remote than there was in the mainframes back then and these charts are nothing more than a broad guess. Trust me, 850s were lower than -5c. 

[LRD]

2 minutes ago, Jason M said:

Cheers.

I think one of the problems on this thread sometimes is that (some) people don't take the trouble to understand the basic qualities of the various airmasses the UK is subject to and how they interact. For example -2C at 850 with a cold undercut from a PC (Polar Continental) airmass is literally about as different as can be from a PM airmass. People see a northerly and assume certain things, but as we know a northerly can have PM qualities or it can be a true Arctic airmass. Without understanding about these factors its not possible to really know what the charts are telling you. In Dec I tend to add 9c to the 850s of a PM to get a rough (and it is rough) approx of 2m temps. So -5 could easily equate to +4c (and I'm being generous here). With a PC airmass in Dec you can add as little as +6c so -5C at 850 is giving +1c 2m temp. Factor in lower due points with a PC flow and its suddenly 'white' not 'wet'. By Feb with added solar input these numbers can change a fair bit.

 

The airmass in Dec 1981 was essentially polar maritime and had uppers of -5 virtually all the way through. I'm not denying there were probably other factors that contributed to a December that was snowier where I live than 2010 was but 850s are the thing under discussion here

Perhaps if someone has the time or inclination they could do an analysis of why it was so snowy and cold that month under-less-than-specatcular 850s in a northerly/PM airmass

[weirpig]

Latest Icon   shows quite a bit of snow  for our Irish members  on or around the 23rd.   

[LRD]

6 minutes ago, Jason M said:

Don't over rely on the accuracy of those charts. Back then the best selling home computer was the ZX81 which had a 1K (yes, 1K) memory !!! And yes, I had one   There is more computing power in your TV remote than there was in the mainframes back then and these charts are nothing more than a broad guess. Trust me, 850s were lower than -5c. 

Ok if what you say is true, post a link or a chart that proves those 850s were under -5 in dec 1981 Because these archive charts are the only place/evidence that I know exists

If you see the archive charts for other cold spells on those archive charts it seems to get those basically correct - Jan 87 for example will show uppers of around -15

FWIW I'm not expecting snowmaggedon next week for everyone (especially where I live) but some places will get lucky

Edited December 21, 2020 by LRD

[Matt H]

16 minutes ago, terrier said:

I’m sorry but I’m really struggling here to see the excitement I think a few people are getting confused with these blue areas over the U.K. at the moment.  Yes we are better placed than last winter admittedly.  But are we heading for a deep freeze with deep crunchy snow for many.  From what I’m seeing at the moment definitely not. Now before I get called a mild lover far from it. We have even had mr murr on here this morning saying the uppers are very marginal in this set up we are seeing. Do I think some will see some snowfall yes. But away from Scotland and the highest northern hills I think we are looking at cold rain and sleet. Unfortunately we just don’t have the cold available to the east at the moment. So let’s see what transpires over the next week. But just remember these areas of blue over the U.K. don’t necessarily mean deep cold and bucket loads of snow. And as Steve has already said this is a very marginal situation I’m afraid. So don’t expect much in the way of snowfall away from the far northern hills and Scotland would be my guess. 

Same time tomorrow?

[Paul Sherman]

2 minutes ago, LRD said:

The airmass in Dec 1981 was essentially polar maritime and had uppers of -5 virtually all the way through. I'm not denying there were probably other factors that contributed to a December that was snowier where I live than 2010 was but 850s are the thing under discussion here

Perhaps if someone has the time or inclination they could do an analysis of why it was so snowy and cold that month under-less-than-specatcular 850s in a northerly/PM airmass

Just a possible explanation but have a look at Nigel Boltons post over on Ukww in their Model Chat Thread a page back and an interesting observation was that 40 years ago there was 300 miles more Ice Sheet North of Scotland than there is today meaning the cold was not mixed as much as it is today (eg it travels over more warmer seas in todays climate) Its an interesting theory if nothing else and may produce answers why our Northerlies these days are so Insipid

[LRD]

4 minutes ago, Paul Sherman said:

Just a possible explanation but have a look at Nigel Boltons post over on Ukww in their Model Chat Thread a page back and an interesting observation was that 40 years ago there was 300 miles more Ice Sheet North of Scotland than there is today meaning the cold was not mixed as much as it is today (eg it travels over more warmer seas in todays climate) Its an interesting theory if nothing else and may produce answers why our Northerlies these days are so Insipid

Great, thanks although I can't see that 300 miles more ice would have made an awful lot of difference

Besides surely uppers of -5 in 2020 are the same as uppers of -5 in 1981. Aren't they? Or are they? 

Edited December 21, 2020 by LRD

[ALL ABOARD]

10 minutes ago, Jason M said:

Don't over rely on the accuracy of those charts. Back then the best selling home computer was the ZX81 which had a 1K (yes, 1K) memory !!! And yes, I had one   There is more computing power in your TV remote than there was in the mainframes back then and these charts are nothing more than a broad guess. Trust me, 850s were lower than -5c. 

I'd dispute this. 

I'm pretty sure the archive charts are run from a modern computer backwards. 

 

Otherwise how would we have charts dating back to the 1600's. 

[Paul Sherman]

1 minute ago, LRD said:

Great, thanks although I can't see that 300 miles more ice would have made an awful lot of difference

Besides surely uppers of -5 in 2020 are the same as uppers of -5 in 1981. Aren't they? Or are they? 

Have not looked into it to be honest but think Nigel used to work for the Met Office so clearly there might be something behind his thoughts of why Northerlies are watered down in the last 30-40 years. Its Interesting for sure

[cheeky_monkey]

3 minutes ago, LRD said:

Great, thanks although I can't see that 300 miles more ice would have made an awful lot of difference

Besides surely uppers of -5 in 2020 are the same as uppers of -5 in 1981. Aren't they? Or are they? 

probably means warming at the surface layers would be greater??

[Battleground Snow]

Hopefully when it comes to next week uppers gate will be a distant memory.

Icon has the trough further south, but not east yet. 

[Jason M]

Just now, LRD said:

Ok if what you say is true, post a link or a chart that proves those 850s were under -5 in dec 1981 Because these archive charts are the only place/evidence that I know exists

If you see the archive charts for other cold spells on those archive charts it seems to get those basically correct - Jan 87 for example will show uppers of around -15

Of course I can't and I expect they were within a few degrees of being right. Did you know that on 13th December 1836 the -4C 850 line was over the northern tip of Scotland?  I'm not making that up, its right there on metieocel . Wish I knew how to copy the image!! How likely was that to be exactly right?

In 1981 two days was about the best range for an accurate forecast (the old countryfile forecasts were usually hilariously wrong). By 1987 things had moved on greatly we had gone from a ZX81 to proper home computers and 5 day forecasts were often in the right ballpark! 

I'd wager John Holmes would add some thoughts around this. 

[Chesil View]

11 minutes ago, Jason M said:

Don't over rely on the accuracy of those charts. Back then the best selling home computer was the ZX81 which had a 1K (yes, 1K) memory !!! And yes, I had one   There is more computing power in your TV remote than there was in the mainframes back then and these charts are nothing more than a broad guess. Trust me, 850s were lower than -5c. 

I,d be wary about making the assumption you make in your last sentence Jason. The biggest snowfall I have experienced in my lifetime 25 foot drifts and 12-18 inches undrifted back in Feb 1978 occured with uppers of -2. These were confirmed in a met office article at the time.

 

[Jason M]

5 minutes ago, ALL ABOARD said:

I'd dispute this. 

I'm pretty sure the archive charts are run from a modern computer backwards. 

 

Otherwise how would we have charts dating back to the 1600's. 

Yes, but they can only operate on the data input which back in the day was substantially less good for obvious reasons. 

[LRD]

2 minutes ago, cheeky_monkey said:

probably means warming at the surface layers would be greater??

Yes that is certainly a possibility. I guess it'll be the acid test for -5s next week

[ALL ABOARD]

Just now, Jason M said:

Yes, but they can only operate on the data input which back in the day was substantially less good for obvious reasons. 

No I understand what your saying. 

But the computer programming is run backwards in a complex manor. 

There was nobody I can assure you sitting around in the 1600s knowing what a minus 5 isotherm was. 

[Kasim Awan]

In the shorter term, ICON has a trough development early Xmas eve. Notice the lift in 850ThetaE values off the NE coast, which highlights the trough..

If this gains support it could produce a few cm in places.

Edited December 21, 2020 by Kasim Awan

[LRD]

13 minutes ago, Jason M said:

Of course I can't and I expect they were within a few degrees of being right. Did you know that on 13th December 1836 the -4C 850 line was over the northern tip of Scotland?  I'm not making that up, its right there on metieocel . Wish I knew how to copy the image!! How likely was that to be exactly right?

In 1981 two days was about the best range for an accurate forecast (the old countryfile forecasts were usually hilariously wrong). By 1987 things had moved on greatly we had gone from a ZX81 to proper home computers and 5 day forecasts were often in the right ballpark! 

I'd wager John Holmes would add some thoughts around this. 

Well you said you can guarantee that the 850s were below -5 so I thought you had some hard evidence of that. Meantime all we have is the archive charts. So we can only work with what we have to work with. Besides I thought it was what another poster said about it being today's computers working this backwards ito generate those archive charts so we're not relying on ZX81s or whatever

No believe it or not I've not looked at what the weather was doing in Dec 1836

Almost 3 years before Dec 1981 the archive seems to have nailed this ok

Edited December 21, 2020 by LRD

[phil nw.]

Just a quick one on uppers and snow.

A flick through the archive charts on the the 62/3 Winter will show many days where we had quite modest uppers.We saw a lot of Polar continental air from easterlies/n.easterlies with southerly tracking lows hence lower dew points=air off the cold continent/Scandinavia.

Of course the persistence of snow cover meant the UK retained a lot of surface cold as the Atlantic couldn't get in.

Anyway i digress so to bring this back yo the present-the London temp.ens.

Steady as we go.Cold into January.Good runs earlier btw maintaining the pattern of upstream ridging and a diving trough from the north west.

 

[Jason M]

Just now, ALL ABOARD said:

No I understand what your saying. 

But the computer programming is run backwards in a complex manor. 

There was nobody I can assure you sitting around in the 1600s knowing what a minus 5 isotherm was. 

Exactly 100% right. The issue is the data quality as there just wasn't the data. Lets leave it there as the next model runs are out shortly.

[Downburst]

As I was going over the Cork and Kerry mountains, I spied 2 cm of snow and it was a melting....

 

[Kasim Awan]

Just now, phil nw. said:

Just a quick one on uppers and snow.

A flick through the archive charts on the the 62/3 Winter will show many days where we had quite modest uppers.We saw a lot of Polar continental air from easterlies/n.easterlies with southerly tracking lows hence lower dew points=air off the cold continent/Scandinavia.

Of course the persistence of snow cover meant the UK retained a lot of surface cold as the Atlantic couldn't get in.

Anyway i digress so to bring this back yo the present-the London temp.ens.

Steady as we go.Cold into January.Good runs earlier btw maintaining the pattern of upstream ridging and a diving trough from the north west.

 

Inversions erode the <1000ft temps just like evap cooling etc can erode the >500ft temps. In both instances, more moderated uppers can support snow.

[Neilsouth]

The media(TV) is making nothing of the upcoming cold, just cooler.. Are the models further ahead than what is currently being forecasted? ( This is a model related question )

Also what happened to people posting constant updates of the latest models, chart by chart? Seems a bit quiet to me :D

Edited December 21, 2020 by Neilsouth

[phil nw.]

Just now, Kasim Awan said:

Inversions erode the <1000ft temps just like evap cooling etc can erode the >500ft temps. In both instances, more moderated uppers can support snow.

There are so many variables Kasim.Thats why forecasters even the pros can get it wrong sometimes.Anyway best leave it there let's be ready for the 12s.

[Jason M]

4 minutes ago, Neilsouth said:

The media(TV) is making nothing of the upcoming cold, just cooler.. Are the models further ahead than what is currently being forecasted? ( This is a model related question )

And that really is the big question isn't it. In a week or so we will know the answer