Model output discussion - The run up to Christmas

[Kasim Awan]

Just now, LRD said:

You can only laugh really can't you? Warm uppers coming in from the low countries. There's no disputing that'd be too warm!

Some worried about this 4-5 days ago and were skeptical about the colder charts being posted. It now seems a fair possibility that a trend to mixing due to low expansion could verify and make things even more marginal. Though we will keep a trend for renwed amplification.

[Met4Cast]

1 minute ago, Frosty Winter said:

Should’ve gone to specsavers...

Indeed, it's very subtle but ICON has the centre of the low to the west of the UK, the UKMO has it to the east, but broadly the pattern is the same. UKMO would be the one you'd want to verify as it'd be colder.

[Ben Sainsbury]

GFS looks very good at T168, cold uppers flooding in from the NE now!

[TSNWK]

Fairly large move at 162 with heights seeping towards scandi.. this is where I think we want the next play..

[booferking]

1 minute ago, mountain shadow said:

Your first statement is just not true. At t+144 the Icon is some 250 miles or so further West then UKMO.

At 144hr icon the low is centered over Northern Ireland and on the ukmo its of the east coast of england think he pulling are leg

[Scott Ingham]

The big story for me is the upgrade in 850s.

Look at the 29th.

Massive upgrade. 

Prospecte definitely look better for snow betweem Christmas and New Year

[Daniel*]

Enjoying trend developing from GFS from winds to swing to NE... less marginal.  

[mountain shadow]

Just now, TSNWK said:

Fairly large move at 162 with heights seeping towards scandi.. this is where I think we want the next play..

Can't see a Scandi High from there. Heights more likely over Greenland but the omnipresent danger of a West based -NAO

[Matt H]

At a risk of reopening some old wounds from earlier, it’s great to see some sub -5C uppers appearing on the charts now.

[Geordiesnow]

2 minutes ago, LRD said:

You can only laugh really can't you? Warm uppers coming in from the low countries. There's no disputing that'd be too warm!

Deja vu from the start of the month where a lack of cold air became a real problem.

Sorry guys but the only snowfall we are likely too see will be very transient and limited in nature, it's the exact same pattern as we started the month with of a large low heading through the UK and where conditions are slack then the snow level could come down but this is even worse as the uppers are slightly warmer than they were then. 

Also the brief northerly is the best chance of some Eastern areas seeing snow falling, maybe a light covering on Xmas morning for east anglia but that is it really. 

[tight isobar]

You only need to frame view the gfs 12z on inter frames,- runs.. to note it’s evolution, is both steadfast-sensicle...   I’m going to refrain now until all 12z- and there supports are run .  @the American model taking charge here

Edited December 22, 2020 by tight isobar

[Tim Bland]

6 minutes ago, Met4Cast said:

Indeed, it's very subtle but ICON has the centre of the low to the west of the UK, the UKMO has it to the east, but broadly the pattern is the same. UKMO would be the one you'd want to verify as it'd be colder.

Agree. Probably easier to see in this view...

[TSNWK]

Still trying to get those heights east to the north of us.. also on this occasion looks a tad less amplified towards Newfoundland which if my logic is correct should reduce risk of west nao.. 

great model watching present..

[That ECM]

I know it’s not our area but southern Sweden is in heavy snow for 48 hours.

[Tim Bland]

2 minutes ago, Frosty Winter said:

At a risk of reopening some old wounds from earlier, it’s great to see some sub -5C uppers appearing on the charts now.

Would -5 be cold enough ...I’ll get my coat

[LRD]

Better but I'd like to see that low pressure head SE now otherwise central Europe will be covered in warm uppers

[Matt H]

1 minute ago, TSNWK said:

Still trying to get those heights east to the north of us.. also on this occasion looks a tad less amplified towards Newfoundland which if my logic is correct should reduce risk of west nao.. 

great model watching present..

Very consistent with the 06z so far, GFS has definitely been leading the way recently.

[Geordiesnow]

4 minutes ago, Frosty Winter said:

At a risk of reopening some old wounds from earlier, it’s great to see some sub -5C uppers appearing on the charts now.

Not cold enough I'm afraid not with winds off the sea. 

It's is astonishing at the lack of cold on our part of the hemisphere, there is just no PV there at all! No wonder we are feeding on scraps. 

[SLEETY]

Big reload from the North incoming  if this run verifies,you would be unlucky not to see some snow from this run/

Edited December 22, 2020 by SLEETY

[Matt H]

2 minutes ago, Tim Bland said:

Would -5 be cold enough ...I’ll get my coat

I resign from model watching today.

[MOF]

4 hours ago, snowking said:

There's been a lot of focus on 850mb temperatures over the last few days and will they/won't they support snowfall, so just wanted to create a post for any of the less experienced members who are probably left scratching their heads about the conflicting views amongst different members and want to know what to believe.

Let's start with why everyone looks for the -6c 850mb (or hPa) line and it's a bit of a history lesson I'm afraid. I believe in terms of these (and other) forums, the magical figure of -6c was really coined as the "snow line" back in the bad old days of the late 90's and early 00's over on the old BBC Snowwatch forums, when things were so desperate that we were crying out for any sort of 2 day northerly toppler just to look forward to (which is why I do find it amusing that some members can't see the potential in the upcoming period, I think unfortunately December 2010 distorted the expectations of some). Anyway, because back in those days we were primarily looking for Northerly topplers for any sort of cold, given an almost complete lack of Easterlies between 1996-2005, the 850mb temperature needed to be significantly below 0c for the airmass to be conducive for snow falling (this is a bit of an oversimplification, but let's keep it light!). Why? Well in the lower part of the troposphere we generally expect that as we travel from the clouds to the surface of the earth, the temperature increases. However we also have to remember that cold air falls, and so what we expect is the colder air from the 850mb level to fall down closer to the surface of the earth over time. The rate of this temperature increase (or "thermal gradient") varies hugely due to an overwhelming number of factors, however if we consider this in a simplified form again, the answer lies, for us at least, in the oceans.

Because from a North or North-Westerly airflow the air is travelling a long distance over a generally warm Atlantic ocean, there is more likelihood of warmer pockets of air close to the surface of any airflow from the N/NW cancelling out some of the colder air dropping from aloft than there would be from say the East, where the air is travelling a far shorter distance over the North Sea and so is less likely to include these warmer pockets of air. These warmer pockets of air can affect many of the parameters that we won't go into in this post, but the two that are worth calling out as they are mentioned a lot are the 2m temperature and the 2m dew point. 

So if we try and summarise the above two paragraphs, in a N/NWly airflow the air is travelling over warmer oceans and so warmer pockets of air can be expected to be found closer to the surface. To override these warmer pockets of air we need the cold air aloft (at the 850mb level) which falls down towards the surface of the earth to be cold enough to override these warmer pockets of air and make the air from cloud to surface cold enough throughout to support all of our lovely snowflakes falling from the clouds remaining as snowflakes rather than falling through a warmer pocket (or layer) of air and melting into horrible rain, which from experience is much more difficult to make snowballs from.

I think the primary reason we chose the -6c line was that back in those bad old days where Wetterzentrale was the choice of most members to consume their daily dose of GFS from, the dashed isotherm line which indicated the 850mb temperatures were spaced out at approximately every 5c (although for some reason I seem to remember Wettzentrale often showing -6c instead of -5c as one of the dashed isotherm lines, correct me if I'm wrong if anyone can remember), and so we would often look for the -5/-6c isotherm on the 850mb temperature charts as our guaranteed snow line, as it allowed for enough headroom for some warmer pockets of air at the surface to be overridden by the falling colder air from above. At this time I think it's fair to say that none of our collective weather knowledge was anything like it was today, and so looking for something simple like a single "snow line" helped us in our search for snow.

 

So, on to the next part, what exactly is the 850mb (or hPa - they both essentially mean the same thing) temperature chart. Again I will keep this as simple as I can, so apologies to anyone offended by the oversimplified statement I may be about to make. Well to answer the first part, it's exactly what it says - it is the temperature of the air at the point in the atmosphere where the air pressure is equal to 850 millibars (mb) or hectopascals (hPa). But the exact height above the surface of the earth of where pressure is equal to 850mb can change, and that is what all of the pretty colours on the "height" charts we see are. So we've been talking about the 850mb level of the atmosphere, let's take a jump to the other common charts we see - the 500mb charts, such as the one below:

The colours here represent how high above the earth the 500mb pressure level is. The more towards blue/purple the colour is, the lower the height (or closer to the surface of the earth) the 500mb pressure level is, and vice versa for the greens/yellows/oranges. This also has a knock on effect on how high/low the 850mb pressure level below it is.

So what we are saying in the context of our upcoming cold spell is that under that huge low pressure system, the height above the surface of the earth of both the 500mb and the 850mb pressure level is lower. What this means is that the colder air aloft, at the 850mb level, does not have as far to fall down towards the surface, and therefore more colder air is able to come down and help override any warmer pockets/layers of air towards the surface. This means that we don't necessarily need the 850mb temperature to be below -6c to support snow falling to the surface. We also then need to bear in mind that as that low pressure falls further South, and we change our feed of air from being from the North/North-West across the Atlantic ocean to instead coming from the East from the near continent, fewer of these warmer pockets of air are likely to exist.

Just to give you an example of this from the most recent 6z run, here are two 850mb temperature charts, the first one at +138 hours, and the other at +168 hours:

It would stand to reason that if it was as simple as colder 850mb temperature = colder surface temperature, then we would expect to see a lower 2m dew point temperature in the corresponding +168 chart right? Well see for yourself:

You can clearly see that despite the 850mb temperature being 1-2c warmer at +138 than at +168, the dew point temperature is 1-2c lower at +138c. 

You can again see the reason for this by looking at the 500mb height charts:

We can see here that the lower heights at +138 aid the cooler 2m dew point temperatures.

So in summary for what is a very long post, you do not require -6c or colder 850mb temperatures, even in a North-Westerly airflow sometimes, in order to guarantee all of the relevant parameters for snow being the right side of marginal, it is significantly more complex than that, and that is why the upcoming period could well deliver a lot of surprises - the word could being crucial in there!

Hi, I've been lurking on this forum for a long time (4 years or so), picking up bits of info when I can so I can work out how to read the weather charts, but after reading this post I felt like hat I had to sign up just to show my appreciation. Absolutely spot on and perfect to help other less experienced watchers gain knowledge.

I'm based in the Lakes so hoping for some of the white stuff over the the next few weeks/ months.

Thanks. 

[mountain shadow]

3 minutes ago, SLEETY said:

Big reload from the North incoming  if this run verifies,you would be unlucky not to see some snow from this run/

Possibly but not certain. 

West based -NAO dangers are evident.

[Matt H]

1 minute ago, Geordiesnow said:

Not cold enough I'm afraid not with winds off the sea. 

It's is astonishing at the lack of cold on our part of the hemisphere, there is just no PV there at all! No wonder we are feeding on scraps. 

Have you read the excellent post from @snowking earlier about 850hpa temperatures and snowfall? Definitely worth a read as it provides a very good explanation about why uppers aren’t always necessarily the most important factor when it comes to snowfall. It was very informative as they don’t have to be as low as I initially thought for decent snowfall.

[Daniel*]

5 minutes ago, Geordiesnow said:

Not cold enough I'm afraid not with winds off the sea. 

It's is astonishing at the lack of cold on our part of the hemisphere, there is just no PV there at all! No wonder we are feeding on scraps. 

If that isn’t cold enough for wintriness  it’s time to find a new hobby. The east coast perhaps too marginal.

 

Edited December 22, 2020 by Daniel*

[sheikhy]

Beauty of a 12z so far!!looks better at 228 hours as well!!