Model output discussion - The run up to Christmas

[Faronstream]

37 minutes ago, Geordiesnow said:

  and not once is there pool of - 20 air in Scandinavia and the uppers in general are between - 8 and - 12 so warmer than average. 

 

-20 as 850hpa temperature in most of scandinavia is not even normal, that's around -10 degrees colder than average so 9-10 is normal for early jan here.

[northwestsnow]

EC ever so slightly east of UKMO with the dropping trougb at 96...

( I think)..

 

[captaincroc]

17 minutes ago, Frosty Winter said:

Blatant cherry picking but perturbation 24 is insane with a deep area of low pressure diving south with -12C uppers tucked in with it as it would bring memorable blizzards for many. Would cause absolute chaos!

I've been thinking the past week, what weather would I pick if I could pick a chart...and selfishly it would be something like this.  A beast from the east is great but being to the west of the Peaks/Pennines always causes a shadow so only the perfect synoptics will give us a dumping

...these charts however would be just amazing to see and something I never have experienced in mt 39 years. A true blizzard (gale force winds and heavy snow) from the north.  If this happened then a dream come true.  

[Daniel*]

Quite something to have a chart like this at day 4 extreme low 500mb heights. 

[Harsh Climate]

Still big differences between ECM and GFS at 120h, make of it what you will.

[swfc]

22 minutes ago, SteveB said:

Did you not read Snowkings post? It's pinned at the top of this page. 

Guess it's his opinion tbh. We live in a democracy and all that. I'm guessing there's know professional forecasters on here altho well respected? Anyway great output today and posts people. Let's hope the nhp and weak vortex and deliver what's showing going forward

[Captain Shortwave]

The ECM has not been overly keen on producing any weak cut-off highs to the north over previous runs, something the GFS (And potentially UKMO if it went further). Will we see one form this run?

Day 5 with the deep U.K. trough and the amplified ridge to the west.

[Kasim Awan]

There's no point fixating on a particular T850 value as the hallmark for snow.

There are around 25 factors that influence the T850s required for snow.

For example, low heights, which move the altitude of 850hpa down by about 200 metres. This automatically drops the T850 requirement by 1-2C. For example, a -4C 850 temp in high heights would produce a 1500m temp of -4C, however in low heights T850 is near 1300m so a -4C T850 would produce a Temp at 1500m of -6C..

In the set up modelled for 27th-29th, surface inversions will form a cold layer up to 2000ft asl. This reduces the T850 requirements, as they do not need to be at the depth of cold needed to ensure the entire troposphere is freezing due to the presence of an inversion. So uppers of -2/-3C can suffice if this permits the temp to remain below 0 from the T850 level to the top of the inversion! Given we have both low heights & light winds 27th-29th, these two mechanisms will work to reduce the T850 requirements.

4th March 2016. Low lying Cheshire got 4". T850s of -2C, evap cooling sets up a superficial inversion (the temps don't decrease near the surface, but the top-down lapse rates drop to near 0 permitting -2C uppers to be just tidy indeed). Even 0C uppers can suffice if evap cooling drops the top-down lapse rate to 0 so that effectively the air does not warm as the snowflake falls near the surface). So theoretically uppers of 0C and a surface inversion could produce heavy icy snow / blizzard. But...

Marginality - precip too light and you will fail to get the evap cooling needed to reduce the top-down lapse rate, keeping the surface warm. Strong winds, shore wind, the Fohen effect, urban heat island, and about a gazillian other factors can do the same.

So hopefully this will clear up the T850 debate once and for all. It is a lot more complex than a single figure. Ultimately the potential is there for "high" T850s to produce amazing events.

It is up to meteorologists to determine the contribution of ALL these factors and how this will impact the precip state. 

[Tim Bland]

models firming up on this low for Monday now. ECM and UKMO have a similar path right through the UK. Sowhere Is going to get a dumping from this. ...

[Blessed Weather]

41 minutes ago, Geordiesnow said:

How is it misleading? The poster says its showing the lack of cold air and clearly there is a lack of cold if Scandinavia is running at above average upper air temperature wise. Perhaps posting a chart for the 7th January is not the best idea but I just looked at the GFS 12Z run upto 200 hours  and not once is there pool of - 20 air in Scandinavia and the uppers in general are between - 8 and - 12 so warmer than average. 

The only snow risk I can see in the short to medium term is when the low heads over the UK and the winds are slack with very low thicknesses. After that, too much wind so too much mixing and the air not really cold enough for most. 

It feels like daylight robbery that such a great set up is failing to deliver uppers of - 10 and lower. 

850hPa temps might well be above long-term average, but to use a chart without any explanation and accompanied by the statement "Really struggling to see any proper cold" is, in relation to what the actual 850hPa forecast is for 7th Jan, misleading. Imho.

[That ECM]

4 minutes ago, Captain Shortwave said:

The ECM has not been overly keen on producing any weak cut-off highs to the north over previous runs, something the GFS (And potentially UKMO if it went further). Will we see one form this run?

Day 5 with the deep U.K. trough and the amplified ridge to the west.

Well there’s your answer

[Howie]

1 minute ago, Kasim Awan said:

There's no point fixating on a particular T850 value as the hallmark for snow.

There are around 25 factors that influence the T850s required for snow.

For example, low heights, which move the altitude of 850hpa down by about 200 metres. This automatically drops the T850 requirement by 1-2C. For example, a -4C 850 temp in high heights would produce a 1500m temp of -4C, however in low heights T850 is near 1300m so a -4C T850 would produce a Temp at 1500m of -6C..

In the set up modelled for 27th-29th, surface inversions will form a cold layer up to 2000ft asl. This reduces the T850 requirements, as they do not need to be at the depth of cold needed to ensure the entire troposphere is freezing due to the presence of an inversion. So uppers of -2/-3C can suffice if this permits the temp to remain below 0 from the T850 level to the top of the inversion! Given we have both low heights & light winds 27th-29th, these two mechanisms will work to reduce the T850 requirements.

4th March 2016. Low lying Cheshire got 4". T850s of -2C, evap cooling sets up a superficial inversion (the temps don't decrease near the surface, but the top-down lapse rates drop to near 0 permitting -2C uppers to be just tidy indeed). Even 0C uppers can suffice if evap cooling drops the top-down lapse rate to 0 so that effectively the air does not warm as the snowflake falls near the surface). So theoretically uppers of 0C and a surface inversion could produce heavy icy snow / blizzard. But...

Marginality - precip too light and you will fail to get the evap cooling needed to reduce the top-down lapse rate, keeping the surface warm. Strong winds, shore wind, the Fohen effect, urban heat island, and about a gazillian other factors can do the same.

So hopefully this will clear up the T850 debate once and for all. It is a lot more complex than a single figure. Ultimately the potential is there for "high" T850s to produce amazing events.

It is up to meteorologists to determine the contribution of ALL these factors and how this will impact the precip state. 

You really know your stuff mate!

[sheikhy]

Just now, Tim Bland said:

models firming up on this low for Monday now. ECM and UKMO have a similar path right through the UK. Sowhere Is going to get a dumping from this. ...

That looks even further east than the 00z!!!literally down the centre of the uk!!!

[Scott Ingham]

Outstanding post 

For any newbies this why the drum has been bashed.

This set up is unique.

This set up has all the hallmarks for a marginal set up that can dump lots of unexpercted surprise snow (although marginality is becoming less of an issue with time and model runs)

[I remember Atlantic 252]

1 minute ago, sheikhy said:

That looks even further east than the 00z!!!literally down the centre of the uk!!!

but are these 850's good! they look pants, even for frontal ppn  with winds not off the sea?

 

[Harsh Climate]

12z esembles out and again astonishing agreement with most perputations clustering at -5 850s or just below for many days

 

[northwestsnow]

Watch that waa go next slide...

Edited December 23, 2020 by northwestsnow

[snowice]

3 minutes ago, I remember Atlantic 252 said:

but are these 850's good! they look pants, even for frontal ppn  with winds not off the sea?

 

Need to park the uppers argument going around in circles for days.

[winterof79]

11 minutes ago, Kasim Awan said:

There's no point fixating on a particular T850 value as the hallmark for snow.

There are around 25 factors that influence the T850s required for snow.

For example, low heights, which move the altitude of 850hpa down by about 200 metres. This automatically drops the T850 requirement by 1-2C. For example, a -4C 850 temp in high heights would produce a 1500m temp of -4C, however in low heights T850 is near 1300m so a -4C T850 would produce a Temp at 1500m of -6C..

In the set up modelled for 27th-29th, surface inversions will form a cold layer up to 2000ft asl. This reduces the T850 requirements, as they do not need to be at the depth of cold needed to ensure the entire troposphere is freezing due to the presence of an inversion. So uppers of -2/-3C can suffice if this permits the temp to remain below 0 from the T850 level to the top of the inversion! Given we have both low heights & light winds 27th-29th, these two mechanisms will work to reduce the T850 requirements.

4th March 2016. Low lying Cheshire got 4". T850s of -2C, evap cooling sets up a superficial inversion (the temps don't decrease near the surface, but the top-down lapse rates drop to near 0 permitting -2C uppers to be just tidy indeed). Even 0C uppers can suffice if evap cooling drops the top-down lapse rate to 0 so that effectively the air does not warm as the snowflake falls near the surface). So theoretically uppers of 0C and a surface inversion could produce heavy icy snow / blizzard. But...

Marginality - precip too light and you will fail to get the evap cooling needed to reduce the top-down lapse rate, keeping the surface warm. Strong winds, shore wind, the Fohen effect, urban heat island, and about a gazillian other factors can do the same.

So hopefully this will clear up the T850 debate once and for all. It is a lot more complex than a single figure. Ultimately the potential is there for "high" T850s to produce amazing events.

It is up to meteorologists to determine the contribution of ALL these factors and how this will impact the precip state. 

Its like platting snot.... I'd give up

[Tim Bland]

6 minutes ago, sheikhy said:

That looks even further east than the 00z!!!literally down the centre of the uk!!!

Yes, now precip charts are out it’s clear it’s a big shift east. I’m going to say this is probably going to clip eastern areas at best the way it’s going! As for ‘surprise’ ‘unexpected’ snow ...ECM had been modeling this for 3 days now lol. It always makes me laugh how we talk about a snow event for days and then alL of a sudden there is a risk of a ‘surprise ’ event.  Even BBC have it on their weather bulletins today 00z vs 12z ECM below 

[mountain shadow]

10 minutes ago, Tim Bland said:

models firming up on this low for Monday now. ECM and UKMO have a similar path right through the UK. Sowhere Is going to get a dumping from this. ...

Yup. 

Cairngorm and Ben Macdui will do very well indeed.

[swfc]

Ec better Heighths to the far northeast svalbard region at the 168 hr mark. Looks a good run going forward

[Daniel*]

ECM still very different to GFS reluctant to send low south in promoting the E/NE winds. FI is currently day 5.

[Weathizard]

Will this wedge of heights be enough to deflect colder 850s to us at 192?

[Updated_Weather]

Aidan from MetO responded to my tweet regarding where they see High pressure more likely.