Model Output Discussion 12z 27/11/2012

[Weathizard]

Whoever said the ECM is an outlier is wrong.

The ECM is colder than the mean but it isn't an outlier and the control run is even colder.

http://www.knmi.nl/e...E_06260_NWT.png

Sometimes when an operation run shows a new trend and is followed by the control run the ensembles slowly begin to back the operationals. This obviously doesn't always happen but if tomorrows 0Z ECM is similiar to the 12Z then I can guarantee it will have more support.

That'll teach me for going on what some have said on here, Apologies haven't even been able to look at the ENS yet been at Football all evening, lets hope the ECM sticks to its guns in the morning.

[swfc]

Fort William, that's your idea of the far north, really?

--

Looks the possibility of some considerable marginal snow in Scotland come Monday

how can you have considerable marginal snow?

[Eskimo]

Marginal conditions often give the biggest snowfalls. One example is 25th Feb 2010 nearly two foot of snow fell in Scotland.

I think he was referring to the choice of wording used.

I had this funny feeling the ECM would backtrack its ideas of doom and gloom to joy and tribulations, lets keep this theme going. However I do feel like if the ECM were to verify, we would want some sort of warm-up and a respite.

[chionomaniac]

How come it is so difficult for some to actually understand what 'Model Output Discussion" actually means?

[LomondSnowstorm]

how can you have considerable marginal snow?

Quite easily, I'm sure we'll all had considerable marginal snow: temperatures around 1C, dewpoint just hovering at 0C, uppers around -2/-3C but with very heavy snow accumulating to great depths. I'd go as far as to say most considerable snowfalls are marginal. Any snow from this undercut is likely to be 'marginal', that's just the nature of frontal snow.

With this kind of overhead profile:

Snowfall couldn't be described as anything other than marginal, temperature wise at least.

Edited November 27, 2012 by LomondSnowstorm

[karlos1983]

How come it is so difficult for some to actually understand what 'Model Output Discussion" actually means?

Apologies,

Looking forward to the 00z when I awake tomorrow, thanks Nick, Steve etc for your informative comments, I'm learning so much, need to try and not get so emotionally attached to each run and comment.

[Dean E]

Sorry mods to go slightly off topic and a bit blunt, but Like I said yesterday, don't just look at this one run and write off...

You can clearly see that the GFS is still having issues with the block in my opinion and is trying to throw in some 'More average' output, like this 'Post-tropical' mess.

There are plenty of factors that indicate that we are about to enter a very blocked and cold December, as GP and others have said. The models are just struggling with the transition period.

Also, way to early to be talking of snow potential! We haven't established the cold just yet!

Keep it cool guys and the models will come into line

SM

[scottishandy]

Shortwaves and modelling.

I have no real experience to answer the questions on these but will try and lay out some thoughts from what I do know in laymans terms.

So if you look at model output from T00 to T384 or whatever the nearer timescales are much more likely to be correct as they are formulated from live or very recently collected data. As model time goes on the computer software works out a possible evolution for conditions at T00, therefore every model run is possible or the supercomputers would be a bit of a waste of money!

What we attempt to do in this forum is work out what is likely/probable and decide which particular model run is most likely to come to fruition. This is done very well by some posters here by a mixture of experience and knowledge of teleconnections (external drivers to our little suface patterns).

So back to the shortwave and it's impact on what we see in model output and reality in our weather. The reason we often see fantastic/extreme synoptics in the far time frame (beyond about T144 to T168 generally) is that the data fed into the computer models at T00 is diluted out and the computers then rely on the various other factors fed into them eg stratosphere, mjo, ssts etc etc. This will then give a long wave pattern (500mb heights, look for the colours on the map, being the driving force behind the surface pressure patterns) such as this -

Not the best example ever but shows a fairly straight northerly from Svalbard with the cold air being sucked straight down toward Scotland.

As this pattern approaches T00 then it is likely that the live data will indicate disturbances in the surface pressure patterns, these disturbances are referred to as shortwaves. These can be anything from a simple wave in the isobars to a small LP system and they often deflect the flow of coldest air from where we want it to be, or where it was modelled at T300.

So taking the current output for tomorrow the following chart shows a small LP just west of Norway.

This LP serves to divert the direct cold flow of T850 air around it so we are fed by a cool northerly instead of a cold north easterly, which would drag the cold air currently shown NE of Scandanavia to our shores.

If the model had this chart at T300ish it probably would have shown a long fetch NErly quickly dragging the colder air towards us as it had no way of knowing that this small disturbance/shortwave would form that far out.

If you flick though the frames on Netweather T850 viewer to T36, you will see that the cold to the NE never gets a chance to get past the warmer mini block created by this shortwave.

The other thing that SW's do is to change the shape of the 500mb troughs and ridges. The reason we are forecast to get a block breakdown at around T132 is beacuse what looks like a strong ridge now, yellows up to Greenland/Iceland, gets cut of by the little low shown here just at the south tip of Greenland. 10 days ago the charts modelled for today were not far off what we have got but this little LP/SW/piece of energy was not modelled.

If you go to the viewer on H500 and follow through you will see this LP sending a SW E and eating away at the block. This then means that the block is much weaker, not linked to the artic HP, so the next LP which comes against it pushes it right out the way.

There are many, many ways which SW's can change our weather prospects, for better as well as for worse, as a pattern change approaches which is why following the models in the run up to a cold spell can be really frustrating or fun, depending on your point of view.

This is why some folks talk about them and seemingly obsess on them. Enjoy the ride and follow the experinced posters as they commentate on the implications and likelihood of these dreaded SW's.

[Jonan92]

Run de Controle showing something more similar to ECM, a greenland blocking instead of scandi

Edited November 27, 2012 by Jonan92

[weathergeek]

@Scottishandy thank you so much for taking the time to write that for us newbs!

H x

[scottishandy]

No mention of the T96 FAX? Lots of troughing and interesting activity on the right side of the 528dm line, looks good to me!

[Tim Bland]

I like how we see different Synoptics from diiferent models but they all seem to lead to a chance f snow for many at 240hrs. With the teleconections in place we would be very unlucky not to see a widespread cold spell in the next 10 days IMO. Great post on the previous page showing how each GFS run increases the northern blocking in the short term. Looking good for high ground and the NE for Monday. Then later in the week it's either a Northerly or easterly..we hope. Loved Steve M's posts when the ECM was coming out, kept me entertained in a traffic jam

Re the above fax, I saw a bbc forecast earlier which suggested a trough moving south on sat and he said snow pushing south. I expect greatest risk is Cumbria then disapaiting as it pushes south. Could also be lots of showers for eastern counties

Edited November 27, 2012 by Tim Bland

[Eskimo]

No mention of the T96 FAX? Lots of troughing and interesting activity on the right side of the 528dm line, looks good to me!

Snow looks a certain possibility from that.

[Man With Beard]

Hi all, I watch these models and forums on and off and this season's events have drawn me right in, brilliant model watching! Nothing quite like a battle between the Arctic and our normal weather and this is a classic!

On topic: I think there's more mileage left in this weekend's events that some seem to think. Similar previous events have taught me two things. First, even when dryish conditions are forecast, localised snow events always seem to crop up somewhere at T24-T48. Second, when a breakdown low does not come predominantly from the south, the low often splits or stalls at the last minute - positioning can often alter 500 miles even from T72 charts. So although the models may have got it right, there are enough precedents for the UK to end up in the freezer at T120 / T144 based on what they currently show. I wouldn't call it till at least T72 personally.

[Weathizard]

Hi all, I watch these models and forums on and off and this season's events have drawn me right in, brilliant model watching! Nothing quite like a battle between the Arctic and our normal weather and this is a classic!

On topic: I think there's more mileage left in this weekend's events that some seem to think. Similar previous events have taught me two things. First, even when dryish conditions are forecast, localised snow events always seem to crop up somewhere at T24-T48. Second, when a breakdown low does not come predominantly from the south, the low often splits or stalls at the last minute - positioning can often alter 500 miles even from T72 charts. So although the models may have got it right, there are enough precedents for the UK to end up in the freezer at T120 / T144 based on what they currently show. I wouldn't call it till at least T72 personally.

Quality first post, I agree, however I can think the Atlantic will break through, albeit temporarily. Welcome to Netweather sir, I'm sure you'll enjoy the ride that the Model Discussion brings, and hate it at times!

[Eskimo]

When that low crosses the UK on Monday, will it sink south east and establish an easterly flow?

Edited November 27, 2012 by PerfectStorm

[nick sussex]

Sometimes you look at a fax chart and wonder!

If I posted this and you hadn't seen any other output what happened next?

I really don't think we've seen the last of the changes in the shorter term with this slider low.

In the medium term its certainly encouraging given the NH pattern so I think lots to look forward to.

[samadamsuk]

Hey all. what time does the ECM update ? I just looked through it and at t168 onwards it's a text book easterly ???

[Mark Neal.]

Hey all. what time does the ECM update ? I just looked through it and at t168 onwards it's a text book easterly ???

6-7am. Yes was lovely easterly showing up was a outlier though apparently.

[Weathizard]

Sometimes you look at a fax chart and wonder!

If I posted this and you hadn't seen any other output what happened next?

I really don't think we've seen the last of the changes in the shorter term with this slider low.

In the medium term its certainly encouraging given the NH pattern so I think lots to look forward to.

As has already been alluded to by the first time poster earlier, I absolutely agree with this again Nick, even T72 isn't nailed at the moment, small differences make a massive difference at a latter period, that fax chart is very good, 528 dam line over most of England/Scotland? The human input makes the fax charts very important in my opinion, they are the best forecasting method, but its all up in the air so who knows. Adios for the evening guys, been a fun day, ups and downs!

[samadamsuk]

6-7am. Yes was lovely easterly showing up was a outlier though apparently.

Ah ok - thanks Mark. Still, imo a major model showing solution at that range is still notable - especialy as the ensembles run at a lower resolution

Sam

[kumquat]

Encouraged by Steve Murr and some (subtle) Nick Sussex posts, and taking on board the longer range hints from Snowking's ECM outlooks - All of that coupled with Ian Ferguson's small hints in the West Country weather output for (albeit marginal potential) for snow at the weekend - in the South West.

I'd like to join in with the idea that I'm sure we've seen within the last couple of years, that whole pattern move south and west as the timeframe comes nearer, run after run. I do kind of hang onto the coattails of these posters, but I've also seen, first hand, how the pattern can change from T72 downwards - backing south and west. Been surprised by snowfalls down here in the west country (even SOUTH of the "M4 corridor". Not saying it's going to happen - but I would not be surprised. Great model watching these past two weeks - so thanks to all for that!

[Mucka]

00z not up to much, shortwaves everywhere interrupting the flow of cold uppers and block weaker by 96. Not sure where it will sit in the ensembles but it is reversal of the trend we saw yesterday. Nothing wintry from this in the short term except for the far North of Scotland and high ground

Hope is on the mild side of the ensembles and other models are better.

Edited November 28, 2012 by Mucka

[Snowy Easterlies]

i suspect this place will become a lot less busy over the next few days now

00z is a downgrade in the very early time-frame lets not kid ourselves here, very cold 850s get to the north sea before retreating again and pressure is higher to the south and west which a very common synoptic pattern, so everything gets shunted further north and may continue to do so on future runs

just looks like a normal low pressure pattern to me, don't be fooled by those dark blue 500mb charts, uppers are only at about -3 next week maybe some hill snow thats about it

height try build post T168 but is slower then the 18z to do this

Edited November 28, 2012 by Snowy Easterlies

[LomondSnowstorm]

i suspect this place will become a lot less busy over the next few days now

00z is a downgrade in the very early time-frame lets not kid ourselves here, very cold 850s get to the north sea before retreating again and pressure is higher to the south and west which a very common synoptic pattern, so everything gets shunted further north and may continue to do so on future runs

Oh please, we're not starting this again are we? Every single morning the GFS 00Z seems to look poorer than the previous day's output and every day the output gets better as the day goes on and other models come out. Even with it being more progressive (and by the way it looks overprogressive compared to the NAM upstream) we still see heights trying to rise in the mid Atlantic once the trough passes through:

Bloating troughs and shortwaves aside which the GFS remains notorious for overdoing this looks pretty consistent with yesterday's output, if not quite as nice in practice. Let's see where the UKMO goes.