Model Output Discussion - cold spell to end Autumn

[Mucka]

Both GFS and UKMO looking good at 120

Just a case if they can set up blocking far enough West

Edited November 28, 2017 by Mucka

[That ECM]

2 minutes ago, karlos1983 said:

UKMO 96 & 120

 

Liking this from ukmo. Ditto mucka. 

Edited November 28, 2017 by That ECM

[Gavin Hannah]

Bit of a difference now in the LP in the Atlantic.  Tripple pronged with lope heading east and putting a bit more pressure on the HP to our west. Overall, the pattern is broadly similar, but this is where it will start to veer in a slightly different direction.. Will it upgrade or downgrade??

Edited November 28, 2017 by Gavin Hannah

[frosty ground]

Cutting this close, the Atlantic is trying to break through..... 

[Met4Cast]

The negative tilt of the high and the shortwaves coming out of America smell undercut to me.

I think the next few runs will show the high moving Northwards into Greenland as the shortwaves slide underneath the block - Unless of course we see a GFS classic overblowing the lows.

[Guest]

MODEL SWINGS AND UNCERTAINTY

Well it has been another extraordinary period of model uncertainty. Models often struggle to nail down future pattern changes and the timing of them but the output in the last few weeks seems to have been particularly volatile. I say “seems” as emotional reaction to even quite minor changes greatly exaggerates the underlying variability. The vast majority on here are “coldies” and “snowies” (including me) but we do ourselves no favours by setting expectations too high and moving from delight to desperation when some great looking charts are suddenly downgraded. I know that many of you say that you enjoy the “chase” but that can be done in a more civilised and less dramatic way which will help the general mood and be far less confusing to some of the readers who are trying to learn and understand what we are on about.  Let me demonstrate this now.

In my last couple of posts I’ve banged on about many comments in relation to the UKMO output. I tried to demonstrate that this model has been far more consistent than the others during the last couple of weeks. It is the other models that have lurched from one evolution to another, often with very different outcomes shown from one run to the next. The recent GFS output is a case in point. Following several runs showing blocking remaining in the right place to deliver another (or further) Arctic reloads, last night’s 0z swung to a much milder solution and dropped the northerlies almost completely (from next week onwards). Then this morning’s 6z run goes back to show almost continuous cold. The ECM, having been the most consistent model for several days showing strong amplification made a major turnaround and thoroughly downgraded the northerlies on last night’s 0z run. To a lesser extent, GEM and JMA also switched to milder solutions. The UKMO has hardly changed at all. It has maintained its output showing renewed Arctic incursions separated by fairly brief milder interludes. The only minor changes have been on the precise timing and duration of each episode.  

Some of the comments about extremely minor changes have been greatly exaggerated. For example with the UKMO, the very minor changes in shortwave activity which was shown on one run and removed on the next run would not “definitely make all the difference going forward” and I repeat what I said in several earlier posts, one cannot assume how the UKMO will go after D6 as they only go up to T+144 (and the T+168 of the western Atlantic almost always leaves one guessing).  Sometimes, as Nick Sussex and several other “short wave specialists” often point out, these minor changes can have significant impacts on the set up. In my view much of the recent drama regarding a minor LP in the central Atlantic was the equivalent of fake news! Yes, when shortwaves occur in a volatile and mobile pattern driven by a powerful Jet Stream with favourable alignment then an innocuous looking minor feature can suddenly deepen and/or phase with a more major LP and override or undercut HPs and alter the broader pattern. With the blocked Atlantic (whether central ridging, east based or west base –NAO) these features have struggled to have any impact at all. They have not been aligned with the Jet Stream and have hardly ever deepened at all and “predicted” ones from any of the models have often come and gone on consecutive runs. With much of the energy going over the top of the block, the path and strength of the Jet Stream is much more important there (to the west of and around southern Greenland for example). Nick F made an excellent post this morning and explained why some of the models chopped and changed. This was due to the meandering or kinks in the Jet Stream with the models struggling to decide whether the UK will be on the warmer or colder side of the loop (or alternating from one to the other). So despite a blocked Atlantic with MLB and long periods of amplification into the Arctic producing mostly meridional patterns (with the UK benefitting from regular Arctic incursions) we still need to keep a careful eye on the path of the Jet Stream. It still looks like the persistent mid Atlantic ridge may be very hard to shift in the foreseeable future.

So it’s a case of how much longer this pattern can be maintained. There is a lot of uncertainty in terms of the impact (or lack of it) from changes in the stratosphere, on other teleconnections and whether the Jet can break through and take its more traditional path across the Atlantic. As I always say, this is way beyond my pay grade and I’ll leave that up to the small number of real experts that (mostly) only occasionally post on this thread. At this stage we simply do not know whether it’s merely a question of time before “normal service is resumed” or whether the blocked patterns can be maintained for an extended period (well beyond mid-December).

That’s more than enough from me for now. I’ll be back with another full report with loads of charts later this week or over this weekend.

[karlos1983]

3 minutes ago, Daniel Smith said:

The negative tilt of the high and the shortwaves coming out of America smell undercut to me.

I think the next few runs will show the high moving Northwards into Greenland as the shortwaves slide underneath the block - Unless of course we see a GFS classic overblowing the lows.

not seeing much negative tilt there Daniel....

EDIT: actually, you might be right, struggling to work these twins out.. 

Edited November 28, 2017 by karlos1983

[frosty ground]

There is a break at the surface pressure, but to much heights have sunk, we could get a balmy south westerly later on (for a short time at least?

[-uksnow-]

Just now, Daniel Smith said:

The negative tilt of the high and the shortwaves coming out of America smell undercut to me.

I think the next few runs will show the high moving Northwards into Greenland as the shortwaves slide underneath the block - Unless of course we see a GFS classic overblowing the lows.

To be honest I think we've overcome the first hurdle now, but the look of 3 lows spinning around eachother in the Atlantic al la GFS seems unlikely to me, it never handles that kind of energy well. UKMO seems the better option keeping them split slightly. Think the trend so far is our friend, but i expect post 120hr improvements on successive GFS runs when it gets a handle on the atlantic lows..

[Cheshire Freeze]

The issue with the 12z vs 6z GFS is that the 12z has the core of the heights slipping south to produce Euro heights, whereas the 6z got just enough heights N...it made all the difference

[igloo]

slowly but surley the GFS is starting to go the same way as the ECM in the mid term  it looks like another case of this model being 24h behind 

[frosty ground]

1 minute ago, CreweCold said:

The issue with the 12z vs 6z GFS is that the 12z has the core of the heights slipping south to produce Euro heights, whereas the 6z got just enough heights N...it made all the difference

Heights wise there is not much difference



The issue (might be) the arctic air heading south west! 

[Cheshire Freeze]

1 minute ago, igloo said:

slowly but surley the GFS is starting to go the same way as the ECM in the mid term  it looks like another case of this model being 24h behind 

The 12z GFS thus far is miles better than the ECM of earlier...Lack of heights around Greenland on ECM..

[karlos1983]

In terms of Greenland heights this is actually turning into a better run, so that is really good news, the detail we can work on! I'm happy so far this afternoon. 

[Mucka]

UKMO 144 looks pretty good to me, I would still prefer a sharper trough and cleaner retrogression of course but this should good enough to keep the Atlantic out.

Edited November 28, 2017 by Mucka

[Met4Cast]

6 minutes ago, karlos1983 said:

not seeing much negative tilt there Daniel....

EDIT: actually, you might be right, struggling to work these twins out.. 

BOOM!

 

[Scandinavian High.]

Looking very good to me high going north.

[radiohead]

Cold spilling down from the north as an Atlantic front pushes ever close.

[Cheshire Freeze]

Anything from here on in is purely eye candy or lack of- depending on how it goes.

Until those Atlantic lows are sorted out and the upstream pattern off the ESB set, this is going to be very very finely balanced.

[feb1991blizzard]

This isn't going to throw a spanner in the works - its a belter, although close run thing to get here, this actually ties in with Met Office text forecast.

[Day 10]

Cold air starting to dig down.

[Met4Cast]

Cold air flooding Southwards

Confidence of a Greenland high I'd put at 70% given the strong signal for one now. Confidence is actually higher in the extended than it is in the medium with regards to how we get there.

[Captain Shortwave]

The GFS 12z should be just about okay, that Atlantic low should move through the south of the UK at worst (Possibly missing south)  with cold air cutting in behind.

 

Edited November 28, 2017 by Captain Shortwave

[Cheshire Freeze]

Just now, feb1991blizzard said:

This isn't going to throw a spanner in the works - its a belter, although close run thing to get here, this actually ties in with Met Office text forecast.

Extremely.

How the preceding pattern plays out will dictate whether we see freezing temps and snow or whether we're cracking the BBQ out and up in the teens Celsius. It's that finely balanced. 

[frosty ground]

Just now, Captain Shortwave said:

The GFS 12z should be just about okay, that Atlantic low should move through the south of the UK with cold air cutting in behind.

All fun but this run could produce a major snow event across the south as rain turns to snow as the cold air pushes south cold an active weather front.

That would be a potentially snowy Low. How it interacts with the Arctic air would be a forecasters nightmare.