Model output discussion - 1st December onwards

[nick sussex]

Thanks Nick F for your reply.

The ongoing debate re the MJO is quite interesting and I suppose sums up a lot of science with differing views.

I've looked at a lot of the research over the last week and I think some of the best is by MacRitchie and Roundy 2009 and 2012. I think a lot of the confusion or argument lies around the role of Kelvin Waves.

The suggestion was that the KW's leave behind potential vorticity which aid the MJO.

The smoothed out version used by MacRitchie is still partly based on the Wheeler Hendon (2004), but uses a pre-filtering technique .

I'll repost this bit in their study:

 This prefiltering removes Kelvin wave signals, which contribute significantly to the standard RMM PCs in a manner that would invalidate the composite analysis because it might specify some of the Kelvin wave signal (Roundy et al. 2009).

I think this is a good thing, even though they leave behind that PV we don't really want to track the actual wave, even though that PV helps the MJO. I think an analogy could be throwing together a meal, we're more interested in how it tastes at the end rather than the ingredients that put it together if that makes any sense!

 

 

 

Edited December 27, 2015 by nick sussex

[Nick F]

30 minutes ago, Alpine glow said:

Hi. Have been watching the model forum for many years now and tried to learn things on the way.

may I ask a couple of questions regarding the current progged set up without causing too much disruption? Just want to validate a couple of things in my mind really.

i'm wondering about the SW low andwhy it's so important. Am I right in thinking that if it cuts under and merges with the scandi High, it will produce snow? Is this because the milder air is merging with the cold air. Also, is it this kind of set up (if it happens) that produces a channel low?

if it doesn't undercut, is the Scandi high enough to produce snow with the right ingredients? I think this is the closest I've ever understood anything and it feels good to understand a little no matter how basic.

whilst I'm here, I'd just like to say the all posters no matter what the style have always been great for me which is what makes the forum the best on the Internet

 

Further to what Perfect Storm has mentioned above ...  apart from wrap around fronts from any lows moving east to the south bringing snow, the cold easterly flow can develop convection over the relatively warmer North Sea and bring sleet/snow showers if the cold pool is deep enough.  

We don't necessarily need lows to the SW to undercut for cold snowy easterlies. If low pressure develops over southern/central Europe (which often occurs in response to cold pools dropping SW from Russia) and the Scandinavian high is far enough west, even under what looks like fairly high pressure conditions over the UK, you can still get snow showers or longer spells of snow if the cold air coming over the North Sea from the east is deep enough.

If the easterly is not cold enough, it will bring low cloud and cold drizzle. Cold air advecting from the east can be 'diluted' by milder air being drawn around lows coming in from the west to our south, which is exacerbated if cold air is not already present across the UK, so the set-up does need to some 'fine tuning' to get the ideal cold easterly flow for snow.

This kind of undercutting set-up could bring a channel low, if the low tracking east is far enough north, but there is the danger that, if sufficiently cold air is not already in place, milder air wrapping around a channel low will make it the wrong side of marginal for snow.

Edited December 27, 2015 by Nick F

[Blake]

Did just message PM but seems to have gone offline... a new thread before the 12z perhaps if any mods reading as we anticipate the change to cooler conditions in a week or so.

[Cloud 10]

34 minutes ago, Nick F said:

06z GEFS teleconnections all trending in the right direction:

AO

 

 

 

Yes,the 06z has gone bonkers for a neagative AO.

 

 

Regarding the MJO,if it is indeed heading for phase 7,can anyone shed some light as to why the composite anomaly doesn't match up very well with the projected ensemble mean?

 

Perhaps there is some sort of lag which needs to be taken into account?

 

[Man With Beard]

Well not only is it great to finally have some winter weather to talk about, but we also have the possibility of what I call the "ultimate combo" - a deep cold pool on a collision course with an Atlantic low somewhere near the UK.

I dont like sitting on the fence, so this is where my money is. Personally I don't think the D8 charts will verify. If the cold pool is still moving west at this point, I would expect based on past model experience for the pattern to be further south, which would put the whole of the UK in the firing line. I think the cold spell will be won our lost before that. Getting a cold pool to move west is like hitting a golf ball through a set of trees and hoping not to hit a branch - even the smallest disturbance will make this cold pool blow up in situ. We need pressure to stay a bit higher over Scandi at T120, and no new disturbances to crop up in T72-T120 timeframe. Unfortunately, this is asking quite a lot -but if we can get through to T144 with the cold pool still heading west (won't know for sure until Wednesday/Thursday), even just slightly, then I think we will see a snow bomb over the UK!

[Methuselah]

2 minutes ago, Man With Beard said:

Well not only is it great to finally have some winter weather to talk about, but we also have the possibility of what I call the "ultimate combo" - a deep cold pool on a collision course with an Atlantic low somewhere near the UK.

I dont like sitting on the fence, so this is where my money is. Personally I don't think the D8 charts will verify. If the cold pool is still moving west at this point, I would expect based on past model experience for the pattern to be further south, which would put the whole of the UK in the firing line. I think the cold spell will be won our lost before that. Getting a cold pool to move west is like hitting a golf ball through a set of trees and hoping not to hit a branch - even the smallest disturbance will make this cold pool blow up in situ. We need pressure to stay a bit higher over Scandi at T120, and no new disturbances to crop up in T72-T120 timeframe. Unfortunately, this is asking quite a lot -but if we can get through to T144 with the cold pool still heading west (won't know for sure until Wednesday/Thursday), even just slightly, then I think we will see a snow bomb over the UK!

I bet that's tomorrows' Daily Express's front-page headline!

[KyleHenry]

19 minutes ago, Cloud 10 said:

 

Yes,the 06z has gone bonkers for a neagative AO.

 

 

Regarding the MJO,if it is indeed heading for phase 7,can anyone shed some light as to why the composite anomaly doesn't match up very well with the projected ensemble mean?

 

Perhaps there is some sort of lag which needs to be taken into account?

 

AO to go negative through retrogression which would allow NAO to go neutral/ slightly negative  for a short time.

Logically NAO would then rebound into positive phase for approx 10 days before it could go negative without any interference. 

[marksiwnc]

Liam dutton just tweeted cold and drier in new year.  #hopeforsnow

 

[SMU]

Just a quick one before the 12s

The GFS is keen to develop a shallow wave down through eastern Norway at T84 - which at 96 tilts to project the cold west / south west into europe

we could do with this system getting a tad further south ..... Its here

(small 1040 kink )

then at 96- pushes SW...

s

 

 

[shaky]

No differences so far up to 96 hours!!

[MR EXTREMES]

 

interesting little feature spawned north of the azores channel runner or into Europe would be ideal getting a little excited but have seen it all go pete tong at 96hrs before fingers crossed

although don't like the way the heights are poised into scandi looks pretty but need it more horizontal

as someone mentioned earlier looks like heading into central southern Europe into spain if it keeps going maybe another bite of the cherry needed although good to see some form of blocking.

Edited December 27, 2015 by emotional rollercoaster

[SMU]

GFS t132 has MUCH better westward allignment of the cold over the continent - picking up more troughing to the south of the high .....

[MR EXTREMES]

knife edge or what running through the model it does seem to show the scandi height better shape and backing west retrogression to Greenland maybe

[IDO]

GFS op now going with this morning's ECM; the first LP system rides up to the north rather than undercutting, that should help with the subsequent shortwave, to send that under, as it has no where else to go.:

  UKMO likely similar: 

[shaky]

Ukmo looks excellent!!sending everything under at 144 hours!!

[SMU]

Yes UKMO 144 would more than likely see some snow across the UK - then hopefully the undercutter slides through...

 

look at that DEEP cold pool in central europe !!

s

[IDO]

3 minutes ago, shaky said:

Ukmo looks excellent!!sending everything under at 144 hours!!

Yes too much Atlantic energy on the GFS, so little disruption and the LP system warms out the uppers, and shifts the pattern east rather than undercutting:

Fine lines, though GFS is known to en-power the Atlantic!

 

[shaky]

I cannot believe we are looking at these type of charts considering just a few days ago it didnt look like we were gona see anything cold let alone snow till middle of january maybe!!

[Carlrg]

That Scandi ridge's halfway to Iceland.

Edited December 27, 2015 by Carlrg

[Radiating Dendrite]

Think the GFS may be the last to the party with the undercut as it does like to over do the Atlantic somewhat. 

Pleased to have the UKMO on board.....let's see what the 12z ECM has for us. Could be Euro vs GFS showdown - how many times does that happen! 

[Blake]

3 minutes ago, Radiating Dendrite said:

Think the GFS may be the last to the party with the undercut as it does like to over do the Atlantic somewhat. 

Pleased to have the UKMO on board.....let's see what the 12z ECM has for us. Could be Euro vs GFS showdown - how many times does that happen! 

OR the GFS is hinting at blocking to our north with the Low off Greenland undercutting a few days later than the Euro...? Would prefer that myself, looks to me that from 9th onwards we get the cold and snow from the 12z so far...

[blizzard81]

1 hour ago, marksiwnc said:

Liam dutton just tweeted cold and drier in new year.  #hopeforsnow

 

Don't know about the drier bit? Brave call!

[radiohead]

3 minutes ago, blizzard81 said:

Don't know about the drier bit? Brave call!

Can't be much wetter!

[blizzard81]

27 minutes ago, THE CHOSEN ONE said:

Yes UKMO 144 would more than likely see some snow across the UK - then hopefully the undercutter slides through...

 

look at that DEEP cold pool in central europe !!

s

I don't agree re snow at 144. Highest ground only and the deep cold is still over eastern europe, not central europe. Although, it is difficult without the 850's. Why doesn't ukmo show these anymore?

Edited December 27, 2015 by blizzard81

[Mucka]

RE comments on UKMO undercut.

It doesn't really undercut between 120 and 144. What happens is the low rides up the high with some energy (not really enough) disrupting SE (the low at 144 to the NW of the UK is actually moving NW at this point and not SE). Yes we would have another chance with the next low if it were far enough South but we really need this potential trigger low to be a little further South and produce more undercut than shown on this afternoons output thus far as this UKMO run would never pull in the cold upper air needed to produce snow (pushed away too far North). I will post some comparisons from GFS ensembles when they come out as some are sure to show better undercut and of course othes will show all the energy heading up the block with little disruption of energy SE.

Edited December 27, 2015 by Mucka