There's a remarkable drop shown by the 06Z GFS op run in the 850mb temps between T+246 and T+252. It's so remarkable that I'm struggling to believe it. And then at T+258 it's gone again. I can only think that the 0Z chart somehow didn't get overwritten.

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Some lobe trends. Keep in mind two single points may not be the best way to measure the lobes, for example the GFS6Z lobe is alot weaker than the ICON6Z lobe despite the numbers now being similar

Jan Mayen island/ Centre circ 0Z 05 Feb. Z500:


GFS6Z: 516/510
GFS0Z: 520/510

ICON6Z: 515/508
ICON0Z: 510/505

Anyway upshot is don't despair, ICON's lobe has weakened as much as GFS's lobe has strengthened.
 

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ARPEGE6Z does look worse though than ARPEGE0Z. Lobe a bit stronger there.
GEM6Z has a stronger lobe than the 0Z too.

Only ICON seems to have gone in the right direction :S
 

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Jan Mayen island Z500 ensembles 6Z vs 0Z. You can see the full 6Z set has a stronger lobe. Not great.

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Why is everyone mentioning Lobe's here? I don't get it?

I think this small chance of an easterly is failing due to what's happening over the other side of the Atlantic!?

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Could just call the GFS 6z a pre cold spell wobble 😂. Just goes to show how difficult it is in our part of the world to get everything to fall in place. Seen some cracking charts though in this forlorn chase

GFS det has been all over the place since the upgrade. Ens have never taken to this easterly with the Det a huge outlier on most runs. 

I thought that the ensemble suite ran using the same software?

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Could just call the GFS 6z a pre cold spell wobble 😂. Just goes to show how difficult it is in our part of the world to get everything to fall in place. Seen some cracking charts though in this forlorn chase

But what does "LOBE" mean 

I am convinced that our colder spell or easterly failing next week if it was to occur is down to what is happening in N. America?

Signs there could be another SW - maybe a wind reversal to give us a colder end to the month!?

http://www.weatheriscool.com/prod/gfs/eps/strat/u10serie_cfsRawCorrGefs.png 

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But what does "LOBE" mean

A secondary low, to give it its traditional name.

Just the latest in a long list of alternative names for secondary lows (see "shortwave" et al from years past!)

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From my understanding the Deterministic has had an upgrade but the ensemble members run with the old model.

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But what does "LOBE" mean 

I am convinced that our colder spell or easterly next week if it was to occur is down to what is happening in N. America?

Signs there could be another SW - maybe a wind reversal to give us a colder end to the month!?

http://www.weatheriscool.com/prod/gfs/eps/strat/u10serie_cfsRawCorrGefs.png 

Upper level trough or vortex (some models have a closed circulation and some don't) that forms in the greenland sea area around T+63 hours as a split off from the main cold core low.

It is absolutely vital for what happens later on, and is the reason the GFS has disagreed with the other models. A strong lobe results in reduced heights, downstream, in scandanavia which delays HP building and results in the anticyclone forming further south when it does build.

You can predict exactly what is going to happen wrg to whether or not the scandi high/cold spell happens based on how strong the lobe is. The weaker, the better, on the GFS6Z it was stronger, hence the downgrade.


Also, I should be clear its not really a shortwave. The surface pattern is not that important, its the upper level pattern which determines the behaviour of the scandi high.
 

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From my understanding the Deterministic has had an upgrade but the ensemble members run with the old model.

You may be right.  That was the case when they upgraded the Op but I thought they might have brought it in line by now.  At the time I queried what the value was in an ensemble suite running a different version of the software, because it just injects another variable into the mix.

Anyway, back to the models and the 06z ensemble suite is even less keen on a severe easterly - we have just 4 or 5 of the perturbations supporting it in around 6-7-8 days' time and just one at the end.

I'm still very sceptical, given the lack of cross-model support - still too much energy to the north.

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6z ensembles are a huge downgrade compared to the 0z! 

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Why is everyone mentioning Lobe's here? I don't get it?

You need to keep your ear to the ground, mate

 

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Another failed midwinter easterly? I blame lobal warming.

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6z ensembles are a huge downgrade compared to the 0z! 

Yup, complete collapse to the milder scenario, with only the op run and a couple of other stragglers preserving a brief cold plunge, and I would expect those to have vanished by the 12Z. So predictable.

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Upper level trough or vortex (some models have a closed circulation and some don't) that forms in the greenland sea area around T+63 hours as a split off from the main cold core low.

It is absolutely vital for what happens later on, and is the reason the GFS has disagreed with the other models. A strong lobe results in reduced heights, downstream, in scandanavia which delays HP building and results in the anticyclone forming further south when it does build.

You can predict exactly what is going to happen wrg to whether or not the scandi high/cold spell happens based on how strong the lobe is. The weaker, the better, on the GFS6Z it was stronger, hence the downgrade.


Also, I should be clear its not really a shortwave. The surface pattern is not that important, its the upper level pattern which determines the behaviour of the scandi high.
 

So you can predict exactly what is going to happen re scandi if you can predict exactly how the lobe will develop - which just takes the forecasting problem back to a day earlier. Not a dramatic advance in weather prediction IMO.

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So you can predict exactly what is going to happen re scandi if you can predict exactly how the lobe will develop - which just takes the forecasting problem back to a day earlier. Not a dramatic advance in weather prediction IMO.

I mean to be clear this is a very specific scenario. All I did was look at the forecasts with the cold spell, and those with out and tried to isolate the cause. It always came back to how that lobe was resolved, and there is a very easy to understand mechanism as to how the lobe effects the build too:

Strong GS lobe -> Low heights pushed over scandi -> HP builds further south -> Cold air shunted south

So what this means for this specific scenario is you don't have to worry about anything else, all the uncertainty is contained in how this lobe is resolved. So, for example, although ICON6Z doesn't look like a big upgrade, it really is because the Lobe is alot weaker. There almost seems to be a threashold where, if crossed, you get the cold weather. As things stand now we are '1 upgrade' away from a cold spell but the uncertainty is starting to reduce now, and I suspect the 12Z will be the last chance for the lobe to stay weak enough to get the cold scenario.


I should also point out that Z500 is one of the easiest things for models to predict. Upper level meterology is much easier than all the turbulant nonsense that goes on near the surface; so when you get big uncertainty in this upper level geopotential field; the consequences are pretty straight forward. Strong lobe = bad, weak lobe = good.
 

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Also having worked out the crucial area I raise two points about the lobe formation zone:

- Not many observations where it happens
- Highly mobile sea ice

So this is why the uncertainty is particularly high. I mean something as simple as a major sea ice drift before the 5th could impact the lobe formation and the models will not account for it.
 

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Thanks for the analysis, Q. Most enlightening 👍

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