And all the self-proclaimed experts and critics and manic depressives and fortune tellers who frequent these forums haven't said much either I suppose?

Sometimes, in situations of uncertainty, silence is the wisest option.

Originally Posted by: nsrobins 

I thought this was an interesting post.

It is perhaps a viewpoint underpinned by an implicit (& apparently widespread!) assumption that the weather is a deterministic system. A deterministic system is one in which no randomness is involved in the development of future states. A deterministic model will thus always produce the same output from a given starting condition or initial state. In this context, the poster proposes that if we had the data and the computing power we could calculate what the weather would be at a future point.

But I think that we all understand innately that weather is far from deterministic. There are chaotic elements - the proverbial flap of a butterfly's wings etc. - which influence outcomes. As forecast horizons extend the scope for these to have an impact increases. Given a specific set of starting conditions, multiple outcomes are possible. The weather that we get is one realisation, but there may have been other equally plausible outcomes at the point the forecast was made.

Criticism of NWP outputs on this board (and others!) often follows a pattern. Posters will say that the models are "struggling to tie down" a feature, or that the models are "always poor in this type of setup". With the benefit of 20/20 hindsight, posters will say the model was "wrong because it failed to pick up ... [height rises over the Azores?]".  Any verification gap is thus attributed to modelling weakness/failure, overlooking the possibility that something (chaotic) happened between the point at which the forecast was made and its realisation.

What is the point of all of this? I guess it's a plea for acceptance that the models aren't "searching for a solution". We shouldn't criticise them when they "flip-flop" from one outcome to another, especially where longer time periods are involved. There simply isn't one path that the models have to "lock-on" to. Weather has chaotic elements. If the models this afternoon don't show the extended cold spell they were showing yesterday it might not be because they are inaccurate. Something might well have changed!

NWP models are wonderful tools for forecasters. I'm sure that they have massively improved forecasting accuracy in recent years, and that these improvements will continue. But we should always remember that weather is not a deterministic system, and modulate our expectations and responses accordingly.

Originally Posted by: IanT 

And all the self-proclaimed experts and critics and manic depressives and fortune tellers who frequent these forums haven't said much either I suppose?

Sometimes, in situations of uncertainty, silence is the wisest option.

Originally Posted by: nsrobins 

Pfft, where's the fun in being silent?  The fun is in the rollercoaster journey- might as well shout and scream and throw up along the way rather than sit cross-armed and unmoved 

Plus, viewing models at this time of year is a great way to stay in shape 

Originally Posted by: SJV 

Pfft, where's the fun in being silent?  The fun is in the rollercoaster journey- might as well shout and scream and throw up along the way rather than sit cross-armed and unmoved 

Plus, viewing models at this time of year is a great way to stay in shape 

Originally Posted by: SJV 

22/23 of the 12 Z MOGREPS ensembles have the cold spell in place next weekend.

Originally Posted by: TomC 

22/23 of the 12 Z MOGREPS ensembles have the cold spell in place next weekend.

Originally Posted by: TomC 

22/23 of the 12 Z MOGREPS ensembles have the cold spell in place next weekend.

Originally Posted by: TomC 

All they've done really...is not said much!

Originally Posted by: John p 

Sounds fun.  Excuse my ignorance but how do you get access to this data?

Originally Posted by: Whether Idle 

Ok maybe not 'silent', perhaps I meant 'reserved'  😊

Originally Posted by: nsrobins 

Very good!

I would add that an additional problem is that I am pretty certain that there are more gridpoints in the models than there is actual data with which to prime the T+0 position.  Thus they start from an approximation of the state of the atmosphere, however good that approximation might be.   That of course is why they run the ensemble suites, to see if small changes in the opening position lead to different outcomes - which of course over time they always do.

As you say, and as I commented earlier, they'll always be the "butterfly" effect of a chaotic system which will make for declining accuracy over time.

Originally Posted by: Gandalf The White 

Good Mean from the ECM seems to represent th Op all the way through.

http://www.wetterzentrale.de/pics/Reem1441.html

Originally Posted by: Ally Pally Snowman 

22/23 of the 12 Z MOGREPS ensembles have the cold spell in place next weekend.

Originally Posted by: TomC 

Best news of the day. No wonder the Beeb graphics have Northerlies for the whole country on the 17th. 

Originally Posted by: Ally Pally Snowman 

Very interesting.  What is their definition of a cold spell?  

Originally Posted by: kmoorman 

Where is that

Originally Posted by: Gooner 

I'm sure you are right.

A (related & also widespread...) misinterpretation of the ensembles is to attempt to derive a probability by expressing the number of perturbations showing a particular outcome as a percentage of the number of ensemble members. If 15 of the 20 ensemble members show a cold outcome at t+whatever, that doesn't mean that there's a 75% probability of the cold outcome. At best all you can claim is a conditional probability where the condition is that all of the assumptions made by the model remain extant through the forecast period, ie no exogenous influences. Where an anomalous outcome is sought it might be prudent to assume that any post-forecast pre-realisation changes will likely tend to diminish the probability of realisation. I think we see (or fail to see!) this very frequently.

Originally Posted by: IanT