I now skip past any posts on teleconnections. Someone on Netweather put it very well in that there is little point in looking for the "triggers" of potential cold (or any weather) as it's best to just focus on the the ops and ensembles - more of the latter than the former. I can only assume that it's possible to draw some macro scale conclusions from teleconnections, but it's not much use for the micro scale of South Wales or even the UK.
Originally Posted by: overland
That's the sensible approach!
Truth be told the search for a "holy grail" has been going on for at least 25 years, if not longer. 20 years ago it was the SSTs in May which people looked at (including the Met Office at the time), then there was the saga of the OPI (October Pattern Index) ten years ago. Some Italian scientists came up with that one, and it had near-perfect hindcasting ability. Of course going forwards it was completely useless, and was never heard from again.
The MJO is massively overrated and is just a way of looking at rainfall charts. Now it may be the case that rainfall in a certain area leads to a greater chance of the Pacific jet behaving in a certain way and a possible effect on us downstream, but there's so much of the atmopshere between there and here that the signal, weak to begin with, gets diluted all the more. And as the MJO is just a way of looking at rainfall, predictions of it are just as likely to be wonky as any other model parameter (pressure, for example).
There are some events which have more of an effect. We know full well that SSWs, particularly a split in the vortex rather than a displacement, have a good chance of "shuffling the deck", so to speak, but not always in our favour. We also know that the elusive Scandinavian High can form without an SSW, and even end up leading to one (see 1987).
If only it was as simple as saying an easterly QBO plus MJO in phase 7 or 8 and an SSW would lead to a cold spell! Mind you, it doesn't seem to matter where the AO/NAO, QBO, MJO, 60N/10hPa zonal winds or even sunspots are, it all seems to deliver record-breaking cold... to America! 😂
TBH I regard the detailed posts over teleconnections elsewhere as little more than snake oil. If it really were possible some people would be very, very rich by now, and the Met Office would no doubt be crowing from the rooftops.
In a way, though, it's comforting to know that despite 40+ years of computer modelling, pattern-matching and more, we still don't really know what the weather will be doing in 3 weeks' time! And I daresay as long as I'm alive (another 25 years?) that won't change either...