Met4Cast

As good as the GFS det was in the extended it doesn't have much support at all.

It has been like pulling teeth, hasn't it? Particularly given for the majority of this winter we've had a weak stratospheric vortex and it hasn't been driving the patterns, the troposphere has very much been in charge. We were in a very favourable position in terms of teleconnections in mid January for a continuation of cold/blocked patterns but then we saw a SSW (not a major one) reflect back, reshuffling the pattern away from something favourable and instead we ended up very mild and stormy. A fascinating winter from a stratosphere & teleconnective point of view at least, if not from a cold/snow point of view.

The GEFS 00z run has backed away from the idea of an SSW, looking very different from the previous runs. 61% of members still suggest a reversal but this is down from 80%+ yesterday.

It’s perhaps quite telling that despite a huge amount of spread on the EPS, only 1 or 2 ensembles suggest the -10C isotherm. There is a complete lack of a signal for deeper cold within the model suites.

A Quick Trop Response (QTR) simply means a rapid response to a SSW. I.e impacts from the SSW reach the trop faster than what would ordinarily be expected. But.. I’d argue a QTR cannot be instantaneous, i.e the same day an SSW happens. Imo you’re looking at about a week, rather than hours. As Simon Lee said though, the idea of a QTR doesn’t appear in scientific literature on SSW’s so the actual definition is open to interpretation. Might be helpful if we agree on a definition for future use of the term though..

johnholmes Quick Trop Response.

Trying to diagnose a potential way forward is proving impossible. The aforementioned equatorward fluxing of +AAM anomalies suggests a strengthening Iberian/S European high however the MJO likely transitioning into a weak amplitude phase 8 combined with an increasingly unstable polar profile (SSW) suggests a likelihood of high latitude blocking. Two rather opposing signals fighting for dominance, no wonder modelling is struggling to the degree that it is. From a forecasting point of view, the mid-extended range at the moment is nigh on impossible. Getting cold into the UK with an Iberian high in the game though really will be a struggle, the ECM shows this reasonably well with both HLB and an Iberian heigh. @Kasim Awan might have to change his name again.

Daniel* It seems a "QTR" isn't something that appears in literature so it's difficult to actually define. In my mind a QTR simply refers to rapid impacts from downwelling -ve SSW anomalies, something which still takes time & therefore cannot be instantaneous.

Whilst pointless analysing beyond day 5, the GFS is not interested in the idea of a wedge of heights to the north and if anything the 18z has flattened the pattern more than the already flat 12z.

You can't see a trop response to an SSW on the same day the SSW is happening, that is impossible. The trop could already be in a position where blocking is happening during an SSW but that would be unrelated to the downwelling impacts of the SSW. Complicated, but I don't think this can be classed as a QTR to be honest. Surely a QTR is a direct response to downwelling easterlies? Pre-primed trop patterns that happened to produce blocking as an SSW occurs, whilst you could argue is loosely related to the SSW happening in the first place, isn't a response to the SSW in of itself. Either way. Given the huge amount of spread I think it's largely irrelevant what any det run is showing at day 10 currently.

The ECM isn't showing a QTR, best estimates place the SSW on the 18th possibly 19th February, that would be an instantaneous trop response which unfortunately, isn't possible. In any case - I remain unconvinced of anything prolonged or sustained, the SSW is certainly a wildcard but I think it'll be rather difficult to see anything proper in terms of cold before the SSW takes place, and even after then I'd be cautious on any possible impacts re: UK specific cold hopes. Notice how through December these anomalies were propagating poleward, this led to the early January cold spell. Compared to now these anomalies are moving subtly equatorward, i.e no HLB. The sudden change on todays UKMO & ECM towards wedges is an interesting one, but given the ridiculous spread within all ensemble suites at the moment it's not worth putting too much emphasis or hope into that just yet. It may not seem like it but I'm really trying to find a reason to be optimistic about cold, but I can't just see it currently. Post SSW might be a different story though.

Even with a decent NH profile we still manage to see the Iberian heights poking it’s nose in

Bank! If it’s not going to be overly cold we may as well get some early warmth in across the UK instead!

Blessed Weather Interesting, thanks for the link I'll give it a read later this afternoon! The SSW seems to be triggered (assuming we do indeed see one, here) by the MJO but also (perhaps more likely) by the recent strong +EAMT event increasing heatflux right up into the stratosphere. There certainly seems far more evidence supporting a SSW this time around compared with the previous attempts this winter but without a QTR (this is another uncertainty) we'd be looking towards late February/early March for any potential impacts. I'm not seeing anything within extended NWP modelling to suggest an overly exciting QTR, usually you'd expect to see at least a couple of outrageous runs from GEFS ensembles but they've been relatively muted, the same is true for the EPS. MJO forecasts are currently quite uniform across. modelling, slow meander through phase 7 before a low amplitude phase 8 follows, albeit these are of course not factoring in any potential impacts from the (again, assuming here) possible SSW. Certainly quite a complicated picture unfolding but in any case, I'm not sure many would be happy with winter finally arriving in March just when some much needed warmth is wanted..

feb1991blizzard The MJO runs on a cycle of roughly 60-90 days, it won’t be back into favourable cycles again until April (ish). AAM tendency likely won’t return in any meaningful way until late March (assuming a coherent MJO cycle). So in a word.. no.

This is just awful The SSW will likely come at a time of falling AAM as the MJO returns to the Indian Ocean, not particularly good timing for cold/blocking conditions to be a quick response..

I want to go back to this post from the 22nd January (the quote shows a different date, had to bodge the quote due to that thread being locked, original post here:) Unfortunately much of this has rung true. There remains a lack of poleward propagating +AAM anomalies hence, despite the huge surge in momentum in the tropics/sub tropics & despite a relatively favourable MJO signal there remains little HLB on offer. Now, we have lost the Iberian heights and we are seeing some brief cold weather thanks to cyclonic wave-breaking but this is temporary. Going forward, there is some evidence within NWP modelling for height rises to the NE but these will be coming up against a broader Atlantic trough. It's possible we could see energy undercutting this but given the current situation regarding background forcing I'm not overly optimistic, it seems plausible that the height rises to the NE will be too far east to really advect cold air into the UK & allow for significant trough disruption SEwards into southern Europe. A SSW seems likely later in the month so the polar field is likely to become increasingly unstable but in the short-mid term, if you're lucky enough to get snow in the coming couple of days I'd enjoy it whilst it lasts..

The GEFS were never all that interested but the EPS have massively backed away from cold now too.

The GEFS aren’t all that interested in colder weather in the extended with the mean comfortably above average. The EPS on the other hand are a complete dogs dinner, I don’t think i’ve ever seen such a large spread!

Paul I mean it's an improvement but it just feels like how it was before but with extra steps..

The main period of interest for me is later this month, 20th February as a broad timeframe for something more interesting in terms of winter weather. AAM/GWO is firmly routed in Nino attracter phases and the recent strong +AAM tendency is starting to show signs of propagating poleward through this month. At the same time, the MJO will be moving slowly through phase 7 and possibly into phase 8 giving a little more credence to blocking amplifying towards Greenland, GEFS ensembles are broadly pointing this way. Meanwhile the stratosphere is once again becoming an interesting topic of conversation, the GFS in particular is quite bullish on a major SSW, reflected by the strong heat flux (likely triggered by the recent and to some extent on-going strong +EAMT event) A major SSW at a time tropospheric patterns are aligning to produce patterns favourable for blocking/cold outbreaks, we've been here before this winter and saw how a SSW disrupted the pattern, perhaps we can get a bit lucky this time around and see a SSW super impose onto the trop amplifying colder patterns? Those hoping for an early spring come March might be in trouble.

Harveyslugger Ha! Far too early for those kinds of specific details i’m afraid.

I think if you’re in the Midlands northwards you can be relatively excited for what’s on offer next week. A scandi trough/south shifted jet stream combination will allow for cold air to seep south across the UK with incoming lows bumping into it. Those further south, how does cold rain sound? Seems unlikely that this is a scenario where we’ll see lows trend increasingly further south until they miss the UK given the lack of any forcing from a block. Quite a standard affair really.

Just to extract this part of my post from yesterday, the GEFS 00z ensembles are certainly more aligned with this than anything prolonged or sustained. A lot of spread in the extended but evidence of a relatively quick return to less cold conditions following a brief cold spell. Timing re: the surge of AAM momentum and the MJO isn't particularly well aligned. We're seeing AAM tendency falling now and with the MJO delayed through phase 7 it seems unlikely the tropics & sub tropics will align in a way that's favourable to sustained blocking. We might see some snowfall out of this, but I really cannot see this colder spell being anything notable.

feb1991blizzard Yes, single deterministic runs approaching day 15 aren’t really worth getting excited about.. Especially given the current lack of ensemble support for that solution.