Uncertainty

Google Scholar SCHOLAR.GOOGLE.COM Have a look through this Feb. Backs up what you’re saying that a full progression from p3-8 at decent amplitude often precedes SSWs. I personally feel that the ideal trop config is a strong scandi-Greenland dipole, followed by high amp p6/7. Much like we had in Jan 18. The west based -NAO / Scandi trough we have now does the reverse (perhaps why we are seeing a strong uptick in SPV strength in the coming weeks). Also there looks to be a significant +EAMT event over the next week or so, these often precede SSWs but no sign of one in the next 3/4 weeks at least.

The next steps we take this winter have been very challenging to forecast - unlike the present cold / blocked spell which was very well highlighted by seasonal, sub-seasonal and analog modelling. For me, given the weakening east-based La Niña, a second ‘bite at the cherry’ in early January has always been plausible. The ec 46 has been consistent on this since the period came into range and today’s model continued the trend. There have however, been two highly notable shifts in the output between runs: 1) The mjo There is now a reasonably strong signal to take the mjo into the WP around the new year. This would, all other things being equal, suggest a renewed effort to reinforce blocking signals and - more sceptically - serve as the precursor to an ssw later down the line, especially if high amplitude can be achieved in p6/7. Compare this to the last forecast (perhaps sullied by tropical systems in the IO?) which had a more -AAM look and a stay in phase 3/4 In my experience, a protracted stay in these phases usually correlates to an extended westerly phase for us. January Phase 5, 6 and 7 Nina composites, on the other hand, suggest a blocking signal. I am not suggesting a 2+2 = 4 relationship with these mjo charts, more the simple fact that a reasonable amplitude run through p5-6-7 is far more preferable for blocking than an extended stay in phases 3 and 4. The other major change is upstairs, where a significant uptick in zonal wind speed is noted. It doesn’t last, but date records could be threatened. This is not what one usually sees before a notable cold spell! The trop charts for early Jan do not presently indicate a fully coupled strat and trop though, and blocking is still very much evident. W4 W5 The precip charts for early Jan also indicate Hp to the NW and perhaps low pressure over S Portugal then the med. So our interesting winter isn’t over yet. I’ve always been keen on an ssw this winter - the wqbo/low solar, favourable walker circulation combo and past analogues back this up. But it’s clearly not going down w/o a fight. Were one to occur, soon after the anticipated early Jan blocking wanes, then we could end up with our first -NAO or even colder than average winter for many years. Equally the power up could proliferate through the levels; the mjo WP progression could falter and we’re back at pv of doom purgatory once more. As ever, the Xmas period could well be crucial in determining which direction the fork in the road takes us. Original post: https://community.netweather.tv/topic/98087-model-output-discussion-the-cold-has-arrived/?do=findComment&comment=4765372

The next steps we take this winter have been very challenging to forecast - unlike the present cold / blocked spell which was very well highlighted by seasonal, sub-seasonal and analog modelling. For me, given the weakening east-based La Niña, a second ‘bite at the cherry’ in early January has always been plausible. The ec 46 has been consistent on this since the period came into range and today’s model continued the trend. There have however, been two highly notable shifts in the output between runs: 1) The mjo There is now a reasonably strong signal to take the mjo into the WP around the new year. This would, all other things being equal, suggest a renewed effort to reinforce blocking signals and - more sceptically - serve as the precursor to an ssw later down the line, especially if high amplitude can be achieved in p6/7. Compare this to the last forecast (perhaps sullied by tropical systems in the IO?) which had a more -AAM look and a stay in phase 3/4 In my experience, a protracted stay in these phases usually correlates to an extended westerly phase for us. January Phase 5, 6 and 7 Nina composites, on the other hand, suggest a blocking signal. I am not suggesting a 2+2 = 4 relationship with these mjo charts, more the simple fact that a reasonable amplitude run through p5-6-7 is far more preferable for blocking than an extended stay in phases 3 and 4. The other major change is upstairs, where a significant uptick in zonal wind speed is noted. It doesn’t last, but date records could be threatened. This is not what one usually sees before a notable cold spell! The trop charts for early Jan do not presently indicate a fully coupled strat and trop though, and blocking is still very much evident. W4 W5 The precip charts for early Jan also indicate Hp to the NW and perhaps low pressure over S Portugal then the med. So our interesting winter isn’t over yet. I’ve always been keen on an ssw this winter - the wqbo/low solar, favourable walker circulation combo and past analogues back this up. But it’s clearly not going down w/o a fight. Were one to occur, soon after the anticipated early Jan blocking wanes, then we could end up with our first -NAO or even colder than average winter for many years. Equally the power up could proliferate through the levels; the mjo WP progression could falter and we’re back at pv of doom purgatory once more. As ever, the Xmas period could well be crucial in determining which direction the fork in the road takes us.

GEM - more than a bit wacky this evening… snow to rain and back again!

The eps mean takes the trough considerably further E than the op Nevertheless, the situation we appear to be headed towards is potentially a nasty one rain wise, the JMA tonight has over 100 hours of long fetch south westerlies by the end of the run. Some runs have waving weather fronts swathing the country from Sunday to Wednesday and beyond. Concerning, but too early for details. Alternatively, as a number of GEFS and eps ensembles show, we get the low through quicker. Representative member below: In this scenario, the rain could turn readily to snow - much like the the Jan 2021 event. Notice the Arctic / Greenland high combo here too. It might not be ideal, but unlike a PV of doom scenario, there’s a wildcard. If it verifies stronger and pushes further S, it could even open up the possibility of a white Christmas. But, to take a paraphrase a quote from James Kirk: ’I’ve never trusted Arctic highs. And I never will. I’ve never forgiven them, for January 2019’

The early signs were already there from the 12z GEFS, the tilt of the low looking so much more negative. The cfs has been subtly trending towards renewed Greenland heights in w2 and 3. It did surprisingly well (if a little hasty) with the scandi - Greenland retrogression signal we’ve just had but was quick to break it down. Now this (note this is an average of the past 8 runs - @blizzard81has noted the most recent of these…) Many of us on here have been plugging this spell since the late summer, and in general the seasonals caught it pretty well. Given they have shifted towards a more blocked January, there’s every reason to think this East based Nina / Wqbo-inc solar combo could deliver the coldest winter since 12/13 or even 09/10. All eyes on the progression of that mjo in to the WP. If we get that, then it’s game on. Original post: https://community.netweather.tv/topic/98087-model-output-discussion-the-cold-has-arrived/?do=findComment&comment=4759361

10 members grouped with the op, arguably another 10 are similar. So it certainly has support. But the big news is the extended eps… Big shift away from +NAO! Will have to see the mean later on, but yet again we swing towards extended cold! Original post: https://community.netweather.tv/topic/98087-model-output-discussion-the-cold-has-arrived/?do=findComment&comment=4759338

The early signs were already there from the 12z GEFS, the tilt of the low looking so much more negative. The cfs has been subtly trending towards renewed Greenland heights in w2 and 3. It did surprisingly well (if a little hasty) with the scandi - Greenland retrogression signal we’ve just had but was quick to break it down. Now this (note this is an average of the past 8 runs - @blizzard81has noted the most recent of these…) Many of us on here have been plugging this spell since the late summer, and in general the seasonals caught it pretty well. Given they have shifted towards a more blocked January, there’s every reason to think this East based Nina / Wqbo-inc solar combo could deliver the coldest winter since 12/13 or even 09/10. All eyes on the progression of that mjo in to the WP. If we get that, then it’s game on.

10 members grouped with the op, arguably another 10 are similar. So it certainly has support. But the big news is the extended eps… Big shift away from +NAO! Will have to see the mean later on, but yet again we swing towards extended cold!

850 spreads for Monday suggest it is now v unlikely the cold air is shifted from N U.K. by the Atlantic low. In fact they look to me that at present it is only southern areas in with a decent chance of a frontal snow event. Given UKMO I would still say midlands / far southern N Eng still in with a shout. No doubt though that todays theme has been south.

The southerly shift of the low and an underestimation of the Greeny heights apparent from the change in the eps, both charts for a week today. 5/12 12z tonight’s run Furthermore, a number of op runs are now flirting with the idea of the low running far to the S of the U.K. Whilst this would deny many in the S a v significant snow event, it would likely prolong the cold spell to day 10 and beyond. The GEM highlights this possibility. In the extended, the GEFS and GEPS are keen to reintroduce a milder scenario. The EPS are less convincing on this and there is more than just a hint of blocking prevailing than that suggested by the other two. Note the main two +NAO regime clusters both have high heights over central Greenland so hardly typical zonality. Cluster 3 has euro troughing, 4 might be a nw/se split and 5 has surging amplification with euro low heights. Very hard to know where we heading towards Christmas but the odds of the medium term period remaining cold have strengthened significantly today.

Hmm… big changes as early as 120. V cold air to our east incoming…

Bit of a shift from last months update! Are you suggesting the Jan / Feb period to be the most sceuro blue? E.g does March dampen the signal? Are you able to post the monthly breakdowns ( I know the rules on paywalls - even just a snippet would be most welcome!) Interestingly, Cansips moved to something similar esp Jan and Feb to some extent

This face sums up the 18z! Just the hundred odd moving parts to make it happen, but brilliant anyway!

For all the mid range uncertainty, day 15 on both the GEFS and EPS are remarkably similar. Troughs over N Alaska, France and Eastern US. Notwithstanding what comes before, not a bad position for coldies heading into the Xmas period…

Clusters: 18 now support the 12z idea vs over half yesterday. Op is probably in the coldest cluster but the other two have frontal snow written all over them. Looking further ahead, Perhaps a spell of more mobile (possibly snowy depending on the orientation/strength of remnant blocking) weather for a few days after day 10 before blocking reasserts to the NW? 1 and 3 have euro troughing and even cluster 2 is hardly pancake territory. GEFS still really keen on a prolonged Greenland high. IF that euro trough verifies, this could be a protracted spell of cold weather.

It’s backed off big time from the whole thing collapsing John. The Greenland high is now more locked in position further east and the lows will be forced under and hopefully elongate under it - bringing the whole of the U.K. and poss N France into the frame. The JMA looks similar and ends like this! At 192 on the ecm, the low to the sw could be a huge snow maker. With cold air now firmly in place… An excellent morning for cold fans. The GEPS ensembles did entertain the 12z ec solution so can’t completely discount, but the pendulum is swinging back hard…

For all the sofa hiders… Massive flip from the ecm! Game on!!!

Save ya James May gifs till the morning! Where’s that jet streak headed w of Greenland? That’s right…It’s reinforcing the GH. I think we have our two ends of the envelope tonight. Gfs /UKMO on the one side and the eps on the other. Here’s what’s at stake! Or this! aaaarrrghhh why do we do this to ourselves

Both are the GEFS and GEPS really extend the blocking signal out beyond 300 hours. The latter in particular are quite remarkable. This is the GEPS at the very end of its run Such a strong signal for continued Greenland heights! At 348, the GEFS still has us in a slack easterly flow And are far colder and for longer than the 18z as a result. The ecm op is going to be very very snowy, were that scenario to manifest then someone would be under a blizzard. And the Greenland heights survive. Wild run, unlikely to verify, but amazing to witness. This cold spell is looking more and more likely and significant.

Fair play to the eps. Up to 1000 hours out. For people who question how much I value the ec46 - subseasonal model analysis is my passion - and the 46 remains the best tool we have for this challenging forecast domain. As a full product, the ecm suite is the best we have. Even a day or so ago, it was the only major model progging the GH. Now it’s hard to find a single perturbation from any suite that doesn’t have one. The ecm had a few big fails over the last 18 month, I highlighted many of them, but it’s pulled a blinder this time. As @Daniel*has pointed out, it got today’s Synoptics high on perfect ten days out. If it’s anywhere near right in just 8 days time, we’re heading to the freezer.

Later on, The GEFS have a stronger, more persistent and less west based GH than the 12z. However, I still haven’t seen a credible pathway to a widespread lowland snow event. Though we’d be unfortunate not to get one out of this incredibly anomalous block. Maybe the gfs para last night but it hasn’t backed it up. Nevertheless the block is still heavily favoured to manifest. The question is now, precisely how will we get our cold air advection? This saga isn’t over folks, it’s only just beginning… 18z 12z It gets even better by d13 Euro low anomaly further SE. GH still standing strong and east based. Improvement.

Ooof Up there with the best charts I’ve ever seen!

The GEFS have a stronger GH than the 6z. The trough getting stuck over us ala the para / GEM is a depressing option but by no means a given. The quick cold ecm op option from the 0z does look a little less likely this afternoon, but… the 12z looks to be flying that flag again… I have a feeling the next frame is going to be interesting!

UKMO 144 Stunning chart… Slack easterly flow, really quite cold!