Stratosphere watch 2023-24

Latest EC46 shows a very significant deceleration now in U-60N through December. More members also going for a major SSW (reversal) through early January 2024 but not a significant increase on yesterday. The main story is how many are now going for a weak stratospheric polar vortex through the month of December, look how far below the climatological mean (the thicker red line) the ensemble mean (the thicker blue line) goes.

The most recent available run of the extended GEFS is yesterday's 0z but even they start to show a sign of a deceleration through the month of December. Not as quick or as stubborn as the EC46 but definitely signs there.

Interesting times.

A stretch of the stratospheric polar vortex is the start of its demise that is expected later on in December and into the New Year. A typical stretched SPV can result in mild weather for us because of the normal progression of minor SSW events where the warming starts over Siberia and displacing the PV towards western Europe which is favourable for explosive cyclogenesis in the North Atlantic. This brings mild and wet conditions into western Europe even though the NAO can go temporarily negative (infamously known as west based -NAO).

All these events have to begin somewhere. They take off via persistent Aleutian troughing and Ural blocking. This might mean the Atlantic can become active as a vortex lobe gets displaced over Greenland. We are expecting that anyway courtesy of the MJO cycle transitioning to phases 4/5. Phase 4 MJO in December during El Niño typically translates to an unsettled pattern for Ireland with deep Atlantic trough but running up against ridging to NE backing into the Urals - this combined with Aleutian troughing here is ideal for propagating wave activity upward to perturbing the SPV. Phase 5 is similar but can be less Atlantic dominated with more enhanced ridging from Europe, a very mild setup.

When the convection returns to the West Pacific and the MJO goes into a strongly amplified phase 8, this can lead to a major build in pressure northward from the south into a high latitude blocking feature over Scandinavia. The stratospheric polar vortex at that point in time would be probably primed for a major SSW event (reversal). Whether it would be just a displacement or a split is dependent on how much wave activity occurs.

The extended EC46 has been flip flopping a bit on its day to day runs (to be expected) on the strength of the signal for a deceleration in zonal winds but the latest is identical to what I've shared previously. The trend remains the same.

The medium range GEFS continue to be less bullish on as significant a deceleration with little change on the wide variation by day 16. More have definitely veered towards the weaker side from previous updates but you would not be able to make a stance on where it sees the zonal wind going just by this forecast alone.

Here's the most recent available extended GEFS trends from the past 3 days. Initially we get the deceleration of the zonal winds via the expected Canadian warming. As always with Canadian warmings, the weakening is very modest as these warmings hardly make a difference to the placement of the SPV from the North Pole which this is based on (60N). The zonal winds make a recovery to average on the ensemble mean and show a deceleration again in the longer term. As the top left mentions, the GEFS have a strong vortex bias which must be considered. So the fact it's showing a deceleration at the long range is a good sign even if it ain't as bullish as the EC46.

Another way of seeing this is through the % of ensembles in the extended GEFS that go to particular strengths and percentiles. Below average passes the median % and goes over 70% by the end of 2023 with a small % going for record weak SPV. Above average or strong SPV is in the minority by then.

This is an excellent place to be in as far as the stratosphere is concerned. The SPV is primed to get perturbed a lot through the remainder of 2023 if the forecast verifies and we get persistent Aleutian troughing/Ural blocking. The active MJO being forecast by many models at the moment (not all mind, some have it much weaker) also favours disruption to the SPV. This is probably the most promising stratospheric outlook I've seen since 2017-18 but don't start getting excited yet, a lot of things can go wrong if you're not well used to that by now. We're talking about the risk of a major SSW event developing through early 2024 and we've only just began December.

Lots to talk about and keep an eye on anyway. Great to see.

No changes again in the medium range GEFS as per the previous post. Largely in line with the extended GEFS now.

No changes mostly on EC46 either so there's not much reason to update here though there is less members going for a reversal if you were to look at the smaller details.

As I had expected, the operational runs have gotten more realistic since the above one with a slower evolution into a big deceleration.

However, yesterday's extended GEFS have massively upped the signal for a reversal into early January with as much as 48% of the ensemble members going for it by the second week. Nearly half the suite at such a lead time is nuts.

Everything remaining the same this morning, just the usual variations as models run daily. Well on track for some kind of significant deceleration into January, whether that becomes a major SSW (reversal) is anyone's guess.

There is some slight signal in some of the GEFS output of a wave-2 attack on the SPV. This is the first time I've seen this season. What wave-2 means for anyone unaware is two ridges (or two areas of warming) occur on either side of the SPV which can evolve into a PV split as they gather momentum but can still be a displacement if not strong enough. This is a tiny chance of verifying as it's only some of the ensembles doing this but thought I should note it anyway purely for the discussion.

This GEFS 500hPa anomaly from Cohen shows perfectly the pattern needed to drive a wave-2 attack on the stratosphere. If we manage this, there's little to suggest we won't get a major SSW in early 2024 but persistence would be required for a full on split.

Good heat flux for disruption signalled by the GFS (very basically a measure of how much wave activity is being propagated from the trop to the strat).

Better on the GFS this morning. Both the 0z and 06z OP runs got U-60N down under 10 m/s as far out as they go.

The 0z GEFS has a ridiculous spread generally between 0 and 40 m/s. It's only one single member doing it but the first medium range GEFS I have seen to show any perturbation going for a reversal.

Terrible medium range GEFS this morning with a big uptrend in U-60N. Look at the difference in the number of ensembles going below 20 m/s for instance versus what I shared yesterday morning. Very few.

On the other hand, there could be the possibility of nacreous or polar stratospheric clouds over the north of Ireland on Wednesday and Thursday with the GFS showing some lobes of -80C from the displaced SPV.

EC46 is the most bullish so far to suggest a big dip/deceleration in U-60N. This is a change from its recent trend towards less and less perturbations getting to reverse levels.

The most recent available ECM operational run only goes out to 29th December and at that point is showing a rather minor wave-1 warming over Siberia displacing the SPV towards eastern Europe. Besides that, there's not much else you can delve from this chart alone. Would need more frames but my best guess would be the possibility of a technical reversal of the U-60N through the first into the second week of January followed by a rather quick recovery as it would probably be a displacement event with how it looks here at t240.

The interesting part is where it is displaced. If it's displaced towards Scandi, that would open the doors for some very significant Atlantic ridging possibly if not sustained provided there would be downwelling. If a more major warming were to occur, the SSW would likely get displaced back to the North Atlantic which is not as interesting but also opens the door to the possibility of a split as the SPV would be more prone to getting stretched.

The medium range GEFS has seemed to have sort itself out after showing a poor trend the past week with an acceleration likely in U-60N through the final few days of December as the SPV returns temporarily to the North Pole after the recent displacement towards Europe. This is then succeeded by a deceleration through early January. Most keeping it within the 10-25m/s range as far out as they go on this latest 18z run.

I would mention the fact the GFS looks quite "meh" on the wave-2 amplitude front to give sufficient planetary wave activity to evolve into a split but the data we have to hand from Stratobserve... well.. the GFS goes the complete opposite to most of the modelling. Look at how cold it keeps the average stratosphere temperature for 60-90N 10hPa whilst the rest of the output warms things up. Laughable. Simultaneously as you'd expect this translates to an intensification in U-60N which is also the opposite to what the rest of the modelling goes for minus some minority ensemble members.

Some interest (food for thought if nothing else) shared by the World Climate Service. Using the top 10 analogs that had similar polar cap heights to the EC46 initialised on the 19th December, 8/10 of them seen a major SSW occur.

I'm pretty torn to be honest. More runs needed.

Everything remains the same, nothing has changed as of the most recent post.

Only thing I'll update on is the GFS 12z is the first run that I've seen an evolution that would hint at the possibility of a split but it's at the very end. See where I've marked, you can see a second lobe over NA but note the absence of a secondary ridge.

The GFS continues to be different to the rest of the models with a slower progression and colder stratosphere.

There we have an official split at the very end of the GFS 0z this morning. Merry Christmas, ho ho ho!

It's Christmas Day but I couldn't resist archiving this one from the latest GFS 06z.

It goes onto produce an absolute beast of a major SSW. A major wave-1 split with a reversal of -24.3 m/s at the very end. This is getting to the top percentile of major SSW events.

Fantasy land but it must be mentioned that both the GEFS and EPS are in favour of a strong wave-2 driven pattern which would split the vortex come the second week of January.

Anyway, I just wanted to bank this as it's a rare beauty. Merry Christmas to you all and enjoy the rest of your day best you can in spite of this mediocre weather!

No, this is the stratosphere watch thread. Please see the first post of the thread for in-depth explanation on how the stratosphere affects our tropospheric weather patterns.

Where the troughs (lows) and ridges (highs) are placed in the stratosphere is not necessarily where they'll be in the troposphere but they do tend to feed off one another to drive the weather.

I still think we're not any closer to seeing the extent this stratospheric warming will go. There is no consistency in the GFS operational runs. 0z showed a wave-1 displacement with the U-60N just barely reversing briefly to easterly. 06z not looking like it'll have any reversal though does split the SPV. Whether it's a technical major SSW (reversal) or not won't matter in the grand scheme of things unless it's a massive event like the GFS 06z OP plugged yesterday. All it means is it won't be classified as a major if so.

Big steps back today from the EPS and GEFS with regards to a major SSW (reversal). Just a quarter showing reversal now on the EPS.

71% of the ensemble suite showed reversal yesterday...

As expected given this morning's EPS and the fact the EC46 is the extended version of this and runs at 0z, EC46 has backtracked on the major SSW with a reversal no longer showing on the ensemble mean.

A very significant deceleration is highly likely nonetheless which is a SSW, just not a major one.

Besides echoing everything I said earlier, the fact it's a minor event opens the floodgates to something more extreme down the line. As mentioned before, major SSW events act as a "cap" or cut off from tropospheric wave activity upwelling to perturb the strat and depending on the point in the season can re-intensify to strong levels aloft. This is not the case with a minor event.

So hypothetically, we could get a major SSW down the line on top of the minor event that is now expected starting later next week. Either that or it could do a last minute correction on models, which has happened before, to a major event. What this means for impacts, you never know.

Reminder that the medium range output is not related to what's happening in the stratosphere.