Andrej / severe-weather.eu

Weather/climate analyzer and forecaster.
Info on global weather models, weather patterns, winter, stratosphere, polar vortex, custom graphics, and more...

[Andrej / severe-weather.eu @Recretos]

@ben_tessab Yes, I am posting the revised plots tonight 🙏

[Andrej / severe-weather.eu @Recretos]

Time to answer the question: What can an early-November Polar Vortex collapse (SSW) do to Winter 2026/2027? 🧵👇

These maps are not a forecast or actual temperatures. They show the DIFFERENCE between winters with an early-November collapse and winters without one, in the same El Niño+wQBO setup.

Since there is no real observational data with such an early SSW and nino/QBO combo, I decided to look for data in two independent model sets:

🔹 Climate models (CMIP6, 80 runs): 32 model winters with an early collapse vs. 565 without. All of them with a strong El Niño + westerly QBO, like this year.
🔹 ECMWF SEAS5 seasonal forecast hindcast (1981–2024): 109 forecast members with a collapse vs 444 without. Each is compared only with the other members of the same forecast, so the ocean and El Niño/QBO are identical.

Why 32 vs 565 is ok in the climate models: the big group is the baseline, or the normal El Niño winter without a collapse, as all are the same base state of strong Niño+wQBO, not random winters. It is supposed to look "average", and subtracting it leaves only what the collapse adds. The small group only makes the result noisier, which is why the dots mark where the difference is statistically significant.

December is where both agree:
🇺🇸 Central & Eastern US 1–3 °F colder, while northern Canada around Hudson Bay runs milder
🇪🇺 Scandinavia & NE Europe 1–2 °C colder, with high pressure to the north and lower pressure over the Atlantic

After December, the signal weakens. The climate models fade it out by January. SEAS5 keeps some cold over the central US and over Scandinavia/eastern Europe into January and February.

Bottom line: an early collapse mainly loads the dice for December. It does not lock in a cold winter. But these are just climate simulations, not actual forecasts, as said, only meant to find an underlying signal.

Use these with caution, and do not present them as an actual forecast, as they are not. These show the difference in signal, and if the mean base anomaly is +3, a -1 difference still leaves it at +2, for example.

For phase 2, I am planning to use a larger CMIP6 set, and I am also thinking of making my own custom simulations, as this is a fascinating situation in development.

@judah47 @RyanWeather @webberweather @TWOweather @bam_weather

[Andrej / severe-weather.eu @Recretos]

@TlmImad75406 @Statisticizer This is my image, did you remove the text from the image?

[Andrej / severe-weather.eu @Recretos]

@solopribuilds You forgot to add "please explain it like I'm five.." 🙄

[Andrej / severe-weather.eu @Recretos]

@PvForecast Yes, if I forecast "rain tomorrow" everyday, I am bound to get a few correct 😃

[Andrej / severe-weather.eu @Recretos]

The 10mb temperature will reach a new record high for this time of year. But the issue currently is with the coupling of this event, as it's in a still-underdeveloped polar circulation and with a strong forcing from a Super El Niño, preventing any visible direct impacts from this event. We look into late month and early December for any potential clues, but it's not that likely with a rapid recovery of the Polar Vortex and strong seasonal radiative cooling spinning it back up.

My diagnostic/forecast plots are here if you're interested in this topic :) https://t.co/TJQdJXknpn

[Andrej / severe-weather.eu @Recretos]

Nothing will grasp an SSW signal realistically beyond 3 weeks. What we are looking at here is true statistical chances and deviations from the usual model operating range. The 2026/2027 forecasts so far have, one after another, shown a weaker Polar Vortex beyond what is usual weak range in the ensemble mean. And that is the signal in the noise.

[Andrej / severe-weather.eu @Recretos]

The new seasonal forecast is just 2 days away, so I decided to look at the stratospheric forecast from the October run.

While it missed the SSW event in early November, I decided to take the weaker end of the members that indicated some weakening in this period.

A total of 25 members (practically half, ot of 51 total) dropped below 15m/s in the Oct 25th-Nov 6th window. 21 of those 25 (84%) have their proper Stratospheric Warming reversal from late December to mid-February.

In the other half of the members, 96% reached a reversal in the December-February period. The difference is that the members with the stronger November were focused on an SSW earlier in the season, while the weaker November members showed a reversal more in the second half of January and into the first half of February.

I will prepare updated plots as soon as new data is available.

[Andrej / severe-weather.eu @Recretos]

Hi! CMIP6 is the Coupled Model Intercomparison Project Phase 6: a coordinated set of experiments in which climate modeling centers around the world (NCAR, Met Office, MPI, IPSL, CNRM, and many more) run their global climate models under the same rules, both for past climate and for future scenarios.

The data is freely available online from ESGF (official archive) or Copernicus Climate Data Store. But keep in mind these are not like a normal ECMWF or CFS forecast, but are more like simulations of the global climate in different scenarios.

[Andrej / severe-weather.eu @Recretos]

Stratospheric Warming Update (Nov 2nd) Part 2:

It's hard to talk about impacts while we are still not sure of the wind reversal 3 days ahead... But in the end, if the wind ends up bottoming out at 1 or +1, it makes little difference down the line. The #polarvortex disruption itself is remarkable for this time of year in any way.

But I did another research run on the data and looked at all modeled sources, like CMIP6 models and seasonal hindcast model members, to extract a weather signal from early November SSW events in modeled projections.

I am going to write a more in-depth article on the researched impacts of an early November SSW. But this is *NOT a forecast*. This research shows the early November SSW signal when compared to the same years with no SSW. So basically it shows us the difference in the atmosphere and what such an event is trying to force, not what the actual outcome is going to be.

But as a glimpse, we can see an interesting signal for December, matched with a strong El Niño and a west QBO setup, for a colder trend over parts of the U.S. and Europe.

As said, this is the early Nov SSW signal alone, so what it's trying to do "in the background". This is also modeled ocean-atmosphere data, not real observations, because no such events appear in observational records this early in the specific global setup as we have in 26/27.

[Andrej / severe-weather.eu @Recretos]

Stratospheric Warming Update (Nov 2nd).

We are still looking at a proper Polar Vortex flow collapse with a wind reversal and the earliest official Stratospheric Warming on record. You can monitor all this yourself in my Polar Vortex hub (including the 3D shape); link in the comments below. 👇

The models are still not in perfect agreement, which is also a bit unusual, given that the first event window is already between November 4th and 6th. There is a second reversal window between the 8th and the 11th, but that falls within the same event window (20 days).

Only the AI models are currently not on board for any reversal (the AI ENS is not far off), which could be by design. But the ECM is lately pulling back the reversal, and usually the GFS follows with some delay. So definitely a race down to the finish line with this event! 🙂

[Andrej / severe-weather.eu @Recretos]

@LeopoldHeinrich Sadly, it does not always work that way, especially in a record east-based Super El Niño atmosphere. 🙂

[Andrej / severe-weather.eu @Recretos]

@PvForecast Nothing out of the norm in sight for now. But these events rarely appear on extended ranges anyway.

[Andrej / severe-weather.eu @Recretos]

Looking at what's ahead for the Polar Vortex:
The heat flux forecast suggests the tropospheric wave drive has peaked for now, and we are slowly returning to normal territory by mid-November.

Radiative cooling in the polar stratosphere is strong at this time of year. With no upward wave guide and sharp radiative loss, it naturally accelerates the polar night jet via thermal wind balance, allowing the #polarvortex to re-intensify.

I am excited to see what the seasonal SEAS5 data will show, as it will have the current SSW event in its model spin-up, so it will be interesting to see what it thinks will follow, and if it will still show another disruption in January.

[Andrej / severe-weather.eu @Recretos]

@AlexWeather25 Yes, the last seasonal range guidance has suggested a stronger disruption in January, but we have to wait a few days for the new data now, which will also take the current SSW into consideration right at the start of the forecast.

[Andrej / severe-weather.eu @Recretos]

@AlexWeather25 Yes this is the latest data. it mainly depends on the tropospheric pattern to provide the wave activity flux if we want to have another go at the Polar Vortex, and current data does not suggest a supportive pattern in the medium range.

[Andrej / severe-weather.eu @Recretos]

@AlexWeather25 The heat flux forecast suggests the tropospheric wave drive has peaked, and we are slowly returning to normal territory by mid-November. With the continued seasonal cooling and no new wave activity for now, the Polar Vortex is expected to recover by late November.

[Andrej / severe-weather.eu @Recretos]

@AlexWeather25 Hey Alex, thanks for all the kind words, and please just call me Andrej! 🙏🙏

As for the activity, it still belongs in the same event group and, at this stage, isn't anything special.

[Andrej / severe-weather.eu @Recretos]

@Met4CastUK Oh, that one is from @RyanWeather I think

[Andrej / severe-weather.eu @Recretos]

@judah47 @HamoudaMosti The immediate medium-range coupling is very weak, with nothing real in sight, as expected for a Super El Niño forcing in the lower levels and an SSW in an underdeveloped circulation. Of course, this is the immediate range, more so than the usual window up to 30 days.

[Andrej / severe-weather.eu @Recretos]

@JitendraBlogger @rustybrick It feels more likely they will just drop out EU publishers in general, except for the very largest ones, which might get deals. Because Google is not obligated to send you traffic or to include you anywhere in its own services just because you exist.