It’s very early, but as another meteorological summer draws to close, now is the time to start thinking winter!

This short article is not a winter forecast nor 1st update, that will likely come late September/early October but this is an appetizer, a peak at what I look for at this end of summer period.

At the time of release, westerly winds are returning to the polar stratosphere heralding the annual return of the polar vortex, albeit in it’s embryonic stage.

Early Factors

• Warm or Cold September for W Europe?
• Possible return of La Nina
• Very Warm North Pacific/Atlantic
• -QBO
• -IOD
• Indio-Pacific SST profile (could determine MJO strength/position)

Other Elements

• Volcanic eruptions in Russia’s Far East

If your hoping for a colder winter, well there are 3 ‘potential’ positive factors currently in place that are likely to continue through autumn.

La Nina, East QBO, -IOD = INCREASED CHANCE FOR HIGH LATITUDE BLOCKING & SSW EVENT

In short, the QBO (Quassi Biennial Oscillation) are a belt of winds which blow west or east within the equatorial stratosphere. West winds (like in 2024-25) tend to be associated with a stronger polar jet stream and polar vortex. An easterly (upcoming winter) can have frictional effects on both jet and vortex leading to weakening and buckling of the jet which allows polar air south towards equator.

La Nina (if, how strong & where?)

While there’s growing model support for the redevelopment of La Nina through autumn, that’s one thing but how strong and where is important.

The Met Office model is one of the more bullish for cooling within the equatorial Pacific, particularly over the CP. WHERE those coldest waters set up is important regarding factors such as the latent heat release from the MJO…

The combination of La Nina and east QBO correlates to a weaker jet stream (& more buckled) and weaker polar vortex as well as greater chances of a sudden stratospheric warming event.

Another positive factor if you wish for a colder or early onset to winter, is a -IOD or negative Indian Ocean Dipole.

The combination of La Nina, east QBO and -IOD all tend to lean towards weaker polar vortex and increased chance for a -AO/NAO (Arctic Oscillation, North Atlantic Oscillation).

Recent Volcanic Eruptions in Kamchatka

What’s important the remember, the depth of atmosphere is greatest at the equator and least at the pole. It’s when ash and materials get released and ejected through the troposphere into stratosphere, particularly sulfur where effects such as an increase in high latitude blocking can occur.

Last case of high latitude eruptions enhancing a -AO/NAO is probably 2010 in Iceland.

There have been multiple large-scale eruptions on Russia’s Kamchatka Peninsula in the past 12 months. August last year saw Shivelich blow, releasing ash into the stratosphere followed by the first ever recorded eruption of Krasheninnikov back in May.

The height of the stratosphere various from 11-12 miles over the tropics to only 3.4 miles above the pole. The Shiveluch volcano is the northernmost point in Kamchatka’s Far North (56.6N). Pretty much the same at Scotland and so the stratosphere is roughly 6-7 miles above.

How much effect these recent eruptions may or may not have is unknown but certainly as element worth noting.

Some Negatives…

Over the last couple of decades, hot summers have traditionally NOT been followed up by a cold winter. In fact you have to go back 30 years to the summer-winter of 95-96 when we last saw a hot summer followed by cold winter. That being said, the parameters are VERY DIFFERENT to then with all the background warming.

I’ve said for several years now, those analogues we love to look at and teleconnections we go by, really aren’t as effective now as they once were due to the lack of SSTA differences.

Additional Thoughts

For those winter lovers, let’s hope we don’t have another warm to hot September, their more often than not followed by warm winters.

As for the tropics, well Pacific and Atlantic activity (esp recurves) are well known for driving heat north not only into the mid latitudes but polar region. This can force a -AO/NAO and intrusions of cold south during September, October, even November with even weakening of the polar vortex. Busy ends to the Atlantic hurricane season have been linked to cold December’s.

If you want me to put myself on the line as of late August, I would lean towards a cold December, possibly January followed by milder end to January and February.

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