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Preliminary thoughts for the Netweather 2022-23 Winter Forecast

A preview of thoughts looking at the main parts of the jigsaw coming together to make the Netweather 2022-23 Winter Forecast which will be issued soon. This is not a forecast for each winter month, but a look at the main climatic drivers that may play a role in shaping the coming winter.

Preliminary thoughts for the Netweather 2022-23 Winter Forecast
Blog by Nick Finnis
Issued: 12th November 2022 15:17
Updated: 12th November 2022 16:01

The Netweather Winter Forecast will be issued towards the end of next week or next weekend, a bit earlier than last year. Winter forecasts are increasingly pushed out earlier and earlier, even though longer lead times typically reduce accuracy. But with the cost-of-living crisis this winter, with businesses and households alike perhaps more interested than ever before to know if we could be in for a cold or mild winter, it is perhaps best to get the word out sooner rather than later.

However, before it’s issued, I thought I’d do a preview looking at the main parts of the jigsaw coming together to make the forecast, such as the climatic drivers and teleconnections. But, also, perhaps some hint on what we could expect, but not draw any conclusions yet, you’ll have to wait for the Winter Forecast for that. But I will drop hints on the likelihood of an average, cold or mild winter.

The winter forecast will discuss the main global climate drivers and teleconnections which can persist throughout winter. This can give indications, when used in combination, of a broad idea of how this winter may pan out. In this blog I will briefly look at how these drivers may affect weather patterns in the coming winter.

But also touch on one or two ‘wild cards’, such as the Hunga Tonga volcanic eruption, potential for Sudden Stratospheric Warming and climate change, that make an already difficult forecast to make, given all the different drivers, even more challenging!

Also, I will take into account what the Numerical Weather Predictions model seasonal outputs are showing for the three winter months, sometimes these seasonal outputs a few or more months prior to the winter season can develop trends and a few have been occasionally successful in predicting and verifying weather patterns.



This winter will be our 3rd winter in a row with La Niña. This consecutive pattern has only been observed two other times since 1950. This year’s La Niña is expected to weaken and could even slide into neutral ENSO territory by late winter.

While three back-to-back La Nina episodes are not unprecedented, they are rather uncommon and analogs suggest that the third winter in this extended oceanic event have tended to feature northern blocking bringing and increased chance for colder spells for the UK.

However, climate change influences are making drawing composites of analogs of winters with similar ENSO, QBO, stages in solar cycle, etc, increasingly unreliable. It is certainly becoming increasingly less likely to achieve a below average month in winter these days, even when analogs point to potential for below average conditions in the past. Even an average month for temperatures is a push. The UK this year has experienced 10 consecutive above average months temperature-wise, November’s mildness so far increasingly likely to make that 11. We have to go back to May 2021, the coldest May since 1996, for the last below average month.

But despite the growing influences of climate change on historically typical responses of the atmosphere to climatic drivers in certain phases, there is still some use in looking at the state of these climatic drivers and teleconnections as we head into winter and how they may influence how the winter pans out. But when they point to cold, based on analogs, perhaps tempering such expectations.

ENSO

So, La Niña will be one of the big drivers of patterns going into the winter as it continues to be unusually persistent as we move into the winter season.

Typically, a weak La Nina has less imprint on the atmosphere and so favours a more-wavy jet and thus higher incidence of high latitude blocking in the northern hemisphere during winter, with a greater chance of cold weather for the UK. While a strong La Nina has more imprint and favours a strong and zonal jet stream with a +NAO (North Atlantic Oscillation) that brings mild weather to the UK.

However, the position of the La Nina cold SST anomalies in the tropical Pacific can also cause variability in the upper patterns over the northern Pacific and subsequently over polar regions and downstream over North America and the North Atlantic. Central-based cold anomalies in the ENSO region of the Pacific can have different effects on extra-tropical patterns than east-based cold anomalies. East based La Nina tends to favour high latitude blocking more than central-based.

La Niña is present with the most recent Oceanic Niña Index (ONI) value (Aug-Oct) of -1.0C, similar to the start of winter last year. However, forecasts are for Niña to weaken as we head through winter. Coldest anomalies are currently biased towards the east – Nino region 1 + 2 over last 4 weeks was -1.8C, Nino 3 and 3.4 were -1.1C. As discussed, a more east-based La Nina more likely to favour blocking, more particularly early on in winter, based on analogs of previous east-based La Nina winters.

Forecasts point to La Nina weakening through winter 2022-23

However, direct connections to impacts on the weather months ahead are often (and usually) confounded by a myriad of other long-term and shorter-term drivers or teleconnections that can dominate through the winter season, which I will look at.

Some of these shorter-term teleconnections around the globe can overwhelm the effects of La Niña, and no single one will dictate the pattern. The problem is that these shorter duration phenomena become unpredictable beyond more than say 3-4 weeks in advance. So, producing an accurate winter seasonal outlook on short-term predictors can be difficult and prone to large error at this range.

Like ENSO (this year La Nina), we can use these teleconnections and compare them to past years with similar ENSO background state to see if any are similar, using analogs, because this gives us a good idea of what conditions we may see this upcoming winter.

The ENSO state is usually my starting point when it comes to constructing the winter forecast, but a forecast based to too heavily on ENSO analogs could easily fail early on if we do not take into consideration other drivers, such as the tropical stratosphere, the position of the solar cycle, shorter term cycles such as the MJO and other miscellaneous events, such as volcanic eruptions.

Hunga Tonga volcanic eruption

One potentially significant event that may influence this winter’s patterns, which didn’t appear in last winter’s forecast, is the Hunga Tonga submarine volcanic eruption. Although it took place way back on January 15, 2022, it injected approximately 45 million metric tons of water vapor into the atmosphere, an increase of 10% and subsequently cooled the southern hemisphere stratosphere by reflecting incoming solar radiation. The increase in water vapour has also reached the northern hemisphere stratosphere too, evident as cooling between 0 and 10 degrees north of the equator. However, nature tends to try and balance out an imbalance in the atmosphere, so where there is an increase in cooling in the southern hemisphere and low latitudes of the northern hemisphere stratosphere it is likely to offset this with a warming of the stratosphere towards northern latitudes of the northern hemisphere to try and rebalance the atmosphere. Indeed, analogs of years that featured a colder-than-normal stratosphere in the southern hemisphere winter (our summer) indicate predominance of blocking in December – January but this blocking waning through February.

Tropical stratosphere – QBO

The QBO is currently in its positive (westerly phase) at both 30 and 50mb and will continue to ascend throughout the coming winter season. With the QBO in westerly state throughout winter 2022-23 and unlikely to turn easterly again until at least summer 2023. Analogs of wQBO / La Nina winters suggest that any blocking is more likely to occur earlier than later this season.

Polar Stratosphere

Current model output in the medium to longer range is signalling the persistence of ridging or blocking over Scandinavia and, also over Alaska / Aleutians area. This poleward ridging and associated wave breaking if it persists could, over time, weaken the polar vortex, which is currently strengthening towards the seasonal norm - as the polar stratosphere begins to cool as polar nights become longer. The models have occasionally hinted at a stretched stratospheric Polar Vortex at range and we’ll be keeping an eye on the stratosphere forecasts. But a Sudden Stratospheric Warming (SSW) tends occur, on average, in January or February rather than December and can’t be predicted more than a few weeks ahead.

Solar Cycle

Perhaps one of the big failures of last winter’s forecast was not anticipating a big increase in solar flares in December 2021 compared to prior months, despite rising out of the solar minimum that occurred in late 2019, with sunspot activity increasing. Last winter’s forecast was partly based on the assumption that there is a lag effect on the atmosphere after the solar minimum, with years near and shortly following a solar minimum tending to have a weaker stratospheric polar vortex and are more prone to blocking. However, there didn’t appear to be any lag effect noticeable in winter 2021-22 of the solar minimum in 2019-20, certainly high latitude blocking was absent most of the winter, only briefly appearing in December over Greenland, but its influence on UK weather was negligible.

MJO

The Madden Julian Oscillation (MJO) has been quite active recently, which during a La Nina isn’t too common. The MJO is often rather muted during La Nina, which suggests perhaps La Nina is already starting to weaken in the west of the ENSO region. The MJO recently travelled across the tropics as quite a coherent wave through phases 6-7-8 into the Western Pacific, which can often promote more amplified upper flow patterns that can lead to high latitude blocking. Indeed, we are seeing in the models the manifestation of Aleutian and Scandinavian ridges as we head through to mid-November. This looks to bring a surge of cold arctic air south across central and eastern North America, while western Russia looks to turn cold too. But for northwest Europe it’s more likely to remain mild and unsettled as the tropospheric polar vortex over Greenland strengthens and extends towards NW Europe. 

Since 1975, only 4 La Nina Novembers saw the MJO move into both phases 7 & 8 at an amplitude of greater than 1. Those are 1996, 2000, 2010 and 2016. November 1996 was followed by a cold December, November 2010 was very cold towards the end, followed by a very cold December. The Decembers of 2000 and 2016 were mild. 

However, although the MJO cycle lasts between 30-60 days and a rough prediction can be made on when it may reach certain phases based on which phase it is propagating through currently, forecasting more than a month ahead can be tricky, as its behaviour can be erratic even a few weeks ahead. The MJO is now in the Circle of Death COD, it is forecast by most models to move back out into phase 5 and 6 at a decent amplitude later this month then perhaps heading towards phases 7 and 8 by early December. Phases 7 tends to promote Scandinavian blocking in December, after a lag of around 10 days reaching this phase, phase 8 blocking more towards Greenland. But forecasting the MJO’s subsequent cycles and their impacts on extra-tropical weather patterns in the northern hemisphere into January and February is impossible.

NWP seasonal charts

ECMWF - November seasonal outlook was updated recently and goes for blocked and dry for northern Europe, including the UK. This would suggest average temperatures with a mixture of mild days but also colder and frosty ones, depending on the position of the blocking.

UKMO - October seasonal outlook (November not available yet) suggests more average heights over NW Europe for DJF, with blocking over NW Atlantic and Russia. Would indicate a bit of everything.

NCEP / GFS - October seasonal outlook goes for low heights to the NW and higher heights to the south for DJF, this suggests mild and mobile westerlies.

 

GLOSEA update for DJF looks blocked and dry over much of Europe, which like EC, could mean a mix of mild and colder prospects depending on where blocking locates

 

Meteo  France October seasonal outlook goes for higher heights over northern Europe, so dry and blocked.

Preliminary thoughts for the winter forecast

  • A moderate basin-wide La Nina going into winter, but with time, coldest anomalies biased to the east initially = potential for episodes of high latitude blocking early winter on in winter.
  • Westerly QBO = analogs of wQBO/La Nina again indicate any blocking more likely to occur in early winter
  • Increasing solar activity as we head towards Solar Maximum in 2025 = bringing increased chance of westerly winds and milder winters to Europe.
  • Weakening La Nina through winter could mean a coherent rather than muted MJO cycles reaching ‘colder’ phases at times. But the cold phases favouring blocking link doesn’t always play out.
  • Wild card – increased potential high latitude blocking in northern hemisphere this winter as a balancing out response to colder-than-normal southern hemisphere stratosphere triggered by Hunga Tonga eruption.
  • Most outlooks from seasonals going for greater chance of higher-than-normal heights over northern Europe / UK over winter, though GFS and UKMO suggest more normal heights range, GFS mildest and more mobile looking. So would heavily weigh in to the winter forecast the possibility of prolonged periods of blocking, but location is very uncertain.

Full winter forecast, including what I think may happen each month, will be issued later next week or next weekend. For now, the above are thoughts on many of the factors that may shape and influence winter 2022-23 in the UK.

Initial thoughts are for an average winter, with any blocking and colder spells in early winter while late winter, particularly February, more likely to be milder.

Tags: UK Weather  Climate

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