Winter 2014/15 initial thoughts

Introduction

TheWeatherOutlook winter forecast for 2014/15 will be issued in late November and made freely available on the site. It is based on a number of factors such as weather patterns in recent seasons and this autumn. It is too early to say what the winter forecast will suggest this year but a few early thoughts are possible. Before reading on remember the caveats. Firstly, long range weather predictions for this part of the world tend to be inaccurate, and secondly, this is not the TWO winter forecast.

Winter 2013/14 was exceptionally wet and very mild but will the snow return this time around?

Seasonal forecast models

The number of seasonal forecast models has increased in recent years but they still offer quite a low skill level for this part of the world where the weather is influenced by a number of different air masses. They are worth looking at as we head towards the winter to see whether trends can be gleaned from the output. Recently I've looked at the output from the International Research Institute (IRI), Coupled Forecast System (CFSv2), UK Met Office and Jamstec models.

The model outputs are updated at regular intervals and tend to be expressed in terms of anomalies for temperature, pressure and precipitation. As usual I'll not go into details but on balance they don't seem to be offering a strong signal. Jamstec is suggesting a colder winter but the others are leaning towards milder than average.

Most of these models will update again at least once before the winter begins.

Conclusion

No strong signal, but possibly leaning towards a milder than average winter across the UK.

Pattern matching

As we’re only half way through the autumn it is too early to draw conclusions here. The run of warmer than average months which August interrupted has resumed and October has brought exceptional warmth. At the moment it's not clear whether the polar vortex will become established across Greenland as we head into winter. If it does then the odds will favour milder and wetter conditions.

Conclusion

Mixed. Gut feeling suggests things are quite finely balanced. Although there seems to be a correlation between mild autumns and mild winters I wouldn't draw any conclusions at this stage.

ENSO

A weak El Niño is expected.

Conclusion

No strong signal, but my view is this favours a milder than average winter.

Atlantic SSTs

Currently Sea Surface Temperatures are above average around the UK and below average further west in the mid-Atlantic.

The May SST signal seemed to suggest a +NAO this winter.

Conclusion

Quite strongly favouring a milder winter.

October Pattern Index (OPI)

This has been discussed widely in recent weeks and is based on research done in Italy. A strong correlation has been identified between the October synoptics and the Arctic Oscillation (AO) in the following winter. This year it is suggesting a strongly negative AO.

Conclusion

Favouring a colder winter but the index is relatively new and although hindcasting has achieved excellent results I would like to see how it performs as an actual forecasting tool. 

Solar Activity

Solar activity has recently returned to low levels after an increase earlier in the autumn.

Conclusion
Possibly tipping back towards favouring more blocking and the possibility of a colder winter. Note that the link between solar activity and winter weather patterns in western Europe is very controversial.

Siberian snow cover

The link between Siberian snow cover and the west European winter is disputed but above average levels are thought by some to increase the chances of high pressure blocking and the colder backing further west.

Conclusion

No bias towards a colder or milder winter at the moment.

Other teleconnections (pressure patterns)

In addition to the NAO, a number of other teleconnections are often considered when making long range forecasts. These include the Pacific Decadal Oscillation (PDO), Madden-Julian Oscillation (MJO), and Quasi-biennial oscillation (QBO). The QBO has attracted quite a lot of attention recently despite being discovered in the 1950s. It is currently switching back to an easterly phase which is thought to increases the chance of blocking and cold easterly winds at our latitude during the winter months.

Conclusion

No strong signal for forecasting. The interaction between these teleconnections isn't well understood and even when taken in isolation there is little consistency in their state and the west European winter.

Stratospheric temperature forecasts

In simple terms stratospheric warming is believed to encourage northern blocking. The problem is forecasting stratospheric temperatures, and fully understanding the time lag and how to use this as a forecasting tool for a small part of the Earth such as the UK.

Conclusion

I remain interested in this but am very unclear about the ability to use it as a forecasting tool several months ahead! Therefore, I'm again going to conclude there's no useable forecasting signal for the coming winter.

Recent weather

Summer 2014 was often warm and dry as was 2013. The first half of autumn has been very mild with variable amounts of rainfall. The current medium range outlook is a mostly unsettled one with close to average temperatures but there have been some very tentative signs of high pressure blocking showing up in the medium range computer models.

Cold winters do tend to come in clusters, but even during these periods mild winter do occur. It's very unclear to me whether we are still in a period when the chances of a particular winter being cold are higher than average. It's possible we have now flipped back to a milder phase.

On balance weather patterns during recent seasons have in my view been more likely to be followed by a mild rather than cold winter. Again this is quite a subjective view which others may disagree with.

So what about winter 2014/15?

I'd be very surprised if it's as mild and wet as last year's! That's not really saying much of course as 2013/14 was exceptional. At this stage of the autumn if I was forced to make a call I'd go for milder and wetter than average season.

Despite this I'd not be surprised to see a blocked cold pattern developing for a time during December and as we saw in 2010 these set ups can be difficult to break down.

January may end up quite stormy again with significant snowfall over higher ground in the north. This type of pattern rarely delivers snow to lowland parts of the UK, especially in the south.

February could turn out to be the joker in the pack. The Atlantic tends to back off later in the winter and if the Arctic Oscillation (AO) is negative as the OPI suggests colder spells could develop.

Issued 31/10/2014  © Brian Gaze

Brian Gaze's blog

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