TWO winter forecast indicators

[TheWinterSoldier]

Update 15/09/16: http://www.theweatheroutlook.com/twooth ... er+weather

Recent UK winters
Sitting on the edge of the Atlantic Ocean means the UK's climate is a temperate maritime one. Colder winters do occur but they are the exception not the norm and are usually caused by high pressure developing to the north. This allows cold arctic or polar continental air masses to push towards the UK.

Winter scene

Snow has been in short supply over much of lowland Britain in recent years

Between 1991 and 2007 most UK winters were milder than the long term average with snow in lowland Britain often notable by its absence. Since 2008/09 the picture has been mixed and four winters have been colder than the long term average. However, during the last five years four of the winters have been in the mild category.

Does this mean the law of averages favours a colder winter this year? No, if anything it suggests the opposite due to the persistence of background forcing mechanisms which load the dice in favour of a mild outcome.

2008/09
Colder than average. The UK mean temperature for the winter was 3.2°C, which is 0.5°C below average.

2009/10
Much colder than average. The UK mean temperature for the winter was 1.6°C which is 2.1°C below average. In many parts of the UK this was the coldest winter since 1978/79.

2010/11
Colder than average. The UK mean temperature for the winter was 2.4°C, which is 1.3°C below average.

2011/12
Milder than average. The UK mean temperature for the winter was 4.5°C, which is 0.8°C above average.

2012/13
Slightly colder than average. The mean temperature over the UK for winter was 3.3 °C which is 0.4 °C below average.

2013/14
Much milder than average. The UK mean winter temperature of 5.2°C which is 1.5°C above the average.

2014/15
Close to average. The UK mean winter temperature was 3.9°C which is 0.2°C above the average.

2015/16
Much milder than average. The UK mean winter temperature was 5.5°C which is 1.8°C above the average.

UK winter 2016/17 forecast indicators
1) Computer forecasting models. These now look months ahead but their accuracy for the UK and north west Europe in general is low, in large part due to the number of different air masses which affect our weather. Currently they are leaning towards the possibility of higher pressure to the north of the UK during the early part of the winter and perhaps a more neutral set-up later in the season. Arguably there is currently a weak signal for a colder winter.

CFS December 2016

Recent runs of the CFSv2 model have shown the potential for higher pressure to the north of the UK during the early part of the winter. This suggests an increased chance of cold spells.



2) El Niño Southern Oscillation (ENSO). Latest projections suggest a neutral ENSO unlike last winter when a very strong El Niño was taking place. Not a strong indicator this year.

3) Other teleconnections (pressure patterns)

These include the Pacific Decadal Oscillation (PDO), Madden-Julian Oscillation (MJO), and Quasi-biennial oscillation (QBO). There is uncertainty about the QBO but there have been suggestions it will be in an easterly phase. May increase the chance of a colder winter.

4) Solar activity is below its 2014 peak but still not at the low levels which are thought likely to increase the possibility of a cold winter. Possibly leans towards a milder winter.

5) Weather patterns during recent seasons.

In the last five years four winters have been milder than average. In 2017 eight of the first nine months (including September which is now almost certain to be mild) have been milder than average. Persistence patterns favour a milder winter.

Winter scene

Will it be possible to build a snowman this winter?

Summary
The latest forecast indicators are much more mixed than they were at this time last year when a mild winter was strongly favoured. Currently there are suggestions of a relatively cold winter but the persistence of above average temperatures this year and in recent winters provides an offsetting counterweight to these. In conclusion things seem finely balanced at this early stage.

During the autumn months look out for further updates on TWO which will be discussing the winter prospects and the latest data.