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After a long run of mild winters through lasting through much of the 1990s and 2000's a run of colder winters started in 2008/09 and finished in 2012/13. Since then winters have been mild and at times exceptionally so. The last notably cold month was March 2013 when the mean UK temperatures was 2.2C which is 3.3C below the average. Before that December 2010 was the first calendar month since February 1986 to record a sub-zero Central England Temperature (CET).
The list below summarises temperature anomalies in UK winter since 2008/09.
2008/09Colder than average. The UK mean temperature for the winter was 3.2°C, which is 0.5°C below average.
2009/10Much 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.
2010/11Colder than average. The UK mean temperature for the winter was 2.4°C, which is 1.3°C below average.
2011/12Milder than average. The UK mean temperature for the winter was 4.5°C, which is 0.8°C above average.
2012/13Slightly colder than average. The mean temperature over the UK for winter was 3.3 °C which is 0.4 °C below average.
2013/14Much milder than average. The UK mean winter temperature was 5.2°C which is 1.5°C above the average.
2014/15Close to average. The UK mean winter temperature was 3.9°C which is 0.2°C above the average.
Very mild. The third warmest in the series from 1910. The UK mean temperatures was 5.5C which is 1.8C above the average.
Very mild. The UK mean temperature was 5.0C which is 1.3C above the average.
Recent climatology may therefore suggest that winter 2017/18 will be milder than average. Is that what the computer models and other forecast pointers are suggesting?
Last winter was mild and every month so far this year has recorded an above average Central England Temperature (CET). Will that trend continue during the coming winter?
At this early stage there isn't a clear picture emerging from the long range computer models. The publicly available data from the UK Met Office GloSea model goes out to the end of December and strongly favours above average aggregate temperatures over the October, November and December period. The American Climate Forecast System (check the latest CFS v2 charts on TWO) v2 is updated regularly but on balance it has been suggesting another mild winter.
On the other hand the Jamstec and International Research Institute (IRI) for Climate and Society models show a different picture. The former indicates below average temperatures in the UK and most of the rest of Europe during the meteorological winter. The IRI model isn't showing a bias towards any one outcome in the UK, but it strongly favours anomalously mild weather in Greenland. That often means an increased likelihood of cold weather in western Europe.
Chart suggesting above average temperatures this winter in Greenland (Source IRI)
Regardless of what they show, the skill level of seasonal models for the UK and north western Europe is still low. In other words they are not very accurate.
The NAO is essentially a measure of pressure patterns across the North Atlantic. During the winters when a negative NAO develops, blocking areas of high pressure form at high latitudes and displace the cold Arctic air down to mid latitude locations such as the UK. The NAO influenced by sea surface temperatures (SSTs) in the North Atlantic and a correlation between SSTs in late spring and the following winter has been established. This year it appears they favour a close to neutral NAO this winter which may suggest a possibility of colder periods.
The QBO index is determined by the strength and direction of equatorial zonal winds in the tropical stratosphere. When the winds are in a westerly phase the index is positive and when in an easterly phase, it is negative. A correlation between the strength of the jet stream across the North Atlantic and the QBO has been identified. A negative (easterly) QBO favours a weaker jet stream which in turn means a greater chance of cold spells during the winter months.
The QBO has recently switched to from positive to negative. The mean period of each phase is 28 or 29 months, so there is a good chance of this continuing through the winter. If that happens, it would be a factor supportive of a colder winter.
An El Nino takes place when SSTs in the central-east Pacific are anomalously warm and La Nina when they are colder than average. ENSO has an impact on global weather patterns, although the link to the UK is quite weak. A weak El Nino event is believed to increase the chance of colder weather during the second half of the winter.
Forecasting the ENSO conditions several months ahead is prone to error, but at the moment ENSO neutral is favoured with a 50% to 55% chance as we head into the Northern Hemisphere winter of 2017/18.
The link between solar activity and the weather remains controversial. There is a suggestion that colder winters are more likely to occur in the UK shortly after a solar minimum is reached. The next solar minimum is expected in 2019 and 2020 so although activity is on a downwards path the minimum remains some time off.
At this stage there isn't a clear signal for winter 2017/18. Recent climatology favours milder than average conditions but some of the background signals tentatively suggest a different outcome. Therefore the initial TWO view is that the chance of a colder winter, or at least one bringing the risk of snow at times, is higher than it has been for several years.
Regular updates will be issued before the TWO winter forecast is released at the end of November.
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