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After a long run of mild winters 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 they have been mixed. Late February 2018 brought a spell of severe wintry weather dubbed the Beast from the East, but despite that the last notably cold month was March 2013 when the mean UK temperature was 2.2C which is 3.3C below the average. Prior to that December 2010 was the first calendar month since February 1986 to record a sub-zero Central England Temperature (CET).
The list below shows 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.
Close to average. The UK mean temperature was 3.6C which is 0.2C below average.
To summarise, recent winters have been varied. Despite winter 2017/18 being close to average it included a spell of bitterly cold weather in late February and heavy snow affected parts of the country in December. The wintry conditions continued into March but that is classified as the meteorological spring.
Last winter was close to average overall but there were large variations during the three month period. Many parts of the UK saw more snow than they had for several years. Will that trend continue during the coming winter?
At this early stage there isn't a clear picture from the long range computer models. Publicly available data from the UK Met Office GloSea model goes out to the end of December and it favours above average aggregate temperatures over the October, November and December period. The ECMWF and Meteo France seasonal models show a similar outlook.
The American Climate Forecast System (check the latest CFS v2 charts on TWO) v2 is updated regularly but recent runs have suggested a mild winter.
The Jamstec and International Research Institute (IRI) for Climate and Society models paint a different picture as they did this time last year. The former indicates below average temperatures in the UK and most of Europe during the meteorological winter as can be seen on the map below. The IRI model isn't showing a bias towards any one temperature outcome in the UK, but it favours anomalously mild weather in Greenland. Mild weather in Greenland is often associated with cold weather in western Europe during the winter months.
Temperature anomaly forecast for winter 2018/19 (Source Japan Agency For Marine-Earth Science And Technology )
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 they favour a positive NAO during the coming winter and that correlates strongly with a mild southwesterly based pattern.
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 switched to a negative phase in June 2017. The mean period of each phase is 28 or 29 months, so there is a good chance of it remaining negative through the winter. If that happens, it would be a factor supportive of a colder winter.
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 El Nino is favoured with a 70% confidence level as we head into the Northern Hemisphere winter.
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 close to or shortly after a solar minimum is reached. The coming winter is expected to coincide with the solar minimum in the current cycle.
There isn't yet a clear signal for winter 2018/19. The North Atlantic SST pattern which is thought to increase the likelihood of a positive NAO and the possibility of a strong El Nino both suggest a milder winter. However, some of the seasonal computer models do not support that outlook. Low solar activity and a negative QBO are also factors that are thought to increase the chance of a cold winter.
Recent climatology is not clear cut, although the last four winters have been close to average or mild overall. However, one of the key factors this calendar year has been the tendency for blocking areas of high pressure to develop. That can be traced back to the second half of last winter when a massive Sudden Stratospheric Warming (SSW) event took place. If blocking high pressure systems continue to form more frequently than normal during the coming autumn and winter, the chance of very cold periods of weather in the UK and much of mainland Europe will be increased.
Taking all of the above into account the initial TWO view is that there is an increased chance of winter 2018/19 being close to or colder than average.
Regular updates will be issued before the TWO winter forecast is released at the end of November.
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