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Winter 2019/20 weather

Long range forecast signals


30/07/2019

Summary of winters since 2008

After a long run of mild winters lasting through much of the 1990s and 2000s a run of colder winters started in 2008/09 and finished in 2012/13. Since then they have been mixed but mainly mild. 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.

2008/09
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 was 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

Very mild. The third warmest in the series from 1910. The UK mean temperature was 5.5C, which is 1.8C above the average.

2016/17

Very mild. The UK mean temperature was 5.0C, which is 1.3C above the average.

2017/18

Close to average. The UK mean temperature was 3.6C, which is 0.2C below average. 

2018/19

Milder than average. The UK mean temperature was 5.2C, which is 1.4C above average. 

To summarise: Since 2010/11 there has not been a significantly colder than average winter. There were wintry spells in both 2012/13 and 2017/18 but most of the cold weather occurred in the meteorological spring and so does not impact the statistics presented here.   

Berkhamsted snow, December 2017

Winter 2019/20 indicators

Last winter was notably milder than average. Computer models frequently suggested the likelihood of cold periods during January and February but generally they did not materialise, although a more wintry spell developed for a time. Will things be different this time?

Seasonal models

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, Meteo France and DWD seasonal models show a similar outlook.

Model Temperature Precipitation
UK Met Office GloSea (UK) O/N/D Above average Weak bias towards above average
Meteo France (France) O/N/D Above average Below average
ECMWF O/N/D Average or above average Average or above average
DWD (Germany) O/N/D Above average Close to average
International Research Institute (IRI) N/D/J No anomaly Above average
Jamstec (Japan) D/J/F Below average in the north, above in the south Above average
C3S multi system (European combi) O/N/D Above average Weak bias towards above average
CFS v2 (USA) D/J/F Below average  Below average
BCC (China) D/J/F Above average Below average

O/N/D = October, November, December

N/D/J = November, December, January

D/J/F = December, January, February

The Climate Forecast System v2 is available on TWO. View the latest CFS v2 charts.



The Jamstec and CFS v2 appear to be suggesting below average temperatures in the UK during the meteorological winter taken as a whole. The currently available BCC run, which also covers December, January and February, suggests  milder than average conditions.

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. However, it showed something similar at this time last year.

The skill level of seasonal models for the UK and north western Europe is low, in other words they are not very accurate.

North Atlantic Oscillation (NAO)

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.

This year the likelihood of a neutral or negative NAO is considered to be higher than the norm.

Quasi-biennial oscillation (QBO)

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 positive phase in November 2018. The mean period of each phase is 28 or 29 months, so there is a high likelihood of it staying positive through the winter. If that happens, it would be a factor supportive of a milder winter.

Grand Union canal in Berkhamsted, February 2018

El Niño Southern Oscillation (ENSO)

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 to be neutral through the Northern Hemisphere winter. 

Solar Activity

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 minimum in Solar Cycle 24. Thereafter activity is expected to steadily recover as Solar Cycle 25 begins.



Summary

At the moment there isn't a clear signal for winter 2019/20. The QBO forecast favours a milder winter and the limited amount of data currently available from seasonal models points towards mild conditions early on. At the moment most of the models only run out to the end of the year.

Low solar activity and the possibility of a neutral or negative NAO are factors that should increase the chance of a cold winter.    

Recent climatology favours a milder than average winter. However, in recent seasons there appears to be an ongoing bias towards a more meridional rather than zonal flow. If that trend continues this winter it would increase the chance of cold spells.  

Taking the above into account the initial TWO view is that the chance of a colder season may be a little higher than average.

Regular updates will be issued before the TWO winter forecast is released at the end of November.

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