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UK winter 2020-21 weather

An initial look


07/09/2020

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 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

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

2019-20

Much milder than average. The UK mean temperature was 5.3C, which is 1.5C above average. 

The last two winters have been notably mild. Much of the UK saw very little, if any, snow. According to the Met Office, winter 2019-20 was the fifth mildest in a series from 1884, and the fifth wettest since 1862.  

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, February 27th 2020

A sprinkling of snow fell in the Chilterns on February 27th, 2020

Winter 2020-21 indicators

Last winter was notably milder than average. Much of lowland UK saw very little snow. In much of southern England no falling snow was recorded until the 27th February. Will things be different this time?

Seasonal models

Publicly available data from most of the seasonal models goes as far as the end of January 2021. Therefore, February which is the last month of the meteorological winter is not included. The exceptions are the IRI, Jamstec and CFS.

Model Temperature Precipitation
UK Met Office GloSea (UK) N/D/J Above average Weak bias towards above average
Meteo France (France) N/D/J No bias Mostly no bias
ECMWF N/D/J Above average Mostly no bias
DWD (Germany) N/D/J Mostly no bias Mostly no bias
International Research Institute (IRI) D/J/F No anomaly Above average
CMCC N/D/J Mostly no bias, colder in the north west Above average
Jamstec (Japan) D/J/F Above average Above average
C3S multi system (European combi) N/D/J Above average No bias
CFS v2 (USA) D/J/F Above average  Close to average
BCC (China)  Not available Not available


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.



Unusually at this stage all of the models listed are either showing no bias or above average temperatures. 

In terms of precipitation it is also striking that none are favouring a drier than average late autumn or winter period.

It is important to appreciate that 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 positive 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.

At the 30hPa level the QBO switched to a weakly negative phase in January 2020. It remained like that until July 2020 when it became positive again. The recent behaviour of the QBO appears to be very unusual because the cycle normally lasts for 28 or 29 months.

At the moment it appears the expected to transition to a stronger positive phase has failed. On balance the latest developments would probably reinforce the likelihood of a milder winter.

Grand Union canal in Berkhamsted, February 2018

Coming to the UK this winter?

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 there is a 55% chance of a La Nina event persisting through the Northern Hemisphere winter. 

The TWO view is a weak La Nina increases the chance of a cold winter in western Europe. A strong event probably diminishes it.

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 early phase of Solar Cycle 25. It is considered to have started between August 2019 and January 2020.

The position of the solar cycle may increase the chance of a cold winter in western Europe.



Summary

At the moment there isn't a conclusive signal for winter 2020-21 but the seasonal models are quite bullish about it being milder and wetter than average.

The prospects for the QBO and NAO are uncertain. On balance they do very little to suggest an increased chance of a cold winter.

The possibility of a weak La Nina may be a factor which increases the likelihood of a colder winter. The same is true of the phase of solar cycle 25. Nonetheless, both of these factors are tenuous at best.    

Recent climatology favours milder conditions. The last two winters have both delivered big positive temperature anomalies with little snow in most lowland parts of the UK. Through 2020 warmer than average months have been a lot more frequent than cooler ones.  

Taking the above into account the initial TWO view is that the chance of a milder season is higher than average.

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

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