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Winter 2020-21 final preview

Update 3


20/11/2020

Overview

For context and background information please read:

Winter 2019/20 weather, Long range forecast signals issued on September 7th 2020.

UK winter 2020-21 Update 2 issued on 19th October 2020

 

Berkhamsted railway station

Berkhamsted railway station in the snow

Autumn

The Central England Temperature (CET) for September was very close to the 30 year average. On balance TWO considers a cooler month to be more likely to be followed by a colder winter.

October brought close to average temperatures. The CET was 10.5C which is 0.1C below the 30 year average. There isn't a known meaningful correlation with the winter ahead. A cold winter may be slightly more likely if October is warm and dry.

November up to the 19th has been a very mild month. The provisional CET is 9.9C which is massive 2.8C above the 30 year average. 

Seasonal models

Publicly available data from most of the seasonal models now covers the three month winter period.

Model Temperature Precipitation
UK Met Office GloSea (UK) D/J/F Above average Above average
Meteo France (France) D/J/F Above average Mostly no bias
ECMWF D/J/F Mostly no bias Mostly no bias, slightly above in the north west
DWD (Germany) D/J/F Above average in the south Mostly no bias
International Research Institute (IRI) D/J/F No bias Mixed
CMCC D/J/F Below average No bias
JMA (Japan) D/J/F Mostly no bias, colder in far south and north Mostly no bias
C3S multi system (European combi) D/J/F Mostly above average Mostly no bias
CFS v2 (USA) D/J/F Above average  Mostly above average


D/J/F = December, January, February

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



The current seasonal model updates, in general, favour above average temperatures. The precipitation signal is weaker but there is a slight bias towards a wetter than average season.

The combination suggests the possibility of higher than normal positive pressure to the south of the UK and a positive North Atlantic Oscillation (NAO) pattern dominating.  

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. There are suggestions of it being negative at times during the early part of the winter.

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 a positive phase is expected to continue through December, January and February. On balance the latest developments would probably increase the likelihood of a milder winter.

Berkhamsted totem pole in the snow

Berkhamsted totem pole in the snow

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 95% chance of a La Nina event persisting through the Northern Hemisphere winter. The percentage chance has increased since the previous update.

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. Up to October 19th there were 204 days this year without sunspots. As a comparison there were 281 days in 2019 when no sunspots were observed. At the time of writing the only activity of note is sunspot AR2783 but it is quiet and there is considered to be no threat of strong solar flares. 

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



Summary

Since the second update there have been some changes. In general the seasonal models continue to favour a mild winter in the UK but the precipitation signal remains weaker. On balance a wet season is favoured, especially in the north, with drier conditions more likely in the south. With a preference for above average temperatures there is a suggestion of  higher than normal pressure to the south of the UK.

Other factors such as NAO, QBO and La Nina are consistent with the previous update.

As noted previously, recent climatology favours milder conditions. Most months in 2020 have been warmer than average and November to date has been very mild.  

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

The TWO winter forecast is released at the end of November.

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