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Winter 2019/20 weather, Long range forecast signals issued on September 7th 2020.
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.
The first half of October brought below average temperatures. The outlook for the last part of the month is more mixed. A correlation between warm and dry Octobers and colder than average winters has been discussed. Nonetheless, it is a very uncertain picture and not one which is universally accepted.
Publicly available data from most of the seasonal models now covers the three month winter period.
D/J/F = December, January, February
The Climate Forecast System v2 is available on TWO. View the latest CFS v2 charts.
In the previous update data from some of the seasonal models was only available until the end of January 2021. The listed ones now cover the entire meteorological winter which finishes on February 28th 2021.
With the exception of the CFS v2, which updates frequently, all of the models are either showing above average temperatures or no bias.
In terms of precipitation only the DWD is favouring a drier than average winter period. It is possibly suggesting higher than average pressure across the southern half of the UK.
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.
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.
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 transition to a negative phase has failed with a positive phase now reasserting. On balance the latest developments would probably increase the likelihood of a milder 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 there is a 85% 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.
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.
The position of the solar cycle may increase the chance of a cold winter in western Europe.
Since the first update there have been a number of changes. However, the seasonal models continue to favour a mild winter in the UK. The precipitation forecast is less clear. On balance a wet season is favoured but a number of the models suggest it will be drier than the norm. 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.
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 dominated, although the first half of October was relatively cool.
Taking the above into account the initial TWO view continues to be 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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