By Brian Gaze
For context and background information please read:
Winter 2022-23 weather, an initial look issued on September 30th 2022.
Heavy snow in Berkhamsted, December 2009
The Central England Temperature (CET) for September was above the 30 year average. On balance TWO considers a cooler month to be more likely to be followed by a colder winter.
The October CET was well above the 30 year average. Taken across the UK it was provisionally the 7th warmest on record. It was also quite a wet month with the UK recording 115% of average rainfall.
A correlation between warm and dry Octobers and colder than average winters has been discussed. Warm and wet ones do not fit into this bracket.
Publicly available data from most of the seasonal models covers the entire period of the meteorological winter.
D/J/F = December, January, February
The Climate Forecast System v2 is available on TWO. View the latest CFS v2 charts.
Since the first review, issued on September 30th 2022, the seasonal models have firmed up in suggesting:
1) Above average temperatures
2) No bias for precipitation levels
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 negative NAO is considered to be slightly higher than the norm, but a predominantly positive phase is still favoured.
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.
An easterly QBO is expected to continue through the winter. This is believed to increase the likelihood of a colder than average 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 75% 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. However, in general terms La Nina is believed most likely to increase the chance of cold snaps during the early part of the winter.
SSTs around the UK are above the norm. This will increase the likelihood of above average temperatures and help to make northerly air streams less cold than would otherwise be the case.
This can be thought of in similar terms to ENSO, but it relates to sea surface temperatures in the Indian Ocean rather than the Pacific. A negative phase, as is currently the case, may correlate with more blocked weather patterns across the North Atlantic and in turn increase the risk of cold snaps in north western Europe.
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.
Solar Cycle 25 is considered to have started between August 2019 and January 2020. Levels of solar activity through 2022 have been higher than some predictions.
The higher levels of solar activity may increase the chance of a mild winter in western Europe.
The Winter Analogue Index (WAI) 30 day tracker at the time of publication has the 10 best Northern Hemisphere matches as follows:
1) 2001 (Mild)
2) 1952 (Cold)
3) 1989 (Very mild)
4) 1999 (Mild)
5) 2011 (Mild)
6) 1990 (Cold)
7) 1977 (Close to average)
8) 2004 (Close to average)
9) 1967 (Cold)
10) 2021 (Mild)
At this stage the WAI has a bias towards average or mild winters.
Since the first update the seasonal models have firmed up on pointing towards a mild winter in the UK. The precipitation signal has weakened and there is currently no strong bias towards above or below average amounts.
Other factors such as NAO, QBO and La Nina are consistent with the previous update.
The WAI may also offer support for a milder than average winter.
Recent climatology favours milder conditions. All months so far this year have produced positive temperature anomalies and winter 2021-22 was mild.
Taking the above into account the current TWO view is that a milder season is favoured, but there are still pointers which support a cold scenario, particularly through the first half of the winter.
The TWO winter forecast is released at the end of November and it will also consider how weather patterns developed during the last weeks of the meteorological autumn.
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