By Brian Gaze
For context and background information please read:
Winter 2023-24 weather: Is a cold winter on the way? Issued 16th September 2023.
The third update, Winter 2023-24 third update, was issued on 18th November 2023.
The Central England Temperature (CET) for September was 17°C which set a new record, eclipsing the 16.9°C achieved in 2006. On balance, we consider a cooler month to be more likely to be followed by a colder winter.
The October CET provisional to the 23rd is 12.9°C, which is 2.4°C above the 1961-90 average. It has also been a wet month with Storm Babet in particular leading to severe flooding in places.
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.
Therefore, at this stage the autumn weather patterns would appear to be more supportive of a milder rather than colder than average winter.
Data from most of the seasonal models goes as at least as far as the end of the meteorological winter, which is 29th February 2024.
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 16th September 2023, there have been changes to the seasonal model predictions. However, the key points are:
1) Forecasts continue to favour above average rather than below average temperatures
2) The signal for a wetter than average winter has weakened
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 slightly higher than the norm. Earlier suggestions of a negative NAO from the ECMWF seasonal model have been lost in the October refresh.
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 Niño takes place when SSTs in the central-east Pacific are anomalously warm and La Niña 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 Niño 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 "strong" El Niño event persisting through the Northern Hemisphere winter.
Our view is a strong El Niño reduces the chance of a cold winter in western Europe, although it possibly leads to an increased chance of cold snaps towards the end of the season.
SSTs around the UK are mostly 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 positive IOD phase, as is currently the case, may correlate with wet and mild weather patterns across western Europe. The positive IOD phase is forecast to continue into the winter.
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 expected to peak in 2024. Higher levels of solar activity may increase the chance of a mild winter in western Europe.
Analogue years
The Winter Analogue Index (WAI) 30 day tracker at the time of publication has the 10 best Northern Hemisphere matches as follows:
1) 2007 (Mild)
2) 1953 (Rather cold)
3) 2011 (Mild)
4) 1977 (Rather cold)
5) 2001 (Mild)
6) 1982 (Close to average, very mild Jan, cold Feb)
7) 2021 (Mild)
8) 1997 (Very mild)
9) 2003 (Mild)
10) 1989 (Very mild)
At this stage the WAI has a bias towards mild winters.
Since the first update the signal from the seasonal models for a milder than winter has firmed up. However, the signal for it to be wetter than average has weakened.
Other factors such as NAO, QBO and El Niño are consistent with the previous update.
The WAI supports a milder than average winter.
Recent climatology favours mild conditions. All months so far this year have produced positive Central England Temperature (CET) temperature anomalies compared to the 1961-90 averages. Winter 2022-23 brought a notable cold spell in December, but overall it was slightly milder than the average.
Taking the above into account, our view is that a milder season is favoured.
The TWO winter forecast is released at the end of November and it will also consider how weather patterns developed during the second half of the meteorological autumn.
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