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Winter 2017/18 update 4

Latest forecast pointers


18/11/2017

Overview

For more context and background information please read:

"Winter 2017/18 weather, Forecast pointers" issued on July 25th 2017

"UK winter 2017/18 - Update 2", issued 13th September 2017

"UK winter 2017/18 - Update 3", issued 30th October 2017

The information here is intended to provide an update on developments.

Snow in Berkhamsted

Autumn so far

When producing the winter forecast the September weather patterns are considered. A cool and cyclonic month is counted as a weak factor in favour of a colder winter. September brought spells of wet weather and the Central England Temperature (CET) finished 0.2C below the average. It is important to emphasise that a correlation between warm Septembers and milder than average winters is not accepted by many, but the TWO view is that it has existed in recent decades.   

October was a mild month with mixed conditions. Storms Ophelia and Brian brought disruption but there was a good deal of quiet and benign periods. The CET to the 29th is a mild 2.2C above the average. The October weather patterns will be counted as a weak factor in favour of a colder winter.

The first half of November brought relatively quiet weather. The provisional Central England Temperature to the 17th is 7.9C which is 0.6C above the 30 year average. 

Seasonal model updates

The picture from seasonal models which cover December, January and February is mixed.

UK Met Office GloSea
A milder and wetter than average winter continues to be favoured. The UK Met Office produced GloSea charts below are for the December, January and February periods. They show the probability of temperature and rainfall being above, close to, or below the 30 year average. The predominance of yellow and orange shading on the above normal temperature and precipitation charts suggests that this is the most likely outcome.

Met Office GloSea 2m temperatures Dec, Jan, Feb
Met Office GloSea Precipitation Dec, Jan, Feb

Jamstec (Japanese meteorological agency)
Milder than average in the south but colder than average in the north of the UK. Generally wetter than average.

IRI (International Research Institute, Columbia University)
No signal for temperature. Weak signal for a wetter than average season in the north. No signal for the southern half of the UK meaning no deviation from the climatological likelihood of different outcomes.

CFS v2 (NCEP, USA)
Mixed but current updates hint at the possibility of colder conditions early in December with milder conditions more likely during the second half of the winter. (Check the latest CFS v2 charts on TWO).

ECMWF and Meteo France seasonal models
Both favour a mild winter. Data from those models combined with the UK Met Office GloSea output is called the C3S multi-system and that also favours a mild winter.

The key things to note since the last update are:

GloSea is consistent.

Jamstec has reverted to favouring colder conditions in the north.

IRI still strongly favours an anomalously mild winter in Greenland. That often means an increased likelihood of cold weather in western Europe.

The European models (GloSea, ECMWF and Meteo France) all favour a mild UK winter.

Note: The skill level of seasonal models for the UK and north western Europe remains low; in other words they are not very accurate.



Indexes and teleconnections

See the July update for more background information.

North Atlantic Oscillation (NAO)

Possibly close to neutral this winter. That suggests a reasonable chance of colder outbreaks.

Quasi-biennial oscillation (QBO)

The QBO is currently in a negative (easterly) phase and that is expected to continue this winter. If that is the case the chance of a colder winter and easterly winds would be slightly increased.

Berkhamsted snow, December 2009

El Niño Southern Oscillation (ENSO)

La Niña conditions are forecast (~65-75%) during the Northern Hemisphere winter 2017-18. If that is the outcome the chance of a colder than average winter would be increased.

Solar Activity

Since the last update activity has declined further but the next solar minimum is not expected until 2019 or 2020. There is a suggestion that colder winters are more likely to occur in the UK shortly after a solar minimum is reached. However a "quiet" sun arguably leads to an increased chance of colder winters. 

Summary

Some of the background signals suggested an elevated risk of colder periods developing during winter 2017/18. However some seasonal forecast models such as GloSea have been consistently pointing towards a milder and wetter than average season. Synoptic developments during November so far have been mixed but a tendency for higher pressure over Greenland has been present. The TWO winter forecast will be issued at the end of November.

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

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