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UK winter 2017/18

Weather update 2


13/09/2017

IMPORTANT UPDATE

The discussion on this page was issued on 13/09/2017. More recent updates are available:

"Winter is coming - update 3" was issued on 30/10/2017.

"Winter 2017/18 - Update 4", issued 18th November 2017

Overview

For more context and background information please read our first "Winter 2017/18 weather, Forecast pointers" which was issued on July 25th 2017. The information here is intended to provide an update on developments during the last couple of months.

Snow in the Chilterns

August and September

The first seven months of 2017 all recorded an above average Central England Temperature (CET). Provisional data suggests August bucked the trend with a CET of 14.3C which is 0.2C below average. The first half of September has also brought cool and unsettled spells.

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. 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.   

Seasonal model updates

The emerging picture from seasonal models which cover the December, January and February period is for a milder and wetter than average season.

UK Met Office GloSea:
Milder and wetter than average favoured

Jamstec
Milder than average in the south east. Colder than average in the north west. Wetter than average.

IRI
No signal for temperature (meaning all outcomes are equally likely). Wetter than average.

CFS v2
Latest update suggests milder and wetter than average conditions in December and January. February looks colder and drier. (Check the latest CFS v2 charts on TWO).

The key things to note since the last update are:

GloSea is consistent

Jamstec has shifted towards a milder season across most of Europe

IRI continues to favour an anomalously mild winter in Greenland. That often means an increased likelihood of cold weather in western Europe.

 

Met Office GloSea forecast temperatures anomalies, Dec, Jan Feb
Met Office GloSea forecast precipitation anomalies, Dec, Jan Feb


GloSea precip and temp (September update, source: UK Met Office)

Regardless of what they show, the skill level of seasonal models for the UK and north western Europe is still 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 may continue through the rest of the autumn and winter. If so the chance of a colder winter would be slightly increased.

Berkhamsted High Street snow, December 2009

El Niño Southern Oscillation (ENSO)

At the moment ENSO neutral is favoured with a 50% to 55% chance as we head into the Northern Hemisphere winter of 2017/18. That means it would not be a strong forcing mechanism on the UK's winter weather patterns.

Solar Activity

Despite a downward trend towards the next solar minimum which is expected in 2019 and 2020, there has been quite a lot of activity recently. There is a suggestion that colder winters are more likely to occur in the UK shortly after a solar minimum is reached.

Summary

It is still much too early for a clear signal for winter 2017/18 to emerge. As noted in the first update, recent climatology favours milder than average conditions but some of the background signals may suggest otherwise. At this stage the TWO view continues to be that the chance of a colder winter is slightly higher than in recent years.



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

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