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Winter forecast prelude

Winter 2018/19 update 4


13/11/2018

Overview

For more context and background information please read:

Winter 2018/19 weather, Long range forecast signals issued on July 28th 2018

UK winter 2018/19 - Update 2, issued 14th September 2018

A cold winter incoming? - Update 3, issued 19th October 2018

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

Snow in Berkhamsted

Autumn weather so far

When producing the winter forecast the autumn weather patterns are considered.

This year the September Central England Temperature (CET) finished at 13.7C which 0.1C above the 30 year average. The provisional UK mean temperature was 12.4C which is 0.2C below the 30 year average. Rainfall was 108% of average but in the south and east it was drier. As a result the month will be considered to marginally favour a cold winter.

The CET for October was 10.6C which is exactly in line with the 30 year average. The first half of the month was quite warm but it turned colder later. Towards the end of the month snow fell in parts of the UK, mostly in the north and over high ground. The month will be considered as a neutral input.

The first third of November was mild. To the 11th the provisional CET is 9.6C which is 1.8C above the 30 year average. However, there are signs of it turning colder during the rest of the month. On balance a mild November is counted as a weak factor in favour of a mild winter. The well known saying, "Ice in November to bear a duck, the rest of the winter'll be slush and muck" may have some validity but on balance the TWO view is that wintry spells in the late autumn are more likely to set the tone for the winter that follows.

Seasonal model updates

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

UK Met Office GloSea:

The temperatures signal is not very well defined. On balance upper level air temperatures are considered more likely to be above or below average than close to average. There is also a suggestion of a positive pressure anomaly to the north of the UK which would favour colder periods.

Meteo France

Colder than average in the north and west of the UK, but no signal for the south and east. Wetter than average in the south and east, but no signal for the north and west.

ECWMF

Above average temperatures are favoured in most of the UK. No precipitation signal for most, but perhaps drier relative to the average in the north west.

Jamstec

Colder than average in all of the UK and much of northwestern Europe. Drier than average in the north but wetter in the south.

IRI

No signal for temperature across the UK but warmer than average in Greenland. Drier than average.

CFS v2

The current update is mixed and suggests colder periods are more likely during the first half of the winter. (Check the latest CFS v2 charts on TWO).



Since the last update GloSea has perhaps trended colder, but ECWMF continues to favour a mild winter. The November Meteo France and Jamstec seasonal updates are not available at the time of publication.

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)

Positive this winter. That suggests an increased chance of mild and wet periods.

Quasi-biennial oscillation (QBO)

The QBO is currently transitioning from a negative (easterly) phase to a positive (westerly) one. On balance a positive phase is thought to lead to an increased chance of a mild winter.

Berkhamsted snow, December 2009

El Niño Southern Oscillation (ENSO)

El Niño is favoured with 80% chance and continue as a weak event through the Northern Hemisphere's winter. A correlation between a weak El Niño and an increased incidence of cold periods during the late winter has been established. The TWO view is that a strong El Niño increases the chance of a mild and wet winter.

Solar Activity

Solar activity is at very low levels and at the time of publication with only one small sunspot (AR2726) visible. There is a suggestion that colder winters are more common in the UK shortly after a solar minimum is reached. However a "quiet" sun arguably leads to an increased chance of colder winters.

Recent climatology

Since the Sudden Stratospheric Warming (SSW) event in February there has been an increased incidence of high pressure blocking. That led to the Beast From The East cold spells in late winter and early spring, as well as the long hot summer. Medium range computer models are pointing towards high pressure continuing to have a lot of influence on the UK's weather in the coming weeks. Therefore, recent climatology is considered as a factor increasing the chance of a cold winter.

Summary

Since the last update in October the TWO view on the prospects for winter 2018/19 have not changed. There is considered to be an elevated chance of it being colder (or possibly much colder) than average.

The winter 2018/19 forecast covering December, January and February will be issued at the end of November.

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