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For more context and background information please read our first "Winter 2018/19 weather, Long range forecast signals pointers" which was issued on July 28th 2018. The information here is intended to provide an update on developments during the last couple of months.
With the exception of February and March all months to date this year have recorded an above average Central England Temperature (CET). Summer 2018 equalled the UK's hottest on record, although the core of the heat was in June and July. The August CET was 16.6C which is only 0.9C above the 30 year average. The CET for the first 13 days in September is provisionally 0.5C above the average. Medium range computer models are pointing towards a warm and anticyclonic period developing in the south with more changeable and cooler conditions in the north.
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. On the other hand a warm and anticyclonic month is considered to favour a mild 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.
The picture from seasonal models which cover the December, January and February period is a mixed one.
UK Met Office GloSea:Milder than average favoured. A bias towards average or above average precipitation levels in most of the UK is shown.
JamstecColder than average in the north west. Drier than average.
IRINo signal for temperature (meaning all outcomes are equally likely) across the UK but warmer than average in Greenland. Wetter than average.
CFS v2The current update suggests milder and wetter than average conditions. (Check the latest CFS v2 charts on TWO).
The key things to note since the last update are:
GloSea is consistent
Jamstec is consistent.
IRI is consistent. Like last year it favours an anomalously mild winter in Greenland. That means an increased chance of cold outbreaks in western Europe.
GloSea Dec, Jan and Feb temperature and precipitation (September 2018 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.
See the July update for more background information.
Positive this winter. That suggests an increased chance of mild and wet periods.
The QBO is currently in a negative (easterly) phase. That may continue through the rest of the autumn and winter but it probably has now peaked and is returning to a positive (westerly) phase. If it remains negative this winter the chance of cold spells would be increased.
At the moment El Niño is favoured with a 65% to 70% chance as we head into the Northern Hemisphere winter of 2018/19. A correlation between a weak El Niño and an increased incidence of cold periods during the late winter has been established. However, the TWO view is that a strong El Niño increases the chance of a mild and wet winter.
The solar minimum is expected next year and levels of activity are expected to be low through the coming months. There is a suggestion that cold winters occur more frequently in the UK shortly after a solar minimum is reached and that low levels of solar activity generally increase the likelihood of colder periods.
It is too early for a clear signal for winter 2018/19 to emerge. Recent climatology favours milder conditions but some of the background signals and the increased propensity for high pressure blocking to form since the Sudden Stratospheric Warming (SSW) event in February suggest otherwise.
At this stage the TWO view continues to be that there is an increased chance of winter 2018/19 being close to or colder than average.
Regular updates will be issued before the TWO winter forecast is released at the end of November.
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