It is worth referring to last winter in brief to provide a comparison point for this year. Many of the key signals have deviated somewhat this autumn from how they were this time last year. The winter of 2016/17 saw a near-neutral ENSO state. After a cold November, the North Atlantic Oscillation (NAO) switched into a positive state for much of the winter. December and February were both mild, especially in the north, but the NAO was neutral during January, when temperatures dropped a little below normal in the south. It was also an anticyclonic winter. The Arctic had another record breaking warm winter, with exceptionally low sea ice extent around Svalbard and the Russian Arctic, and so what few northerlies we had were not very potent and the easterly in early February was also tame.
There is a strong consensus among the forecast models from NMME, NOAA and the UK Met Office that the winter of 2017/18 will stay in a weak La Nina state. ENSO moved into a positive state during the summer but not enough to trigger an official El Nino, and shifted to a weak negative state through the autumn. There is some weak support for a change to a near-neutral ENSO state by the end of February but this is uncertain and many Met Office model ensembles keep us in a weak La Nina through Spring 2018 as well. While winters with a strong La Nina have often been mild and westerly dominated (e.g. 1989, 1999, 2000, 2008) some very cold winters, most famously 1963, featured a weak La Nina.
It is worth looking at analogues for years with a weak winter La Nina (generally -0.5 to -1.0) that did not have a strong La Nina state in the preceding summer. 1962, 1963, 1967, 1968, 1985, 1996, 2001, 2009 and 2012 are the years that particularly stick out. The second image below shows that these winters, on average, had a much reduced mean westerly flow relative to normal over the UK and more high latitude blocking than normal between Iceland and Scandinavia. Thus the ENSO state points to a potentially cold winter, with the cold tending to come more from continental air masses than from northerlies associated with high pressure over Greenland.
Breaking it down by month, we see an anomalous easterly flow over the British Isles during January and a tendency for anticyclonic weather with reduced westerlies in February. The Decembers produced less of a strong signal with a weak tendency towards high latitude blocking and near average strength westerlies over the British Isles, albeit slightly veered from normal.
The QBO is currently in an easterly phase, as has been the case since early in the autumn. An easterly phase of the QBO is correlated with a weak jet stream and more high latitude blocking than usual, which is also consistent with the signal from ENSO. Meanwhile, however, the stratosphere between 25N and 90N is running a little warmer than average. There are no indications of a Sudden Stratospheric Warming event during November 2017 and into December, but a SSW during January and/or February combined with the easterly QBO would increase the likelihood of a period of significant high latitude blocking developing. Already as of mid-November 2017 we are seeing an unusually negative Arctic Oscillation (very high pressure near the North Pole) and this is looking set to bring generally cold weather to the British Isles towards the end of November and into early December.
The Madden-Julian Oscillation is currently in a neutral phase but the forecast models are suggesting a shift to phase 3 or 4 into December. This points to anticyclonic and cold conditions early in December but then turning milder, and less settled in the north, with an enhanced westerly flow over the British Isles. MJO phase 7-8-1 is correlated with high latitude blocking during January and February and so the MJO can be expected to play more of a role later on in the season.
Arctic sea ice is anomalously low but rather less extreme than it was at this time last year. Sea ice extent is well up on last year in the Norwegian and Russian Arctic, though still somewhat down on the 1981-2010 long-term average. Temperatures north of 80N are running above the 1981-2010 normal but again rather less exceptionally so than last year according to the ERA-40 reanalysis.
Winters following below average Arctic sea ice extent in late autumn, but without the extreme negative Barents/Kara anomalies seen in 2012 and 2016, have tended to have slightly more high latitude blocking than average, promoting more input of cold air masses. The caveat here, however, is that Arctic air masses in particular tend to be less cold, and also more strongly modified, as they approach the British Isles, when sea ice extent is anomalously low. The tendency for high pressure over Eurasia may be linked with the Warm Arctic Cold Continents pattern that has been a recurring feature of recent years.
The less extreme warmth of the Arctic means that any northerlies may be more potent during winter 2017/18 than they were during the winters of 2015/16 and 2016/17, but still rather less potent than in 2010 and in most years prior to 2005.
Most of the above analogues are pointing towards a weakly negative North Atlantic Oscillation and Arctic Oscillation but with a tendency for high latitude blocking to predominate over Scandinavia and Russia rather than around Greenland. Many forecast models broadly agree with this (CFS, Netweather, ECMWF) but the Met Office model is a marked exception, suggesting a positive AO and NAO with an enhanced, albeit not especially mild, westerly flow over the British Isles.
In contrast, the Met Office probability and ensemble mean maps for Dec 2017 to Feb 2018 are pointing towards a similar sort of winter to 2014/15, with an enhanced westerly flow over the British Isles but not an especially mild one, coming in off the persistent cold anomaly in the North Atlantic Ocean, and a positive NAO and AO. Slightly above average precipitation is also suggested but with a weak signal.
The Netweather long-range model is pointing towards a cyclonic/westerly December, with below average pressure to the north of Britain, but with a reduced westerly flow relative to normal over the British Isles in January and to a lesser extent February, with an emphasis on above average Eurasian blocking in January. Precipitation is, correspondingly, forecast to be above normal in December, below average in western Scotland in January, and above normal in the east in February.
The CFS model is broadly in agreement, pointing towards a positive NAO and AO in December and a mix of anticyclonic, westerly and south-westerly types, and then more blocking to the north in January and February promoting some easterly outbreaks, but also some milder interludes in between.
The long range forecast guidance from ECMWF is more in line with the majority of other signals, suggesting a negative AO and a near neutral to slightly negative NAO, and an emphasis on Arctic and Russian blocking. Pressure is projected to be below normal over Britain with temperatures near the long-term normal, and so rather cool for recent years but not remarkably cold. Interestingly the ECMWF long-range forecast points to above average chance of a cold winter over Siberia, suggesting less penetration of the westerlies into Siberia than usual, also pointing to blocking to the west of Siberia.
The CanSIPS forecast model is suggesting an anomalous north-westerly flow in December and then cyclonic/westerly weather in January and February, but this model has been fluctuating a lot in recent months.
The JAMSTEC model outlook resembles what I would expect given the Met Office sea level pressure outlook: slightly colder than average over Scotland, Ireland and parts of Scandinavia, much warmer than average over most of Eurasia, and wetter than average for most of Europe.
Most of the models are pointing towards a winter with an above average incidence of blocking to the north and east of Britain, particularly over Scandinavia and Russia. However, there is also some support for an enhanced westerly flow with low pressure in charge, including the Met Office seasonal forecast ensemble. Note that the models that go for a westerly winter still do not have particularly warm conditions over the British Isles, with the cold anomaly in the North Atlantic perhaps making its presence felt as per winter 2014/15.
In general the signal for westerly dominated conditions is strongest for December, but the current model outputs are suggesting a blocked start to December with frequent northerly and north-westerly winds. Thus we will probably see a change to more unsettled conditions as December progresses, though still with potential for further cold shots mainly for the north, most likely from the north and north-west rather than east, especially with the Madden-Julian Oscillation heading towards phases 7 and 8 which are correlated with blocking to the north and north-west of the UK.
For January we have much stronger support for eastern blocking and potential for some easterly incursions, and a continued suggestion of relatively cyclonic conditions. In that kind of setup we have to bear in mind that the UK often lies “at the end of the line” for easterlies and so sometimes we see very cold air masses accumulate over central and eastern Europe that never make it as far west as the British Isles.
For February the signal is weaker but this month is the most likely of the three to have anticyclonic spells, which could mean increased chance of the cold continental air making it to the British Isles at times, especially if we get a Sudden Stratospheric Warming event, but the lack of a strong signal for easterlies implies that it probably won’t be a particularly cold month overall.
Temperatures are most likely to be slightly below the 1981-2010 long-term average during this winter as a whole, but if we get notable Sudden Stratospheric Warming (SSW) events later in the season (which is currently difficult to predict at this stage) there is potential for it to be significantly colder than average.
December is likely to start off significantly colder than average, but in the second half of the month temperatures are expected to end up near normal in the south and below in the north, and so the month as a whole is most likely to be colder than average but not on a par with the exceptional December of 2010.
January and February are both most likely to have near average temperatures but in January there is 50% chance of cold weather establishing from the east for a significant part of the month. This will be most likely to happen if we see an SSW occcur early in the month, in which case we may end up with a very similar atmospheric situation to what triggered the prolonged snowy spell in the middle of January 2013.
For February, there is somewhat more uncertainty, but again it could become wintry should there be an SSW in late January or early February.
A wetter than average winter (but not exceptionally so) is suggested by the majority of the seasonal forecast model outputs, but some of the other signals, notably ENSO, are consistent with a drier than average winter for western Scotland. December is likely to come out with close to average rainfall with a dry first half cancelled out by a wet second half, but there is about a 20% chance of the second half ending up very wet, and also about a 30% chance of the wetter second half coming to little, leaving a fairly dry December overall. January is likely to be wet over much of England and Wales, especially the east of England (50-60% chance) but the month is likely to be drier than average in western Scotland. For February the evidence is more mixed but above average rainfall is most likely for eastern and central England.
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