Continue below for the in depth forecast and explanation of the factors which are expected to influence the weather 2020/2021 Winter
After a cool October and November with a southerly tracking jet stream and above average rainfall away from the north-west of the UK, the jet stream shifted northwards for the winter of 2020/21. December 2019 was mostly mild and unsettled but a couple of colder frosty interludes ensured that the mean monthly temperature wasn't far above average. January was a mild westerly-dominated month and largely snow free. February was a very disturbed "westerly" month with record high rainfall in many parts of the UK, with particularly high totals from Storm Dennis. Rather like in the UK's previous wettest February (that of 1990), frequent polar maritime incursions mid to late month meant that despite mild temperatures overall, the frequency of sleet/snow falling for February was relatively high in many parts of the UK, but snow on the ground was rare at low levels and short lived when it did occur.
The NMME ensembles are predicting a strong La Nina state during the winter, though with disagreement on its extent (between -0.5 and as low as -2.5), but with some hints of it weakening towards the end of the meteorological winter quarter. The Met Office ensemble set is agreed on a La Nina winter with ENSO between -1.5 and -2.5, while the ECMWF ensembles are a little more conservative, going for between -1 and -2 with a slight easing of the La Nina towards the end of the winter. Thus, there is high confidence that this will be a La Nina winter.
While it is quite common for weak La Nina winters to be cold and snowy (e.g. the winters of 2012/13 and 2017/18 were both cold and snowy by recent standards), strong La Nina winters have tended to be predominantly mild and westerly-dominated, as becomes apparent when looking at the composites going back to 1950. The closest ENSO analogues for 2020/21 are the winters ending in 1950, 1951, 1956, 1974, 1976, 1989, 1999, 2000 and 2011. A couple of exceptions stick out: the winter of 1955/56 had an unusually cold February with mainly easterly winds, possibly helped by the weakening of the 1955/56 La Nina by February 1956, and the winters of 1950/51 and 2010/11 both had very cold Decembers. The intensity of the cold weather in December 2010 was especially remarkable given the recent warming trend in the global and regional climate.
The mean sea level pressure anomaly chart for these composites for December-February gives the following:
This pattern translates to an increased westerly flow, rather veered from normal (such that the prevailing winds are westerly rather than west to south-westerly):
The "veered from normal" element comes from the Decembers, where there was a strong tendency for below average pressure over Scandinavia (anomaly centre -6mb) and higher than average pressure over the North Atlantic, suggesting a relatively high frequency of northerly and north-westerly winds. The Januarys of these years had a stronger than average Icelandic low and a slightly enhanced westerly flow over the UK, but only weak signals overall (maximum anomaly centre -2.5 west of Iceland). But the Februarys - despite the inclusion of 1956 in the composites - had a strong cyclonic/westerly tendency, with a mean pressure anomaly of about -6mb over Iceland.
Thus, the ENSO composites point towards a similar-ish January and February to what we had in 2020 (though probably not as extreme for wet and windy weather as February 2020 was), but with a fair chance of northerly and north-westerly winds featuring more frequently in December 2020 than was the case in December 2019.
The QBO (Quasi-Biennial Oscillation) is currently in a near-neutral phase. When it is in a strong easterly phase, this increases the chances of high latitude blocking and sudden stratospheric warming events in the winter months, whereas a westerly phase is correlated with mild, wet, westerly-dominated winters. The current trend is towards a positive QBO, which, if maintained, would increase the chances of winter 2020/21 being mild, wet and westerly-dominated, especially during the back end of the winter.
The Madden-Julian Oscillation is currently weakly in phase 3, and while there is some uncertainty in the forecasts, it may move towards phase 6 as we head towards mid-December. Phases 7 and 8 are correlated with high latitude blocking and an increased frequency of winds from the north and/or east, hinting at increased potential for northerly and/or easterly outbreaks during the latter part of December (more likely from the north given the ENSO composites). The MJO has relatively short-term predictive value so the impacts on January and February's weather are impossible to predict at this range.
The Arctic sea ice extent shows much below average extent over the Barents and Kara Seas to the east of Svalbard, and also between Alaska and Russia. This pattern has broadly been seen in every November from 2015 onwards, but by far the closest match is 2016. The winter of 2016/17 saw an anticyclonic December and January over the south of the UK with temperatures a little above average in December 2016 and a little below in January 2017, but it was mild and westerly-dominated in the north, and February 2017 was a mild month despite a rather feeble easterly in the second week.
The Met Office ensemble is going for a more "westerly" winter than average, which is also reflected by the predicted cold anomaly in the mid-North Atlantic and extensive warm anomalies over Eurasia, especially in Russia and Siberia. Also noteworthy is that the anomalous westerly flow is predicted to be stronger in Jan/Feb/Mar than in Dec/Jan/Feb, implying that March is projected to be somewhat more anomalously "westerly" than December. This was typical of many winters in the 1990s and early 2000s, with a relatively high frequency of northerlies in some of those Decembers, followed by an enhanced westerly flow over Eurasia during January, February and March.
The ECMWF ensemble mean outputs are remarkably similar to those from the Met Office, especially for predicted temperature anomalies. Again, an enhanced westerly flow is predicted over the UK but with an increased tendency for enhanced westerlies in Jan/Feb/Mar than in Dec/Jan/Feb, hinting at the likelihood of a rather less "westerly" December. It is interesting to note that both the ECMWF and Met Office models are not predicting an especially mild winter over the UK: this could be due to a relatively cold December, or alternatively a relatively high frequency of polar maritime air masses as happened in the westerly-dominated but only slightly warmer than average winter of 2014/15, or perhaps some of both.
The main difference between the ECMWF and Met Office model ensembles is in the distribution of precipitation anomalies. Both are suggesting a wetter than average winter, but the Met Office ensemble points to near average precipitation for the south-west and wet elsewhere, while the ECMWF ensemble suggests relatively near average precipitation in Scotland, and above average precipitation especially towards the south-west.
The Netweather.tv atmospheric circulation forecasts, based on the long-range CFS, are suggesting a fairly anticyclonic/westerly December, more southerlies than average during January, and near-average pressure in February, though these CFS-based forecasts haven't had the best of track records in recent seasons.
The long-range CFS based forecasts from NOAA give mixed signals for precipitation but there is general agreement on a wetter than average January, with December's precipitation signals rather more mixed. Two of the three sets of ensembles go for near average temperatures in December and February (the other one goes for mild temperatures) and there is general agreement on a warmer than average January.
The JAMSTEC seasonal forecast model also strongly agrees with the outputs from the Met Office and ECMWF, going for a wetter than average winter over much of north-western Europe combined with anomalous warmth over Eurasia, nearer average temperatures over the UK and colder than average temperatures across much of the North Atlantic Ocean and into Greenland. Again this is consistent with a dominant westerly type, but perhaps with a relatively high frequency of colder polar maritime air masses for the UK.
Based on current forecast model outputs, December looks set to start off fairly cold with low pressure over the south and some north and east winds. The longer-range signals suggest milder westerlies towards mid-December but strong potential for a relatively high frequency of northerly winds during the latter part of December. In general both the ENSO analogues and the long-range forecast models from the Met Office, ECMWF and JAMSTEC suggest a likelihood of a relatively cold December with some high latitude blocking at times. The cold weather early in the month is unlikely to bring widespread snowfall, with late December showing potential to be the snowiest time of the winter.
There are strong suggestions that January and February will be dominated by westerly winds, which will result in mild temperatures especially in the south. February has potential to have closer to average temperatures in the north and west of the UK with a stronger cyclonic influence, a mid-Atlantic ridge of high pressure and more in the way of polar maritime air masses. This could lead to a relatively high frequency of sleet/snow falling and some short-lived snowy incursions mainly in the north and west of the UK, but lying snow at low levels tends to be rare, and short-lived if it does happen, in the south in this setup.
For the winter quarter of 2020/21 as a whole, mean temperatures are forecast to be close to the 1981-2010 long-term normal over much of Scotland and Northern Ireland, typically with small positive anomalies of up to 0.5C. It will be a milder winter than average further south and east, with positive anomalies of 1.0-1.5C expected over the south-east of England.
Mean temperatures are forecast to be close to or just below the 1981-2010 long-term normal during December, with negative anomalies of between 0 and 1C over most of the country. It is expected to be milder than average around midmonth but with some colder weather early and late in the month.
For January and February, it is expected to be milder than average in the south-east with a likely positive temperature anomaly of around 2C in both months. February in particular has potential to have much closer to average temperatures over Scotland and Northern Ireland, with positive anomalies of less than 1C.
Overall there is fairly high confidence (about 70-80% chance) of a large majority of the UK having a wetter than average winter.
December will start off wetter than average in the east and drier than average in the west, but some changes of pattern are expected later in the month. Confidence in rainfall anomalies for December is relatively low (indeed, unusually, lower than for January and February), but the most likely outcome is for near-average rainfall for the UK as a whole, but tending to be drier than average in the north-west and wetter than average in the east.
January and February are forecast to be wetter than average, especially in the west of the UK. Coastal parts of the north-east are most likely to have close to or below average rainfall due to orographic enhancement in the west and rain shadow in the east being common features of westerly-dominated months. However, these same areas are also among the most likely to be wetter than average in December.
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