Winter Forecast Headlines

• Overall, temperatures are forecast to be slightly below the 1981-2010 long-term average for winter 2021-22.
• December expected to be around the long-term average temperature-wise, January and February slightly below the long-term average.
• Likely to be average rainfall overall, December likely the wettest month with slightly above average rainfall, January around average, February slightly below average.

Continue below for the in depth forecast and explanation of the factors which are expected to influence the 2021/2022 Winter.

ENSO (El Nino Southern Oscillation)

ENSO arises from changes in wind direction and sea surface temperatures in the tropical Pacific which in turn affects the climate of the tropics and sub tropics. When El Niño occurs, the central and eastern tropical Pacific Ocean becomes warmer than average. This leads to more thunderstorm activity here. The rising air in association with this enhanced convection in turns strengthens the Hadley Circulation – where air rises near the equator, spreads towards the poles, then sinks back to the surface in the subtropics. This changes the position and strength of the jet stream in the mid-latitudes which in turn affects weather patterns in North America and Europe.

Unlike El Nino – which shifts enhanced thunderstorm activity from the western tropical Pacific to the central tropical Pacific, La Nina shifts thunderstorm activity away from central Pacific to the western Pacific, as warm waters shift here and cool over the central Pacific. This strengthens the Hadley Circulation over the western Pacific and Maritime Continent (La Nina) rather than central and eastern Pacific (El Nino) – this in turn affects the jet stream residing near Japan and over the North Pacific – which in turn can affect weather patterns across North America and Europe.

La Nina conditions have developed this autumn, with cooler than normal SSTs in the central and eastern tropical Pacific, with La Nina forecast to continue with a 90% chance during the 2021-22 winter. This will be the second winter in a row with La Nina conditions after ENSO went into neutral conditions during spring and summer this year. It is quite common for La Nina to occur in consecutive winters (but not El Nino).

The strength of La Nina can affect the strength and position of the jet stream, troughs and ridges across the North Pacific, North America and downstream across the North Atlantic and Europe. So, there can be a wide range of variability in the weather experienced in the northern hemisphere winter depending on the strength of La Nina. Typically, a weak La Nina favours a more-wavy jet and thus higher incidence of high latitude blocking in the northern hemisphere during winter, with a greater chance of cold weather for the UK. While a strong La Nina favours a strong and zonal jet stream with a +NAO (North Atlantic Oscillation) that brings mild weather to the UK.

However, the position of the La Nina cold SST anomalies in the tropical Pacific can also cause variability in the upper patterns over the northern Pacific and subsequently downstream over North America and the North Atlantic. A large high-pressure system over the North Pacific, known as the Aleutian High, is a common feature during a La Nina winter. An east-based La Nina, where coldest SSTs anomalies are towards the eastern tropical Pacific, can favour a more poleward Aleutian high which extends into polar regions at times and encourages high latitude blocking in the northern hemisphere. Also, an east-based Nina favours a weaker sub-tropical jet which in turn favours a weaker North Atlantic jet stream. Whereas central (or Modoki) based La Nina, with cold SSTs over the central tropical Pacific favours a more flatter Aleutian ridge, which favours lower higher latitude heights and stronger likelihood of a +NAO with strong N Atlantic jet. However, a La Nina can move from an east Pacific based to a central Pacific based event during the winter – which can have ramifications for how the winter patterns develop and this may make forecasting the winter more difficult.

Weak La Nina years often feature an Aleutian High that builds poleward, also Ural and Scandinavian high, cross polar link up of height rises weakening the polar vortex.

A weak La Nina is forecast to persist through the winter, so we can perhaps expect a greater chance of blocking with cold outbreaks than with a strong La Nina or strong El Nino. There are exceptions though, the cold 2010-11 winter featured a strong La Nina, so the theory that strong La Nina equals a mild +NAO winter doesn't always ring true. This is likely to do with the La Nina cold SST anomaly structure across the tropical Pacific basin, i.e. where the cold anomalies are – as mentioned above. In the winter of 2010-11, La Nina was east-Pacific based rather than central as in the case of other strong La Ninas winters that have featured little high latitude blocking and were mild overall. It looks like the coldest anomalies through much of the coming winter will be basin-wide initially before moving towards the eastern tropical Pacific with time.

ENSO can be forecast fairly-well over the whole winter and longer. But ENSO is just one of various other climatic drivers that can affect the weather variability we experience this coming winter. The other drivers, which will be discussed, can work in tandem to increase the chances of a cold or mild winter or work against each other to make it less certain what may happen, depending which phases they are in.

The Quasi Biennial Oscillation (QBO)

The QBO manifests itself as downward-propagating easterly or westerly mean zonal winds in the equatorial stratosphere in a 22 to 28-month cycle. The driving force for the QBO is the vertical transfer of momentum from the troposphere to stratosphere by a broad spectrum of vertically propagating waves including Kelvin and Rossby waves.

Why is the QBO important to our forecasting our winter weather? It can be useful guide to seasonal prediction, as it has been widely recognised that the QBO can impact the Northern Hemisphere winter stratospheric polar vortex through altering planetary waves the propagate poleward and may also affect the tropical troposphere directly and possibly how the solar cycle interacts with the atmosphere.

Right now, the QBO is descending in an easterly or negative phase. Although there is no clear direct link between QBO phase and its effects on surface weather patterns, the easterly or negative phase of the QBO is associated with more a poleward shift of the sub-tropical jet leading to an increase in Rossby wave-breaking events over Eurasia. This leads to a weakening and shifting of the polar vortex and a slowing down and weakening the northern hemisphere polar jet stream, thereby causing a meandering jet stream leading to cold arctic outbreaks over Europe. A westerly QBO, conversely, leads to a stronger polar vortex in the stratosphere.

An easterly QBO is expected through the coming winter, so there is a greater probability of a weakened and displaced Polar Vortex occurring this winter which, as suggested above, may lead to greater probability of cold outbreaks affecting the UK and other parts of Europe this winter than would be the case with a westerly QBO. However, reanalysis of previous eQBO winters show that a weakened and displaced stratospheric Polar Vortex (SPV) is more likely during January and February than in December and that the SPV is displaced towards Eurasia.

Madden-Julian Oscillation (MJO)

The MJO is a large-scale coupling between atmospheric circulation and tropical convection. It differs from ENSO in being a travelling pattern across the warm tropical oceanic areas of the globe rather than the standing pattern of El Nino / La Nina over the tropical Pacific. It is characterised by an eastward progression of large regions of either enhanced or supressed tropical rainfall, this anomalous rainfall area is mostly evident over the western Indian Ocean, then the warm tropical western and central Pacific as the MJO progresses eastward. The MJO wave of enhanced or supressed tropical rainfall is usually less evident when it moves over the eastern tropical Pacific and tropical Atlantic. The enhanced areas of rainfall or convection are followed by a dry phase when convection or thunderstorms are supressed – as the MJO wave propagates east. Each cycle of the MJO lasts about 30-60 days.

The MJO affects the Indian and Monsoon, plays a role in the onset of ENSO events, has an impact on tropical cyclogenesis but also has more far-reaching impacts on northern hemisphere extratropical weather through Rossby Wave propagation. The Rossby Wave propagation depends on the longitude of where the enhanced convection associated with the MJO wave takes place. As the MJO has a significant impact on northern hemisphere weather patterns, including the North America, Atlantic and Europe, there are correlations that can be made between the 8 different phases of the MJO and the lagged impacts on the upper air patterns that might be expected over the North Atlantic and Europe based on composites of previous events. The impact of the MJO on European weather is strongest about 10 days after the MJO is in phase 3 or 6, the probability of a strong +NAO is increased about 10 days after the MJO is in phase 3, but decreased 10 days after phase 6, with a probability of a -NAO increasing after MJO has passed through phases 7-8. However, the impact of the MJO on North Atlantic and European weather patterns depends on the strength of amplitude of the MJO, if the MJO is weak then there is less impact.

As the MJO cycle last 30-60 days, a rough prediction can be made on when it may reach certain phases based on which phase it is propagating through currently, though often in La Nina the MJO is rather muted, so forecasting more than a month ahead can be tricky.

MJO Phase 7 lag effect on 500 height in December

The MJO is currently rather weak in the Maritime Continent, phases 4, the MJO forecast to strengthen and propagate through phases 6 and 7 in early December. This progression can lead to an increase in high latitude blocking in early December – particularly towards Scandinavia. But as the MJO can only be predicted on a relatively short time scale of month at best, and in La Nina it’s often weak as now, its impacts on January and February’s weather are impossible to predict at this range.

Solar Cycle and North Atlantic Oscillation (NAO)

We entered the Solar Minimum in 2017, but as of December 2019 we entered solar cycle 25 and are now rising out of the solar minimum with sunspot activity increasing to a peak around 2025. Solar cycle lasted 11 years, the average for cycle, though was the weakest in terms of activity in 100 years. Solar cycle 25 is equally predicted by scientists to be quiet and weak, with a below average number of sun spots. Sunspot number has increased over the past year; however, the Sun’s geomagnetic field remains very quiet.

In general, given the lag of the effects of reduced sunspot activity, years near and shortly following a solar minimum tend to have a weaker stratospheric polar vortex and are more prone to blocking, with the opposite true in solar maxima winters. As we are just emerging from a solar minimum and the lag effects of it, the increase solar activity is unlikely to play a major role. The cold winter of 2010-11 featured a lot of high latitude blocking and, like this winter, that winter was emerging from a solar minimum with an increase in activity while was also a La Nina winter. The winter of 2017-18 also featured some blocking, when we were entering the most recent solar minimum.

A study by Boberg and Lundstedt in 2002 presented a significant correlation between the electrical field strength of the solar wind and the temporal evolution of the NAO. NAO is a time-averaged pressure differences between regions representing the two pressure centers. A positive NAO index phase is characterized by a strong subtropical high pressure centre and a deep Arctic low pressure centre. During this phase, more and stronger storms cross the Atlantic from southwest to northeast resulting in warm and wet conditions in northern Europe while the northwest Atlantic and southern Europe experience cold and dry conditions. A negative NAO index phase has a weak subtropical high and a weak Arctic low pressure region. As a result of the reduced pressure gradient fewer and weaker storms cross the Atlantic with tendency for blocking highs to form at high latitudes. This phase brings moist and warm air to southern Europe and to the northwest Atlantic while northern Europe experiences cold and dry air.

The graph below suggests a relationship between the NAO index and the electric field strength E of the solar wind. A possible scenario for the suggested interaction is that an electromagnetic disturbance is generated by the solar wind in the global electric circuit of the ionosphere. This disturbance is then dynamically propagating downward through the atmosphere and subsequently influencing the large-scale pressure system in the North Atlantic region. A relationship is also evident on longer timescales when using the group sunspot number as a proxy for the solar wind.

Although we are coming out of a solar minimum, solar activity remains low. The KP-index measures the disturbance of the Earth's magnetic field caused by the solar wind. The faster the solar wind blows, the greater the turbulence. The KP index has been hovering below 3 - which is considered to be weak.

With the weaker than normal electric field strength of the solar wind, as we head into winter, it suggests an atmosphere that continues to favour the development a negative NAO at times this winter, conducive to northern blocking and cold weather for the UK at times.

Stratosphere

Way up in the stratosphere, it is normally very cold, but there are times when it "warms up" relatively speaking. Stratospheric warming occurs when atmospheric waves from the troposphere move into the stratosphere. As these waves move upward into the stratosphere they will often push the polar vortex (PV) away from the North Pole. This brings warmer, midlatitude air northward, and cooler, arctic air southward. A downward motion field also causes the poles to warm further.

As these waves enter the stratosphere, it can warm the stratosphere and causes the mean flow to decelerate, weakening the Polar Night Jet (causing the PV to weaken or split). With the weakening of the polar night jet, its winds then begin to converge towards the centre and descend, resulting in warming due to compression. This event is known as a Sudden Stratospheric Warming (SSW). A SSW is technically defined to have happened when the mean zonal wind at 60°N and 10hPa (approx. 30km high) reverses to easterly. Once winds shift east, ridging patterns are usually evident in the mid-upper latitudes. Disruption of the tropospheric PV continues, which allows cold air to be displaced from high latitudes and spill south. Stratospheric warming is often associated with cold outbreaks, which generally last for weeks.

Stratospheric warming events have a tendency to occur earlier in the winter if we have a -QBO and low solar radiation like this winter. However, there is no imminent signs of a warming showing in the model. In fact the stratospheric polar vortex is stronger than average as we enter winter. So, we may be looking at late December at the earliest if a warming occurs. Then it can take anything between 2 to 6 weeks for the impacts of the SSW to work its way down 30-50km up and impact the stratosphere. Unfortunately SSW cannot be forecast well more than a few weeks ahead.

Northern hemisphere snow cover

Snow coverage over NE Europe is more extensive than this time last year. In fact, it’s similar to this time in November 2010 – which featured a very cold second half to the month. Arctic sea ice is also more extensive over the Kara Sea, Svalbard archipelago and adjacent Arctic sea than this time last year. Generally good snow coverage over Russia and NE Europe too. So, some positive signs in this respect.

Current snow cover for Eurasia

This time last year

Indian Ocean Dipole (IOD)

During positive phase of the Indian Ocean Dipole (IOD)- Westerly winds weaken along the equator allowing warm water to shift towards Africa. This sets up a temperature difference across the tropical Indian Ocean with cooler than normal water in the east and warmer than normal water in the west. During the negative phase, the reverse happens - with warmer than normal water in the east and cooler than normal water in the west.

The persistent and strongly positive IOD in 2019 has been suggested by some papers to have been a key driver in the anomalously positive NAO that was responsible in bringing an exceptionally warm and wet winter in 2019/20 across the UK and northern Europe.

The IOD has recently been weakly negative but is forecast to become neutral through winter, as would be expected as the monsoon shifts south and trade winds change. So, it is likely the IOD will have little impact on this winter’s weather patterns.

Analogs that closely match the likely phases of seasonal drivers combined this winter

This winter will likely feature:

• A weak / moderate east-based La Nina
• Descending eQBO
• Low solar activity coming out of a Solar Minimum

The years in the table below most closely match the 3 main drivers of QBO, La Nina and solar activity:

Matches to the drivers likely during Winter 2021-22
Winter	QBO	La Nina	Solar Activity
1954-55	Easterly	Weak	Min April 1954
1964-65	Easterly	Weak	Min October 1964
1974-75	Easterly	Weak	Min March 1976
1995-96	Easterly	Moderate	Min July 1996
2007-08	Easterly	Strong	Min December 2008
2020-21	Easterly	Moderate	Min December 2019

Below are the 500mb height anomaly composites for December, January and February and analog years.

December composite features a meridional pattern with anomalous ridging over central N and NW Atlantic, troughing over NE Atlantic and NW Europe, ridging over W Russia.

January composite features northern blocking with positive heights centred over Labrador and NW Russia, low heights over Azores and western Europe.

February composite features northern blocking centred over Iceland, low heights Azores through into mainland Europe.

Based on the analogs, December features Atlantic ridging but a generally unsettled look for the UK with Atlantic lows moving NW to SE, with potential for some northerly outbreaks. January and February certainly a cold and wintry look in the mean, with northern blocking and low pressure close to the south and southwest of the UK.

If we took out the eQBO match from the analogs, then the infamous cold winter of 2010-11 would be a good match. Although wQBO was present that winter, it was La Nina, albeit a strong one and we were on the early rise out of a solar minimum.

Global warming

Taking in all the above, it generally points to a rosy picture for a colder than average winter, backed up by analogs of past cold winters that are a similar match with regards to climatic drivers. However, the Earth’s surface continues to significantly warm, with recent global temperatures being the hottest in the past 2,000-plus years. The past six years have been the warmest on record since 1880, with 2016, 2019 and 2020 being the top three, according to a World Meteorological Organization (WMO).

This background warming means warmer than average periods are outnumbering colder than average. September, October and, so far, November have seen temperatures above average and with North Atlantic SSTs anomalously warm basin-wide this autumn, it may moderate any cold polar or arctic incursions from the north or northwest early in the winter, while western Europe may take longer to cool down with an anomalously warm source upstream. With all the anomalous warmth building up through the summer and lingering into autumn, cold weather has often been delayed until the New Year in recent years, with December more often than not mild. So, it will take some pretty cold incursions to overcome this, particularly with the warm Atlantic this autumn. Continental arctic airmasses from the east or northeast probably least likely to be affected by the anomalously warm Atlantic.

What the NWP model long-range seasonal forecasts suggest

In contrast to what the drivers seem to be pointing towards. i.e. a greater chance of northern blocking this winter and thus slightly colder than average temperatures – based on seasonal drivers mentioned above - the long-range seasonal output from ECMWF, GFS/NCEP, UKMO and Meteo France suggest +NAO +AO and mild this winter in the means.

Putting it all together

La Nina conditions developed in the autumn and are likely to continue through the winter, with a 90% chance, according to the NOAA Climate Predicition Center. The cold SST anomalies of the ENSO region are basin-wide as we enter winter, but there are signs that they will become more east-based with time. Historically, La Nina has often led to a front-loaded winter for colder weather. However, with La Nina becoming east-based – it would suggest the colder part of winter could be in the second half, so a back-loaded winter or generally evened out with equal chances of cold episodes between milder episodes this winter.

The winter looks to start with a strong stratospheric Polar Vortex, above average in strength. So far, the strong SPV hasn’t coupled with the troposphere, with an episode of blocking over the Atlantic in late November testament to this. However, model forecasts indicate strong lower stratospheric polar vortex over the pole, northern Canada and Greenland may couple with the troposphere as we head through early December, which will likely lead tend to lower heights over the North Atlantic and a strengthening jet aimed at Europe. But equally height rises are indicated to develop and persist over NE Europe and NW Russia.

The descending easterly QBO would point to stress on the SPV with time, weakening it and shifting it off the pole. When this may occur and where it may shift is uncertain, but likely we will see a strong SPV through December into early January - but which may weaken and displace as we head through to late January. Sudden Stratospheric Warmings tend to occur earlier in the winter with a -QBO and low solar activity.

There are no signs of this occurring in December, with the SPV remaining strong. However, there are hints recently of precursors to a warming, with ensemble guidance indicating Scandinavian/Ural blocking and more poleward fluxes of the Aleutian high - which is more typical with La Nina forcing, than the recent Aleutian low dominance. This poleward wave-driving may be aided periodically by East Asian Mountain Torque events and associated increase in Atmospheric Angular Momentum (AAM) – though these can’t be predicted over long time-leads so are limited as a tool in seasonal forecasting.

These areas of positive heights driving tropospheric waves poleward into the polar stratosphere may weaken the stratospheric polar vortex early in the New Year. So, we could see a SSW in January, at the earliest – however, these events are impossible to predict more than two weeks ahead. If one does occur, the downwelling impact on the troposphere would have a lag of a few weeks and would impact February’s weather.

With low solar activity still, as we start to rise from the minimum of Solar Cycle 25, plus the tendency for colder winters to occur as come out of a minimum (e.g. 2010-11 winter), it favours periodic episodes of -NAO. These maybe a struggle to attain for prolonged periods in December with the SPV coupling with troposphere and the tropospheric vortex becoming strong to the northwest. But if the Scandinavian/Ural blocking persists and perhaps extends to Greenland later in December, with help of MJO forcing, then we could see a brief -NAO period in December. But it’s not anticipated that the SPV will remain strong in the New Year through to the end of the winter, so longer -NAO episodes appear possible in later in January and February. These episodes aided by changes in northern hemisphere upper patterns from forcing from the tropical Pacific as La Nina becomes more east-based, also increasing impact as we head through winter from the descending -QBO.

The winter forecast by month:

December

Temperatures are expected to finish around average. The month will see episodes of mild and cold weather. The first half of the month will likely see a battle between a blocking high pressure system building between Scandinavia and the Urals (with potential to extend west) and a strong tropospheric polar vortex wobbling back and forth between northern Canada and Greenland. A strong jet stream looks to push east across the North Atlantic towards Europe in the first few weeks of December. However, the jet stream may be forced southeast by a growing blocking high to the northeast over Scandinavia, perhaps allowing colder conditions across to spread south at times, depending on how far south the jet tracks. But milder conditions may win out as equally as colder conditions. Potential for a more prolonged spell of colder weather from the east appears possible in the second half of the month, as the lag effects of the MJO moving into more favourable phases of 7-8 early in the month start to have an effect. With the jet stream nearby much of the time, bringing low after low, rainfall is likely to be above average.

January

January perhaps starting off with a strong TPV dominating - with a +NAO / +AO bringing mild and unsettled conditions, bolstered by the MJO moving into milder phases. But we may see the stratospheric polar vortex weaken and become displaced as we head through January, due to stress on it from the descending -QBO and possible favourable tropospheric forcing from poleward wave driving of recurrent poleward Aleutian high extension from increasingly east-based La Nina forcing, MJO forcing, along with continued Ural/Scandinavian blocking. This may even lead to a SSW mid-late month – but this cannot be predicted with any confidence. So, we may see an increasing chance of blocking and a -NAO episode mid to late January - with an increased potential for a cold spell, lasting longer than in December. The month finishing slightly below the long-term average. Rainfall around average.

February

Anticyclonic conditions featuring a lot this month, with a weakened and perhaps displaced stratospheric polar vortex coupling with the troposphere leading to northern blocking / -NAO episodes for the first half of the month at least, perhaps lingering into the second half of the month if a SSW occurs in January. This means colder than average conditions on-the-whole, perhaps milder towards the end of the month, as the SPV rebounds back to the pole - with a shift to a +NAO / +AO pattern - but with anticyclonic conditions rather than low pressure dominating. Temperatures likely to be slightly below the long-term average overall. Rainfall slightly below average.