A roller coaster of temperatures is likely this month, as the jet stream alternates with periods of meridional and zonal flow, due to the volatile nature of the tropospheric polar vortex moving back and forth over Greenland and northern Canada and also later in the month, increased forcing from the tropical Pacific. This will affect the position, wavelengths and strength of the polar front jet stream over the North Atlantic, which could bring alternating milder and colder periods throughout the month.
This volatility in the jet stream will be evident in the 1st week, with mild southerly winds replaced by a colder northerly flow for a few days, before a strengthening zonal jet stream brings mild, wet and windy weather for a few days. The weekend 7-8th looks to see a return to colder but drier conditions, as the jet stream buckles again and winds turn northerly for a time, introducing arctic air, before high pressure builds in from the west into the following week, bringing increasingly dry but chilly conditions.
From mid-month, a return to a more zonal flow is favoured, with unsettled, sometimes wet and windy conditions, with further lows passing near/over the UK through to Christmas period and perhaps beyond to New Year. But there is a signal that jet stream may become more meridional late in the month, due to the MJO reaching the western Pacific by mid-month and increasing forcing poleward / +AAM , encouraging blocking high pressure to build either mid-N Atlantic or over Scandinavia, which could open the way for colder weather.
Probability for temperatures against 1991-2020 average: 50% chance close to average, 30% chance of above or 20% below average.
Probabilities for rainfall: 40% of chance for average, 30% chance of above average, 30% chance of below average.
A strong and zonal jet stream is favoured to dominate much of January, due to a signal for a coupled strong stratospheric and tropospheric polar vortex over Greenland. This leading to areas of low pressure often crossing west to east near or over the UK. As a result, it will often be unsettled with spells of wet and windy weather, with potential for two or more named-storms. With the flow often from the southwest or west for most of the month, a milder and wetter than average month is favoured for the whole of the UK. However, the jet stream may move south enough to bring colder air across Scotland occasionally to bring a risk of wintry hazards, but generally short-lived. A break in the strong zonal flow can't be ruled out, with a high pressure building, perhaps with a cold snap. This chance of colder conditions for all more favoured, if it happens, late in the month.
Probability for temperatures against 1991-2020 average: 60% chance above average, 30% chance of average or 10% below average.
Probabilities for rainfall: 60% of chance for above average, 30% chance of average, 10% chance of below average.
A similar theme to January is favoured through February, with a strong and zonal jet stream dominating northwest Europe for much of the month. This means often unsettled, with spells of wet and windy weather, as low pressure systems move over or near the UK. There is potential for the polar vortex to weaken or shift towards northern Scandinavia later in the month, perhaps as a result of a Sudden Stratospheric Warming, the chance of which look to have a greater probability than normal of occurring this winter, particularly later on (the reasons for this are discussed below). This could bring a greater chance of a spell or two of colder northwesterly, northerly or easterly flows, bringing a risk of wintry hazards, particularly for the north. But overall, milder-than-average and wetter-than-average conditions for the whole month.
Probability for temperatures against 1991-2020 average: 50% chance of above average, 30% chance of average or 20% below average.
Probabilities for rainfall: 40% of chance for above average, 40% chance of average, 20% chance of below average.
Continue below for the in depth forecast and explanation of the factors which are expected to influence the weather this Winter.
It's that time of year again when we try to figure out what to expect for the upcoming winter. A seasonal forecast at any time of year is always tricky. But the winter forecast is the most difficult because you have far more factors to consider than in other seasons. When creating a seasonal forecast for the warm season (Spring and Summer), the primary forcing mechanisms or drivers are from the tropical regions so that one can focus on the state of ENSO. However, in the cold season months, one must consider various factors outside of the tropical regime like the Stratospheric Polar Vortex (which is absent in summer), the transport mechanisms between the tropospheric mid-latitudes and the stratospheric high latitudes, movement of stratospheric winds, and how all of these factors interact with each other.
The winter forecast looks at a range of data including weather model seasonal forecasts, current and recent weather patterns in the northern hemisphere, tropical forcing over the Pacific, tropical stratosphere winds, solar cycle, and various teleconnections are considered. Based on some of the drivers that can be predicted to remain in a similar state through the season, we also use analog years where certain drivers were in a similar state in past winters too. We consider all this data to try and come up with an idea the most likely patterns that will prevail during each winter month. However, confidence in how the weather will behave more than a few weeks away is low, even more so two months or more away. So after a few weeks we can only give a broad idea of what patterns we think may dominate and whether it is more likely to be milder/colder or drier/wetter than average. We can't say what the weather will be doing specifically during a particular week more than 2 weeks away.
With global warming, a milder-than-average winter is becoming the form-horse, so forecasting any potential for colder conditions is becoming even more difficult.
2024 is on track to be hottest year on record and the first year of more than 1.5ºC above pre-industrial levels according to the ERA5 dataset.
It is now virtually certain that the year 2024 will be the warmest in the ERA5 reanalysis dataset, going back to 1940, based on the data available through October. The month was the second-warmest October globally, after October 2023, with an average surface air temperature of 15.25°C, 0.80ºC above the 1991-2002 average for the month. October 2024 was 1.65ºC above pre-industrial level, marking the 15th month in a 16-month period with average temperatures above the 1.5ºC threshold set by the Paris Agreement.
From Copernicus Climate
The North Atlantic, due to climate change, is widely anomalously warm away from the far north to the north of Iceland. Western Russia and eastern Europe is still yet to turn properly cold with extensive snow cover. All this background extra warmth will make it an uphill battle to get colder than average conditions across an island surrounded by warm waters. Northerly winds will blow, easterly winds too, so it will turn cold occasionally, but it is an increasingly uphill battle in recent years to get a below-average month.
The clues to build the winter forecast mostly come from trends in numerical models, atmospheric drivers and analogs. We develop our winter forecast based on these 3 main tools used together to best estimate pressure and temperature anomalies over Europe.
Why use all 3 parameters? Numerical models and analogs are regularly used for seasonal trends across all seasons. However, the atmospheric drivers generally become important in mid-autumn until March. Indeed, they are effective when certain patterns begin to clearly emerge that seasonal models issued monthly may not have picked up on, so it is therefore easier to identify changes in indices which could have a knock-on effect for the rest of the winter.
The C3S multimodel mean sea level pressure (MSLP) for each of the 3 winter months shows a classic +NAO pattern. Temperatures for all 3 months 0.5-1C above average. Precipitation average for England and Wales all 3 months, slightly above for Scotland.
Of the various seasonal model output - we'll focus on ECWMF, NCEP & UKMO. All 3 models are in generally broad agreement of lower pressure towards the north over Iceland and northern Scandinavia with high pressure to the south over western or central Europe and the Atlantic to the west. This generally points to +NAO often dominating the winter months with above average temperatures. However, there are some differences between models on certain months.
MSLP (above): Dec -- increased chance for NW flows at times. Jan -- increased occurrence of W-SW flows
Temperatures: above average all 3 months
Rainfall: average all months for most, NW slightly above average
MSLP (above) - Strong westerly flow all 3 months, with low pressure towards Iceland and high pressure over western Europe. Strongest +NAO signal of all models
Temperatures -- above average all 3 months by 1-2C.
Rainfall -- average over England and Wales all 3 months, above average western Scotland and N. Ireland.
MSLP (above) -- Dec -- higher pressure over much of Europe. Jan & Feb -- higher pressure over western Europe, lower Iceland and northern Scandinavia.
Temperatures -- above average all 3 months
Rainfall -- average all 3 months
The polar vortex is a large area of low pressure and cold air surrounding both of the Earth's poles. It always exists near the poles, but weakens in summer and strengthens in winter. The term "vortex" refers to the counter-clockwise flow of air that helps keep the colder air near the Poles. During winter in the northern hemisphere, the polar vortex will expand, sending cold air southward with the jet stream, which is often associated with large outbreaks of Arctic air being sent southwards into Europe, northern Asia and North America.
The North Atlantic Oscillation (NAO) is measured as the pressure difference between the Azores and Iceland and thus the strength of the westerly flow over the North Atlantic. If the NAO index is positive, the westerly circulation is stronger than usual. With a negative NAO index, the western circulation is weaker than normal, which increases the chances of cold winter weather in the UK.
During December the stratospheric Polar Vortex (SPV) is forecast to become anomalously strong for this time of year. This is generally not a good signal for cold weather in winter in the UK. However, the SPV is not always coupled with the troposphere, as appears to be the case as we head into the start of winter, so this continuing disconnect may mean there is potential for some cold snaps in December, before the strong SPV eventually couples with the troposphere and tends to drive a strong +NAO pattern until it weakens perhaps later in the winter, if a Sudden Stratospheric Warming (SSW) occurs. Though the SPV may not weaken until spring.
To some extent, although these are very weak signals, towards the end of December we see an increasing number of members opting for a steady weakening of the Vortex. But this is closer to normal rather than a potential for Sudden Stratospheric Warming. That is not the case for the time being.
But for now, a notable tropospheric polar vortex TPV anomaly looks to move in across Hudson Bay, eastern Canada and NE USA as December starts. And this TPV looks like it may persist in this area and Greenland for quite some time. How long is uncertain, but it could potentially last for a large chunk of the winter and so will be factored into the winter forecast, as the TPV over eastern North America and Greenland has ramifications for the upper flow patterns down stream over the North Atlantic and Europe.
A strong and persistent TPV over eastern North America, which brings anomalously cold weather to eastern USA, was present in winter 2013-14. That winter, this anomalous tropospheric feature drove a persistently strong jet stream in across NW Europe bringing disturbed, often windy and very wet weather -- leading to catastrophic flooding. That doesn't mean such a winter is on the cards this time, as other factors were also the antecedents to that winter's extreme rainfall -- such unusual conditions in the tropics and also unusually strong stratospheric winds.
The TPV over eastern North America, although, we think, a persistent feature this winter in the northern hemisphere flow patterns, will shift position, wax and wane. When the TPV shifts axis towards the NW Atlantic -- it will likely extend and strengthen the North Atlantic jet stream towards northern Europe -- bringing low pressure close to NW Europe and high pressure over central and southern Europe. When the TPV axis retrogrades back into eastern N America, the jet stream will likely retract and increase the chances of high pressure building over NW Europe or Scandinavia, while low pressure could develop over southern and eastern Europe. The timings of when this happens are very difficult to pinpoint for a whole season. But we feel this could be a general theme of alternating spells of either: +NAO with strong westerly flow and areas of low pressure bringing mild, sometimes wet and windy weather; or more settled spells with high pressure more in control, perhaps allowing occasional snaps of colder weather.
We have discussed how the polar vortex can influence the NAO and thus our winter weather. It has been known for some time that the temperature in the stratosphere has a major influence on the NAO. In the winter months there often appears to be a high degree of 'coupling' between the troposphere (the lowest air layer of the Earth) and the stratosphere (the air layer above it). A 'Sudden Stratospheric Warming' can occur most winters or every other winter which can weaken the stratospheric Polar Vortex through vertical wave movements occurring in the Northern Hemisphere during the winter. This is where movement of air from the troposphere reaches the stratosphere, causing air to accumulate in the stratosphere, causing warming and a high-pressure area in the stratosphere. As a result, the stratospheric Polar Vortex, which is normally very strong and with strong westerly winds in winter, is seriously disrupted, causing a reversal of stratospheric winds to easterly. We will discuss later how La Nina combined with the Quasi Biennal Oscillation (QBO) and maximum in the solar cycle and QBO can increase the chances of a SSW.
We are currently experiencing much higher sea surface temperatures (SSTs) than normal in the North Sea and almost the entire North Atlantic Ocean. Only north of Iceland there is an area with sea temperatures below normal. This situation with so much heat in the oceans adjacent to Europe has existed for a long time now. There is no clear correlation that this creates a different air pressure pattern in the Northern Hemisphere. What it does have an influence on is air temperatures, when the flow is off them during winter, meaning temperatures will more likely be a few degrees higher.
With a wind direction from the west or southwest and the east, the warm seas around ensure that cold airmasses that cross them will be moderated more while warmer airmasses, such as from the southwest, are warmer than normal. This is not a favourable for below average temperatures. Though it is worth noting that the colder SST anomalies around and north of Iceland may mean northerly winds have more 'bite' to them. At the same time, SSTs in the extratropical Atlantic aren't an important 'driver' of winter weather, as they do not couple to the atmosphere like the tropical Pacific SSTs do, but more about this later when discussing teleconnections.
Cold air tends to sink to the lower parts of the atmosphere near the ground. This causes an increase in air pressure at the ground. And if a high-pressure area is already present, the high is strengthened by the cold. This effect is most visible when there is snow somewhere. Large snow areas such as Greenland or Siberia often have high-pressure areas in winter that, as it were, reinforce themselves. The more snow cover, the stronger this effect.
To increase the chances of cold weather in northern Europe during winter, you would want the snow cover in the Northern Hemisphere - and especially above Russia and Eastern Europe - to be higher than normal. Unfortunately, there is currently less snow cover than normal in many places, especially in the source areas discussed that are important bring cold weather patterns to the UK. There is currently less snow than normal in Scandinavia, Eastern Europe and large parts of Russia. And that does not seem to be changing immediately.
Strong winter high-pressure areas from the source areas are therefore less likely to occur in the coming period. Once again, we should not ignore this parameter, but there are more important parameters. These are the so-called teleconnections, which will be discussed below.
Around the equator there is a jet stream in the upper air, which changes direction every so often (from 'westerly' to 'easterly' and vice versa). The Quasi-biennial Oscillation or QBO describes the wind component of this jet stream and its strength. If the QBO is negative, the jet stream is easterly. And if the QBO is positive, then the jet stream is westward.
We are currently in an extremely positive QBO phase -- which is expected to last through this winter. The QBO is not considered a major factor - but a westerly QBO does make it less likely we see prolonged blocking through winter. Also, research has shown that the QBO works together with other teleconnections, such as the ENSO and solar activity, which will be discussed below.
This is by far the best-known teleconnection, which many have heard of. The El Nino Southern Oscillation is a measurement of the deviation in sea water temperatures in the tropical Pacific Ocean, between South America and Asia.
We are currently in a cold neutral ENSO state and the ENSO is expected to remain cold neutral or verging on weak La Nina status through winter. However, it should be noted that La Nina, even a moderate one, was forecasted a few months ago to develop by now. But these forecasts were wrong and La Nina may not develop at all now. The anomaly in region 3.4 on October 23rd was -0.5C, with indications at the time a weak La Niña was developing. But on November 20th, the anomaly has warmed to -0.1C. This is likely down to a zonal/westerly wind pattern across the tropical Pacific through much of November. easterly trade winds that upwell colder waters absent, and so La Niña has failed to develop so far. However, there are signs from models that the zonal winds may subside through December, which may allow the tropical Pacific too cool sufficiently to allow a weak La Nina to develop.
ENSO forecasting by models at range can be difficult, so model seasonal forecasts based on a La Nina developing may now be unreliable for the winter outlook. La Nina, especially strong ones that are central rather east Pacific based tend to favour +NAO during winter. So, there is, perhaps a greater potential for colder weather this winter due to neutral ENSO or weak La Nina climatology coming into play.
La Niña occurs when below-average sea surface temperatures (SST) develop in the central and eastern tropical Pacific Ocean. Over Indonesia, rainfall tends to increase, leading to more thunderstorm activity here, while rainfall decreases over the central and eastern tropical Pacific Ocean. The normal easterly winds along the equator become even stronger. In general, the cooler the ocean temperature anomalies, the stronger the La Niña. El Nino is the opposite of La Nina - with above-average sea surface temperatures (SST) in the central and eastern tropical Pacific Ocean. Over Indonesia, rainfall tends to become reduced while rainfall increases over the central and eastern tropical Pacific Ocean.
The rising air in association with the enhanced thunderstorms of where the warmer waters are during El Nino or La Nina, in turn 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.
A neutral ENSO is when tropical Pacific SSTs are generally close to average. However, there are some instances when the ocean can look like it is in an El Niño or La Niña state, but the atmosphere is not playing along (or vice versa). Research shows that the ENSO, in collaboration with the QBO, has a major influence on the chances of stratospheric warming occurring during the winter. In the event of a La Niña in combination with a +QBO decreases the chance of an SSW.
The expectation for winter 2024/2025 is that we are on the edge between a neutral phase and weak La Niña. Because the limit value for La Niña is -0.5C and it remains to be seen whether we will achieve that. What is quite certain is the +QBO we are dealing with. And as number 3 shows above, if La Niña does develop, it in combination with a +QBO decreases the chance of an SSW.
If a La Niña does occur, the chances of colder winter weather regimes in Europe are highly dependent whether the location of coldest SSTs are in the eastern tropical Pacific based or a central tropical Pacific.
In an eastern based La Niña, the centre of negative sea surface temperature anomalies is close to the coast of Peru. During a central-based La Niña, the seawater areas with the most negative deviation are located further out in the Pacific Ocean, more towards Asia.
A La Niña with an eastern focus increases the chances of a negative NAO regime in Europe. This increases the chances of colder winter weather.
On the other hand, a La Niña with a central focus actually creates greater chances of a positive NAO index, which reduces winter chances in Europe. (With an El Niño, all of the above is reversed.) The expectation is that the La Niña - if it arises - will have an eastern center of gravity -- which increases the chances of a negative NAO index, especially in January and February.
However, given the uncertain over whether the La Niña will occur, with the model forecasts of an impending La Niña being postponed repeatedly, for the time being there appears to be low confidence in this signal.
We are still near a solar maximum expected to peak in 2025. There is a link between low solar sun spot activity around the solar minimum and increased incidences of prolonged blocking. It's happened the last 5 cycles in 1977, 1987, 1996, 2010, 2021. But even then, the blocking came either around the solar minimum, but sometimes it was the year after the minimum. However, the correlation during a solar maximum is fairly low, so solar will not be weighed in the analogs.
But the sun's activity can contributes to our chances of winter weather. More specifically: the number of sunspots (SSN) interacts with the QBO and increases or decreases our chances of stratospheric warming.
The expectation for winter 2024/2025 is that we will have a pronounced high solar activity, in combination with a positive QBO. This increases the chances of an SSW. This combined with a possible La Nina/wQBO indicates the potential for a SSW is fairly strong.
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 is currently over the Maritime Continent between the Indian Ocean and Pacific Ocean (phase 4 and 5). There is potential that the MJO may make it to the western Pacific, most recent forecasts increasingly indicate this, so this could increase the chances of blocking high pressure in December, particularly during the second half of the month.
The Indian Ocean Dipole (IOD) index. The effects of this have only been properly mapped out in recent years. The IOD is the counterpart of the ENSO in the Indian Ocean. This also concerns SST differences between two areas of one ocean, this time, the Indian Ocean. In a positive IOD phase, the SSTs off the African east coast are warmer than normal, while the sea water on the other side of the Indian Ocean (near Indonesia) is colder than normal. In a negative IOD phase, the roles are exactly reversed.
The effects on winter weather patterns are as follows:
- With a positive IOD index, the chances of northern high-pressure blocks decrease and thus the chances of cold weather in Western Europe are lower.
- With a negative IOD index, the chances of northern high-pressure blocks increase and the chance of cold weather in Western Europe increases. The limit value to qualify an IOD as 'positive' or 'negative' is +0.5 and -0.5C respectively.
For winter 2024/2025, the IOD is expected to be negative initially (at the time of writing), but to gradually trend towards neutral. Due to the negative IOD index at the beginning of winter, there is, we think, a greater chance of high pressure blocks over northern Europe. Later in the winter the IOD will no longer have any influence.
Not a driver that will be used beyond a month ahead and even a few weeks ahead can be subject to change along with the constraints of limited data on AAM available. But Atmospheric angular momentum (AAM) is a complicated subject to discuss in layman's terms, so I will try to make it simple. Basically, it is a measure of the rotation of the atmosphere around the Earth's axis, is a useful quantity to investigate changes in the global atmospheric circulation.
At the moment, AAM is high due westerly momentum in the atmosphere, which is atypical of La Nina and more typical of an El Nino background state, but also encouraged by the MJO moving through the Maritime Continent inducing frictional and mountain torques. There is uncertainty whether the MJO will fade over the maritime continent before reaching the Western Pacific, but there is potential models maybe underestimating the progression, so AAM may stay high through December, allowing greater potential for high latitude high pressure blocking that can increase the chances of cold spells.
For winter 2024/2025, we arrive at the following backgrounds that apply to the current situation:
Winter 2024/2025 has - based on current teleconnections and expectations discussed above - the following signals:
Cool Neutral ENSO winters since 1970 based on Oceanic Nino Index (ONI) are 1981-82, 1985-86, 1996-97, 2001-02, 2012-13, 2013-14, 2016-17.
Weak La Niña winters since 1970 based on ONI are 1974-75, 1983-84, 1984-85, 2000-01, 2005-06, 2008-09, 2016-17, 2017-18, 2022-23
Cool neutral or weak La Nina years with wQBO 1985-86, 2001-02, 2008-09, 2013-14, 2022-23. This produces a 500 hPa geopotential composite of those analog years as below for January and February. Will not use the composite of analog years for December, as using current model output is likely to be more accurate.
What can be gleaned from the composite of best-fit analog years for January and February is low geopotential heights over the UK being the common theme, but the anomaly shifting south in February compared to January. So stormy in the means both months, but the storm track shifting south in February.
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