A look at the very latest Winter 2024/2025 forecast data shows the growing influence of a weak La Niña phase. Seasonal prediction systems are slowly aligning and starting to point to a specific weather scenario for the upcoming Winter season, but past forecasts issued in September were often too warm over the United States.
The global weather system has many large-scale and small-scale driving factors. The main factor this year will be a weak La Niña event in the Pacific, but we also expect the Polar Vortex to play a larger role in mid-Winter.
>>This article was updated on September 23rd, 2024, with the latest analysis data and forecast graphics, that were issued this month.<<
COLD LA NINA GROWING
La Niña is a cold phase of the large and powerful oceanic ENSO oscillation. It is a region of the tropical Pacific Ocean that shifts between cold and warm phases. The cold ENSO phase is called La Niña, and the warm phase is called El Niño.
These ocean temperature anomalies (warmer or colder) occur in specific regions of the Pacific Ocean, as shown in the image below. The main region is 3.4 and is also the main area where we determine what phase is active and how strong it is.
Below is the latest mid-September ocean anomaly analysis. It shows colder-than-normal surface waters in the central and eastern ENSO regions. These cold anomalies have a “wave-like” shape. This is because of the strong easterly trade winds that push the waters towards the west, creating swirls on the ocean surface.
These cold anomalies are the signs of the next La Niña phase emerging onto the ocean surface.
Below is an analysis/forecast image from NMME, which shows the ENSO ocean temperature drop as the warm phase ended in spring. Negative anomalies and cooling are forecast over the Autumn and Winter of 2024/2025. The forecast average is within the La Niña threshold but shows a weak to moderate event.
The image above shows that even in early spring, the ocean anomaly was still above normal. Below is a high-resolution video animation that shows how the ocean went from warm to cold anomalies across the ENSO regions in the tropical Pacific.
Below is the ocean temperature forecast for early Winter from the latest data, and it shows that a La Niña event is active. It is not a strong and will likely become a moderate event at best. But as you will see in the winter pressure patterns, it will leave its mark on the jet stream position.
This data gives us enough confidence that we will see some influence from these anomalies over the coming cold weather season. The question now is how exactly La Niña changes the winter weather patterns?
WINTER AND THE POLAR JET STREAM
Several winter seasons have had an active La Niña phase in recent decades. This allows us to analyze past data to see how the winter weather patterns usually develop under a La Niña.
Overall, each ENSO phase significantly influences tropical rainfall, pressure patterns, and the complex energy exchange between the ocean and the atmosphere. The image below shows the circulation pattern of La Niña and its connection to the ocean and the atmosphere.
This way, the ENSO significantly impacts tropical rainfall and pressure patterns, altering the atmosphere-ocean feedback system. The ENSO influence is spread globally through this system, changing the Winter temperature and snowfall patterns over the Northern Hemisphere.
In essence, you could also say that these ocean anomalies act like lights on the dashboard of a car. So, they not only have their own influence but can also show us a lot about the overall state of the atmosphere.
La Niña usually creates a high-pressure system over the North Pacific. That promotes the development of a low-pressure region over Alaska and western Canada and shifts the jet stream downwards in between the two pressure systems.
The image below shows the shift of the jet stream into the northern United States. It shows the average position of the jet stream during La Niña winters and the resulting weather patterns over the United States and Canada.
The displaced jet stream brings colder temperatures and winter storms from the polar regions down into the northern and northwestern United States. Warmer and drier winter weather prevails over the southern states.
In the northern part of the country, colder and wetter events are more frequent, as the jet stream directs the storm systems that way. But, this can lock out the southern United States from cold air, creating warmer and more stable weather with fewer storms and cold fronts over the south.
Colder air is more easily accessible to the northern United States, increasing the snowfall potential when moisture is available. The image below shows the average snowfall pattern for weak La Niña years, as expected for this Winter season.
In addition to the northwestern United States and the Midwest, there is increased snowfall potential over the northeastern United States and eastern Canada. This image is typical for a weak La Niña event, but every year has its own variations.
So, as we now know what to expect from the winter patterns based on past weather alone, we will continue to the actual Winter 2024/2025 predictions.
WINTER 2024/2025 – ECMWF FORECAST UPDATE
For the early Winter forecast, we looked at three seasonal models: the ECMWF and UKMO from Europe and a special blend of North American models called NMME. The images are from the Copernicus project and CPC/NCEP.
All these forecasts are an average picture over three meteorological winter months (December-January-February) and show the general prevailing weather patterns.
Even if the models were 100% accurate, it does not mean such weather conditions would last for three months straight. It only suggests how the weather patterns might look most of the time.
To go a step forward, below are the last 4 La Niña winters. Above is the temperature forecast for each winter, issued in the preceding September. On the bottom is the actual temperature analysis for that Winter.
What you can see is that the seasonal forecasts issued in September are generally warmer overall, and have a harder time detecting the cold areas. This is important to remember as we look at the Winter 2024/2025 forecasts below, issued this September.
The latest winter pressure pattern forecast from ECMWF shows the typical La Niña high-pressure system in the North Pacific and a broad low-pressure area from Canada into Greenland. A strong high-pressure area extends from the southern United States across the Atlantic and into mainland Europe.
Such a pressure pattern pushes the jet stream into the northern and northwestern United States. But over Europe, the jet stream is pushed further to the north, because of the high-pressure area.
The surface pressure anomaly also shows the expected La Niña pattern over the tropical Pacific. A broad high-pressure area is forecast over the ENSO regions. This is a sign that confirms the La Niña influence on the tropical surface-level circulation and its presence in the atmosphere.
Looking at the surface temperature forecast over Europe, we see an expected pattern based on the high-pressure anomalies. Most of the continent is warmer than normal, under the influence of the high-pressure system extending into the continent from the Atlantic.
The precipitation forecast for Europe shows a drier signal over the west and southwest due to the high-pressure anomaly. Northern parts of the continent show more precipitation than normal, with normal precipitation forecast over the central parts.
Over North America, the updated ECMWF winter forecast shows mostly warmer-than-normal temperatures over eastern Canada, the southern and eastern United States, and the Midwest. The warm anomaly gets weaker toward the northern and northwestern United States, with a normal area over western Canada.
The blue arrow shows a break in the above-average anomalies, indicating a potential route for cold air movement.
Below is a single-month forecast for January 2025, and it shows much less warm anomaly areas both over the United States and Europe. Here, the normal to below-normal area is expanded from the northwestern United States into the central parts and the Midwest.
The latest precipitation anomaly forecast again shows a typical La Niña-type pattern over North America. More precipitation is forecast over Canada and the northwestern and northeastern United States. Less precipitation is forecast over the southern United States, as usually seen in a La Niña pattern.
Because the precipitation forecast doesn’t show us everything, we also produced a snowfall forecast from the latest seasonal forecast data.
ECMWF WINTER 2024/2025 SNOWFALL FORECAST UPDATE
Below is a special snowfall forecast from the ECMWF data. The September data release now also includes February, so we have the snowfall forecast average for the whole Winter period.
Looking over Europe, we see below-average snowfall. More snowfall than normal is forecast over Scandinavia due to the influence of a low-pressure system and the jet stream over northern Europe.
We see an interesting forecast for North America. Most of the central, southern, and eastern United States are forecast with below-average snowfall. This is perhaps a bit surprising, especially for the Midwest and the far Northeast, which can get more snow in a weak La Niña winter.
More snowfall is forecast for the northwestern United States and parts of the north. Most of southern Canada also has above-average snowfall for this period.
We will soon update our forecast article, linked at the bottom, dedicated to snowfall predictions. There, we analyze snowfall potential in more detail and provide a month-by-month breakdown.
UKMO WINTER 2024/2025 FORECAST – NEW
As you can never trust a single forecast model, we use the UKMO long-range forecasting system along the ECMWF. It was developed by the United Kingdom Met Office, which is where the initials UKMO come from.
This forecast shows an expected La Niña pattern, which we have already seen in the previous model forecast. A high-pressure zone sits over the North Pacific and the Aleutians, with a low-pressure area indicated over Canada and into Greendland, equal to the ECMWF above.
Another high-pressure area is over Europe. Overall, this forecast is pretty much aligned with the ECMWF.
Looking at surface temperatures we see a warmer winter than normal over much of Europe. Unusually high anomalies are forecast for the northern parts of the continent.
Over North America, the UKMO model forecasts above-normal temperatures across the southern and eastern parts of the United States. But it shows an area of normal to colder temperatures across western Canada and the northwestern United States.
Just like in the ECMWF forecast, the blue arrow shows the most likely path of potential colder anomalies, indicated by the gap in the higher anomaly area.
The UKMO forecast for precipitation is similar to the ECMWF. The model shows more precipitation over northern and parts of central Europe but less over the southern parts under the high-pressure area.
We can also see a very similar precipitation forecast over North America. Drier-than-normal conditions will prevail over the southern half of the United States, while more precipitation is forecast for the northwestern United States.
This shows an almost equal forecast to the ECMWF. The main snowfall potential in this forecast is over the northern United States, the upper Midwest, and southern Canada.
NMME WINTER 2024/2025 FORECAST UPDATE
NMME stands for North American Multi-Model Ensemble. It is a special forecast produced by combining several different North American long-range models. It is useful as it contains different calculations and weather solutions, providing an average forecast from different views.
The latest pressure anomaly forecast shows a strong, high-pressure system in the North Pacific resulting from the active La Niña. A low-pressure system is indicated over Canada and into Greenland, but a bit more to the north than usual.
As the low-pressure area is shown to move further north, that gives room for a high-pressure system to exapnd from the south, over most of the United States.
The updated temperature forecast for North America nicely shows the effect of the low-pressure move to the north. Normal to colder temperatures are forecast over western Canada and potentially over the northwestern United States. Warmer temperatures are forecast for much of the United States and eastern Canada.
The latest precipitation forecast below shows a similar pattern than last month. More precipitation is forecast over the northwestern parts of the United States and the Midwest, while drier Winter is expected over much of the southern United States.
Overall, this still shows a textbook weak La Niña Winter pattern unfolding, but the main low-pressure area is shifted more to the north than usual.
WINTER 2024/2025 OFFICIAL OUTLOOK UPDATE
Below is the NOAA’s official Winter 2024/2025 temperature forecast for the United States, issued in September. It shows the winter temperature probability, with colder chances in the northern United States. The southwestern part of the country and the east coast have a higher probability of warmer than normal weather.
But take note of the trough of “equal” temperatures extending down low into the Midwest. That can be interpreted as a potential route of winter cold air outbreaks from the north into the east-central United States.
The official precipitation forecast also shows a classical La Niña pattern. We see an equal-to-high probability of more precipitation (and snowfall) over the northern half of the United States. The southern United States is forecast to have a drier-than-normal winter season.
In a typical La Niña winter, there is usually a problem with the persistence of drought conditions in the south and southwest.
Overall, the forecasts show a very obvious La Niña influence. But, one very important factor can change the course of Winter at any time: the stratospheric polar vortex. Unlike the La Niña, which works on a long time scale, the polar vortex is more of a subseasonal driver.
THE POLAR VORTEX IMPORTANCE
The Polar Vortex re-emerges every Autumn and plays a key role in weather development over the Winter and into Spring. For this reason, we have to take a quick look at its importance and early forecast.
The Polar Vortex is a large cyclonic area that spins over the entire Northern Hemisphere, from the ground up to the top of the stratosphere, reaching over 50km/31miles in altitude.
Below is an image that shows a 3-dimensional model of the Polar Vortex, extending from the lower levels up into the stratosphere. The vertical axis is greatly enhanced for better visual purposes. This image shows the actual structure of the Polar Vortex, otherwise invisible.
In the example above, the polar vortex underwent a temporary warming event. These events can disrupt the polar vortex, weaken its circulation, and change the weather patterns below.
We use the westerly stratospheric winds around the polar vortex to analyze its strength. This is basically like a jet stream but in the polar stratosphere. Strong stratospheric winds mean a strong polar vortex and weak winds can indicate a disrupted polar vortex. But why does it matter?
A strong Polar Vortex usually means strong polar circulation. This usually locks the colder air into the polar regions, resulting in milder seasonal conditions for most of the United States and Europe.
In contrast, a weak Polar Vortex can disrupt the jet stream pattern, making it harder to contain cold air that can now escape from the polar regions into the United States or Europe.
That is why we look at the wind speed within the stratospheric Polar Vortex to determine its strength. The stronger the Polar Vortex, the stronger the winds it produces in the stratosphere, and the stronger its impact.
It is the same for the opposite scenario, with a weaker Polar Vortex having weaker stratospheric winds. So, by looking at the wind speed forecast in the stratosphere, we can assess the state of the Polar Vortex and find early hints of its impacts.
The weak Polar Vortex event is what you want to see if you like colder weather and snow, across the eastern United States and Europe. A Polar Vortex is not easy to disrupt and usually needs a strong Stratospheric Warming event (SSW), which raises temperature and pressure in the stratosphere, collapsing the polar circulation from above.
Historically, a La Niña winter has a 60-75% chance of producing a Stratospheric Warming Event (SSW). It has produced them in the past and also in recent winters. The image below shows the typical SSW event frequency by month and by the ENSO event.
As you can see, a La Niña phase has a higher chance of producing a Polar Vortex collapse event. It also produces one a bit later in Winter, compared to an El Niño. Overall, this means that a La Niña event is unfavorable for a strong Polar Vortex, at least on average.
We will keep you updated on the developing weather trends in the coming seasons, so make sure to bookmark our page. Also, if you have seen this article in the Google App (Discover) feed, click the like button (♥) there to see more of our forecasts and our latest articles on weather and nature in general.
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Winter 2024/2025 First Snowfall Predictions: Under the Jet Stream shift of the cold La Niña