Stratospheric Warming Watch: Polar Vortex Collapse has begun, with cold air and snowfall impacts expected
A Sudden Stratospheric Warming event has begun, marking one of the earliest collapse events of the Polar Vortex. The latest forecasts indicate this collapse, impacting the lower levels, bringing cold weather and snowfall to certain areas across the United States, Canada, and Europe.
A strong Polar Vortex typically locks cold air in the polar regions, creating a mild winter. However, when the Polar Vortex is disrupted or collapses, the cold air can escape, unleashing proper winter weather in the mid-latitudes.
This article explores the latest stratospheric warming data and the collapsing Polar Vortex. It demonstrates how these stratospheric events will release cold and snow across the United States, Canada, and Europe, leading to a significant pattern change in December and, as currently forecasted, snowfall for the holidays.
To understand the Polar Vortex, you don't need to be a weather expert. Imagine it as a large low-pressure area, acting like a spinning "wall" over and around the polar regions, from the surface to the stratosphere (over 50km/30miles high), containing the cold polar air inside.
The Polar Vortex is separated into an upper (stratospheric) and a lower (tropospheric) part. Both play distinct roles, so they are monitored as separate systems. Together, they form the winter circulation and shape our daily weather at the surface.
We closely monitor the Polar Vortex's strength, shape, and size, categorizing it into two main "modes":
- A strong (stable) Polar Vortex indicates a strong polar circulation, locking colder air in the polar regions and preventing its escape, resulting in milder winter conditions for most of the United States, Europe, and other mid-latitudes.
- A full Polar Vortex collapse occurs when a disrupted Polar Vortex struggles to contain cold air, allowing it to escape from the polar regions into the United States and other mid-latitude regions.
A weak or disrupted Polar Vortex is ideal for proper winter weather with cold and snow, increasing the chances of a cold winter season.
For a better understanding of the Polar Vortex, a high-resolution video is available, showing the Polar Vortex in true 3D, providing the best possible representation of what it actually looks like if you were to see it.
The video emphasizes that the Polar Vortex is a single large circulation, connected through the atmosphere. It's not just one winter storm across the northeastern United States.
The latest analysis of the Stratospheric Warming event reveals that the Polar Vortex is already starting to deform. It typically has a circular shape, but a high-pressure area is pushing it out and changing its structure, with rising temperatures around the cold core.
The cold core of the Polar Vortex usually covers a large area centered over the North Pole. However, as the Polar Vortex mass is displaced, temperatures over the polar regions are much higher than normal.
The pressure anomaly in the stratosphere shows a developing high-pressure anomaly, causing significant issues for the Polar Vortex. A 3D analysis of the Polar Vortex reveals its full structure and how its mass is being deformed.
The strength of the Polar Vortex is measured by the winds it produces. The more organized the stratospheric Polar Vortex, the stronger the winds around its core.
The latest analysis and forecast of the stratospheric winds and temperature around the Polar Vortex indicate a significant weakening over the next two weeks, with the high-pressure area expanding in the stratosphere and temperatures rising due to stratospheric warming.
An important wind threshold is the 0-line, which marks a wind reversal in the stratosphere from westerly to easterly, indicating a major Sudden Stratospheric Warming (SSW) event. However, surface weather impacts can occur without a wind reversal if the Polar Vortex is sufficiently disrupted.
A typical schematic illustrates how a stratospheric warming event works, showing pressure anomalies across the atmosphere during the SSW event. The main event in the stratosphere affects the surface layers downwards over a certain period, currently around day -5 to -7.
This energy disrupts the Polar Vortex by creating areas of higher temperature and pressure in the stratosphere. The top-down collapse of the Polar Vortex then allows cold air to escape the polar regions and cover the United States and/or Europe.
The peak Polar Vortex disruption is forecasted around Thanksgiving, marking the start of a strong weather pattern change as we head into meteorological winter.
The latest forecasts indicate that any major weather impacts from this stratospheric warming event are still 2-3 weeks away, aiming for early December. The peak warming/disruption is expected to occur later this month, around the 25th, with a strong high-pressure area in the mid-stratosphere.
The true extent of the stratospheric warming event can be seen in the latest temperature anomaly forecast for the same level, showing a strong warming anomaly with temperatures in the mid-stratosphere nearing 30 degrees C above normal.
The impact of this event on surface weather is being closely monitored. Signs of progress from the stratosphere to the surface are evident in the forecast graphic, showing a high-pressure area with a strong warming anomaly and a displaced Polar Vortex.
Further analysis using pressure anomaly graphs for the lower 50km/30miles of the atmosphere reveals the progress and movement of pressure anomalies through the atmosphere, indicating a strong stratospheric disruption and the first direct signs of a connection between the stratosphere and lower levels.
Past events suggest that warming events can have a significant impact all the way down to surface levels. By examining historical data, we can anticipate the effects of such strong atmospheric events.
The surface pressure pattern 0-30 days after a stratospheric warming event, based on a combined image of several past SSW events, provides an idea of the typical surface impacts. A blocked pole leads to a weakened jet stream and low-pressure systems, unlocking cold air from the Arctic and sending it towards the United States, Canada, Europe, and other mid-latitudes.
Data reanalysis reveals that only three stratospheric warming events occurred this early in the season in the past 70 years: 1958, 1968, and 2000, with stratospheric warmings in November. Two had full wind reversals, and 2000 had only a weakening.
The surface temperature anomaly for December, from PSL, shows a cold weather corridor over Canada and the United States, except for the southwest. Europe shows a weak response with some cooling in the north and north-central parts.
As the 2025/2026 meteorological winter season approaches, updated forecasts will determine if the same cold weather response is expected following the upcoming early SSW event.
The time for cold weather and snow is approaching, as a stratospheric warming event typically means high-pressure anomalies from the stratosphere can reach the surface, disrupting weather systems and releasing cold air from the polar regions into the United States, southern Canada, and Europe.
The surface pressure anomaly forecast for the first half of December shows expected high pressure building around and into the polar regions, with the main surface core of the Polar Vortex being displaced into North America.
This Polar Vortex core spins counter-clockwise, bringing a strong northerly flow from the Arctic into Canada and the United States, while a southwesterly flow moves into the far North Atlantic.
The main impact region is currently in North America. The 500mb pressure anomaly forecast for December shows a strong Polar Vortex core being pushed into eastern Canada and the northeastern United States, affecting the entire continent.
A core of the lower Polar Vortex in this region creates a sustained northerly cold flow into the United States from western and southern Canada, indicating a cold and snowy start to meteorological winter across the United States and Canada.
The temperature forecast for the first week of December suggests a strong cold (polar) air outbreak across the northern, central, and northeastern United States, aligning with the pressure pattern and the lower Polar Vortex core acting as a "pump" for the cold air.
Individual scenarios also predict a significant cold air spill into the United States from the north to central, south, and then spreading east. However, these are just potential scenarios and should be taken as examples.
In late December, the cold air anomaly continues to linger and spread over southern Canada and the United States, indicating a stable northerly flow pulling colder air from the deep polar region and transporting it down into the United States during the holidays.
With the local Polar Vortex core over Canada, this pattern could persist into late December, increasing the chance of a white Christmas across many parts of the United States.
An extended-range scenario currently shows a significant cold-air release across southern Canada and most of the United States in mid-late December, directly related to a strong local core of the lower Polar Vortex, bringing down a cold air anomaly.
The snowfall forecast for December predicts a good spread of total snowfall from Canada into the northern, western, and eastern United States, including the southwest, with snow cover spreading over the Plains. This is an ensemble average, focusing on coverage.
This indicates a good chance of a snowy Christmas holiday, reaching farther south in the United States than in some recent winters.
While this is a far-extended range forecast, past scenarios following a stratospheric warming event provide confidence in the predictions.
