Stratospheric Warming Alert: A Massive Shift in the Polar Vortex is Forecast for early February
A disrupted Polar Vortex core will bring Arctic air into North America in the coming days. But the latest forecasts show a potential full collapse of the Polar Vortex, following a Stratospheric Warming event in early February. High-resolution model data shows a breakdown of the polar circulation, forecast to create a prolonged release of cold air across North America and also Europe in February.
One of the roles of the Polar Vortex is to lock the cold air into the polar regions. But when disrupted or collapsed, the cold air can escape, creating proper winter weather across the mid-latitudes.
Currently, we are tracking the latest stratospheric disruption and the cold air release into the United States. We are also monitoring the critical development of a Sudden Stratospheric Warming (SSW) event, which can redefine the weather patterns over North America and Europe for the whole of February, and even into early Spring.
Polar Vortex: Driving the Cold Flow
Since we have a lot of new readers, or readers not that familiar with the Polar Vortex, we usually include a very short rundown on what the Polar Vortex is and why it matters each Winter.
In simple terms, the Polar Vortex is the broad winter circulation over the northern (and southern) hemispheres. You can imagine the Polar Vortex as a spinning wall over the polar regions, rising from the surface into the stratosphere (over 50km/30 30miles high) and trapping the cold polar air inside.
As you can see in the image below, the Polar Vortex is divided into two layers that we monitor year-round during wintertime: the stratosphere and the troposphere. The first is at a higher altitude, and the second is at a lower altitude. The Polar Vortex rises through both layers, but with different shapes, strengths, and impacts.
For this reason, we separate the entire Polar Vortex into an upper (stratospheric) and a lower (tropospheric) part. A strong Polar Vortex can lock the colder air into the polar regions, preventing it to spill out into lower latitudes. This usually creates milder conditions for most of the United States and Europe.
But when the Polar Vortex gets disrupted or collapses, it can no longer contain cold air, releasing it out and towards the mid-latitudes. This allows the cold air to flood the United States and other mid-latitude regions. Below is an example of how a disrupted Polar Vortex brings cold polar air into the United States and Europe.
This weak/disrupted Polar Vortex is exactly what you want to see if you like cold weather across the United States, Canada, and Europe. The disruption itself usually comes from the rise in pressure and temperature in the stratosphere, called a Stratospheric Warming (SSW) event, or from other dynamics.
To better understand what the Polar Vortex is, we produced a high-resolution video below. It shows the latest Polar Vortex forecast in 3D. It shows the current disruption of the Polar Vortex structure and an even stronger event coming later this month and in February.
We will first look at the current Polar Vortex disruption and how it relates to the upcoming Arctic air event over the United States.
Polar Vortex Core and Arctic Cold
Below is the latest analysis for the mid-stratosphere at the 10mb level (30km/18.5miles height). It shows a deformed, oval-shaped Polar Vortex, which is disrupted by the strong stratospheric high-pressure area. This elongates the Polar Vortex, stretching its core directly into North America. Image by weatheriscool.com
While this is the analysis for the higher levels, this does indicate changes below, as the Polar Vortex is one large connected system.
You can see this in the forecast graphic below, which shows the pressure anomaly forecast for the lower stratosphere at the 50mb level (20km/12.4 miles). There is a strong high-pressure anomaly over the polar regions and a split Polar Vortex core, with the main one over North America. This supports the northerly flow at surface levels into the United States, bringing a proper polar vortex airmass.
We can see the full Polar Vortex structure in the 3D analysis below. You can see the disrupted shape in the upper and middle stratosphere, with several areas of disruption and stratospheric warming waves. This causes the lower parts to push out the core into North America, creating a strong northerly flow with an Arctic airmass to arrive in the coming days.
This is much better seen in the temperature forecast below for the next 5 days. You can see a direct polar air corridor into southern and eastern Canada, extending into the whole of the United States, apart from the southwest and Florida. This is a strong cold airmass, which is exactly what we expect from a Polar Vortex core in late January.
In the forecast, we can also see colder air over Europe, reaching into the northern and north-central parts. A warmer airmass is spreading over the south, and milder conditions will prevail over the west and northwest.
This spread of cold will bring a significant winter storm event, typical of a proper Polar Vortex core. In the image below, you can see the official forecast for the Winter Storm Severity Index for days 1-3. It shows a high coverage of intense winter conditions in the coming days over the central, southern, and eastern United States.
But looking into next week, there is another shift forecast in the mid-stratosphere, seen in the pressure anomaly below for the 30mb (23km/14miles) level. You can see a strong, broad Polar Vortex core in the stratosphere, even stronger than the one expected in the next few days.
This has the potential to bring down an even colder airmass, as despite being high in the stratosphere, it indicates where the main position of the Polar Vortex core is, reflecting on the lower levels.
And this is confirmed with the forecast for next week, which shows an even stronger spread of cold air over the central and eastern half of the United States, now also reaching deep towards the southeast. This event is likely to be the coldest so far this season, which aligns well with the strength of the stratospheric Polar Vortex core anomaly.
But to track further changes, we have to look at the overall strength of the Polar Vortex. We usually do that by looking at how strong the winds are around it in the mid-stratosphere. Stronger winds mean stronger circulation and vice versa.
Below is the latest analysis and forecast of the mid-stratospheric winds around the Polar Vortex. The dashed lines are the last few forecasts. You can see that all forecasts show the current Polar Vortex disruption, with its core coming into North America. But notice the forecast for late month and into February.
All the forecasts show a prolonged weakening/disruption of the stratospheric Polar Vortex into February. This usually happens during a Sudden Stratospheric Warming event, and one is now forecast to start in early February.
Sudden Stratospheric Warming and Polar Vortex Collapse
As mentioned, a stratospheric warming event means a rise in pressure and temperature in the stratosphere, putting stress on the Polar Vortex. These events can disrupt or even fully collapse and split the Polar Vortex.
Below is the pressure and temperature forecast for the start of February in the mid-stratosphere at the 10mb level (30km/18.5miles height). You can see two high-pressure areas (H) in the mid-stratosphere, and a very strong warming wave occurring where it comes together with the Polar Vortex (L), deforming and splitting its cores.
This is an example of a stratospheric warming event where the Polar Vortex is starting to collapse, but the stratospheric winds are not reversed, so it would not be classified as a major SSW event. But this is just a single GFS model forecast.
Below is the mid-stratosphere pressure anomaly forecast for the same level and period, but from more reliable ECMWF data. In this image, you can clearly see the powerful high-pressure anomaly in the mid-stratosphere at the 10mb level, collapsing the Polar Vortex, splitting it into two separate cores, and changing the course of weather in February.
The true extent of the stratospheric warming event is evident below in the latest temperature anomaly forecast at the same 10mb level and period in early February. You can see a strong warming anomaly over the polar regions, with temperatures in the mid-stratosphere more than 50 degrees C above normal, which is a significant event.
The pressure and temperature anomalies in the stratosphere show a proper, strong SSW event, exactly the type that is usually the driver behind some of the coldest weeks in a winter season.
Below is also a 3D forecast of the Polar Vortex for the end of the month, just before the major SSW change. You can clearly see a highly disrupted upper structure with warming waves on top. It also shows a deformed core, with a warming event in the upper stratosphere. You can also see how the Polar Vortex leg connects directly into North America, bringing along cold.
In the forecast graphic below, you can see the wind direction/speed forecast for the stratosphere at the 10mb level. In normal conditions, there is just one large circle/circulation. But here we see separation in the very core, creating two main parts. And you can also see that the Polar Vortex has been turned into smaller subvortices.
This is also reflected below in the ECMWF extended ensemble forecast for the 10mb Polar Vortex wind strength forecast. You can first see the current disruption, and then a much bigger prolonged collapse, with many members going below 0, which means a reversal and disintegration of the Polar Vortex.
This forecast is a great indicator of what is to come, especially since this forecast has a slight positive bias. That means it shows slightly higher strength than it turns out in the end, so its increasingly likely that it will reach actual reversal, as indicated in the high-resolution run.
But what can we usually expect after a full reversal and collapse of the Polar Vortex? Below is the sea-level pressure pattern 0-30 days after a stratospheric warming. This is an average image of several different events, but it gives an idea of what we can usually expect in the weeks after the event.
An obvious feature is the blocking high over Greenland and the polar regions, as a result of the Stratospheric Warming event and the collapse of the Polar Vortex. You can also see a low-pressure zone extending from the eastern United States across the Atlantic all the way to Europe.
This means a northerly flow with a polar airmass for the United States, and also cold potential for a large part of Europe.
The image above is the average result of many different events, produced by one of the CLS research groups at NOAA. In reality, each individual event can be much stronger or much lighter, for example, giving a slightly different outcome.
And for that reason, we can look at the weather trends in February and find a potential Stratospheric Warming signature impact.
February Cold, following a Stratospheric Warming Event
Below is the 500mb pressure anomaly forecast for early February, and it shows an almost exact pressure pattern as the one seen above, for the 0-30day period after a stratospheric warming event. We see a blocking high over the polar regions, and a low-pressure area from the United States into the Atlantic and towards Europe.
You can see an overall disrupted look, with a collapsed circulation. This is exactly what we would expect to see, following a stratospheric warming event and a Polar Vortex collapse. But, we do have to be aware of the delay period between the Polar Vortex collapse and the weather impacts at the surface.
The early February temperature anomaly forecast for the United States and Canada shows a return to a warmer anomaly in the early days of the month over the northern United States and southern Canada. While these are “warm” anomalies, they don’t mean actual warm temperatures, but just higher than normal for this time.
But a cold air anomaly is set to remain over the eastern and southern United States. This is just a transitional pattern as weather patterns readjust to the overall dynamics that led to the stratospheric warming event.
Across Europe, the temperature anomaly forecast for early February indicates a warmer airmass spreading over the continent. Colder air is forecast over the northern and northwestern parts, while the rest of the continent is expected to see normal to above-normal temperatures in this adjustment period.
Looking towards mid-February, the ECMWF ensemble temperature forecast shows the return of the colder pattern, with a stable cold air corridor from southern Canada down into a large part of the United States. This is a 7-day average, but the trends support the cold air release in the first half of February, following the collapse of the Polar Vortex.
While this cold anomaly might not look overly impressive compared to some of the forecast maps higher above, keep in mind this is a day 15-22 forecast trend. At this range, we hardly get such strong cold signals, so this is a good indication of cold potential.
Even over Europe, the temperature forecast for this period shows the colder air starting to return from the north, driven by a northerly flow and lower pressure. This is expected to occur after a Stratospheric Warming and a Polar Vortex disruption, as it releases the cold air from the polar regions into the mid-latitudes.
Of course, we have to keep in mind that this is the extended range forecast. But what gives us good confidence is that this is not some random forecast, but a pattern following a Stratospheric Warming event.
This article is based on the latest available model/ensemble data, and we will continue to release weekly updates on this developing Polar Vortex situation.
We will keep you updated on the global weather pattern development, so don’t forget 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.
The main forecast images in this article are from WeatherBell and weathermodels.com, using a commercial forecaster license.
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