April weather across the Northern Hemisphere will be shaped by the Polar Vortex collapse event from last month. Unseasonably cold weather events will continue over North America, with a weakened circulation and a disrupted jet stream pattern.
The Polar Vortex is a very powerful short-to-medium term weather driver during every cold season. For this reason, we monitor the activity high above in the stratosphere.
The Polar Vortex underwent a strong collapse event in late March, with the effects now coming down from the higher levels of the atmosphere. The already weak circulation will receive a new wave of disruption.
THE RISE AND FALL OF THE POLAR VORTEX
Every year as we head into autumn, the polar regions receive less and less sunlight. With less solar energy, cooling begins over the north pole.
But as the polar temperatures drop, the atmosphere further south is still relatively warm as it continues to receive light and energy from the Sun.
You can see the winter solstice in the image below. The polar regions receive little to zero solar energy, compared to regions further south, which still continue to receive plenty of sunlight and warmth.
But, as temperature drops over the polar regions, so does the pressure. With colder temperatures over the pole, the temperature difference towards the south increases.
This causes a strong global pressure difference from the poles to the equator, and a large low-pressure (cyclonic) circulation starts to develop across the Northern Hemisphere. It extends from the surface layers, far up into the stratosphere. This is known as the Polar Vortex.
You can see the layers of the atmosphere in the image below. The troposphere (blue layer) and the weather are on the bottom and the stratosphere (green layer) with the ozone layer above it.
We produced 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. You can see in the image below what the actual structure of the Polar Vortex looks like.
The Polar Vortex behaves like a very large cyclone, covering the whole north pole, down to the mid-latitudes. It is connected through all atmospheric levels, from the ground up, but usually has different shapes at different altitudes.
In the next image below, we have the bottom of the polar vortex at around 5km/3miles. The closer to the ground we go, the more deformed the polar vortex gets because it has to interact with the mountains and overall terrain.
Be aware of its “arms” extending into the lower latitudes, bringing along colder air and snowfall. These arms also pack a lot of energy and can create strong winter storms, like a Noreaster’ in the United States or powerful cyclones in the North Atlantic.
We are typically focusing more on the Stratospheric Polar Vortex, as it plays a very important role in the weather development throughout the season. It is known for its strong influence down from the stratosphere.
A strong Polar Vortex usually means strong polar circulation. This usually locks the colder air into the Polar regions, creating milder conditions for most of the United States and Europe.
In contrast, a weak Polar Vortex can create a weak jet stream pattern. It has a much harder time containing the cold air, which can now escape out of the polar regions, into the United States and/or Europe. Image by NOAA.
Typically, a polar vortex circulation is disrupted due to a rise in temperature and pressure in the stratosphere. That is called a Sudden Stratospheric Warming (SSW) event, which collapses the Polar Vortex.
To put the Polar Vortex into a perspective, we produced a high-resolution video, which shows the recent collapse event of the Polar Vortex.
Video shows the 10mb level (30km/18.5miles) from the GFS model. We can see the Polar Vortex first being split apart, followed by a strong displacement and disintegration.
The main takeaway from the video should be that the Polar Vortex is not just one single winter storm or a cold outbreak that moves from the Midwest into the northeastern United States.
It is a large cyclonic area that is spinning over the entire Northern Hemisphere, from the ground up to the top of the stratosphere, reaching over 50km/31miles in altitude.
POLAR VORTEX COLLAPSE EVENT
We typically use the 10mb level, when analyzing the polar vortex in the stratosphere. That is around 30km (17-20 miles) in altitude.
This layer is considered to be in the mid-stratosphere and is generally a good representation of the strength and status of the stratospheric polar vortex.
The strength of the polar vortex is most often measured by the power of the winds that it produces. This is typically done by measuring the westerly zonal (west to east) wind speeds around the polar circle (60°N latitude).
In the image below we have the seasonal average zonal wind speed for the Polar Vortex at 10mb level. The black line is the long-term average, and the blue line is the analyzed polar vortex strength in the 2021/22cold season.
For the majority of the cold season, the polar vortex was stronger than normal. But in March we can see the rapid collapse of the polar vortex. When the zonal (westerly) winds are reduced below zero in a sharp/rapid event, that is considered a breakdown of the circulation.
But not just winds, the temperature is also of major importance. The colder the polar vortex gets, the stronger it becomes.
Below we have the 2021/2022 season analysis of the stratospheric temperature at 10mb from NASA. The red/pink line is the 21/22 season, which shows the colder than normal polar vortex at the 10mb level in the stratosphere.
But notice the pink line, which shows a rapid temperature from February into March. You can see three distinct warming waves, each stronger than the last one. The final “strongest” peak, was produced in the collapse of the Polar Vortex.
But what exactly was happening up in the higher levels of the atmosphere? And how is it still influencing our weather down here?
Looking at the polar vortex in mid-March below, we can see the vortex was already under pressure from the European and the North Pacific sector. It had an oval shape, and it already began the separation in its core, creating two centers.
Taking a look at the temperature profile at the 10mb level (30km/18.5mi), we can see a warming wave developing over eastern Siberia and a weaker one over northern Canada. Here we can also see the separation of the core into two separate entities.
The collapse process can also be seen in the 3D structure of the polar vortex. The vortex at this point was breaking apart, developing two cores.
The vertical pressure anomaly reveals a connection between the stratosphere and the lower levels. It shows the stratospheric low-pressure areas separated into two cores in mid-March.
Currently, the polar vortex is defeated, and the polar circulation is reversed. The remains of the stratospheric polar vortex are over the eastern north hemisphere, with a high-pressure system taking over.
The 3D structure shows that only the lower level circulation remains. The upper stratospheric part of the polar vortex is gone until the next cold season 2022/23.
The influence of such an event can be seen below. This graphic shows the vertical temperature anomaly. We can see the three warming waves, coming down further from the upper stratosphere.
The final warming event can be tracked now still progressing down, forecast to reach the upper levels of the troposphere.
As you will see now, we can track these stratospheric events in the lower level circulation. That means that it will effectively also influence the weather that we feel.
FROM THE POLAR VORTEX TO THE SURFACE
This first graphic is probably the most important. It shows the vertical pressure anomaly from the surface up into the upper stratosphere. The red square marks the polar vortex collapse event.
We can see by the arrows, that the high-pressure anomaly resulting from the collapse is propagating down. It is connecting to the weather patterns, most strongly in the coming days and into the mid-month.
We can see an anomaly extension around the 10th of April. That coincides with a negative phase of the PNA, seen in the forecast below.
PNA stands for the Pacific North American index, explaining the pressure pattern over North America. A negative phase means lower pressure over western Canada and the western United States.
The weather pattern in the next few days can be seen below and is very disrupted already. High-pressure anomalies are present over the polar circle, with low-pressure systems scattered around. Main low cores are forecast over Scandinavia and the eastern United States.
This pattern is what a weak polar circulation looks like, with a disrupted jet stream. Such weather patterns can generally bring unseasonable (warm/cold) weather to the mid-latitudes.
Such a pattern in the next few days will translate into colder air over northern Europe under the low-pressure system. But the low-pressure system over the eastern United States is bringing down colder weather into the eastern half of the country. Warmer weather remains over the southwestern United States, under the high-pressure.
Taking a closer look at this cold shot over the United States, we can see the colder than normal temperatures spreading over much of the central and eastern United States. Warm weather in the southerly flow prevails over the west coast and western Canada.
The air will be cold enough to bring fresh snowfall into much of the northern United States. This cold outbreak will spread snowfall down from Midwest into the Ohio Valley.
MID-MONTH WEATHER PATTERN
Going into next week, we will be witnessing a shift in the weather pattern. The polar vortex collapse wave will reach further down. We can see stronger high-pressure anomalies building around the polar circle. A strong low-pressure system will descend into western Canada and the western United States.
This is the negative PNA pattern, that was forecast, linked to the overall pressure changes from the Polar Vortex Collapse event.
The temperature forecast for this period shows the cold air following the strong low-pressure system over western Canada and the United States. At the same time, a southerly flow will be established ahead of this low-pressure system, bringing warmer temperatures into the eastern United States and Canada.
Warmer weather will also prevail over western Europe, as colder conditions remain to the east.
Looking closely at this period over the United States, we can see a strong cold air spilling down from Canada into the western United States next week. We can see a strong temperature contrast, with the northerly flow in the west and the southerly warm flow in the northeast.
With such a strong cold air outbreak, more snowfall is expected from the Rockies into the Midwest. The final amount is not yet clear, as the event is too far out, but we can see the early distribution pattern.
NOAA OFFICIAL 8-14 DAY WEATHER OUTLOOK
This is all nicely captured below on the official NOAA/CPC 8-14 day weather forecast. Colder than normal temperatures will prevail over much of the United States, as the low-pressure system crosses the country. Warmer temperatures will move out into the far eastern and southeastern states.
Looking at the official NOAA precipitation forecast, we have a signal for more precipitation over the eastern half of the United States. Less precipitation is expected under the drier cooler northerly flow in the west-central United States, and in the southwest.
WEATHER PATTERNS EVOLVE INTO LATE MONTH
Looking into the second half of the month, the high pressure will remain around the polar circle. Two main low-pressure areas will be again over the eastern United States and northeastern Pacific. The low pressure over the eastern United States is actually the same system that will be moving in from the west next week.
At the same time, a ridge builds again over the western United States, while one remains over western Europe.
In the temperature anomalies for this period, we can see the colder air following behind the low-pressure system in the eastern United States. At the same time, a warmer airmass spreads into the western United States, with the developing ridge.
Looking closer at the United States pattern, we have an example forecast below, showing a possible scenario. The idea is that the low-pressure system next week will move further towards the east, bringing down more cold into the eastern half of the country.
At the same time, a building high-pressure ridge will expand a warmer air mass in the western half.
The precipitation anomaly forecast below shows that the cold air is mostly dry. The main precipitation area is pushed towards the southeastern and eastern parts of the United States.
We do have to repeat, that this is further out in the forecast period, so it serves as an example of a possible scenario. The ensemble forecast supports the disrupted circulation, which is supportive of further cold air intrusions into the United States.
Further down we will also cover the weather trends for late April and early May, and after that, we will look at the seasonal weather into early Summer.
LATE APRIL – EARLY MAY WEATHER PATTERN
The latest extended-range forecast from ECMWF was released this Monday, so we can look at some late April and early May trends.
The pressure pattern forecast for late April shows higher pressure over the lower latitudes. A broad low-pressure zone remains over Siberia and a weaker one is signaled over Canada.
Looking at the temperature forecast for Europe in the same period, we can see mostly warmer than normal temperatures. Colder than normal air is likely to remain mostly over Scandinavia.
The temperature forecast for North America shows the colder temperatures remaining mostly in Canada. An extension is likely into the northern United States. The rest of the country is expected to be warmer than normal under the high-pressure zone.
Going into early May, the high-pressure zones remain. But we have a better signal for low-pressure areas over Canada and northwestern Europe.
Temperatures for this period over Europe are more neutral/normal. It can also be a hint for a likely cold air spill into central Europe around this period.
Over North America, we can see the colder anomalies over Canada. But we can also see a larger neutral extension into the Midwest and towards the central United States. This is likely another cold outbreak event in development, triggered by the low-pressure zone over Canada.
NOAA/NCEP also releases regular weekly forecasts, produced by their CFS model. Below is the temperature anomaly forecast for the same period as the ECMWF extended range above.
The late April forecast on the left also shows the colder air over northern parts of the United States. And going into early May, we have the same hints of a new colder air outbreak down into the Midwest and the eastern United States.
We will now quickly look at the latest seasonal forecast below, extending our look at the weather trends in the coming months.
SPRING TO SUMMER WEATHER OUTLOOK
We typically use the ECMWF long-range model (SEAS-5), as it is often referred to as “the best” solution for long-range forecasting.
The forecast period we will be focusing on is May-June-July (MJJ 2022). This period covers the remaining month of meteorological spring and the first two months of meteorological summer.
In the pressure pattern forecast from ECMWF below, we can see a La Nina high-pressure system remaining in the North Pacific. It extends into the western United States.
A similar high-pressure pair is found over the northeastern United States and over western Europe. A low-pressure area is hinted over northern Europe.
The global temperature distribution of course follows this pattern. Over North America, we see warm pooling over the western United States. That is the warmer airmass under the high-pressure anomaly.
Europe features mostly warmer than normal conditions over the south-central and western parts. Northern Europe however is seen neutral, under a likely low-pressure area.
Looking closer at Europe, we see warm anomalies over most of the continent. But the exception is northern Europe, which will be under the influence of a low-pressure system.
Over North America, we can now better see the warm pooling over much central and western United States. The southeastern United States however does feature a neutral area, hinting at a possible partial northerly flow on the eastern side of a ridge.
Warm anomalies are also forecast over much of central and eastern Canada, and also the northeastern United States. That region is under the influence of the high-pressure system in the region.
The precipitation forecast shows drier conditions over most of the central and northwestern United States. But parts of the southeastern and southwestern United States, and eastern Canada have a higher chance of wetter conditions.
Normal to wetter conditions will prevail over northern Europe under the low-pressure zone. But the rest of the continent is expected to be drier than normal, under the high-pressure anomaly.
This forecast raises concerns about severe drought conditions in much of the western United States. The drought conditions are already present strongly over the west and south, as seen in the analysis below by the United States national drought monitor service.
Looking at the NOAA official temperature outlook, most of the United States is warmer than normal. The core warm anomalies are so far focused on the southern and southwestern United States.
The official precipitation forecast is quite similar to the model forecast. We see an equal-to-higher probability for more precipitation over the northeast. But most of the western United States, however, is forecast to have a drier than normal season from late spring and into summer.
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