The weather has been influenced by the collapse of the Polar Vortex over the past few weeks. These effects are now diminishing rapidly, according to the latest forecasts. In May, new weather patterns will emerge in North America and Europe. This is important for the development of weather into Summer.
The Polar Vortex is a powerful weather driver with a short-to-medium range. We monitor its activity in the stratosphere. The Polar Vortex experienced a strong collapse in March. The effects lasted for a few days.
We are starting to see a shift of weather patterns that is forecasted, setting a tone for the next Summer.
RISE AND FALL OF POLAR VORTEX
As we approach autumn, the sun reaches the polar regions less each year. Cooling begins at the north pole because there is less solar energy.
However, as the polar temperatures drop the atmosphere further south (towards to the Equator) remains relatively warm because it continues receiving light and energy from Sun.
The image below shows the winter solstice. The polar regions receive very little solar energy compared to the regions further south which continue to receive plenty sunlight and warmth.
The pressure decreases as the temperature drops in the polar region. The temperature difference towards the south is greater when there are colder temperatures than at the pole.
This creates a strong global pressure differential from the poles towards the equator and large low-pressure (cyclonic), circulation across the Northern Hemisphere. It extends far beyond the stratosphere and the surface layers. This is known as the Polar Vortex.
We created a 3-dimensional model the Polar Vortex that extends from the Stratosphere to the lower levels. The vertical axis was enhanced for visual purposes. The Polar Vortex’s actual structure can be seen in the image below.
The Polar Vortex behaves as a large cyclone that covers the entire north pole and the mid-latitudes. It is connected at all atmospheric levels. However, it has different shapes at different altitudes.
Below is the bottom of the polar vortex, approximately 5km/3miles. The polar vortex becomes more deformed the closer we get to the ground. This is because it interacts with the uneven terrain on the continents.
Be aware of its cold “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.
Because the Stratospheric Arctic Polar Vortex plays an important role in weather development throughout the season, we tend to focus more on it. It is well-known for its strong influence from the stratosphere.
A strong Polar VortexStrong polar circulation is usually a sign of high temperatures. This causes colder air to be trapped in the Polar regions. This results in milder season conditions for most of Europe and the United States.
In contrast, a weak Polar VortexIt can cause disruptions in the jet stream pattern. It is much more difficult to contain the cold air as it can escape from the polar regions into the United States or Europe. Image by NOAA.
A rise in temperature or pressure in the stratosphere can disrupt polar vortex circulation. This is known as a stratospheric heating event, which can disrupt the Polar Vortex or even cause it to collapse.
We produced a high resolution video that shows the recent collapse of Polar Vortex to help put things in perspective. You can see the Polar Vortex being first split apart, followed closely by strong displacement and disintegration.
The main message of the video is that the Polar Vortex does not consist of a single winter storm or cold outbreak that moves from Midwest to Northeastern United States.
It is a large cyclonic region that is spinning across the entire Northern Hemisphere. From the ground up, it reaches the top of the stratosphere and reaches more than 50km/31miles.
POLAR VORTEX COLLAPSE
When analysing the polar vortex in stratosphere, we usually use the 10mb level. This 10mb level is located at 30km (17-20 mi) elevation.
This layer is located in the mid-stratosphere. It represents the strength and status for the stratospheric Polar Vortex.
The strength of the vortex is usually measured by the strength of its winds. This is done by analyzing the westerly zonal (west to east) wind speeds around the polar circle (60°N latitude).
Below is the annual average zonal wind speed of the Polar Vortex at 10mb. The black line represents the long-term average and the blue line the analyzed polar vortex strength during the last cold season.
The polar vortex was strong for the majority of the cold seasons. The polar vortex is rapidly eroding in March. A breakdown of the circulation occurs when the zonal (westerly winds) drop below zero in a sudden/rapid event.
The temperature is also important. The stronger the polar vortex, the colder it is.
Below you will find the 2021/2022 season analysis from NASA of the stratospheric temperatures at 10mb. The 21/22 Season is represented by the red/pink color. This shows that Winter is colder than normal due to the polar vortex located at the 10mb level of the stratosphere.
The pink line shows a rapid temperature change in March. There are three distinct warming waves. Each one is stronger than the previous. The final “strongest” peak, has produced the breakdown of the Polar Vortex.
But what was really happening at higher levels? How can it affect our weather at the surface of the earth?
The polar vortex of mid-March can be seen below. It was already under pressure from both the North Atlantic and North Pacific sectors. It was oval-shaped with a core separation that created two centers within the circulation.
If we look at the temperatures at the 30km/18.5mi level, we can see that there is a warming wave over eastern Siberia. There is also a weaker wave over northern Canada. We can also see the further splitting of the polar vortex into 2 distinct cores.
You can see the collapse process in the 3D structure. As seen in the previous photos, the vortex was currently breaking down and forming two cores.
The vertical pressure anomaly indicates a link between the stratosphere & the lower levels. It shows that the stratospheric lowpressure areas are split into two distinct cores in mid March. From the surface, a strong high pressure area extends into the stratosphere.
The polar vortex had been defeated early in the month and the polar circulation was reversed. The stratospheric remnants of the polar vortex were pushed to the east by a high pressure system.
The 3D structure revealed that only the lower level circulation of the whole polar vortex circulation was left. The upper stratospheric portion of the polar vortex has been lost until the next cold season in 2022/23, which begins in Fall.
Below is an illustration of the effect of such an energetic event. This graphic shows the vertical temperature anomaly. The three warming waves can be seen coming down from the upper stratosphere.
The final warming event is now visible leveling off, forecasted to stabilize at the upper levels in the troposphere.
As you’ll see, these stratospheric activities can be tracked down to the lower levels. This means that it can influence the weather we feel.
NEW MONTH, NEW WEATHER
This is the most important graphic in this story. It shows the vertical pressure anomaly that extends from the surface to the upper stratosphere. The Polar Vortex collapse process is marked by the red arrows, which influence down to the surface levels.
The arrows indicate that the high pressure anomaly due to the collapse propagated downward. It is linked to the weather patterns, particularly in mid-to late April. But the Polar Vortex’s effects are beginning to recede.
However, we can see a positive upward anomaly forecast in May. This coincides with a positive phase in the AO, as shown in the forecast below.
AO stands for Arctic Oscillation Index, which describes the pressure pattern and circulation at the North Pole. A positive phase indicates a stronger and more stable lower-level Polar circulation.
This is a strong sign that the subsiding Polar Vortex collapse effect is at work. We can see the shifts from the positive to the negative sides starting in March and continuing through April. Fluctuating after the polar Vortex collapse.
Below you can see the weather pattern at the start of the month. It is showing low-pressure anomalies across the polar regions. The AO index is in the positive phase because of the low-pressure returning from the pole. This increases the polar circulation.
We are in mid-Spring so this can have a different impact than mid-winter. You can also see a ridge in eastern Canada and the United States. Alaska is another low-pressure area.
This pattern will bring colder than usual air into northern Europe and parts of central Europe at the beginning of the month. While warmer than usual weather will prevail in the southeastern United States and the Midwest, a cold front will move over the Midwest.
If you look closely at the cold front that is moving across the United States, you can see the colder temperatures spreading from the Midwest towards the south. The southeastern United States will see warmer temperatures.
The air will be frigid enough to bring snowfall into South Dakota, Nebraska, and other states. Also, snowfall is still possible at higher elevations in the Rockies during this time.
PATTERN FOR THE MID-MONTH WEATHER
We will see a slight adjustment in the pattern as we move into the second quarter of the month. The positive AO low-pressure system will still be over the polar areas. A ridge will form over Canada with one low-pressure trough escaping and one entering the United States.
The pressure pattern across Europe is expected to remain stable throughout this period.
This forecast shows cold air due to strong low-pressure areas in the eastern and western United States. The forecast predicts warmer temperatures in the south, while we have warm anomalies for central Canada.
As colder conditions are still further east, the west will see warmer weather.
If you look closely at the period over the United States, you can see colder air flowing down over the northwest. Another cold anomaly area can be seen over the eastern United States in the wake the outgoing low pressure system.
We find a zone of higher than normal temperatures in the area between the low-pressure system and the high-pressure system. It is expanding from the south to the Midwest and Canada.
NOAA OFFICIAL 8-14 DAY WEATHER OUTLOOK
This is all captured in the official NOAA/CPC 8-14 Day Weather Forecast. Low-pressure systems on both sides of America will bring colder temperatures to the west. This is because the weather forecasts for the region are more accurate. Warmer temperatures are expected for the southern United States and will expand to the north.
According to the official NOAA precipitation forecast, there is a trend towards more precipitation in the north and south-central United States. The northeastern United States and the southwestern United States are expected to see less precipitation.
WEATHER PATTERNS EVOLVE IN LATE MOMENTS
The low-pressure area is still around the polar circle when we look at the second half. The pattern changes in the United States. The low-pressure system moves from the west to the central United States, being replaced now by two ridges along the west coast and the coast.
A ridge is also visible over western Europe. It will keep most parts of central and western Europe in warmer spring temperatures.
The North American continent is still seeing a warm anomaly. However, warmer anomalies are forecast to replace the cold anomalies that were visible the previous week.
We have provided below a forecast of the United States that illustrates a possible scenario. The idea is that the low pressure system will move into central America, bringing with it a cold front.
Warmer temperatures will still be below the ridge in the west United States. Warmer temperatures are also forecast for the far east, although to a lesser extent than in the west.
Below is the precipitation anomaly forecast. It shows that the cold front will likely move from the Midwest into the south-central and southeastern United States. This pattern supports severe weather events. It will be closely monitored as the period approaches.
We must reiterate, however, that this is further out of the forecast period, so it serves to illustrate a possible scenario. This ensemble forecast supports this solution and is the basis of weather forecasting in this range.
LATE MAY WEATHER SPECTRANS
The latest extended-range forecast for ECMWF was published this Thursday. This means that we can now examine some May second-half predictions.
The pressure pattern for the late mid to late may shows lower pressure over polar regions. This will keep the polar circle and most of the coldest air at the closest. High pressure is expected over the western United States and eastern Canada as well as southern Europe.
We can see that the temperatures forecast for Europe over the same time period is generally warmer than normal. Scandinavia will continue to receive colder than usual air.
The North America temperature forecast shows that colder temperatures are likely to remain in the far western part of Canada. A possible extension into the northwestern United States is possible. Expect warmer temperatures in the northeastern United States, eastern Canada, and the Southwest.
The high-pressure areas remain in place until the end the month. However, we have a better indication for low-pressure areas in northern Europe and western Canada. Some of this could be the ensemble forecast average over a longer time.
The temperatures for this period in Europe are still higher than usual. The northern parts of Scandinavia are subject to cold anomalies.
The colder anomalies are still visible over North America. Warmer anomalies now extend further to the north and into the Midwest. Also, the forecasts are for warmer temperatures on the east coast and southwest of the United States.
The CFSv2 model of NOAA/NCEP also produces weekly weather forecasts. Below is the extended temperature anomaly forecast for May, from mid- to late.
The forecast for mid-May on the left shows colder air in western areas of the United States and warmth to the east. We are beginning to see the signs of a cold front moving in from the northwestern United States. Strong warm anomalies are expected for Canada during this time.
May is the last month in Spring. We will now look at Summer, the next weather season.
SPRING TO SUMMER SEASONAL EXPERIMENTAL OUTLOOK
We will use the ECMWF forecast to examine the early seasonal trends for summer 2022. This is the meteorological Summer Season, which includes the June-July and August periods.
The majority of the time, we use the ECMWFFor long-range forecasting, this is often referred too as the most reliable model within the long-range category. A lot can change over the course of a single year or season.
Below is the forecast pressure pattern from ECMWF. We can see that there is a La Nina high pressure system in the North Pacific. It extends to the west/northern United States.
The northeastern United States is home to a stronger high-pressure region. This will have a regional impact on weather development in the eastern United States as well as over eastern Canada, where it is separate from the high-pressure zone.
A high-pressure system is also observed over western Europe. There is also a low-pressure area visible over northern Europe.
The La Nina pattern is also evident in the global temperature distribution. We see the most warm anomalies in North America over the central and northern United States. This is because the high-pressure anomaly causes warmer airmass. Warm anomalies also extend to large parts of eastern and southern Canada.
Europe has a south-central half that experiences warmer than usual conditions. However, the northern regions are neutral due to a cooler low-pressure zone.
Europe is showing warmer weather than usual. Northern Europe, which is expected to be under the effect of a low-pressure storm system, is the exception.
We now have a better view of the warm anomalies in North America. However, the southeastern United States does have a neutral area similar to the historical La Nina summer pattern.
Also, warm anomalies are forecast for much of central and Eastern Canada, as well as the northeastern United States. The high-pressure system is expected to influence this region.
Northern Europe will see normal to wetter weather conditions due to the low pressure zone. The rest of the continent will be dry, which could lead to a drought scenario.
North America is forecast to have drier conditions than most of the northern and central United States. There is a greater chance that parts of the southeastern, southwestern United States, and eastern Canada will experience wetter than usual summers.
This is in line with the Summer La Nina effect, where the east and southwestern United States can receive more precipitation. The northern and central regions are the drier zones.
In this long-range outlook, the south-central United States is likely to experience a hot and dry summer. More storms are forecast for the east and southwest, with higher temperatures and normal to high levels of precipitation.
The official NOAA Summer temperature outlook indicates that most of the United States are warmer than usual. The core warm anomalies have so far been focused on the west half of the United States, with some focusing on the far northeast.
The official Summer precipitation forecast looks very similar to the ECMWF forecast. We see an equal-to higher probability of more precipitation in the east than the southwestern United States. However, the summer season is expected to be drier in the majority of the United States’ northwestern and central regions.
The problem with precipitation during any La Nina season, is usually the persistence drought conditions in the south and west United States. Below is the most recent drought analysis from NOAA. This shows the drought conditions in the country.
The majority of the United States’ western half is in drought. The southern and northern United States have the driest conditions. The drought conditions will be worsened by a hot and dry summer as is currently forecast for the north-central and south-western states.
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Source: Severe Weather