A cold anomaly is moving across the United States at the moment. Looking ahead, however, the weather forecast suggests a shift towards a more summer mode, as the meteorological summer begins on June 1.
The global weather circulation and the jet stream are key components of this weather forecast story. It connects the pressure system across the hemispheres, helping to shape our daily weather.
The pressure patterns in the warm seasons are generally weaker so we don’t experience wild pattern swings. However, the weather is always changing and we will see how cold anomalies can quickly be replaced by Summer patterns.
WEATHER PATTERNS and THE JET STEAM
The jet stream is a powerful, large stream of air (wind), at an altitude of around 8-11km (5-7mi). It flows from From west to eastAround the entire Northern Hemisphere. They interact with pressure systems, affecting the strength of their strength, and shaping our weather at surface.
It is driven by temperature differences between the cold polar regions of the Earth and the warmer tropics. Because of the Earth’s rotation, it flows from west to east.
The image below shows a simplified representation of the global jet stream. The subtropical jet stream plays an important role during the warm season, while the polar jet stream is more prominent during the cold seasons.
The Polar Vortex is directly connected to the jet stream in winter. A strong polar Vortex will mean a strong Jet stream and vice versa. You can also experience very different weather impacts.
The circulation may be a little different in the warmer seasons, but the basic principles of circulation remain.
A strong Polar VortexThis usually means strong polar circulation and a strong jet stream. This often locks the colder air in the Polar regions, creating warmer climates for most of the United States.
In contrast, a weak Polar VortexA weak jet stream pattern can be created. It is much more difficult to contain the cold air as it can escape from the polar regions into the United States. Image by NOAA.
The Arctic Oscillation Index (AO) is a tool that tracks the strength of the lower Polar Vortex as well as its influence. It basically tells us whether the polar circulation has strength or weakness.
There was a special event in the stratosphere that occurred in March. It saw a warming event and the building of high pressure. It ended the polar vortex.
Below is an image showing pressure anomalies in the upper layers and in the stratosphere. You will notice low-pressure anomalies in winter and a strong AO Index.
We had the polar vortex collapse in March and the AO index entered a negative phase for most time since. This event has passed and we are now in a stronger circulation system.
Below is the AO Index forecast. It shows a strong, positive phase that began in early May. It is expected to last at least until the end of the month according to the latest forecast.
Because the AO is closely related to the polar jet stream, we created a special video showing the jet stream over Western Hemisphere. It can be seen moving from the North Pacific to the United States, and then out into the Atlantic.
It will give you an idea of how dynamic it can be, and it can interact with pressure systems.
PATTERN FOR CURRENT WEATHER
The current weather pattern shows us two areas of particular interest. The first is the deep low pressure area that stretches across Canada and the United States. Other areas include a high-pressure anomaly over the North Pacific and an eastern United States weak ridge.
The North Pacific’s strong ridging creates an interesting weather environment. A ridge has a northerly stream on its eastern side. This means that the ridge and low-pressure one have a strong northerly stream.
We can see that a cold area has been established over southwestern Canada, and the northwestern United States. The south-central United States is currently experiencing temperatures that are much higher than normal.
The latest data from the surface temperatures show that the weather is warmer than usual in most of the country, with the exception being in the northwestern United States.
The weather forecast for the future shows that this cold anomaly will continue to spread across the country, bringing with it cooler weather and potentially severe weather.
1-5 DAY FORECAST
North America’s weather will be the most dynamic in the coming days. The negative anomalies in the subpolar and polar regions can be seen, which is consistent with the positive AO forecast.
The low-pressure system is further down, from Canada to southern United States. It is bending jet stream, driving warmer southerly winds towards the east, and bringing in colder air to the north.
This pattern creates warmer conditions in the northeastern and eastern United States. Notice the unseasonable cold anomalies that are coming down. The strong northerly flow behind this system, combined with the influence from the ridge, can bring an unseasonable mass of air down from the North.
If we look closer at the United States, you can see the colder temperatures that are spreading across the west and northwestern United States this weekend. The northeast is experiencing unseasonably warm temperatures, as a result of the southerly flow and the low-pressure system.
The image below shows the cold air sweeping across the United States from the east to the west. It will continue into next week. The anomalies are expected be extremely cold, especially in the Midwest and the north United States.
The temperatures are expected to reach 50 degrees in the south-central United States, and low 40s in upper Midwest. It is not as cold as the winter, but it is unusually cold for this time of year.
Precipitation-wise the forecast below shows that the cold mass of air will be quite dry. The frontal edge will receive most of the precipitation, where the cold air meets warmer air. This increases the severity of severe weather in the south-central and southeast United States.
LATE-MONTH PATTERN FOR WEATHER
As we move towards the mid-month mark, we see further progression of the pattern. The AO index is stable due to low Arctic pressure. It is worth noting the strong ridge expected to push from Siberia towards the Arctic Ocean.
We have the low-pressure above Canada over North America. Just off the east coast of America, a ridge still exists.
The temperature forecast for this period indicates that the cooler airmass will remain in Canada. If the wind direction changes, it can be transported back down to Canada. Also, be aware of the strong warm anomaly when it crawls into the polar regions.
We can see in this image that colder air than normal will be moving into the eastern part of the United States. As the pressure builds, temperatures begin to rise in the west.
Canada can also expect colder temperatures, with the exception of the south-central regions. These cold anomalies will be relegated to western Canada, while warmer temperatures will take over the eastern United States.
NOAA OFFICIAL 6-10 DAY WINTER OUTLOOK
This is captured on the official NOAA/CPC 6-10 Day Weather Forecast. The western United States will see warmer than usual temperatures. A low-pressure system crossing the country will move into the Atlantic.
We can see a break of the warm pattern in the region that is home to the cold anomalies. The expansion continues from the south-central US to the Ohio Valley and the eastern Midwest.
According to the official NOAA precipitation forecast, there is a trend towards more precipitation in the eastern half of the United States, with a dry area in the western half. The southeastern United States will also see more precipitation.
This can also tell us (based on pressure and temperatures) that severe weather events are also possible.
A NEW MONTH BEGINS
We can see a little more variation in the regime when we look at the last days of the month as compared to the earlier days. We don’t have one dominant low as it was replaced with two smaller low-pressure areas.
The high-pressure areas are forecast to remain stable across the eastern half of the United States. This is why we often see this cold west/warmer pattern of pressure.
Below are temperature anomalies that clearly illustrate this pattern. There is a pooling of colder air in western Canada and rising temperatures in the eastern United States. Also, we can see strong signals for warm anomalies in The Polar circle.
The surface temperatures of the United States can be seen closer to see that the anomalies in the northwestern United States are expected to remain stable. The rest of America is forecast to see warmer than usual temperatures, most notably in the east.
It is important to reiterate that this is further out than the forecast period. However, it serves as an example for a possible scenario. This ensemble forecast supports this solution and is the basis of weather forecasting in this range.
NOAA OFFICIAL 8-14 DAY WEATHER OUTLOOK
This information is also available below in the official NOAA/CPC 8-14-day weather forecast. The northwestern United States will see cooler temperatures. The eastern and central United States will see warmer temperatures.
According to the official NOAA precipitation forecast, there is a tendency for more precipitation to the north. The forecasts for the central and southwest United States show less precipitation.
We have entered a new month within the forecast period. We will now take a look to the extended forecast of ECMWF. We will be looking at trends for the first six months of June.
PATTERNS FOR EARLY JUNE WEATHER
The pressure pattern for May suggests a shift to high pressure over western Canada. There are also signs for lower pressure in the northeastern and southwestern United States.
The North America temperature forecast shows that colder temperatures are expected to remain mostly in western Canada and the northern United States. The southwest and south will continue to see warm weather. However, parts of the eastern United States could experience another cold anomaly due to low pressure in northeast.
The forecast pressure pattern for mid-June shows high pressure in western Canada and low pressure in the southwestern United States. This would encourage warm southerly flow to the central United States.
We see that the cold anomalies remain in western Canada if we look at the temperature forecast. As expected from the pressure pattern, warmer temperatures are reaching the central United States.
June is also the first month in the meteorological Summer. We have also added a Summer weather outlook to the ECMWF long range forecasting system.
SUMMER 2022 WEATHER EXPERIMENT
We often use the ECMWFThis is often referred as the best model for long-range forecasting. However, there are many factors that can change each year/season. But generally, the ECMWF model is at the top as far as “reliability” goes.
But, it is impossible to call long-range/seasonal forecasting systems reliable. We are only predicting trends and how weather patterns will change over longer periods of time.
We will be focusing our forecast on the period June-July/August (JJA2022). This period includes the meteorological summer, which is the peak season.
Below, you can see a persistent system of high-pressure in the North Pacific as shown in the ECMWF pressure forecast. It extends to the United States and Canada.
The northeastern United States has a secondary high pressure area. This will have a significant regional impact on weather development in the eastern United States of America and eastern Canada.
This pattern is reflected in the global temperature distribution. North America is dominated by peak warm anomalies. This is the warm air mass that is affected by the high-pressure anomaly. Warm anomalies also extend across large parts of southern and eastern Canada.
The warm pooling can be seen in North America. It covers most of the country’s central and western regions. However, the anomaly zone in the far southern and southeastern United States is similar to historical La Nina summer patterns.
Over much of central and eastern Canada, as well as the northeastern United States, warm anomalies are forecast. The secondary high-pressure system is currently in effect over this region.
The forecast for precipitation over North America shows that it will be drier than the rest of the United States. There is a greater chance of wetter conditions in parts of the southwestern, eastern United States and eastern Canada.
This latest outlook predicts a hot and dry summer across the south-central United States. 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 shows that the majority of the United States is hotter than it used to be. The core warm anomalies focus on the west half of the United States. A warm zone exists in the northeast under secondary high pressure zone.
The official Summer precipitation forecast matches the model forecast. There is a higher chance of more precipitation in the east than elsewhere in the country. However, the summer season is expected to be drier in the central and northwestern United States.
Low precipitation is usually caused by persistent drought conditions in the south and west of the United States. Below is the latest drought analysis by NOAA. It shows the current drought conditions in the United States.
The drought conditions are already severe in the majority of the United States’ west. The southern United States is where the driest conditions prevail. A warmer and dryer summer than usual, as forecasted for the south-central or northwestern states, could sustain or worsen drought conditions.
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Source: Severe Weather