Many of our most memorable fall and winter storms, whether they bring heavy snow, strong winds or a sudden drop in temperature, originate from areas of low pressure that form immediately to the east of the Rocky Mountains.

One of these development zones sits over Alberta, producing what we fondly call an “Alberta clipper,” while another forms farther south over Colorado, responsible for the infamous “Colorado low.”

So, let’s revisit why the lee of the Rockies is such a breeding ground for storm systems and why certain lows grow into major weather makers while others barely organize at all.

We’ve previously discussed how the jet stream, with its sweeping curves and shifting speed, helps shape regions of rising and sinking air. When the jet accelerates, rising motion and low pressure often develop beneath it. When it slows, sinking air and high pressure tend to form. While this plays a supporting role, it doesn’t fully explain why lows so often take shape immediately east of the mountains.

To understand that, meteorologists talk about vorticity, a measure of how much spin an air parcel has. There are several types — absolute, relative and the Earth’s own vorticity — but the fine details can be complicated enough to test anyone’s patience. Instead, we’ll focus on the main ideas needed to understand how lee-side lows develop.

As you move closer to the equator, the Earth’s vorticity decreases. Relative vorticity, meanwhile, refers to the air parcel’s own spin — counterclockwise rotation adds positive vorticity and clockwise rotation adds negative.

The important concept is that absolute vorticity, which combines both the Earth’s vorticity and the parcel’s relative vorticity, stays constant unless something forces it to change. So, if an air parcel moves southward and the Earth’s vorticity drops, the parcel must gain relative vorticity to maintain the balance. If it moves northward, the opposite happens. Increasing vorticity encourages cyclonic (low pressure) development, while decreasing vorticity promotes anticyclonic (high pressure) behaviour.

Upward bound

Now imagine Pacific air flowing eastward toward the Rockies. When it reaches the mountains, it is forced upward. At the same time, the tropopause acts like a rigid ceiling, preventing the air from expanding upward as much as it would like. The result is that the atmospheric column becomes squeezed vertically and must, in turn, spread out horizontally. When the column becomes shallower, its absolute vorticity decreases. Because the Earth’s vorticity hasn’t changed at that moment, the parcel’s relative vorticity also has to decrease. This gives the air an anticyclonic, or southeastward, turn as it flows over the mountains and spills down their eastern slopes.

Once the air begins drifting southeast of the Rockies, however, it is now entering a region of lower Earth vorticity. To compensate, its relative vorticity must increase. This creates a cyclonic bend in the flow, turning the air northeastward. Put together, these shifts form a trough of low pressure stretching along the lee of the mountains — a crucial first step in the development of an Alberta clipper or Colorado low.

The next question is why some of these troughs intensify dramatically while others fade. The Rockies themselves play a major part. These are among the tallest mountains on the continent, and their height forces a dramatic squeeze on the air column. The stronger the squeeze, the more the vorticity must adjust, and the deeper the resulting trough. But a trough alone is not enough to guarantee a storm. If it were, we would be dealing with a constant conveyor belt of major lows sweeping across the Prairies all winter long.

Other factors at play

To develop into a significant system, several additional ingredients must align. Cold arctic air often slides southward along the mountains, while warmer, moister air waits to the south. When the developing low taps into both air masses, a strong temperature gradient forms which is a key source of energy for strengthening storms. The moisture adds even more fuel as it rises and condenses, releasing heat that intensifies the system. When these ingredients line up perfectly, an Alberta clipper can quickly spin up and race eastward, bringing snow, wind, and rapid temperature changes.

Colorado lows, meanwhile, owe much of their punch to their southern position. Like clippers, they draw cold air from the north, but they also have access to warm, moisture-rich air from the Gulf of Mexico. Because the Gulf is one of the most reliable moisture sources for the continent, these systems sometimes tap into deep, sustained humidity. As this warm moist air rises and condenses, it releases a tremendous amount of heat, fueling rapid development. This is why Colorado lows can grow into sprawling, slow-moving storms capable of affecting vast regions at once.

Still, not every setup produces a major event. A storm might have abundant moisture but lack arctic air, limiting snowfall and reducing the system’s strength. A promising low might start strengthening only after it has moved east of us, missing the Prairies entirely. Other times, a lack of cold air shifts the storm track farther west, producing more rain than snow or allowing the system to slide too far south to have much impact.

With so many moving parts like the jet stream position, mountain effects, temperature contrasts, moisture supply and timing, it’s no surprise that forecasting these systems can be challenging.

Whether all the ingredients will come together for a major storm this winter remains an open question, but one thing is certain: the unique geography of the Rockies will continue shaping our storm season, just as it has for generations.

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Nobody knows the answer to that question with any certainty, but there are currently three main factors likely to be the driving force behind the type of weather that’s in our future this winter.

The first is La Niña which is currently in a weak stage and is forecast to last until December or January before becoming neutral. The second possible factor is that the northern Pacific is experiencing well above average temperatures. The final factor is the early season snow cover across Siberia.

It’s going to take a series of articles to cover this ground, so this time we will look at La Niña, what it is and how can it impact our winter weather.

Most of us are more familiar with what the related El Niño event is. It is an unusual warming of the central and eastern parts of the equatorial Pacific Ocean. This warming changes the general circulation of the atmosphere over the Pacific. In particular, the trade winds weaken — and in some extreme cases, reverse direction. This can result in large changes in the location of heat and moisture globally and can give rise to anomalous temperature and precipitation events around the world.

La Niña is the opposite. La Niña means the little girl, and can sometimes be called El Viejo, the anti-El Niño, or simply, a cold event. La Niña occurs when there is an increase in the strength of the normal pattern of trade wind circulation. Under normal conditions, these winds move westward, carrying warm surface water to Indonesia and Australia and allowing cooler water to flow up along the South American coast. When a La Niña event occurs these trade winds are strengthened, which helps to increase the amount of up welling which in turn creates more cooler water along the coast of South and Central America and builds up warmer waters on the western side of the Ocean.

On the western side of the Pacific, the influx of warmer water causes an increase in cloud cover over southeast Asia and results in wetter-than-normal conditions for that region during the Northern Hemisphere winter.

Now, the big question is, what does this have to do with our weather in Western Canada?

Limited impact

These changes in the tropical Pacific are usually accompanied by large changes in the jet stream across the mid-latitudes (our part of the world), that shift the usual location of the jet stream across North America. This shifted jet stream can contribute to large changes in the normal location and strength of storm paths and can result in temperature and precipitation anomalies over North America that can persist for several months. Interestingly, these changes are most strongly felt in the winter.

Similar to last year, the latest La Niña advisory is predicting the current weak La Niña to continue through early 2026 before transitioning to neutral conditions. Since La Niña is expected to be on the weak side, this would likely result in limited impact on our weather. For context, let’s just take a look back and see what has happened during previous La Niña winters.

Sometimes it just easier to show the impacts of different weather events with the use of a map. If you look at the map up at the top of this article, it shows the typical impacts on our weather during a La Niña winter across Canada and the U.S. Typically across our region we see colder-than-average temperatures.

The problem is, that map is for a strong La Niña, and it is looking like we will only see a weak La Niña this winter. If we look back at the last 13 weak La Niñas (including last winter), six of them saw near- to below-average temperatures, six were near- to above-average, and one saw above-average temperatures over the eastern Prairies with below average temperatures over western regions. So, statistically, this does not help us out that much in predicting this winter’s temperature, as it is almost a perfect split between whether a weak La Niña winter will be warmer or colder than average.

What about snowfall? Well, once again, we will use a map to help us visualize it. The snow map you see here shows the difference from average snowfall across Canada and the U.S. during the previous nine weak La Niñas. There was a map showing snowfall data for strong La Niñas, but interestingly, it was nearly identical to this map with only a few minor variations. Looking at the map, you can see that during a weak La Niña winter we will, on average, see above-average snowfall over most of Alberta and parts of extreme southern Saskatchewan along with the north central parts of Saskatchewan. The rest of south central Saskatchewan along with southern and central Manitoba typically sees near- to below-average snowfall.

In the next issue we will look at how the current warmer northern Pacific could make a mess out the typical La Niña weather pattern and will also look into what impacts an early snow pack in Siberia might have on the upcoming winter.

Before I sign off, a few interesting weather-related stories have come out. First, global temperatures in September were the third warmest on record. Secondly, scientists believe that due to warming global oceans, the world coral reefs have reached what is very possibly an irreversible die-off. Lastly, global carbon dioxide levels have now reached heights that we have not seen on Earth for 800,000 years. Levels are currently at 424 ppm. When I first started writing weather articles 20-plus years ago it was around 370 ppm.

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“Above-average sea surface temperatures (SST) across the equatorial Pacific Ocean were indicative of a strong El Nino, with anomalies increasing in the central and east-central Pacific in the past month,” the Climate Prediction Center (U.S. CPC) said.

El Nino is a warming of ocean surface temperatures in the eastern and central Pacific, and can provoke extreme weather phenomena from wildfires to tropical cyclones and prolonged droughts.

Additionally, there is a 35 per cent chance of this event becoming “historically strong” for the November-January season, the U.S. CPC said.

The naturally occurring phenomenon is already spurring calamities across the globe, with the latest target being the neighbourhoods of Paraguay and Somalia. The stakes are seen higher for emerging markets more exposed to swings in food and energy prices.

In Canada, El Nino’s effects are seen mostly during winters and springs, when it’s associated with milder-than-normal conditions in western, northwestern and central parts of the country.

Generally, according to Environment Canada, El Nino does not significantly impact Eastern Canada — including the Maritimes — but has been associated with reduced tropical cyclone activity in the Atlantic Ocean.

— Reporting for Reuters by Harshit Verma in Bangalore with files from Glacier FarmMedia Network staff.

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Climate models suggest that after three years of the La Nina weather pattern in the Pacific Ocean, which generally lowers global temperatures slightly, the world will experience a return to El Nino, the warmer counterpart, later this year.

During El Nino, winds blowing west along the equator slow down, and warm water is pushed east, creating warmer surface ocean temperatures.

“El Nino is normally associated with record-breaking temperatures at the global level. Whether this will happen in 2023 or 2024 is not yet known, but it is, I think, more likely than not,” said Carlo Buontempo, director of the EU’s Copernicus Climate Change Service.

Climate models suggest a return to El Nino conditions in the late boreal summer, and the possibility of a strong El Nino developing toward the end of the year, Buontempo said.

The world’s hottest year on record so far was 2016, coinciding with a strong El Nino — although climate change has fuelled extreme temperatures even in years without the phenomenon.

The last eight years were the world’s eight hottest on record — reflecting the longer-term warming trend driven by greenhouse gas emissions.

Friederike Otto, senior lecturer at Imperial College London’s Grantham Institute, said El Nino-fuelled temperatures could worsen the climate change impacts countries are already experiencing – including severe heatwaves, drought and wildfires.

“If El Nino does develop, there is a good chance 2023 will be even hotter than 2016 — considering the world has continued to warm as humans continue to burn fossil fuels,” Otto said.

In Canada, El Nino’s effects are seen mostly during winters and springs, when it’s associated with milder-than-normal conditions in western, northwestern and central parts of the country.

Generally, according to Environment Canada, El Nino does not significantly impact Eastern Canada — including the Maritimes — but has been associated with reduced tropical cyclone activity in the Atlantic Ocean.

EU Copernicus scientists published a report on Thursday assessing the climate extremes the world experienced last year, its fifth-warmest year on record.

Europe experienced its hottest summer on record in 2022, while climate change-fuelled extreme rain caused disastrous flooding in Pakistan, and in February, Antarctic sea ice levels hit a record low.

The world’s average global temperature is now 1.2 C higher than in pre-industrial times, Copernicus said.

Despite most of the world’s major emitters pledging to eventually slash their net emissions to zero, global CO2 emissions last year continued to rise.

— Kate Abnett is Reuters’ European climate and energy correspondent in Brussels. Includes files from Glacier FarmMedia Network staff.

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“The coastal warming in the eastern Pacific may foreshadow changes across the Pacific basin. Therefore, an El Niño watch has been issued, and the range of possibilities toward the end of the year includes a strong El Niño,” the National Weather Service’s Climate Prediction Center (CPC) said.

Neutral conditions would prevail through spring, the CPC added.

The El Niño phenomenon is a warming of ocean surface temperatures in the eastern and central Pacific, sometimes causing crop damage, flash floods or fires.

“We note that a transition to El Niño, widely expected to occur during the second half of 2023, would be expected to weigh on rice yields across Southeast Asia, Mainland China, and both southern and eastern India,” Fitch Solutions said in a note last week.

Cereal and oilseed crops across Asia are forecast to face hot and dry weather, with meteorologists expecting the El Niño weather pattern to develop in the second half of the year, threatening supplies and heightening concerns over food inflation.

On Monday, the Japan weather bureau also pointed to a 60 per cent chance of an El Niño pattern during summer.

But for some regions, El Niño could be beneficial in the near term. Argentina, which is currently facing drought conditions, could see improved weather.

It could also boost yields for U.S. cotton farmers, who were forced to abandon a big chunk of cropland in 2022 after a crippling drought.

According to Environment Canada, El Niño’s effects on Canada are seen mostly during winter and spring, when it’s associated with milder-than-normal conditions in western, northwestern and central parts of the country.

Generally, Environment Canada said, El Niño does not significantly impact Eastern Canada — including the Maritimes — but has been associated with reduced tropical cyclone activity in the Atlantic Ocean.

This week’s predictions also call for El Niño to damp down the storm outlook for the 2023 Atlantic hurricane season.

— Reporting for Reuters by Ashitha Shivaprasad with additional reporting by Arpan Varghese in Bangalore. Includes files from Glacier FarmMedia Network staff.

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The La Niña weather pattern is characterized by unusually cold temperatures in the equatorial Pacific Ocean.

According to Environment Canada, La Niña winters are often associated with below-normal temperatures on the Prairies and above-average precipitation in British Columbia, Ontario and Quebec.

The U.S., meanwhile, is known to see drier conditions in both its Pacific and southeastern regions during La Niña winters.

“While a majority of NMME models suggest that La Niña will transition to ENSO-neutral in January-March 2023, forecasters are split on this outcome resulting in equal forecast probabilities for that season,” the U.S. National Weather Service’s Climate Prediction Center said Thursday.

El Nino-Southern Oscillation (ENSO)-neutral conditions refer to periods in which neither El Nino nor La Niña is present, often coinciding with the transition between the two weather patterns, according to the centre.

— Reporting for Reuters by Kavya Guduru in Bangalore. Includes files from Glacier FarmMedia Network staff.

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The La Niña weather pattern, characterized by unusually cold temperatures in the equatorial Pacific Ocean, has a 67 per cent chance of persisting from March through May this year, the National Weather Service’s Climate Prediction Center (CPC) said.

The CPC, in its monthly forecast, estimated a 51 per cent chance of a transition to El Nino-Southern Oscillation (ENSO)-neutral conditions during the April-June period.

ENSO-neutral conditions refer to periods in which neither El Niño nor La Niña is present, often coinciding with the transition between the two weather patterns, according to the center.

The El Niño pattern brings a warming of ocean surface temperatures in the eastern and central Pacific every few years, and is the opposite of La Niña.

Earlier this week, Japan’s weather bureau said the La Nina phenomenon is continuing and that there is an 80 per cent chance it will prevail through the end of the Northern Hemisphere winter and an 80 per cent chance the conditions will end in spring.

— Reporting for Reuters by Bharat Govind Gautam in Bangalore.

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And La Nina, in turn, is expected to produce hard cold snaps over the Prairies, above-normal precipitation over southern British Columbia and relatively mild temperatures with more snow over Ontario and Quebec, according to private forecasting agency AccuWeather.

The U.S. National Weather Service’s Climate Prediction Center on Oct. 14 reported La Nina — a weather phenomenon marked by unusually cold temperatures on the equatorial Pacific Ocean — had emerged over the past month.

“Negative anomalies” have been observed at depth across most of the central and eastern Pacific Ocean over that time, the agency said.

It also puts the chance at 87 per cent that a “moderate-strength” La Nina will continue over the period from December this year through February 2022.

Forecasters’ consensus then calls for a return to ENSO (El Nino-Southern Oscillation)-neutral conditions during the March to May 2022 period, the agency said.

AccuWeather, in a separate release Oct. 14 including the company’s annual winter forecast for Canada, said this La Nina’s effect, particularly over Western Canada, will likely be temperatures falling “even lower than they do during the average winter.”

Noting last winter’s weather was also under the influence of a La Nina, the company said a polar jet stream “amplified” by La Nina conditions can again lead to colder air and more frequent storms.

“The upcoming winter is expected to be fairly stormy from southern British Columbia through the Canadian Rockies with many opportunities for significant rainfall and strong winds along the coast,” AccuWeather meteorologist Brett Anderson said.

“Abundant” snowfall is expected throughout much of B.C.’s Coastal Range through the Rockies in western Alberta, he said in the company’s release.

“Based on what I see, I think this winter will be wetter than the past five winters in southern British Columbia,” he said. “I think this winter will certainly put a dent in the ongoing severe drought across south-central parts of the province. Conditions have already improved across southwestern British Columbia this fall as drought conditions have almost disappeared.”

Further east, he said, the polar vortex could be displaced from its normal area above the North Pole and drop into the Prairie region from time to time. “I believe we may see at least three extreme blasts of bitterly cold air dropping down into the southern Prairies this winter,” with temperatures dropping below -30 C at those points.

“This winter will likely end up colder than the winter of 2018-2019 and the coldest winter since 2013-2014 in the region.”

Cold snaps across the Prairies this winter should also force a secondary storm track well far to the south in the U.S., Anderson said, and drive storms through the U.S. Rockies and southern Plains of the U.S., before swinging northward into Eastern Canada.

“The majority of the snowstorms will track up into Ontario and Quebec,” he said.

Further east, the company said, the storm track skewing north and west, combined with “very high” water temperatures in the northwest Atlantic Ocean, favours a milder winter with average snowfall in Atlantic Canada. — Glacier FarmMedia Network

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“ENSO neutral” refers to periods in which neither El Nino nor La Nina is present, often coinciding with the transition between the two weather patterns, according to the National Weather Service’s Climate Prediction Center (CPC).

There is a 67 per cent chance the pattern will be seen in the June to August period, but “La Nina chances are around 50-55 per cent during the late fall and winter,” the CPC said.

The ENSO weather pattern is marked by average long-term ocean temperatures, tropical rainfall and atmospheric winds.

The La Nina pattern is characterized by unusually low temperatures in the equatorial Pacific Ocean and is linked with floods and drought.

The centre predicted last month a transition from La Nina to neutral conditions, with an 80 per cent chance of the ENSO pattern during May-July 2021.

Anomalies known to occur in Canada during a La Nina event include colder-than-normal temperatures on the Prairies and above-average precipitation in British Columbia, Ontario and Quebec, according to Environment Canada.

— Reporting for Reuters by Sumita Layek in Bangalore. Includes files from Glacier FarmMedia Network staff.

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The global declaration of the La Nina event by the United Nations’ agency will be used by governments to mobilize planning in climate-sensitive sectors such as agriculture, health, water resources and disaster management.

The latest La Nina is expected to be moderate to strong. The last strong event was in 2010-11, followed by a moderate event in 2011-12.

La Nina refers to the large-scale cooling of the ocean surface temperatures in the central and eastern equatorial Pacific Ocean, coupled with changes in the tropical atmospheric circulation, namely winds, pressure and rainfall. It usually has the opposite impacts on weather and climate as El Nino, which is the warm phase of the so-called El Nino Southern Oscillation (ENSO).

“El Nino and La Nina are major, naturally occurring drivers of the Earth’s climate system. But all naturally occurring climate events now take place in against a background of human-induced climate change which is exacerbating extreme weather and affecting the water cycle,” WMO secretary-general Petteri Taalas said in a release.

“La Nina typically has a cooling effect on global temperatures, but this is more than offset by the heat trapped in our atmosphere by greenhouse gases. Therefore, 2020 remains on track to be one of the warmest years on record and 2016-2020 is expected to be the warmest five-year period on record,” Taalas said. “La Nina years now are warmer even than years with strong El Nino events of the past.”

WMO’s new ENSO update states there is a high likelihood (90 per cent) of tropical Pacific sea surface temperatures remaining at La Nina levels through the end of 2020, and maybe through the first quarter of 2021 (55 per cent).

In Canada, anomalies known to occur during a La Nina event include colder-than-normal temperatures on the Prairies and above-average precipitation in British Columbia, Ontario and Quebec, according to Environment Canada.

This follows more than a year of ENSO-neutral conditions (that is, neither El Nino nor La Nina). The WMO’s update is based on forecasts from its affiliated global producing centres of long-range forecasts and expert interpretation.

El Nino and La Nina are not the only factors that drive global and regional climate patterns, said the WMO. No two La Nina or El Nino events are the same, and their effects on regional climates can vary depending on the time of year and other factors. Therefore, decision makers should always monitor latest seasonal forecasts for the most up to date information.

For this reason, WMO is now adding to the existing portfolio of seasonal information provided through the National and Regional Climate Outlook Forums and has increased the frequency of its Global Seasonal Climate Update (GSCU) from quarterly to monthly.

The GSCU also incorporates influences of other climate drivers, such as the North Atlantic Oscillation and Indian Ocean Dipole, to assess their likely effects on regional surface temperature and precipitation patterns and as such used to underpin much of the seasonal discussions with the United Nations and other partners.

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