A deep persistent slab avalanche problem has plagued most of western Canada this season. It’s made backcountry travel challenging and contributed to 73% (11 of 15) of avalanche fatalities that have occurred so far this season. Although avalanche activity on this layer has been less frequent over the last few weeks, experience from previous seasons tells us this type of problem can reawaken during prolonged periods of above freezing temperatures and produce very large avalanches.

Natural persistent slab avalanches in the South Coast observed on Apr 12 (Credit: Peter Marshall).

The first major warming event of the year is approaching this week, with freezing levels reaching 3500 m and remaining above most mountain tops for several days. The intensity and duration of this warming will destabilize the snowpack and cause widespread natural avalanche activity. We can expect cornice collapses, wet loose avalanches, and wet slab avalanches across all of western Canada. Travel in avalanche terrain will be dangerous on all aspects during the heat of the day and on sun-exposed slopes when the sun is strong.

Naturally triggered avalanches during intense warming north of Pemberton on Apr 26 (Credit: Jon Goodwin)

This danger will be amplified in regions with persistent and deep persistent slab avalanche problems, which includes much of western Canada. With the warm temperatures, buried weak layers will be under extra stress and could easily be triggered by cornice falls or wet avalanches. Human triggering is also possible, especially on steep slopes where the snowpack is thin. These avalanches will likely be very large and destructive. While they will most likely start in alpine terrain, they could run far and even reach snow-free valley bottoms.

Skier-triggered avalanche in the Rockies from Apr 20 (Credit: Parks Canada).

While the timing of reawakening persistent problems is difficult to predict, avalanche professionals across western Canada are confident that very large avalanches will occur at some point during this warm period. The Purcells and Rockies likely require minimal heat to trigger large avalanches, while other ranges have been experiencing a period of dormancy (e.g., Coast, Yukon, Cariboos, Monashees, Selkirks) making it harder to know exactly when and where the problems will reawaken.

Naturally triggered slab avalanche during intense warming south of Whistler on Apr 27 (Credit: Barny)

The bottom line is the mountains will be dangerous during this period of warm sunny weather. Here are some tips to safely enjoying your time in the backcountry:

Avoid all avalanche terrain during the heat of the day.

Avoid slopes receiving direct sunlight at any time of the day. 

Avoid travelling underneath large avalanche paths or slopes with cornices overhead.

Give cornices a wide berth when travelling on ridgetops.

Be aware that persistent slab problems can result in very large avalanches without warning signs of instability.

Don't let the blue skies lure you into a false sense of security.

Consider postponing plans for high-consequence objectives until future years.

Cornice triggered avalanche from Haines Pass from Apr 20 (Credit: Haines Avalanche Center).

Persistent problems are about to reawaken

The deep persistent problem has dominated our messaging this season and has been the suspected avalanche problem in ten (83%) of this season’s fatalities. As we near the end of this unusual winter, let’s look back at the weather and snowpack conditions that formed this problem and kept it firmly in our concerns. 

Snowpack faceting creates weak snow grains. It occurs with extended periods of cold temperatures and in shallow snowpacks.

We had below average snow depth and several periods of cold weather during November and most of December, which led to the development of a major weak layer at the bottom of the snowpack.

In the southern Kootenays, most of the South Coast, and parts of the NW Coast, a rain crust formed in December. This crust formed above the weak layer at the base of the snowpack which substantially reduced the likelihood of human triggering of the deep weak layer. 

As the snowpack grew and went from significantly below average to average in February, exceeding 2 m in depth, deep persistent slab activity became less common in many of the deep snowpack areas through the Columbia Mountains. The deeper snow meant the layer was too deep for a human to directly trigger. 

The problem continues to remain a major concern in shallow snowpack areas, especially in the Purcells and Central Rockies. 

When the winter snowpack transitions to a spring snowpack, any bridging we observed from the rain crust and the effects of the deeper snowpack may be negated. As we move into spring, deep avalanches are possible in all regions—even those where the problem hasn’t been a major concern this winter.

For background information, check out the following blog posts:

Dec 15, 2022: Persistent to Deep Persistent – Why the Switch?

Jan 27, 2023: Deep Persistent Slab Management and Mindset

Mar 2, 2023: The Deep Persistent Slab Problem is Not Going Away

Before we get into the weather and snowpack data, it is helpful to review the process of snowpack faceting. Faceting is a snowpack metamorphism process by which the snow grains transform into larger, angular grains. Because these larger, angular grains have less surface contact with the adjacent grains, they are considered a type of weak snow crystal and commonly create persistent weak layers in the snowpack. Cold temperatures promote this faceting process. Facets are often described as sugar-like because they are loose and you can sift the snow through your hands like sugar. If you’ve ever tried to make a snowball in a cold place with a shallow snowpack and the snow won’t stick together no matter how hard you try, that is faceted snow. 

Snowpack faceting occurs when the snowpack is subjected to a strong temperature gradient for an extended period of time. Because the ground below the snowpack is effectively always at or near 0 °C, the temperature gradient is determined by the air temperature and the snowpack depth. The general threshold for temperature gradients is 10 °C per 1 m snow depth. If the gradient is greater than this, we can expect faceting, and if it is less, we can expect rounding. 

Simplified temperature gradients in the snowpack showing how rounding and faceting occur. 

There are two basic conditions that typically lead to prominent facet growth in the snowpack: extended periods of cold temperatures and shallow snowpack depths. When both of these conditions exist, we can expect the worst case scenario for facet development. And this is basically what happened through the early winter season for much of the interior.

An example of the weak facets at the bottom of the snowpack in the southern Rockies

Glacier National Park keeps manual height of snow and temperature data from the Mt. Fidelity weather station which sits at treeline elevation (1905 m) and is updated daily. This is highly reliable data that dates back to the early 1960s, so it is useful for research and reviewing historical conditions. It is important to note that this is only a single location and conditions will vary throughout the interior. However, it provides a baseline for looking at our early season conditions in the interior and comparing them to historical values.

Mt. Fidelity manual height of snow, air temperature, and precipitation data for September 2022 to February 2023 provided by Parks Canada. 

One of the most memorable things from last fall was how warm and dry it was, and how late snowfall arrived in the interior mountains. On the Thanksgiving long weekend, it was still hiking season in the alpine—whereas in normal years, many keeners are already finding pockets of snow to ski by mid-October.

When snow finally arrived in late October, it came in heavy and we saw almost 1 m of snow accumulate by November 6. This is the snow that evolved into the deep persistent weak layer. 

The next two weeks (Nov 7-Nov 21) were dry and relatively cold. This was the initial period of faceting and weak layer formation. In addition to the extended dry period, we also saw strong northerly outflow winds for much of this time which redistributed the snow in any exposed alpine terrain. There was also one notable warm-up in the middle of this period with an inversion and some alpine areas saw above freezing temperatures for a brief period on November 15. In wind-sheltered areas at lower elevations, we also observed the growth of large surface hoar. Steep south-facing slopes saw the formation of a sun crust during this period. 

Just before this complicated early season snowpack was buried, it was described as having the following characteristics:  

Around 1 m snow depth in wind-sheltered terrain at treeline in the deepest snowpack areas of the Columbia Mountains. 

Around 50 cm average snow depth in wind-sheltered terrain at treeline in the Purcells and Rocky Mountains.

In the alpine, highly variable snow depth (30-80 cm) due to wind stripping and redistribution from a prolonged period of northerly outflow winds during the drought.

In the alpine, hard wind slabs were faceting out with persistent cold temps.

In deeper snowpack areas, the upper snowpack was composed of up to 30 cm of facets.

In shallower areas, the entire snowpack was generally faceted throughout.

Sun crust had formed on steep solar aspects, up to 2 cm thick.

A brief period of warming and temperature inversion during the drought may have contributed to surface crust formation in some alpine areas.

Widespread surface hoar up to 50 mm could be found in wind-sheltered terrain.

These are just a selection of observations to highlight the general snowpack conditions prior to burial, but conditions likely varied across the interior.

Following this two-week period of weak layer formation, another 50+ cm accumulated in the last week of November, burying the layer. 

Because snowpack faceting occurs in shallow snowpacks during cold periods, it is normal to expect a layer of facets at the base of the snowpack in many seasons. But they don’t always lead to a prominent persistent weak layer, especially one that lasts for most of the season. In many seasons, the facets at the bottom of the snowpack get compacted below the ground roughness if the facet layer is thin or from the weight of heavy snow accumulations through December, may undergo rounding with mild temperatures and deeper snowpacks, or get capped by a firm melt-freeze crust. I expect if we had a normal mild and snowy December, we wouldn’t still be discussing this layer in March. 

December was notable for being much colder and drier than is typical. In the Mt Fidelity dataset, we saw two periods of Arctic air. The first occurred at the beginning of December and saw cold and dry conditions for around eight days. The second occurred just before Christmas and saw temperatures reach near -30 °C for around five days. During both of these periods, the snowpack in the relatively deep snowpack area of Mt Fidelity was only slightly above 1 m and in many other areas, such as the Purcells and Rockies, the snowpack was still only in the range of 50-70 cm. This means the snowpack was continually faceting through both of these periods and further weakening the already weak snow from November at the base of the snowpack. 

A major storm occurred at the end of December, which formed a substantial storm slab above the weak snowpack from November and early December. We began to see large avalanches occurring on the deep layer during this period and issued a Special Public Avalanche Warning on December 28 that warned about the dangerous conditions caused by this deep layer. 

Even after the major storm at the end of December, the snowpack throughout the interior was still well below average for many parts of the interior at the end of the month. Through most of December, the snowpack throughout the interior was very shallow compared to average. Source: BC River Forecast Centre

The Mt Fidelity data is very useful for comparing to historical data, but it is also important to look at regional trends. Several other points were selected, including one in each of the Purcells, Cariboos, southern Kootenays, and Rockies. 

Location of weather stations for analysis.

This data is presented below. It is not compared against historical values like Mt Fidelity, but rather compared against each other. Given that these stations are in a variety of locations and elevations throughout the interior, they show surprisingly similar temperatures—to the point where it is difficult to differentiate between the lines in the graph. We can interpret this to mean the cold temperatures described at Mt. Fidelity occurred throughout the interior. 

The snow height data deviates more than the temperature data, but the trends are the same. Mt. Fidelity is the deepest snowpack of the six stations analysed, so if we are assuming substantial faceting occurred at Fidelity, we can also assume it also occurred in the rest of these locations. In fact, we would expect that the faceting was actually more substantial in the shallower snowpack areas. This snowpack data might be a bit misleading with several of these stations being lower elevation than ideal. The deep persistent problem seems most problematic at upper treeline and alpine elevations, but good snow height data is often not available at these higher elevations, so we use lower stations as a proxy. For the few snow height lines at the bottom of the graph, we might assume the representative snow heights are a bit deeper than presented on the chart below. 

Average air temperature and height of snow data for six stations in interior BC. 

A discussion of deep persistent problems wouldn’t be complete without including the concept of snowpack bridging. An astute reader might be asking about Kootenay Pass and the general lack of a deep persistent problem in that region, despite the data above showing the same general conditions during the early season. The concept of bridging is that strong layers of snow above a weak layer can prevent the stress of a human from reaching and triggering a weak layer. This concept comes into play this season in two ways: melt-freeze crust bridging and deep snowpack bridging. In the case of Kootenay Pass, a melt-freeze crust bridging the weak layer is thought to be the main reason for the lack of a deep persistent problem. 

The first concept of bridging is generally related to melt-freeze crusts but can apply to other firm, dense layers as well. Basically, if a crust is thick and/or firm enough, it can prevent the stress of a human from penetrating deeper into the snowpack. This is common with thick or icy rain crusts. Generally speaking, you can imagine that if a crust is thick and/or firm enough to support the weight of a human when the crust is on the surface of the snow, it is also going to be able to prevent the stress of a human from penetrating deeper into the snowpack and causing failures on deeply buried layers once it is buried. This is often referred to as a capping crust because it is creating a cap on the snowpack making the human triggering of any deeper weak layers much less likely. 

A firm melt-freeze crust can prevent the stress of a human from reaching deeper into the snowpack. This is often referred to as a capping crust. 

While it isn’t immediately apparent in the data presented above, in late December a period of rain during a big storm around Christmas impacted the north and south coast, and the southern Kootenays near the US border. This rain crust was strong enough to create a capping layer above the basal weakness and has generally prevented the November weak layer from creating a deep persistent slab problem in these regions. This is illustrated in the map below, where areas with a capping layer are excluded from the regions of most concern.

This map was published on March 2 in the blog post called “The Deep Persistent Slab Problem is Not Going Away” and excludes areas of the southern Kootenays, South Coast, and Northwest Coast because of rain crust, which formed above the deep weak layer and is effectively capping the layer. Source: The Deep Persistent Slab Problem is Not Going Away

The second important concept related to bridging is deep snowpack bridging. The typical maximum stress of a human travelling under normal conditions, whether it is a skier or snowmobiler, is around 1 m or so in depth. This does not include the component of snowpack penetration, so if your skis or snowmobile track are penetrating 50 cm into the snowpack, it is possible to trigger a layer around 150 cm down. Of course, this depends on many factors, such as snowpack conditions, but provides a rough idea of how deep a layer needs to be before it becomes unlikely that it would be triggered directly by a human. Although extra stress caused by jumping or crashing might make it possible to trigger something a bit deeper.

With a deeper snowpack, the middle of the snowpack can create a bridging effect, making it difficult for the weight of a human to penetrate deep enough to trigger deep layers. 

We saw a trend across regions related to the snowpack depth. The first round of avalanche activity on the layer occurred in deeper snowpack areas in the Columbia Mountains because it was the first area to form a thick enough slab for avalanches to happen. It took longer for slabs to develop in shallower snowpack areas of the Purcells and Rockies and consequently it took longer for the deep persistent problem to develop as well. Later in the season, the deep snowpack areas were the first to see the problem become less severe, while the problem persisted in the shallower snowpack regions. 

February was a snowy month and we saw the snowpack go from well below average to return to average levels. As a result, the deep layer was now more than 2 m deep in the Columbia Mountains, as seen in the Mt. Fidelity data. Once it was buried this deep, it became difficult for a human to trigger the layer directly in most areas and we saw avalanche activity taper. By the end of February, the layer was down almost 3 m and direct triggering became quite unlikely. Of course, there are caveats to this. Areas with a thinner snowpack, particularly rocky alpine regions where strong winds prevent snow accumulation, remain a concern. Caution should still be exercised when venturing into these areas.

This excellent animation shows the process of a skier triggering a weak layer and also highlights thin spot triggering. It is important to remember that even in a region with a deep snowpack, it may still be possible to trigger a deep layer in an area where the snowpack is shallow. Source: Bruce Jamieson, An inside look at how people trigger slab avalanches

Late Spring and Isothermal Snowpack Conditions

An isothermal snowpack means that the snowpack is the same temperature from the surface to the ground, 0 °C throughout. When this occurs, any bridging that we might have been relying on goes out the window and full-depth avalanches become possible again—even in the regions like the South Coast and southern Kootenays where the layer hasn’t been a problem for most of the season. We normally expect the alpine snowpack in the Columbia Mountains to become isothermal in May, but it is possible that it could happen sooner with a hot April or later if we have a cold spring. 

While the snowpack going fully isothermal is expected to result in the layer waking up in many areas, it is likely that the layer will also wake up in some areas sooner when the winter snowpack in the alpine starts to transition into a spring snowpack. Some of the conditions we are watching for include the following:

Freezing levels reaching mountaintop elevations coupled with strong solar radiation

Sustained high freezing levels for multiple days, especially if temperatures remain warm overnight

These types of warming will likely start to wake up the problem in shallow snowpack areas first. As more heat gets added to the snowpack, the problem will likely wake up in the deeper snowpack areas. When the snowpack finally goes fully isothermal in late spring, all bets are off and very large, full depth avalanches become possible again everywhere, even in regions where the layer hasn’t been a problem for most of the season. 

This is a long blog that came together over several weeks of digging into early season data and reflecting on a season dominated by the deep persistent slab problem for many of our regions. We had the perfect conditions of a low snowpack in November and most of December, combined with mainly cold temperatures during the same period, and cold temperatures through most of January and February. As a result, we had prime conditions for the weak layer to form at the bottom of the snowpack and very limited melt-freeze crust formation through the middle of the winter, meaning no strong bridging layer to cap the deep weak layer. The Kootenays and coastal areas remained an exception as a capping rain crust significantly reduced the likelihood of humans triggering the deep persistent problem.

We now have a deep snowpack through most of the interior and likely have a new surface crust from the recent sunny weather (although it’s likely that this crust does not exist on north aspects) which has given us a break from the problem in many areas. The expectation remains that shallow snowpack areas, especially in the Purcells and Central Rockies, may still be places where it is possible for a human to trigger the deep persistent layer directly. And as we head into spring, it is important to remember that cornice falls and avalanches in motion can trigger the layer. Extra caution is warranted during the heat of the afternoon when the snowpack will be the least stable. Finally, when the snowpack becomes isothermal or near isothermal in the late spring, all bets are off and we expect the snowpack bridging to break down. When this happens, we will likely see another round of full-depth avalanches occurring naturally in all regions. 

A Technical Review of the Formation of the 2022-23 Deep Persistent Problem

This year’s avalanche season is plagued by a deeply buried weak layer across much of western Canada, making it one of the most difficult snowpacks to safely manage in decades. The layer of concern formed in November and is buried near the base of the snowpack, which means it varies from over 1 m deep in the Rockies to over 3 m deep in the Columbia and Coast Mountains. This weak layer has caused many high-consequence avalanches since its formation and has already claimed numerous lives.

Where the deeply buried weak layer exists

The map below highlights where the deep persistent slab avalanche problem most likely exists in western Canada. Since the layer is deeply buried, there are some terrain features that make it more likely for humans to trigger it. These include:

Steep terrain where the snowpack is shallow

Areas where the snowpack transitions from thin-to-thick

We’ve also already seen many large avalanches on reloaded bed surfaces. This means that slopes that previously avalanched this season have the capacity to produce more large avalanches when reloaded with new snow. This is important, as it tells us that this buried weak layer can survive multiple avalanche cycles. It means that knowing that a slope already had an avalanche on it does not mean that the slope is safe.

Approximate location of the deep persistent avalanche problem in western Canada

The layer is deep enough that it isn’t likely to substantially change in strength before the avalanche season ends, meaning we should expect this deep persistent slab problem to remain for the coming months. We will likely have to wait until the snowpack undergoes major changes in spring and summer months, and accept that the problem could exist into summer months at high elevations where the snowpack remains cold.

What will change the likelihood of deep persistent slab avalanches

The likelihood of naturally triggered and human triggered avalanches occurring on the buried layer will change as the season progresses. The primary conditions to watch out for include periods of rapid loading from snowfall, wind, or rain, and periods of prolonged or intense warming.

Rapid loading from snow, rain, or wind-transported snow stresses the snowpack, including this deeply buried layer. These conditions also increase the likelihood of smaller avalanches releasing, which could step down to the deep weak layer and cause very large, destructive avalanches. We saw many very large avalanches (size 3 to 4) release in the past few weeks under stormy conditions and we expect this trend to continue for the remainder of the season.

Warm air temperature and sunny skies also modify the snowpack. Warming affects the snowpack by reducing its strength and increases stress on buried layers. Warming also affects how deep a human’s stress reaches into the snowpack. When human stresses reach buried weak layers, they can trigger avalanches. The influence of warming is greatest where the snowpack is inherently thin, and this is also where stresses from a human are most likely to trigger this layer. Warming also causes cornices to weaken and release, which could provide the large load needed to trigger the deep weak layer.

Regardless of trigger type, we expect to see many consequential avalanches releasing on the buried weak layer as we transition to spring conditions.

Human stresses through a snowpack. Avoiding shallow snowpack areas decreases the likelihood of triggering deeply buried weak layers.

We must acknowledge that this snowpack exists and we can’t change that. What we can change is how we manage the terrain to limit the likelihood of being caught in a high-consequence avalanche. This means understanding where you stand with respect to avalanche terrain and being disciplined with any terrain choices. Experienced riders are making a point of avoiding consequential terrain, particularly exposed or rocky alpine start zones where triggering this buried weak layer is most likely.

Your mindset going into the mountains should be that this is not the season for high-consequence objectives. Be the critical thinker in the group during any decision, stay disciplined, and please have a safe remainder of this avalanche season.

Read more about the deep persistent slab problem and the mindset needed to manage it.

Below are some images of deep persistent slab avalanches over the past few weeks.

Selkirk Mountains, photo credit MIN user S.A.M.M

Cariboo Mountains, photo credit Emma Freeman

Rocky Mountains, photo credit Parks Canada Visitor Safety

The Deep Persistent Slab Problem is Not Going Away

We have a deep persistent slab avalanche problem in many interior regions, as well as areas of the northwest and parts of the south coast inland, meaning that we have weak layers near the base of the snowpack. In most areas it’s unlikely that this layer could be easily triggered, but the avalanches it can produce are large and destructive if it is. It’s difficult to forecast for, offers very few clues about instability, produces very large avalanches, and lasts a long time, requiring a level of patience even the pros find testing. 

Given how difficult safe travel with this problem can be, keeping it simple with our terrain choices can help keep us out of trouble. Here are some ways you can opt for safer travel this winter…

Choose low angle terrain without the capacity for large avalanches

Most avalanches happen on slopes between 30 and 45 degrees. Avoiding these steeper slopes can go a long way to helping avoid an avalanche. 

Low angle terrain can still make for great riding. Image taken by an AvCan field team near Revelstoke, February 17. 

Also think about how “connected” your slope is. If you were to trigger an avalanche, is there the opportunity for it to propagate widely and cause a large avalanche? Choosing slopes without that potential can reduce your risk.

Avalanches connect across various slopes. Image from the Purcells, February 11. Read the MIN. 

Terrain traps make the consequences of an avalanche worse. They might be cliffs, gullies, flat areas under steep slopes, or even trees. Avalanches can carry people a long way, so consider what’s below you.

Trees like this can create a terrain trap. Image from the Nelson area. 

It’s always important to space out in avalanche terrain, but the possibility of large avalanches means leaving even more space. Make sure your group is spaced out widely enough that even a large avalanche would not involve multiple group members. 

Make sure you also factor in other people on or around a slope. If a group above you triggered a large avalanche, would you be in danger? If you triggered one, would it harm people below? If you think it might, it’s time to choose a different spot. 

Spreading out in avalanche terrain. File photo, credit Saul Greenberg.

The terrain around you is as important as the terrain you select to ride. Be aware of cornices or other avalanche terrain above you that might expose you to danger, especially as the sun comes out in the spring.

A cornice collapses in the Pine Bowl in the North Rockies in 2021. Cornice collapses can trigger larger avalanches on the slope below. 

Because deep persistent slabs don’t offer much feedback, don’t assume that no evidence means no problem. A deep persistent slab can be triggered with no warning signs, even on slopes that seem well-used. 

A deep persistent slab avalanche in Glacier National Park in January. See the MIN. 

In areas where the snowpack is thinner, the weak layers may be easier to trigger. Avoid thin, rocky and wind-affected areas, and steer clear of terrain where the snowpack may go from thick to thin quickly. Bear in mind that the snow depth isn’t always obvious from the surface. If in doubt, avoidance is the best technique. 

A deep persistent slab avalanche in the Little Yoho Valley area. See the MIN. 

The snow depth test shown in this video is a good example of what we mean. 

Keeping It Simple with a Complicated Snowpack

As we head into the latter part of the winter, we may start to see danger ratings dropping in parts of the interior. But before we start making big plans, we should take a moment to reflect on the current state of the snowpack and the mindset needed for a safe second half of the season.

Deep persistent slab avalanches continue to be our biggest concern throughout the interior. These two avalanches failed naturally in the Purcells, near Golden on Tuesday, January 24. They failed on a layer of facets near the base of the snowpack. MIN photo from A SHERRIFF

A deep persistent weakness exists throughout much of the interior. It has been popping up in different areas at different times and has been unpredictable, which makes forecasting for it difficult. Most recently, we have seen very large avalanches failing near the ground in the mountains around Valemount and Golden. Earlier in the month, we saw the same type of avalanches around Revelstoke and Nelson. We expect the weakness is still lingering through most of the interior and could produce very large and destructive avalanches with the right trigger. The layer seems most problematic at upper treeline and lower alpine elevations. We’ve also seen many reports of remote triggering of this layer, which means you might be able to trigger the layer from below the slope, from a ridge above the slope, or from an adjacent slope. This means you should be giving large, suspect slopes a wider margin of error than normal and be constantly vigilant about what is above you. 

A layer of surface hoar from early January also continues to produce large and unpredictable avalanches. This layer is typically down 40-90 cm through most of the interior, making it prime to be triggered by the weight of a rider. This layer has been most reactive around treeline and the upper parts of below treeline elevations. 

For a refresher on the deep persistent problem, check out the blog from our Senior Forecaster Mike Conlan from early January called “The Persisting Problem”. 

A layer of surface hoar from early January remains reactive and resulted in this persistent slab avalanche in Rogers Pass on Wednesday, January 25. This layer continues to produce large, human-triggered avalanches throughout the interior. MIN Photo from SONDRE100.

Strategic mindset is a term that is now well established amongst avalanche professionals in Canada. The original concept was established by Roger Atkins in a 2014 paper called “Yin, Yang, and You”. It establishes the following seven strategic mindsets: initial assessment, stepping out, status quo, stepping back, entrenchment, open season, and spring diurnal. 

Two of the most common mindsets being used by professionals at the moment are stepping back and entrenchment. While we often associate the term entrenchment with the patience needed during big storms, the term has been used lately when referring to the approach to our deep persistent slab problem. Many professionals stepped back a few weeks ago when the large avalanches began to occur and have been entrenched since then. This means they are skiing the same runs over and over and avoiding pushing out into terrain that has not been used this season. They are sticking to low angle slopes and tight trees where they are certain the problem doesn’t exist. 

The original strategic mindset diagram. The common thing with stepping back and entrenchment is that they both lead professionals to stick to safe terrain. Image source: Yin, Yang, and You, Roger Atkins, ISSW 2014.

In our forecasts, we often use the terms like ‘conservative’ or ‘patient’ mindset. We use these to mean people should to stick to simple, safe terrain and not be lured into bigger terrain features by boredom or ambition. It takes a lot of discipline to spend the whole season with simple objectives, but this is the attitude that professionals are using at the moment. It is the only way to ensure you don’t push yourself too far and end up triggering a large, unexpected avalanche. 

The snowpack is highly variable throughout the interior, which makes it unpredictable. Unless something drastic changes in the snowpack, this could continue for the rest of the season in parts of the interior. One of our main concerns about this situation is that we can’t say exactly where and exactly when the deep layers will be reactive. This uncertainty is why we are continuing to issue warnings about the snowpack for most of the interior in our forecasts and communications.

First consider your mindset when planning your trip. Reassess your mindset during the day and ensure your travel habits are aligned with your mindset. Finally, assess if your mindset was appropriate while you are reflecting on your day. 

Over time, we may start to see the danger ratings drop to moderate for some of the interior. However, it is important to remember that this does not mean there is no danger. The persistent and deep persistent slab problems are expected to continue to be a major concern, even if we start to see the frequency of large avalanche events decreasing. 

Forecasting for a deep persistent problem is complex, because it doesn’t fit nicely into the definition of either moderate or considerable danger. For example, the likelihood of triggering an avalanche might fit best into the moderate definition as the time passes, however the size and unpredictability of that avalanche fits more comfortably into the considerable definition. As forecasters and recreationists, we have to decide how much weight each part of the definition carries. Personally, I weight the travel advice section much higher than the likelihood section. I would also weight very large avalanches in isolated areas much higher than small avalanches in specific areas. With that in mind, I almost always lean more towards considerable than moderate in a situation like the example shown below. This is the classic low probability/high consequence problem associated with a deep persistent slab problem. 

A hypothetical breakdown of Moderate and Considerable danger rating for the current deep persistent slab problem. How much weight would you put on each part of the definitions? 

For more details on moderate hazard, check out this great blog post Managing Moderate from our Senior Forecaster, Grant Helgeson, who provides the following advice: 

Spooky moderate means slopes should be presumed guilty until proven innocent. 

Consider choosing terrain where the consequences of your snowpack assessment being wrong will promote a survivable outcome.

An extra margin of error is needed if we are experiencing changes in temperature.

Few Instability Clues Don’t Mean Stability

One of the hardest things with a deep persistent slab problem is a lack of clues. When we provide advice on managing storm-related problems near the surface of the snowpack, we often advise people to watch for signs of instability like whumpfing and cracking, signs of wind loading, and observations of recent avalanche activity. These are all strong signs that the snowpack is unstable.

With a deep persistent problem, there are typically no obvious clues until it’s too late. The first sign of instability could be a large and destructive avalanche. This makes it very difficult to assess the problem in your local riding area. Bear in mind as you travel that a lack of clues does not mean a deep instability is not present in the area. 

With a deep persistent slab problem, the first sign of an instability could be a large and destructive avalanche. Because the layer is so deep, there may be no clues that an instability exists until it is too late. MIN photo from MLDUQ

One of our biggest concerns for the coming weeks is that clear weather may motivate people to push into terrain that was previously unappealing in poor weather. The temptation might be strong, but we are cautioning people against pushing into untracked or unfamiliar terrain. Big, untracked slopes are one of the most likely places we would expect a deep persistent slab avalanche to occur. One of the most common themes amongst professionals when they talk about managing the current snowpack is the use of familiar terrain. They are sticking to previously skied terrain which they know is well tracked and tested. This approach can also be applied by recreationsists by selecting busy areas that get tracked out everytime it snows. 

Large, untracked slopes may be alluring during periods of clear weather, but resist the temptation to push into bigger terrain, especially where a thin alpine snowpack may make it easier to trigger a deep instability. Photo: Ryan Buhler

Since we will likely be dealing with the deep persistent slab problem for the rest of the season in parts of the interior, here are some tips for managing terrain and keeping a disciplined mindset:

Stick to simple, low angle terrain. Resist the temptation to push into bigger and steeper terrain. 

Remember that large, destructive avalanches can still occur with a moderate danger rating.

Choose terrain that promotes a survivable outcome if an avalanche occurs. Consider: if the slope produces an avalanche while you are on it, what would the worst case result be? 

Use a conservative mindset even if you are not seeing clues of instability. Clues of a deep instability may not exist until it is too late. 

Plan to maintain a conservative mindset, even after evidence seems to indicate these instabilities are no longer a concern.

Be willing to postpone plans for big, committed objectives and accept that they may need to wait for another year. 

The possibility of remote triggering means you should be leaving a wider margin of error around large, suspect slopes. An avalanche could be triggered from a location well away from the slope. 

Follow disciplined group decision making, ensuring that each group member is engaged in terrain selection. Speak up if you’re uncomfortable or think there’s a better option. Every voice in the group matters.

For more information on managing a persistent slab problem, check out these great articles from Powder Cloud called “How To Manage the Persistent-Slab Avalanche Problem” and “Know Where Not To Go.”

Deep Persistent Slab Management and Mindset

The weak snowpack throughout the BC interior remains a concern and warm temperatures arriving on Wednesday, January 25, are expected to further destabilize any existing weaknesses in the snowpack. Given the volatility of the snowpack recently, we recommend avoiding avalanche terrain, especially in the North Rockies and northern Columbia Mountains. 

While the current weather forecast is showing major warming for the coast and parts of the interior, there is some uncertainty about how warm conditions will actually get and the full extent of this warming away from the coastal areas. The North Rockies appears to be the warmest location in the interior. It is also one of the areas with the most concerning snowpack conditions. 

Freezing levels on Wednesday are forecast to reach over 2700 m for the south coast, 2100 m for the north coast, and near 2000 m for parts of the northern interior. Avalanche Canada Mountain Weather Forecast

The south coast will see the most substantial temperature increases, but the snowpack has already experienced warming and rain earlier in January so the impact of this warming event may not be as substantial as in other regions. That said, we can expect the warming to weaken surface snow and cornices. In parts of the coastal inland region, like the Hurley and the Duffy, a heavy trigger like a cornice falling would be the most likely trigger for any deep weaknesses still lingering in the snowpack.

The north coast has recently seen substantial storm snow accumulation and high danger ratings have been in place for several days. With the incoming warming, we can expect that this storm snow will continue to be reactive to triggering and the danger will continue to be elevated. 

An example of a cornice-triggered slab avalanche. Cornice releases are common during periods of warming and sun, and their heavy weight can trigger deep weaknesses in the snowpack. Photo: Mark Bender

The snowpack structure throughout the interior is weak, and this weakness is expected to persist, potentially for the entire season (see further: The Persisting Problem blog). We’re worried that this warming has the potential to destabilize any lingering instabilities and make the snowpack even more susceptible to human triggered avalanches. 

Recent deep persistent avalanches in the interior include the recent fatal accident near Valemount on Saturday, this nearby deep release near Clemina Creek on Sunday, and a large avalanche near Golden on Monday. In addition to these deep releasing avalanches, we have seen an assortment of other concerning avalanches throughout the interior including touchy wind slabs and persistent slab avalanches failing on a layer of surface hoar from early January. On Monday, these problems resulted in two serious incidents in the interior. 

When the weak layer near the ground is triggered, wide propagations are possible. An example of a deep persistent slab avalanche from early January in the Revelstoke area. MIN Photo from MLDUQ.

We’ve been stressing the importance of conservative decision making and patience in our bulletins and our past blog posts (see further: Managing a Persistent Slab blog) and this is going to remain essential as long as the snowpack is challenging. Our Senior Forecaster, Simon Horton, provides the following advice on managing warming and sun:

Reduce your exposure to avalanche terrain when the temperature is rising rapidly. Be even more cautious it is above zero.

Avoid travelling on or under slopes that are getting hit by the sun during periods of warming. 

The importance of aspect becomes more pronounced during warming. South-facing slopes are more susceptible to the impacts of warming, while north-facing slopes may remain colder.

Be mindful of overhead avalanche terrain.

Small avalanches or cornice falls have the potential to step-down and trigger large avalanches when there is a persistent or deep persistent slab avalanche problem.

Use the power of group decisions. Giving everyone a voice helps groups make better, safer decisions. Speak up if you’re uncomfortable or think there’s a better option.

Watch for increasing temperatures through the day. Even temperatures below freezing have the potential to destabilize the snowpack when weak layers are present. Avalanche Canada photo. 

Given the recent deep persistent slab avalanche activity in the northern interior, we are also advising against riding undisturbed and unfamiliar terrain. Good visibility and mild temperatures should not be seen as an excuse to push deeper into the mountains to find fresh tracks. These are the slopes that are most suspect and have the greatest likelihood of producing large avalanches. Now is a time to stick to simple, low angle terrain as well as busy, tracked-out areas. 

This season’s snowpack is already proving a challenge and the warming will exacerbate that. Keep this forefront in your mind when planning and make the decision to ride terrain that will let you come home safe.

Warming and Weakness in a Complex Snowpack

This year’s snowpack is different from most previous years. Professionals with decades of experience suggest this weak of a snowpack is only seen once every ten to twenty years for much of western Canada. Some professionals are comparing this snowpack to 2003, which was one of the worst years on record for avalanche fatalities. This winter presents a very different set of problems than normal and we need to adjust our mindset to remain safe.

We experienced lengthy periods of drought and cold weather that created numerous problematic layers in the snowpack. The map below shows the status of these weak layers, with the caveat that there are small-scale variations which are better described in the daily avalanche bulletins. The setup of the snowpack varies across the provinces but there is a similar theme for most areas – riders have triggered large, scary avalanches with high consequences.

Avalanches can be triggered from a distance

One commonality we’ve seen is that many of the avalanches were triggered by riders from tens to hundreds of metres away from the slope that avalanched. This indicates that the buried weak layers are widespread and spatially connected, meaning you could trigger the layer anywhere on or near a slope. This is known as remote triggering and is a clear sign of an unstable snowpack. It also means that it is difficult to anticipate where avalanches may be triggered from.

The avalanche was remotely triggered from riders on the ridge above in the Selkirk Mountains (Eric Johnstone photo).

There may not be strong evidence that the weak layers exist

We often suggest looking for signs of snowpack instability to provide evidence that your riding area could be problematic. The complication with this snowpack setup is that the layers are deep enough that we are a lot less likely to see clues, like nearby avalanche activity, whumpfing, or cracking snow. If you do experience any of these then of course it is a strong sign to keep things tame, but right now we must remember that the first sign of trouble could be triggering a high-consequence avalanche.

Riders indicated they did not experience signs of instability before the avalanche in the Purcells (MIN user PURDY.LOGAN)

Many avalanches are triggered where the layers are shallowly buried

We may see a decline in the number of avalanches triggered by riders on these layers as they get deeper. Triggering is less likely when weak layers are buried deeper than about a metre because our stresses are absorbed by the upper snowpack. Where we are most likely to trigger deep layers is where they are shallow, or within that upper metre, which often can be found in wind-affected, rocky terrain.

A wide-propagating avalanche that released on a layer near the base of the snowpack in the Monashees (Vincent Jauvin photo)

Getting caught in an avalanche on this layer could be fatal

Given the widespread nature of these layers and their location deep in the snowpack, a triggered avalanche is likely to be large to very large (size 2 to 3 or more). We’ve seen several near misses of riders triggering large avalanches on these layers and narrowly escaping a fatal outcome. Professionals are treating the snowpack with an abundance of caution given the high consequence of being caught.

A rider escaped being caught in a large avalanche in the Monashees (MIN user MLDUQ)

A weak snowpack will stay around for a while

There’s uncertainty in how long these layers will persist, but it is likely that it could be a season-long problem in some areas. The weak layers have large grain sizes with poor bonding, and it can take months for them to shrink and start bonding as a cohesive snowpack. The season may not be long enough to experience this slow transition, meaning we could see large avalanches from these layers for months to come and even into early summer in the high alpine.

Professionals are stressing the absolute importance of “terrain, terrain, terrain” given the high consequence of triggering these layers. This is the year to be highly conscious about the terrain rather than the snowpack. Discipline and patience will be required to not step out when there is new snow or during a beautiful sunny day. A snowpack like this is unforgiving and cannot be micromanaged. Step back, think of the big picture, and ask yourself if you should be there in the first place.

Here are some further risk management strategies we recommend for areas with a persistent and deep persistent avalanche problem:

Adopt a conservative mindset when in avalanche terrain.

Be diligent about terrain choices, sticking to slope angles less than 30 degrees when in clearings, open trees, and alpine terrain.

Follow disciplined group decision making, ensuring that each group member is engaged in terrain selection.

Minimize exposure to overhead hazard, given that these avalanches can be remotely triggered and travel far in runout zones.

Travel one at a time when exposed to avalanche terrain and regroup in safe spots well away from overhead hazard.

Avoid exposure to terrain traps, such as gullies, cliffs, and trees to reduce the consequence of being caught in an avalanche.

Blogues

Persistent problems are about to reawaken

A Technical Review of the Formation of the 2022-23 Deep Persistent Problem

The Deep Persistent Slab Problem is Not Going Away

Keeping It Simple with a Complicated Snowpack

Deep Persistent Slab Management and Mindset

Warming and Weakness in a Complex Snowpack

The Persisting Problem

Travelling in Avalanche Terrain During Extremely Cold Weather

Avalanche Canada Weather Stations

Persistent to Deep Persistent – Why the Switch?