Tracking the formation of the snowpack and conditions throughout the season helps you keep a mental picture of what is going on in the backcountry which is the first step in addressing uncertainty due to conditions. Building your pattern recognition skills by connecting weather and conditions to layers in the snowpack helps build experience and increases your familiarity with different types of conditions. Keeping track of trends also helps you approach each trip in the backcountry with a targeted set of questions.

Understanding and recognizing patterns with regards to avalanche problems also helps you target your observations by targeting information you know you don't know. Understanding the avalanche problem can help you turn more of your unknown unknowns into known unknowns and thus enabling you to attempt gain more information through targeting specific observations in the field.

Our ability to evaluate the avalanche hazard relies on our understanding of how the mountain snowpack develops over terrain. This includes understanding how snow layers become relatively strong or weak, and it includes relating snowpack instability to a loss of cohesion of near surface snow or to propagating cracks through weak layers. This is knowledge of how avalanches form and how avalanches release.

Understanding the mechanism of avalanche release improves our ability to:

• Determine where (distribution) and when (timing) avalanches could occur.

• Determine the probable extent and consequence resulting from avalanche release.

• Better understand which field observations and tests provide valuable clues that describe the avalanche problem.

Dry Loose Avalanche

Considerations

Often referred to as “Point releases” or “Sluff”. They typically entrain loose snow as they move downhill forming fan shaped avalanches. Dry loose avalanches typically occur within a layer of soft snow near the surface of the snowpack. These avalanches can be naturally triggered, or triggered by the weight of a single person.

Significance

These avalanches can be seen as insignificant in the extreme ski/ride culture. Sure, they do tend to be small in both relative and destructive size, but their potential to injure or kill a person greatly increases when that person is above a cliff or a terrain trap. They can also trigger larger slab avalanches by rapidly loading a weak layer.

Tracking

Boot penetration and ski penetration into the loose snow surface are signs that the snow is not consolidated. Test slopes with no consequences can give results that would indicate avalanches on a greater scale. Track snowfall rates. Rapid snowfall rates (greater than 1”/hour) and slow settlement (cooler temperatures) are factors to track.

Storm Slab Avalanche

Considerations

Storm slab avalanches are the release of a cohesive layer of new snow that breaks within the new snow or on the old snow interface. Storm slabs are typically formed during heavy precipitation events that start out cold and end warm, or have lighter snow that is topped with heavier snow.

Significance

Storm slabs are not that fun to ride or walk upon. They are often deemed as “upside down” as ski tips tend to dive under the slab itself. Storm slabs can be quite reactive in storm and typically start to heal following a snowfall. They can turn into persistent slab if they are overriding a persistent weak layer and continue to produce avalanches a few days after a storm. The difference between storm slab and wind slab is the element of wind. Thus, storm slabs typically occur below ridgeline and can even occur in sheltered areas like tree glades.

Tracking

Variation in hand hardness is a great delineation of storm slab. Denser snow that overrides weak layers is a sign of instability. Decreased boot and ski penetration along with areas of rapid settlement are also signs that a newly cohesive slab is overriding weaker snow below. Hand shear tests are an easy way to note if the slab is adhering to the weaker snow below.

Wind Slab Avalanche

Considerations

Wind slab avalanches are the release of a cohesive layer of snow (slab) formed by the wind. Windward snow is transported and deposited on leeward slopes. That snow is mechanically degraded by the wind and packed really tightly together creating strong snow. The problem is that this snow is usually overriding weaker grains which become the source of instability.

Significance

Wind slab avalanches can catch backcountry travelers off guard as they enter terrain above treeline or into exposed terrain for the first time during a tour. They are often triggered on the steepest section of the starting zone, or areas with convexities.

Tracking

Pole probing, hand shears, hand hardness tests, and decreased boot/ ski penetration are all ways to track the formation of wind slab. Wind slab can be predicted by watching weather patterns within a storm, and having historical knowledge of exposed areas. It is important to keep a fine eye on the snowpack as prevailing winds can be channeled and recirculated during high wind events which can cause anomaly slab formation. Be wary of the leeward side ridgelines and cross loaded slopes. It is critical to observe direction of winds to note the associated loading patterns.

Cornice Fall

Considerations

Cornices typically grow on the leeward side of ridgelines and often obscure visibility to the slope below. Thus, it is hard to determine where the ground is under a cornice. Cornice collapses account for fatalities each year especially as the springtime sun enhances the effects of drooping and failure.

Significance

Cornices are unpredictable. They can release at any point, and can break beyond further back onto the ridge top than expected. Cornices can also release slab avalanches below when they fall. The weight of a mature cornice can be enough to penetrate meters into the snowpack.

Tracking

Avoidance is the best practice for managing cornices. Though, immature cornices can be managed as they first start to form. The older a cornice gets, the more you want to stay out of its way. Sun and heavy snowfall are two factors to watch when monitoring cornices. Sun can cause the cornice to droop and eventually fail. Wind and snowfall can continue to build the cornice.

Wet Loose Avalanche

Considerations

The release of wet, unconsolidated snow or slush. These avalanches typically occur near the surface of the snowpack and entrain/ gorge into layers deeper in the snowpack as they gain momentum. They start at a singular point and entrain snow, creating a fan shaped path with chunky debris.

Significance

These avalanches can gorge into the snowpack and release wet slab avalanches, which are larger and more destructive, as they greatly increase weight upon weak layers. They can start above you especially if the sun is warming trigger points like rocks, cliffs, and trees.

Tracking

Avoid terrain when wet conditions begin to penetrate into the snowpack. Timing is critical. Avoid avalanche terrain when the temperature is above 32F/0C, especially as dry winter snow starts to transition to springtime corn snow. Watch for pinwheels and rollerballs as they are typically a precursor to loose snow avalanches.

Wet Slab Avalanche

Considerations

Wet slab avalanches are the release of a cohesive layer of moist or wet snow that is weakened when liquid water percolates through the snowpack and breaks down the bonds between the slab and the bed surface below. They often occur during prolonged warming events but are hard to predict. Due to the fact that they can release to the ground, it is imperative to be realistic about their severe destructive capabilities.

Significance

Wet slab avalanches are one of the main avalanche types in the springtime. They have the capacity to decimate forests and even move the subsurface (land, rocks) downhill with them. Because of this, they are extremely dangerous. Timing is critical with regards to temperature and activity of this potentially large and destructive avalanche.

Tracking

Observe temperature trends, specifically as they rise to and above freezing. Many days in a row above freezing will likely result in wet slab activity. Deep foot penetration through wet surface snow is an indicator that the snow is losing cohesion and also percolating water deeper into the snowpack. Tour planning with accurate timing is critical to make sure you are not stuck in or under slide paths as the day starts to warm.

Persistent Slab Avalanche

Considerations

Persistent slab avalanches are likened to a sleeping dragon. You never know when you will hit a scale and awaken the dragon. It is imperative that you track persistent weak layers within the snowpack as the season progresses. Persistent slabs are generally found in areas that are cold and have long gaps between storms. Almost all snow climates have persistent problems during the early winter months.

Significance

Just because there has not been avalanche activity on a persistent weak layer does not mean that the layer isn't prone to avalanche activity. It simply means that areas of snow are protecting the layer, or the layer is temporarily dormant due to a variety of factors (connectivity of weak layer, height of snow, slab hardness, and grain type). Persistent weak layers are capable of producing avalanches weeks and even months after a storm. Persistent slab avalanches can be large and deadly. It is important to remember that you should not travel in avalanche terrain when persistent slab avalanches are forecasted on the local advisory. Furthermore, make sure to buffer if you are crossing under an avalanche path to avoid being in the runout.

Tracking

Always keep an eye out for shooting cracks, audible whumphing, and collapsing of weak layers as you travel in the backcountry. Snow profiles can reveal soft, coarse grained weak layers being overridden by firm, fine grained layers. The soft, coarse layers can be composed of facets, depth hoar, and/or surface hoar. Never expose yourself to a potential avalanche slope to get information. Rather, use test slopes with no consequences.

Deep Slab Avalanche

Considerations

Consider the conditions to be the same as persistent slab, but even greater uncertainty. The depth of the overriding slab serves to protect the weak layer below. Though, snow does not lay even across terrain, so the difference in slab thickness across a slope is the cause for great uncertainty.

Significance

Deep slab avalanches have been known to lay dormant for months after their weak layer was created. They are the most destructive form of avalanche and are generally considered to be unsurvivable.

Tracking

Following the local forecast and avalanche advisory are paramount when tracking deep slab avalanches. Digging down to the layer of concern can take too much time for most people, so following a professional opinion is helpful. Watch recent avalanche activity, especially as it pertains to the load itself. Snowmobiles, cornices, and large groups are capable of putting stress deeper into the snowpack than the weight of a single person.

Glide Avalanche

Considerations

Glide avalanches are the release of the entire snow cover and are very specific to certain paths. The slope needs to be steep enough and the ground needs to be smooth enough for this rare event to occur.

Significance

It is very unlikely that the weight of a single human, or even a group of humans can trigger a glide avalanche. These avalanches are a force of nature not to be reckoned with.

Tracking

Glide avalanches are nearly impossible to forecast. Glide cracks (the start of tensile failure) are a sign that a glide avalanche can later be a result. They can take seconds to months for the glide crack to penetrate to the surface and release the snowpack. Glide avalanches are typically historical, though they can be more abundant on mountains with permanent icefields such as volcanoes.

Uncertainty and Persistent Problems

One can never be certain when traveling in the mountains. There are far too many variables that are ever changing. Thus, it is important to always leave a buffer for uncertainty.

Start by identifying reliable resources like weather and avalanche information. Follow the trends to gain an understanding of how each avalanche problem is forecasted for your region. As you start to compare avalanche activity to the problem on the bulletin, you will start to discern historical trends.

Following layers of concern (which are typically persistent weak layers) is critical in buffering for the uncertainty of high consequence but low likelihood events. Keeping track of these layers in a field book and planning tours accordingly are critical steps in creating a safe tour plan.

If uncertainty remains high due to the groups confidence in assessment, physical ability, or terrain selection, then it is important to buffer accordingly and simplify the terrain that you plan to travel through. Keeping terrain simple (ie low angle, primarily forested, and away from avalanche paths) is always a tool that can be used when planning tours on days that involve persistent avalanche problems.