Each Avalanche Problem consists of four factors: type, location, likelihood, and size. The combination of these factors describes the avalanche hazard.
- Avalanche Type (or Character) – One of 9 potential avalanche types.
- Location – Where the avalanche is most likely to exist in the terrain, shown with an Aspect/Elevation diagram.
- Likelihood – The chance of triggering an avalanche, shown on a five-step slider graph going from "Unlikely" to "Possible" to "Likely" to "Very Likely" to "Certain."
- Size – The destructive potential of the expected avalanche, shown on a four-step slider graph going from "Small (D1)" to "Large (D2)" to "Very Large (D3)" to "Historic (D4-5)."
The Nine Avalanche Problem Types
Storm Slab
Storm Slabs involve the release of a cohesive layer (a slab) of new snow that breaks within the storm snow or at the interface between new and old snow. They form during storms.
Depending on the storm characteristics and snowfall amounts, the problem can vary from a thin, relatively harmless soft slab to a much thicker, harder, and more dangerous slab. Storm Slab problems typically last between a few hours and a few days. They are commonly distributed widely across terrain that received similar snowfall amounts. Larger, deeper, and sometimes more reactive storm slabs form where wind-drifted snow piles up on the leeward side of terrain features.
Look for feedback, such as shooting cracks, on small, steep test slopes. Shallow Storm Slabs are most dangerous on larger terrain features or slopes with terrain traps, such as trees, gullies, and cliffs. You can reduce your risk from Storm Slabs by waiting until the storm snow bonds to itself and the old snow surfaces below it before venturing into steep terrain.
Wind Slab
Wind Slabs involve the release of a cohesive layer of snow (a slab) formed by wind-drifted snow.
They form when wind transports snow from the upwind sides of terrain features and deposits it into thicker drifts on the downwind side. This forms slabs in somewhat predictable and specific locations, such as below the leeward side of ridges or in cross-loaded gullies. Wind Slabs can range from soft to hard, thin to thick, and are often smooth, rounded, and sometimes sound hollow. Blowing snow and cracking or collapsing in drifted snow are clear warning signs of the problem.
Wind Slabs can be avoided by sticking to sheltered or wind-scoured areas.
Persistent Slab
Persistent Slabs involve the release of a cohesive layer of snow (a slab) when the bond to an underlying persistent weak layer breaks.
Persistent weak layers include surface hoar, depth hoar, near-surface facets, and faceted crusts. These layers can vary greatly over short distances and continue to produce avalanches weeks or even months after burial. Persistent Slabs are characterized by difficult-to-manage and often surprising behavior. They can be triggered long after a storm has passed or release under modest loading events, and may be triggered remotely from flatter terrain below, above, and to the sides of steep slopes. Shooting cracks and collapses are clear warning signs.
Persistent Slabs require a wide margin for error. The most reliable way to manage them is to make conservative terrain choices, which often means avoiding travel on or below steep terrain where the problem exists.
Deep Persistent Slab
Deep Persistent Slabs involve the release of a thick cohesive layer of hard snow (a slab) when the bond breaks between the slab and an underlying persistent weak layer, deep in the snowpack or near the ground. They are similar to Persistent Slabs, but deeper and larger.
Deep Persistent Slabs are destructive events that are especially difficult to manage and forecast for. They are characterized by hard-to-trigger (low likelihood) but deadly (high consequence) events due to the enormous mass of snow involved. They are most commonly triggered from areas where the snowpack is relatively shallow, such as rock outcrops or the margins of the slab.
Deep Persistent Slabs require a very wide margin for error. The most reliable way to manage them is to avoid avalanche terrain where the problem exists.
Wet Slab
Wet Slabs involve the release of a cohesive layer of snow (a slab) caused by meltwater weakening the bond between the slab and an underlying weak layer.
Wet Slabs often occur during intense or prolonged warming events and/or rain-on-snow events. They are generally more destructive and more difficult to manage than Wet Loose avalanches; they can break widely and without clear warning signs. Because the timing and amount of meltwater draining into buried weak layers varies significantly from slope to slope, there is a high amount of variability and uncertainty in predicting Wet Slabs.
Give yourself a wide safety buffer. If the snowpack has refrozen overnight, you can reduce your risk by traveling early in the day or on colder slopes where surface crusts are strong and supportive. Otherwise, conservative terrain selection is prudent during significant warm-ups.
Wet Loose
Wet Loose avalanches involve the release of unconsolidated damp or wet snow.
Wet Loose instabilities develop as sunshine, warming temperatures, and/or rain-on-snow wet and weaken the snow surface, causing it to lose cohesion. Like Dry Loose avalanches, they start at a point and entrain more snow as they fan out and move downhill. However, they entrain heavier snow that is more difficult to manage or escape from, and can trigger larger slab avalanches.
The best way to manage Wet Loose avalanches is to travel when the snow surface is colder and stronger. Time your trips to avoid crossing on or under very steep slopes in the heat of the day. Rollerballs and pinwheels are obvious precursors.
Dry Loose
Dry Loose avalanches involve the release of dry, unconsolidated snow.
These avalanches typically occur within layers of soft snow near the surface of the snowpack. They start at a point and entrain more snow as they fan out and move downhill, and can act as a trigger for slab avalanches.
Dry Loose avalanches are the simplest problem to manage because of their predictable behavior and relatively smaller sizes. They are most hazardous if you are caught and carried into a terrain trap such as a gully, cliff, couloir, or thick trees. Management strategies include sluff management and choosing less consequential terrain.
Cornice Fall
Cornice Fall involves the release of an overhanging mass of snow formed by wind deposits.
Cornices grow through the winter on the leeward side of wind-exposed ridges and summits, and range from small wind lips of soft snow to overhangs of hard snow larger than a school bus. They can break off suddenly and unexpectedly and can sometimes be triggered from a distance. Even a small cornice can be deadly if it carries you over a cliff or rocky terrain below, and the impact from a Cornice Fall can easily trigger slab avalanches on steep slopes below.
Travel cautiously on corniced ridgelines, giving cornices or unknown edges a wide berth, and limit your exposure to slopes below cornices.
Glide Avalanche
Glide Avalanches involve the release of the entire snow cover as a result of gliding over the ground.
They can be composed of wet, moist, or almost entirely dry snow, and can release under both warm and cold regimes. They occur on very specific slopes where the ground surface is relatively smooth, such as slick bedrock or grassy slopes. Glide Avalanches are often preceded by full-depth cracks (glide cracks) that are visible across the slope, though the time between the appearance of a crack and an avalanche can vary between seconds and months.
Glide avalanches are unlikely to be triggered by a person, but natural failures are very challenging to predict. Because they only occur on very specific slopes, safe travel relies on identifying and avoiding those slopes.