Utilize this guide to aid in risk mitigation based on the primary avalanche problems of the day. Most of this information is organized around Canadian Avalanche Association’s (CAA) operations 1 course.
Dry Loose
Description: Cohesionless, dry snow starting from a point. “Sluff or point release”
Persistence: When associated with new snow generally lasts hours to days. When associated with facets, then longer.
Risk Mitigation: Avoid terrain traps, exposure above cliffs, and steep terrain overhead where sluffs can start, debris can concentrate, and small avalanches can have consequence.
Wet Loose
Description: Cohesionless, wet snow starting from a point. “Sluff or point release”
Persistence: Correlates with warm air temperatures (especially overnight), wet snow or rain, and solar radiation.
Risk Mitigation: If water from melting or precipitation is moving through snowpack, avoid terrain traps like gullies and confining terrain features. Watch for rollerballs (“snails”) and softening snow.
Storm Slab
Description: Cohesive slab of new storm snow.
Persistence: Peaks during periods of intense precipitation and stabilizes within hours or days.
Risk Mitigation: Avoid avalanche terrain during periods of intense precipitation and for 24-36hours following. Assess for crack propagation potential in all avalanche terrain during and in the days following a storm.
Wind Slab
Description: Cohesive slab of deep, locally-deposited snow.
Persistence: Peaks during periods of intense wind loading and stabilizes several days following, but cold air temperatures can extend persistence.
Risk Mitigation: Identify wind-drifted snow by observing sudden changes in snow surface texture and hardness. Wind deposits on the downwind side, most commonly on the lee side of ridge tops or gullies.
Persistent Slab
Description: Cohesive slab of old or new snow that is poorly bonded to a persistent weak layer and only slowly strengthens over time. This structure is conducive to failure initiation and crack propagation.
Persistence: Builds slowly then activates within a short period of time. Can persist for weeks or months but generally disappears within six weeks.
Risk Mitigation: Identification and tracking of weak layer distribution as it is typically located on specific aspects or elevation bands, but it can be wide spread. This is a complex problem that is difficult to assess, predict, and manage, so maintain conservative terrain choices. Assessing crack propagation propensity while maintaining a wide margin for error will aid in mitigation.
Deep Persistent Slab
Description: Thick, hard cohesive slab of old snow overlying an early-season persistent weak layer located in the lower snowpack or near the ground. Typically characterized by low likelihood and large destructive size.
Persistence: Develops early in the winter season and active periods followed by dormancy, then activity again. This can persist for the entire season.
Risk Mitigation: This is the most challenging avalanche problem. Triggering is common from shallow, weak snowpack areas, with long crack propagations and remote triggering typical. Tracking the weak layer and providing wide margins for error are essential.
Wet Slab
Description: Cohesive slab of moist to wet snow that results in dense debris with no powder cloud.
Persistence: Peaks during periods of extended warm air temperatures or rainfall. This will persist until either the snowpack refreezes or turns to slush.
Risk Mitigation: Timing is key regarding slope aspect and elevation, and overnight re-freezing of the snow surface can stabilize the snowpack. Snowpack can destabilize immediately with rainfall, strong solar radiation, and/or extended periods of above-freezing air temperatures.
Glide Slab
Description: Entire snowpack glides downslope then cracks, and continues to slide down the slope until it releases a full-depth avalanche.
Persistence: This problem ca appear at any time and persists for the remainder of the season.
Risk Mitigation: Usually localized, visible, and easy to recognize. Avoid slopes with glide cracks and overhead exposure to glide slabs.
Cornice
Description: Overhanging mass of dense, wind-deposited snow jutting out over a drop-off in terrain.
Persistence: Persists all winter on ridge crests, and tends to collapse spontaneously during periods of warming or following intense wind loading events.
Risk Mitigation: Avoid overhead exposure to cornices whenever possible, particularly during storms or periods of warming or rain. Stay on solid ground on ridge crests, allowing a margin of safety away from the roof of the cornice.
