Avalanche Bulletin - Banff, Yoho, and Kootenay National Parks - Thu Feb 20, 2020

• Are usually small, but may gain significant mass on long steep slopes.
• Are typically limited to steep terrain (40+ degrees).
• Stabilize soon after a storm, usually within a few days.

• Avoiding terrain traps and large steep slopes until the surface has stabilized.
• On large steep slopes, occasionally move across the fall line to avoid being caught by your own sluffs from above.

• Are more powerful than dry loose avalanches due to their higher density.
• Are often limited to sunny slope aspects.
• Are commonly confined to the warmest part of the day.

• Avoiding start zones and avalanche paths when the snow becomes moist from daytime heating, from rain, or does not freeze overnight.

• Vary in size from small to medium.
• Occur on steeper lee and cross-loaded portions of slopes (typically 35+ degrees).
• Are often limited to specific terrain features such as lee ridge-tops.
• Can often be recognized by the appearance of the snow surface, changes in surface snow hardness, hollow, drum-like sounds and/or shooting cracks.
• Winds that vary in strength and direction can produce complex and unexpected wind slab patterns.
• Stabilize fairly soon, usually in a few days to a week.

• Recognizing and avoiding areas where wind slabs have formed, until they have stabilized.

• Vary in size from small to very large.
• May be soft slabs, fooling people into underestimating slab potential.
• Tend to occur on moderately steep slopes (35+ degrees).
• May occur in all terrain, but are larger and more frequent in the alpine.
• Stabilize soon after a storm, usually within a few days.

• Conservative terrain choices during and after storms until the storm snow has stabilized.

• Involve wet layers in the snowpack, typically including deeper layers.
• Tend to be large.
• Occur when water forms or penetrates below the surface of the snowpack.

• Avoiding start zones and avalanche paths when the snow becomes wet from daytime heating, rain, or lack of an overnight freeze.

• Slide on buried persistent weak layers, which often form during clear periods and may involve deeper layers from multiple storms.
• Vary in size from medium to very large and may cross terrain barriers to involve multiple slide paths.
• May occur on very gentle terrain, even slopes of 20 degrees or less.
• May be localized to specific elevations, aspects, or regions.
• There are often no visible signs of persistent slab instability.
• Lack of avalanche activity and lack of danger signs are NOTreliable indicators of stability.
• Compression tests and Rutschblock tests may locate persistent weak layers.
• Stabilize slowly, tending to persist for several weeks or longer.
• Often have dormant periods, becoming active again when the weather changes.
• Are prone to lingering pockets of instability that persist long after most areas have stabilized.
• Tend to release above the trigger, making it difficult to escape.
• Are often triggered remotely from a long distance away.

• Very conservative terrain choices.
• Allow extra time for persistent slabs to stabilize and use a very cautious approach to new terrain. Be especially cautious after storms or during warming periods.

• Slide on deeply buried persistent weak layers, which often form during clear periods or rain-on-snow events early in the season.
• Involve thick, hard slabs, sometimes the entire snowpack.
• Tend to be very large, commonly cross terrain barriers to involve multiple slide paths.
• Tend to occur on larger slopes of moderate steepness, typically 30-40 degrees.
• May be localized to specific elevations, aspects, or regions.
• There are often no visible signs of persistent deep slab instability.
• Lack of avalanche activity and lack of danger signs are NOT reliable indicators of stability.
• Stabilize slowly if at all, persisting for months and often the entire season.
• Dormant persistent deep slab instabilities often become active again when the weather changes, especially after storms or with warm spring weather.
• Tend to release above the trigger, making it difficult to escape.
• Are often triggered remotely from a long distance away.

Deep persistent slabs are best managed by:

• Very conservative terrain choices and a very cautious approach to new terrain.
• Be especially cautious after storms or during warming periods.

• May trigger large slab avalanches on relatively stable slopes below.
• Are often associated with recent wind loading and/or temperature changes.
• Can be triggered from ridges, sometimes breaking surprisingly far back onto ridge tops.

Cornices are best managed by:

• Approaching corniced ridges cautiously.
• Avoid travelling on or near overhanging cornices and limit time spent exposed to slopes below cornices, especially soon after wind events and during periods of warming temperatures.