Avalanche Bulletin - Little Yoho - Thu Jan 20, 2022

Issued: Wed Jan 19, 2022 17:44
Valid Until: Thu Jan 20, 2022 17:00

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Climbers be mindful of terrain overhead and avoid gullied features. There have been several natural & explosive triggered avalanches over the past few days... with more wind and snow on the way Thursday. [LP]

Public Avalanche Forecast
Danger Ratings: Thursday alpine treeline below treeline
alpine: 3 - Considerable
3 - Considerable
  • Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.
  • Natural avalanches possible; human-triggered avalanches likely.
  • Small avalanches in many areas; or large avalanches in specific areas; or very large avalanches in isolated areas.
treeline: 3 - Considerable
3 - Considerable
  • Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.
  • Natural avalanches possible; human-triggered avalanches likely.
  • Small avalanches in many areas; or large avalanches in specific areas; or very large avalanches in isolated areas.
below treeline: 3 - Considerable
3 - Considerable
  • Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.
  • Natural avalanches possible; human-triggered avalanches likely.
  • Small avalanches in many areas; or large avalanches in specific areas; or very large avalanches in isolated areas.
Forecast
Forecast Friday Saturday
alpine View Alpine Danger Rating Trend 3 - Considerable 3 - Considerable
treeline View Treeline Danger Rating Trend 3 - Considerable 2 - Moderate
below treeline View Below Treeline Danger Rating Trend 2 - Moderate 2 - Moderate
Confidence: Moderate - Due to the quality of field observations
High The forecast is based on high-quality information and the nature of the issue makes it possible to render a solid judgement. A 'high confidence' rating does not imply fact or complete certainty however, and such judgements still carry the risk of being wrong.
Moderate The information used to produce the forecast is credibly sourced and plausible, but it is not of adequate quality or sufficiently corroborated to warrant a higher level of confidence.
Low The credibility or plausibility of the information used to produce the forecast is questionable, or the information is too fragmented or poorly corroborated to make solid judgements, or there are significant concerns regarding problems with the sources.
Past 24 hour weather
Station Maximum (°C) Minimum (°C) Snowfall (cm) Snow Pack (cm) Wind speed Ridgetop wind direction
Low
  • Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.
  • Natural and human-triggered avalanches unlikely.
  • Small avalanches in isolated areas or extreme terrain.
Moderate
  • Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
  • Natural avalanches unlikely; human-triggered avalanches possible.
  • Small avalanches in specific areas; or large avalanches in isolated areas.
Considerable
  • Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.
  • Natural avalanches possible; human-triggered avalanches likely.
  • Small avalanches in many areas; or large avalanches in specific areas; or very large avalanches in isolated areas.
High
  • Very dangerous avalanche conditions. Travel in avalanche terrain not recommended.
  • Natural avalanches likely; human-triggered avalanches very likely.
  • Large avalanches in many areas; or very large avalanches in specific areas.
Extreme
  • Avoid all avalanche terrain.
  • Natural and human-triggered avalanches certain.
  • Large to very large avalanches in many areas.

Problem 1: Wind Slabs

Which Elevation?
Which Elevation? alpine, treeline
Which Slopes?
Which Slopes? southeast, east, northeast, north, northwest, south, southwest, west
Chance of Avalanches?
Chance of Avalanches? possible - likely
Expected Size?
Expected Size? small - large

Increased wind Thursday will transport recent snow (10-25 cms) adding to the previous windslabs in alpine and treeline terrain. In some areas, these have been enough to initiate deeper layers in the snowpack once triggered.

Travel and Terrain Advice

  • Be careful with wind loaded pockets, especially near ridge crests and roll-overs.
  • Minimize overhead exposure during periods of heavy loading from new snow, wind.

Problem 2: Persistent Slabs

Which Elevation?
Which Elevation? treeline, below treeline, alpine
Which Slopes?
Which Slopes? north, northeast, east, southeast, south, southwest, west, northwest
Chance of Avalanches?
Chance of Avalanches? possible
Expected Size?
Expected Size? large - large

This problem represents both the Dec. 2nd crust/ facet layer (treeline and below) and layers of faceted snow that were formed during the late December cold snap (all elevations). If triggered, either layer will result in large avalanches.

Travel and Terrain Advice

  • If triggered the wind slabs may step down to deeper layers resulting in large avalanches.
  • Forecasters are operating with alot of uncertainty at this time.

Problem 3: Dry Loose

Which Elevation?
Which Elevation? alpine, treeline, below treeline
Which Slopes?
Which Slopes? northwest, north, northeast, east, southeast, south, southwest, west
Chance of Avalanches?
Chance of Avalanches? possible
Expected Size?
Expected Size? small - large

Wind on Thursday may trigger loose avalanches that may then entrain facets in the track and gain mass. Avoid climbing in gullied features with overhead hazard.

Problem Glossary
Dry Loose
  • 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.
Dry Loose avalanches are best managed by:
  • 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.
Wet Loose
  • 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.
Wet Loose avalanches are best managed by:
  • Avoiding start zones and avalanche paths when the snow becomes moist from daytime heating, from rain, or does not freeze overnight.
Wind Slabs
  • 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.
Wind slabs are best managed by:
  • Recognizing and avoiding areas where wind slabs have formed, until they have stabilized.
Storm Slabs
  • 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.
Storm slabs are best managed by:
  • Conservative terrain choices during and after storms until the storm snow has stabilized.
Wet Slabs
  • 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.
Wet slabs are best managed by: 
  • Avoiding start zones and avalanche paths when the snow becomes wet from daytime heating, rain, or lack of an overnight freeze.
Persistent Slabs
  • 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.
Persistent slabs are best managed by:
  • 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.
Deep Persistent Slabs
  • 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.
Cornices
  • 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.
Forecast Details

Weather Forecast

Winds increase Wednesday night with snow beginning on Thursday. 5-10 cms of snow is expected with strong W alpine winds and warming temperatures through the day. Friday looks to be overcast with moderate NW winds and light snow.

Snowpack Discussion

10-15 cms of low density snow with little wind effect sits on top of windslabs in the alpine and in some treeline areas.These overlie facets in many places resulting in wide propagations in recent avalanches. . The Dec. 2 crust/facets are ~130 cm deep in thicker parts, and half that depth in thinner snowpack areas such as Mt. Stephen and Mt.Dennis.

Avalanche Activity Discussion

In the past 48 hrs a small avalanche cycle to size 2 occurred. Avalanche control in Yoho today produced slabs with every shot. Most were failing on a windslab or facet layer 40-60cm deep and then entraining facets in the track. One stepped down to the November facets ~ 180 cm deep.


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