Avalanche Bulletin - Banff, Yoho, and Kootenay National Parks - Sun Apr 21, 2019

Issued: Sat Apr 20, 2019 17:32
Valid Until: Sun Apr 21, 2019 17:00

A very widespread cycle of natural and skier triggered avalanches up to size 3 was observed Saturday. Steep slopes will remain primed for human triggering on Sunday. Make conservative terrain choices until the storm snow has a chance to bond. [CJ]

Public Avalanche Forecast
Spring Conditions:  Saturday
Spring Conditions

The avalanche danger is variable and can range from Low to High. Travelling early in the day is recommended, as conditions can change rapidly in short periods of time due to daytime warming. Pay careful attention to the integrity of surface crusts formed overnight and rising air temperatures during the day. Dry slab avalanche danger may also exist during spring snow storms.

Past 24 hour weather
Station Maximum (°C) Minimum (°C) Snowfall (cm) Snow Pack (cm) Wind speed Ridgetop wind direction
Simpson 2115/2320m Not Available Not Available Not Available Not Available Not Available Not Available
Bow Summit 2040m Not Available Not Available Not Available Not Available Not Available Not Available
Sunshine 2200m Not Available Not Available Not Available Not Available Not Available Not Available
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: Storm Slabs

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

Significant snowfall over the last week combined with two periods of strong to extreme wind has created widespread storm slabs in the alpine. These are primed for human triggering. Minimize exposure to large slopes and cornices in the alpine.

Travel and Terrain Advice

  • New snow will require several days to settle and stabilize.
  • Use conservative route selection, stick to supported terrain features, avoid overhead hazards.

Problem 2: Wet Loose

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

Lower elevations and South aspects will have a crust from Saturday, but intense solar inputs will weaken these crusts on Sunday allowing for wet loose avalanches later in the day. Travel and return early before things heat up.

Travel and Terrain Advice

  • If triggered, loose wet sluffs may step down to deeper layers resulting in larger avalanches.
  • Use extra caution on slopes if the snow is moist or wet.
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

Cool temperatures with freezing levels at valley bottom are expected Saturday night. Generally clear skies (lots of solar heating!) and light W winds are expected on Sunday. Monday looks similar with a bit more wind and slightly warmer temperatures. 

Snowpack Discussion

60-100 cm of snow in the last week with snowpack depths reaching seasonal highs above 2000 m. Strong to extreme W winds with 20-40+ cm of snow on Friday created widespread storm slabs in alpine areas and sluffing in steep terrain. This recent snow sits on a variety of surfaces above treeline. Moist snow at lower elevations and on solar aspects. 

Avalanche Activity Discussion

On Saturday Visitor Safety staff observed widespread natural and skier triggered slab avalanches up to size 3 above 2000 m with 1-2 m thick crowns, especially on North and East aspects. Wet loose avalanches were observed to size 2 at lower elevations and steep south aspects, with sluffing on steep N aspects and several new cornice failures.

For More Details

Emergency: (403) 762-4506
Forecasters: (403) 762-1470

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