Avalanche Bulletin - Glacier National Park - Sun Apr 30, 2017

Issued: Sun Apr 30, 2017 07:31
Valid Until: Mon May 01, 2017 08:00

Large areas of Glacier National Park are CLOSED for avalanche control using EXPLOSIVES. Daily or annual winter permits are required to access winter restricted areas. Access information is available at the Rogers Pass Discovery Centre

Today is the last day of the winter permit system. New snow in the alpine accompanied by strong south winds will make things feel wintery up high and storm slabs will be reactive to human triggering. [CG]

Public Avalanche Forecast
Spring Conditions:  Sunday
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.

More Spring Conditions details.

Past 24 hour weather
Station Maximum (°C) Minimum (°C) Snowfall (cm) Snow Pack (cm) Wind speed Ridgetop wind direction
Fidelity 1905m 5 -2 10 327 Moderate (26-40 km/h) S
Rogers Pass 1315m 9 -1 1 104 Strong (41-60 km/h) S
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
Which Slopes?
Which Slopes? northwest, north, northeast, east
Chance of Avalanches?
Chance of Avalanches? possible
Expected Size?
Expected Size? small - large

Expect to find pockets of windslab on high elevation N aspects formed by the new snow being redistributed by S-SW winds.

Travel and Terrain Advice

  • Be careful with wind loaded pockets, especially near ridge crests and roll-overs.
  • Use extra caution on solar slopes or if the snow is moist or wet.

Problem 2: Loose Wet

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

Surface crust breakdown can be expected with daytime warming. Wet slides will occur once the crust softens.

Travel and Terrain Advice

  • Use extra caution on solar slopes or if the snow is moist or wet.
  • Minimize exposure to steep, sun exposed slopes when the solar radiation is strong.

Problem 3: Cornices

Which Elevation?
Which Elevation? alpine
Which Slopes?
Which Slopes? north, northeast, east, northwest
Chance of Avalanches?
Chance of Avalanches? possible
Expected Size?
Expected Size? small - large

Cornices are huge and hanging over alpine lee slopes in many places. Daytime warming may promote failure of these large triggers. Cornice failure may wake persistent weak layers.

Travel and Terrain Advice

  • Pay attention to overhead hazards like cornices which could trigger the deep persistent slab.
  • Avoid slopes when the solar radiation is strong, especially if they have large cornices overhead.
Problem Glossary
Loose Dry
  • 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.
Loose dry 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.
Loose Wet
  • Are more powerful than loose dry avalanches due to their higher density.
  • Are often limited to sunny slope aspects.
  • Are commonly confined to the warmest part of the day.
Loose wet 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

A mix of sun and cloud today with isolated flurries. Freezing levels should remain below 1700m and winds will be moderate from the SW. More of the same for Mon and Tues, then a dramatic warm-up on Wed-Thurs with freezing levels rising to 3000+m and light precipitation.

Snowpack Discussion

High elevation N aspects still hold a winter snowpack with surface windslab that may be sensitive to human triggering given the right location. On all other aspects, expect temperature and sun crusts until daytime warming softens the surface. The snowpack is isothermal below these surface crusts at tree-line and below tree-line elevations.

Avalanche Activity Discussion

No new activity has been observed. Glide cracks continue to widen with the Spring warming and are very difficult to predict when they will fail.

For More Details

Emergency: 1 (877) 852-3100
Forecasters: (250) 837-7500

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