Avalanche Bulletin - Glacier National Park - Sun Mar 24, 2013

Issued: Sun Mar 24, 2013 08:06
Valid Until: Mon Mar 25, 2013 08:00

Afternoon cloud reduced the effects of the sun yesterday. Today should be mostly sunny- which will increase danger. Cornices are the biggest concern right now; they are large and weak, and likely to fail with the strong sun. [DM]

Public Avalanche Forecast
Danger Ratings: Sunday 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: 2 - Moderate
2 - 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.
Forecast
Forecast Monday Tuesday
alpine View Alpine Danger Rating Trend 3 - Considerable 2 - Moderate
treeline View Treeline Danger Rating Trend 2 - Moderate 2 - Moderate
below treeline View Below Treeline Danger Rating Trend 2 - Moderate 1 - Low
Confidence: Moderate - Intensity of incoming weather systems is uncertain
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
Fidelity 1905m -7 -15 0 335 Calm (0 km/h) Not Available
Rogers Pass 1315m -1 -19 0 158 Light (1-25 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: Cornices

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

Look up lots today and avoid slopes with cornices above them. Cornice failures are expected due to daytime warming or by people traveling along ridges. A section of cornice hurtling down slope is a real concern, it may also trigger slab avalanches.

Travel and Terrain Advice

  • Do not travel on slopes that are exposed to cornices overhead.
  • Stay well to the windward side of corniced ridges.

Problem 2: Dry Loose

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

It may take a while for temps to warm up today, but when they do loose avalanches are possible due to strong solar. This is especially likely on steep solar aspects and around cliff and rocky outcrops. Loose avalanches may trigger deeper slabs.

Travel and Terrain Advice

  • Watch for clues, like sluffing off of cliffs, that the snowpack is warming up.
  • Avoid sun exposed slopes when the solar radiation is strong, especially if snow is moist or wet.

Problem 3: Wind Slabs

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

Isolated wind slabs in the alpine and near ridge crests have surprised a few skiers. These likely exist in steep, aggressive terrain where even a small avalanche could have severe consequences.

Travel and Terrain Advice

  • Be cautious as you transition into wind affected terrain.
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

Lots of sun, warming temperatures, and calm to light SW winds are expected today and Monday. High cloud may form this afternoon and reduce solar inputs. Solar aspects, starting with easterly in the mornings, will receive strong solar. By Monday, air temps will be closer to 0'C and the effects of the sun will be greater. Clouds will increase on Tues

Snowpack Discussion

Cool temps and clouds have reduced solar effects; a sun crust has formed only on steep, solar aspects. Elsewhere, 25cm of dry snow exists above 1400m. Tests on two surface hoar/crust layers in the top 1.5m indicate that they may be triggered by large loads (like cornices) in some areas (most likely on solar aspects). The snowpack below is strong.

Avalanche Activity Discussion

Natural avalanches were triggered during sunny breaks yesterday. 7 size 2 to 2.5 avalanches were observed mid day from solar aspects. Many were loose, with moist debris and a few triggered slabs. Very large cornice failures were reported throughout the Selkirks yesterday. One cornice failure was reported to be 120m long and triggered a size 4!

For More Details

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

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