Bollettino valanghe

Lutak

Haines Avalanche Center
Vedi sulla mappa

Pericolo valanghe

Sat
Alta quota
3 · Marcato
Limite del bosco
2 · Moderato
Sotto il limite
2 · Moderato
Open full map
Non aggiornato — questo bollettino è scaduto 3 years ago. Stai vedendo condizioni passate, non il bollettino in corso.

In sintesi

The avalanche danger is CONSIDERABLE above 2,500′ and MODERATE below this. Wind slabs 1-3ft deep have formed over weak layers above tree-line since Tuesday. Wind loaded slopes greater than 30 degrees are becoming harder to trigger but could have unexpectedly wide propagation with high consequences. Use visual clues to identify drifted snow and avoid these areas to increase your safety margins. Cold temperatures are an additional hazard and slow any emergency response.

 

Problemi valanghivi

  • Wind Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    1–3

    On Tuesday the north wind kicked up to 15-20mph and didn't slow down until Thursday afternoon.  With 12-15" of loose and un-consolidated snow from January 3-4th there was plenty of surface snow that was drifted into start zones, including cross-loaded and top-loaded slopes. The bigger problem is that those slabs formed over widespread persistent weak layers (see Problem 2) that could produce wider propagation and more destructive avalanches. This problem is becoming more difficult to trigger but remains possible for human-triggering.

    The combination makes it very important to resist getting tempted by larger and more exposed terrain where a slide may propagate up-slope or from a shallower areas into a deeper area. These hard slab avalanches are not a problem to try and outsmart. Use wide safety margins to  manage the risk and limit exposure by identifying and avoidng areas of concern. Snow that is getting denser and more consolidated is a clue that you are venturing out on a slab, also shooting cracks and collapsing/whumphing are indicators too.

    Consider others hazards such as terrain traps like rocks, cliffs or trees that could increase the consequences of being caught in a slide, or where snow could pile up quickly

    Here is an example of a wind scoured and wind loaded slopes. Wind slabs tend to form around ridgelines and peaks. They also can form by cross-loading in gullies and across valley walls. Often propagation is from shallow to deeper areas.

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Unlikely
    Dimensione
    1–3

    Buried under wind slabs (see Problem 1) are a combination of weak surface snow, or near-surface facets from last week and surface hoar that formed Jan 6-7th. Cold temperatures are making it worse by driving the weakening of snow grains, known as faceting that make bonding more difficult. Besides last week's persisent weak layers,  snow around melt-freeze layers that act as a vapor barrier are also weakening. This includes the end of year (EOY) crust found at all elevations in this zone and other melt-freeze layers from earlier in the season.

    Understanding that slopes greater than 30 degrees have the potential to propogate an avalanche from a thin area of snow to a deeper is essential for how to mitigate. Talk to your partners about what terrain you should avoid and where you are most likely to initate a slide from. Travel one at a time to limit exposure to slopes on the way up and down. The uncertainty involved with the cold temperatures making the snow weaker will involve slope to slope assessment as we continue to map out layers and determine their sensitivity.

    Another consideration is the possibility of an avalanche stepping down from one layer into another that should contribute to the neccessary wider safety margins.

    This surface hoar grew during the calm, clear and cold weather last weekend. It was buried the evening of Jan 6-7th by 1-3" of snow. Recently formed wind slabs over this layer could produce wider than expected propagation.

Avalanche Discussion

Freezing cold tempearutures are accelerating the weakening of snow grains at the surface of the snowpack, buried below wind slabs, and above and below melt-freeze crusts. This includes the widespread surface hoar event coupled with near-surface faceting and the EOY crust.   Although triggering a slide below the crust remains unlikely, cold temperatures are slowing the healing process and drive faceting, or the weakening of these layers. Depending on what the next loading event (wind, snowfall, warming, rain, etc) is the intensity of faceting this week will produce an highly unstable sliding surface.

We should treat these layers as guilty until proven innocent and continue to monitor, track, and assess stability on a slope-by-slope basis. Melt-freeze crusts have played a role in avalanche accidents across our forecast zones and our state. Use safe travel techniques by only exposing one person to a slope at a time, avoid grouping up in run-out zones and re-consider islands of safety to truly limit your group's exposure. The people you go out with, the way you communicate, and the way you practice risk reduction as a team throughout the day is equally important as the conditions.

What we know about the EOY crust is that above freezing temperatures up to 3,000' due to an inversion at the end of the storm on Decemer 30th in the Transitional zone, caused the formation of a thick ( 1" +/-), widespread EOY melt-freeze crust.  The strength of this layer, thick and thiness is variable due to  duration and intensity of the EOY event and is elevation dependent due to a cooling trend that followed the increase in temperature at elevation.

Other layers to note are from earlier this season, and are now more than a meter deep. These include a few melt-freeze layers that are present throughout the snowpack at all elevations and aspects. These layers formed during other warming, or rain on-snow events and have mostly been found to be healing but could fail and produce a very unlikely though larger avalanche. Our overall warm and wet temperature regime through the season has helped slow the development of persistent weak layers (PWL's) in the snowpack. However at higher elevations and further inland these grain types can exist, and will require continued assessment. 

Help us gather information about weather, snowpack, and avalanche activity by reading and submitting field observations. Useful observations can be anonymous and include: 

  • Natural or human-triggered avalanche activity
  • Signs of instability: Cracking, collapsing, or whumphing
  • Red Flag Weather: Precipitation, wind loading, warming temperatures and rain-on-snow
  • Surface hoar growth, snow depth, persistent weak layers, etc
  • Snow stability tests, or pit result and location, elevation, aspect, etc 

Were you caught, carried, injured or buried in a slide? We are all human and can benefit from lessons learned, please report incidents to Haines Avalanche Center staff confidentially.

Regioni coperte