Lawinenlagebericht

Northwest Mountains

Crested Butte Avalanche Center
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Lawinengefahr

Sat
Hochgebirge
3 · Erheblich
Waldgrenze
3 · Erheblich
Unterhalb
3 · Erheblich
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Veraltet — dieser Lagebericht ist 4 years ago abgelaufen. Du siehst vergangene Verhältnisse, nicht den aktuellen Bericht.

Kernpunkte

Recent storm snow and relentless strong southwest winds have formed sensitive slabs on leeward terrain.  The strong winds stiffened the new snow into slabs in some open areas below treeline.  Avoid traveling on drifted terrain features and maintain awareness for stiffer slabs at lower elevations.  In sheltered areas, use caution and carefully assess for instabilities in the upper snowpack before entering steep terrain.  Cracking and collapsing are sure signs of unstable conditions.

Lawinenprobleme

  • Storm Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Likely
    Größe
    1.5–2.5

    Storm slabs will be likely to human triggers or they may release naturally on wind-loaded terrain during periods of heavy snowfall or wind-loading; simply avoid wind-drifted slopes.  In wind-sheltered terrain, there is uncertainty as to how reactive this avalanche problem will be. While in wind-sheltered areas look for small, steep test slopes to help identify how reactive Storm Slabs are in the area you are traveling.  Digging into the upper snowpack can also help you identify if there are other weak layers beneath the recent storm snow that could surprise you with a larger-than-expected avalanche. 

    2/22: A skier-triggered Storm Slab broke at the base of the recent storm snow on a sheltered northeast-facing slope.  This is good evidence that even sheltered terrain can produce dangerous human-triggered avalanches. 

  • Persistent Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Possible
    Größe
    2–2.5

    Since the end of January, the upper snowpack has become layered and more complicated. Persistent slab avalanche concerns will first develop in the highlighted areas above. Buried in the upper snowpack in these highlighted areas, is a stack of thin crusts with layers of facets between them. These persistent weak layers may produce remotely triggered avalanches, and avalanches that both release deeper and wider than you expect. Given how specifically located this avalanche problem is, changing avalanche conditions could easily catch you off guard while moving from one slope to another.  Uncertainty exists as to when and how much loading it will take before crust/facet layers in the upper snowpack begin to fail.  Collapsing, or whumpf-ing, are a clear sign that these layers have reached their breaking point and you should avoid traveling on or below steep slopes.

    Layers of facets sandwiched between thin crusts are creating persistent slab avalanche concerns in some areas. 

Avalanche Discussion

Over two feet have fallen in the Northwest Mountains and around a foot in the Southeast Mountains with an additional 3 to 6 inches expected today for many areas.  Southwest winds have blown relentlessly, transporting the new snow to leeward terrain forming drifts up to 3 or 4 feet thick.  The winds will continue to transport snow on Friday.  Wind speeds have been strong enough to create atypical loading patterns in some locations like drifting lower in the start zone or open below treeline features.  Visibility has been limited but some natural avalanche activity has been observed on leeward terrain on Whetstone and Mount Emmons.  Irwin Cat-Ski reported explosive-triggered avalanches in drifted easterly terrain. Given the dangerous size of drifted slabs and the continued loading, it seems like a straightforward proposition to simply avoid drifted terrain to greatly reduce your risk of triggering a dangerous avalanche.

There is a Persistent Slab problem listed for both the Northwest and Southeast Mountains forecast zones.  A CBAC forecaster experienced a large rumbling collapse that shot cracks through the top of Climax Chutes.  The group was surprised by the collapse and by the long distance it propagated.  The failure occurred somewhere in the middle of the snowpack 3 feet down on a thick layer of facets.  An observation from Thursday, outside the CBAC forecast area near McClure Pass, stated a skier triggered an avalanche much larger than expected breaking on a crust/facet layer in the upper snowpack.  Luckily the skier was able to ski out of the moving debris before being carried down the slope.  It is not clear yet if these surprises are isolated events or if they point to a broader problem with Persistent Weak layers in the upper snowpack.

A final round of snowfall and wind will add further stress to the snowpack in drifted terrain and sheltered areas that have buried weak layers.  Conservative decisions and dialing back terrain selection will help you manage the uncertainty of triggering a large avalanche.  Predicting when buried weak layers 'wake up', or become reactive, can be difficult and is best managed with a wider-than-normal margin for error.  

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