Bollettino valanghe

Northwest Mountains

Crested Butte Avalanche Center
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Pericolo valanghe

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Non aggiornato — questo bollettino è scaduto 6 years ago. Stai vedendo condizioni passate, non il bollettino in corso.

In sintesi

You can still trigger a dangerous avalanche where you find supportive slabs resting on top of weak snow.  The most concerning terrain features are located near and above treeline where winds have formed large, cohesive slabs.  Signs of instability have become less frequent, so don't necessarily expect feedback from the snowpack.  If you are stepping into avalanche terrain, choose terrain features that have lower consequences like slopes that are smaller in size and are free of terrain traps.

Problemi valanghivi

  • Persistent Slabs

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    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    2

    On or near steep terrain, you can trigger persistent slabs 2 to 4 feet thick that fail on weak snow near the ground.  Persistent slabs often surprise even the most experienced travelers: They give irregular feedback, they can propagate unusually wide distances, and they can be remotely triggered from below or adjacent to the slope.  To avoid this problem,  choose routes that stay off of, and out from underneath, steep terrain holding dense slabs over sugary weak layers.  Poor structure is most common on the slopes highlighted by the distribution rose above, on west to north to east and some southeast slopes.   Shooting cracks and collapses are clear warning signs to stay on slope angles less than about 30 degrees, although their absence isn't a guarantee of safety.   In the Northwest Mountains, slabs are generally thicker and denser: you are most likely to trigger them from shallow points on the slope such as rock bands, pungie trees, or where slabs taper in thickness from wind effects.  

    12/31: A large, remotely triggered persistent slab in Redwell Basin

  • Wind Slabs

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    • Limite del bosco
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    Probabilità
    Possible
    Dimensione
    1

    Fresh Wind Slab formation may be found on upper elevation slopes that are easterly facing.  Expect them to have formed immediately below ridgetops and have a smooth, pillowy appearance.  Shallow Wind Slabs may also be present on windward aspects in gullies or cross-loaded behind ribs.  Expect the Wind Slab avalanche problem to primarily exist in the Ruby Range of the Northwest Mountains where accumulations were greater than 6 inches.  Slabs may be up to a foot thick on terrain features that have an efficient fetch. 

Avalanche Discussion

Yesterday's storm created a small impact on the overall avalanche hazard for a limited geographic area in the Northwest Mountains.  Areas in the Ruby Range, in the Northwest Mountains, received up to 7 inches of new snow that was transported by westerly winds.  Fresh, shallow Wind Slabs have formed on leeward terrain features with easterly aspects being the bullseye.  With limited amounts of new snow, Wind Slab formation will be on the small side and likely be found immediately below ridgetops.  Fresh drifts could be up to a foot deep in places but will taper in thickness further below ridgetop.  The Wind Slab problem largely overlaps with the Persistent Slab problem.  Consider the possibility that a triggered Wind Slab could "step-down" and release a much larger, and more dangerous Persistent Slab avalanche.  The most likely places to find a Wind Slab are precisely the locations with the most worrisome Persistent Slab issues.

Sheltered areas that face west, north, and easterly aspects continue to hold on to the Persistent Slab problem, but the snowpack has been slowly changing since the last major loading event back around December 28th.  For areas in the Southeast Mountains, slabs have little by little began to lose strength due to faceting.  As slabs facet and lose strength, they are less likely to propagate significant distances and the expected avalanche size is becoming smaller.  Even though slabs are weakening, if you trigger an avalanche and are pushed into terrain traps like rocks, trees, or ditches then the consequences remain high.  To see an example of weakening slabs in the Southeast Mountains take a look at this observation from Snodgrass.  Sheltered terrain in the Northwest Mountains has received significantly more snow than the Southeast Mountains and slabs are denser and have yet to lose as much strength.  The Persistent Slab problem, in sheltered terrain, for areas with deeper accumulations remains a more concerning problem due to the potential size even though triggering has slowly become less likely.

Drifted terrain features near and above treeline, in both the Northwest Mountains and Southeast Mountains, are the most dangerous slopes.  The faceting process will not be able to weaken the slabs that have formed.  Expect the continued possibility of triggering a deadly avalanche on drifted features to linger well into the future.  Consider this fact: 40% of avalanche fatalities occur at a MODERATE danger level.  The avalanche hazard is improving, but do not let your guard down because signs of instability like collapsing and cracking are decreasing.  Continue to expect surprises if you are traveling around drifted features at upper elevations.

 

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