Bollettino valanghe

Northwest Mountains

Crested Butte Avalanche Center
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Pericolo valanghe

Wed
Alta quota
3 · Marcato
Limite del bosco
3 · Marcato
Sotto il limite
3 · Marcato
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Non aggiornato — questo bollettino è scaduto 4 years ago. Stai vedendo condizioni passate, non il bollettino in corso.

In sintesi

A storm system will deliver up to a foot of snow to some parts of the Northwest Mountains by sunset and the avalanche hazard will slowly increase as snowfall accumulates.  The new snow will accumulate on a snowpack that largely remains wet from the recent warm weather.  As the snow stacks up today greater than about 8 inches, you can trigger an avalanche in the storm snow.  An avalanche in the storm snow could gouge into the wet snowpack below creating larger than expected debris piles, particularly at low elevations.  High elevation northerlies have a dry snowpack and there is a chance you can trigger a large slab avalanche that breaks several feet deep.  Use caution where you find an unsupportive snowpack below the storm snow, and on northerly-facing terrain.

Problemi valanghivi

  • Wet Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    1.5–2.5

    Consecutive days of intense meltwater production and several nights with poor refreezes are causing natural and triggered wet slab concerns.  Several large natural wet slabs released over the past few days, caused by meltwater weakening buried weak layers.   Today's storm system could exacerbate this problem as the wet weak layers need to carry the burden of a new snow load.  The most concerning terrain face northwest to north to northeast at lower elevations where last night delivered another poor refreeze.

    3/27:  This destructive wet avalanche on Gothic Peak started as a wet loose and then propagated as a narrow wet slab as it ran further downslope.  

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Unlikely
    Dimensione
    1.5–2.5

    On the northerly aspects, there is a rare but dangerous chance you can trigger a hard slab avalanche that breaks up to several feet thick on weak faceted snow that was buried last month.  The new snow will add burden to buried weak layers and cause a slight uptick in the likelihood of you triggering a large slab avalanche. Slabs can be triggered remotely, from below or on adjacent slopes.  Given the thickness and hardness of slabs, you are unlikely to get direct feedback, such as collapsing or cracking, until it's too late.  In the Northwest Mountains, slopes holding a shallower snowpack (less than about 2.5 meters deep) are most suspect.  This could be in steep, rocky terrain or in terrain that has variable depths due to wind erosion.  When in doubt, head for lower consequence terrain or slope angles in the lower 30-degree range and less. 

    This large persistent slab avalanche was remotely triggered on 3/18. 

  • Storm Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    1–2

    The current storm system is set to deliver up to 12 inches of snow by the end of the day for the Northwest Mountains.  As storm snow accumulates throughout the day the hazard will slowly increase.  Warm temperatures at the onset of the storm will likely provide a reasonable bond between the storm snow and the old snow surface.  Look for signs of instability like cracking to identify a developing problem in the storm snow.  Consider the possibility of a small avalanche in the storm snow entraining wet snow below at low elevations.  Investigate the bonding between the old and new snow on small test slopes and use caution where it resides about an unsupportive, wet snowpack.

    An example of an avalanche breaking in storm snow.  Although good bonding is expected between the new and old snow, take the time to investigate the new interface.

Avalanche Discussion

There is a healthy amount of uncertainty about how the snowpack will behave with the incoming storm.  The most significant avalanche hazard will follow where the most snow accumulates in the Northwest Mountains AND at lower elevations in both forecast zones due to the poor refreeze.  The uncertainty comes from the combination of a snowpack still draining meltwater from the recent warm weather and a substantial new snow load.  Take a cautious approach where you find more than about 8 inches of new snow, slopes included in the Wet Slab/Persistent Slab distribution roses as this is where you could trigger a dangerous slab avalanche, and on low elevation terrain where the snowpack remains unsupportive and wet.

Below treeline terrain had a very poor refreeze last night which has created wet and mushy conditions for today.  Last night was the warmest low temperatures below treeline in the past week.  The threat of Wet Loose and Wet Slab avalanches is greatest at lower elevations.  A small triggered avalanche on steep terrain could release a Wet Slab on shady terrain, or simply gouge into the snowpack and create large avalanches.  If you find an unsupportive snowpack below the storm snow, your safest option will be to choose lower angled terrain.

Areas in the Northwest Mountains could accumulate a foot of new snow today.  This will create storm snow problems for you to manage where you find it resting on a supportive snowpack.  The new snow started to accumulate during relatively mild temperatures which often promotes good bonding and winds are forecasted to remain light today which reduces the chances of encountering a wind drifted slab.  Use a conservative mindset at upper elevations to ensure that the snowpack below the storm snow is well frozen and that there is a good bond between the new and old snow.

Today is a good time to dial back terrain choices and slope angles to account for the dramatic changes to the snowpack.

 

 

 

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