Bollettino valanghe

Northwest Mountains

Crested Butte Avalanche Center
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Pericolo valanghe

Sat
Alta quota
2 · Moderato
Limite del bosco
2 · Moderato
Sotto il limite
1 · Debole
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Non aggiornato — questo bollettino è scaduto 4 years ago. Stai vedendo condizioni passate, non il bollettino in corso.

In sintesi

Wind-sheltered terrain with a deep and uniform snowpack offers the safest riding today.  Light snowfall and an uptick in westerly wind speeds today may form sensitive wind drifts at upper elevations.  The new drifts will form on top of older, wind deposits from a few days ago that produced a couple of human-triggered avalanches (Ruby, Schuylkill).  The potential for deep failing avalanches is very slight but a small chance remains.  To guard against a very rare but dangerously large avalanche, choose routes that minimize exposure to shallow and rocky areas.

Problemi valanghivi

  • Wind Slabs

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

    Observers triggered a handful of fresh wind slabs 6" to 12" thick on Wednesday.  West winds are forecasted to increase later today accompanied by light snowfall, which will thicken recent slabs or form a fresh batch.  Wind drifts are most common below ridges, in crossloaded gullies, and below abrupt rollovers.  Be especially cautious in large or consequential terrain.  Cracking snow and blowing snow are signs of instability.  Most slabs are expected to be small, although there could be isolated exceptions in terrain with large fetches.  

    1/25: A skier triggered wind slab that broke about a foot thick on Ruby Peak.  

  • Deep Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Unlikely
    Dimensione
    2–3

    On most slopes in the Northwest Mountains, faceted weak layers are buried deeply and unreactive to human triggers.  Some slopes have unusually shallow or variable snow depths where riders may be able to trigger a hard slab several feet thick or more.  Be cautious of windward terrain features with variable depths, rock bands, and slopes that previously avalanched in early December where you are most likely to find a thin trigger point.   The safest riding is where the snowpack is uniform and consistently deeper than about 5 feet (150 cm).  

     

    This avalanche was triggered by explosive work in the Irwin Cat-Ski tenure on Thursday, 1/26.  It is representative of the type of terrain (shallower than normal, full of trigger points, and unsupported from below) where an unlikely chance remains for you to trigger a deep avalanche.

Avalanche Discussion

The danger level for the Northwest Mountains is MODERATE (level 2 of 5) due to the formation of wind-drifted slabs at upper elevations.  Slabs began forming around 2 days ago with observers on Wednesday reporting sensitive conditions on leeward terrain features (Ruby, Schuylkill).  In the Southeast Mountains, observers noted several recent natural Wind Slabs on leeward upper-elevation terrain that reached large size (Hunter Hill).  A layer of faceted snow from the recent cold, dry period may keep drifted slabs sensitive to the weight of a person, and with more snow/wind in today's forecast, Wind Slabs may incrementally increase in distribution and size.  Seeing blowing snow and feeling stiffer deposits of snow will help you identify the problem so you can avoid it.

The Persistent Slab problem, in the Northwest Mountains, changed in name today.  Forecast staff decided it was time to transition to a Deep Persistent Slab problem.  The nature of the problem has not changed much for roughly a week, but a new weak layer has formed at the surface due to the recent cold and clear weather.  Modest snowfall is in the forecast for the next week which may create the need to describe a new avalanche problem at the surface if slabs develop above the current weak snow surface.  Yesterday, a large avalanche was triggered with explosives, at the cat ski operation above Irwin, indicating that a very small chance lingers for deep failing avalanches in the Northwest Mountains. Travel advice remains the same; avoid areas that are shallower than average and littered with trigger points like cliffs, rocks, or sparse vegetation.  Favor terrain with a deep (greater than 5 feet) and uniform snowpack on planar slopes. 

In parts of the Southeast Mountains, it's an unhappier story, where the snowpack has remained thinner and weaker this season.    For example, stability tests near Red Mountain on Tuesday produced unstable results on both of our prominent weak layers in the lower snowpack, both at ideal depths for human triggering.   Although there's a lot of variability across the Southeast Mountains right now, stability involving deep layers is generally improving.  You'll want to assess each slope on a case-by-case basis.  This map of modeled snow depths around our forecast area paints a generalized picture of the deeper and more trustworthy areas vs. the have-nots.  The snow depth for finding the most troublesome structure seems to be around 3 to 4 feet (90 to 120 cm).  Portions of the Southeast Mountains with greater snow depths (areas bordering the Northwest Mountains or the heads of Brush and Cement Creek) still offer far more trigger points than terrain in the Northwest Mountains thus creating an increased likelihood of triggering a deep failing avalanche.

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