Bollettino valanghe

Northwest Mountains

Crested Butte Avalanche Center
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Pericolo valanghe

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

In sintesi

Yesterday's windy storm and today's warmup are conspiring to make dangerous conditions.  The most dangerous terrain is on steep, wind-loaded slopes, where it will be easy to trigger fresh wind slabs and there is greater potential for larger, deeper failures.   In wind-protected terrain, sluffs will become heavier and more reactive as the day warms up, and the moistening storm snow could also fail wider as a slab.  The highest quality powder and safest riding is in wind-protected, north-facing terrain, where these problems are less likely, but not impossible. 

Problemi valanghivi

  • Storm Slabs

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

    1 to 2 feet of snow and strong winds have formed sensitive slabs in the new snow up to several feet thick.  Slabs will be largest and most reactive in wind drifted terrain at higher elevations.  They may also become easy to trigger on sunbaked slopes as the new snow moistens or consolidates.  Look for cohesive, slabby feeling snow and cracking in the snow to alert you to instabilities.  A triggered avalanche in the storm snow could release an avalanche deeper in the snowpack, particularly on aspects highlighted by the Persistent Slab rose.  Choose slope angles less than 35 degrees out of the wind and sun to avoid the most problematic slopes.  

    A recent slab in drifted terrain.  Similar avalanches are expected today. 

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    2–3.5

    Large, stubborn slabs several feet thick or more exist over faceted weak layers in the upper snowpack.  Remote triggers and natural avalanches failing on these persistent weak layers have been gradually diminishing since widespread activity last week, though this latest storm may cause an uptick in activity again.  The greatest number of avalanches have occurred on easterly-facing slopes where faceted crusts exist. Similar structures exist on west-facing terrain.  Recent warming on lower elevation and more southerly aspects has strengthened the upper snowpack with crusts, and triggering persistent slabs is less likely in that terrain.   Weak layers on north-facing terrain are less developed and have been less active.  

    Early warning signs for this avalanche problem are rare and feedback is infrequent. You can reduce your chance of triggering this avalanche problem by avoiding recently wind-loaded terrain and unusually shallow terrain.  Safer riding is where the snowpack is deep, wind-protected, and on slope angles less than 35 degrees, without steeper slopes above. 

    A recent persistent slab on Schuylkill Ridge.  

  • Loose Wet

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

    The new snow will easily sluff off of steep, sunbaked terrain once the now surface becomes moist or wet.  In some terrain, sluffs could trigger slabs in the new or drifted snow.  Wet loose instabilities are easy to identify: look for small sluffs, rollerballs, or pinwheels cascading off of rocky or very steep terrain to alert you to changing conditions.   Stay away from terrain traps and consider overhead hazards once wet loose conditions develop. You can avoid the problem by moving to colder aspects or lower-angle terrain.  

    Wet loose avalanche activity that ran earlier this week. Similar activity is expected today.  

Avalanche Discussion

The clash of a potent winter storm yesterday with today's spring-like weather will make for dangerous and challenging conditions.  Storm totals range from 9" to 24", favoring the Northwest Mountains.  There are two main considerations driving instabilities within the storm snow.  The first, and most hazardous, is wind loading.  Yesterday's alpine winds were blowing 20 to 40 mph, gusting 40 to 70mph, which is easily strong enough to transport the recent snow into thicker and more reactive drifts.  Secondly, today we'll see strong solar radiation with temperatures rising above freezing near and below treeline.  This will moisten and settle the new snow, causing the potential for slab formation in wind-protected terrain and promote wet loose sluffing.  Conditions can change rapidly; monitor the snow surface and use test slopes to observe how the snow is behaving before riding in bigger or steeper terrain.  Due to higher storm totals in the Northwest Mountains, storm snow instabilities have the potential to be larger and more common, and we have a higher danger rating below treeline. 

The joker in the deck is our persistent slab problem.  The warmup earlier this week helped consolidate the snowpack and formed crusts on some of the more problematic aspects, which is a favorable trend after numerous persistent slabs ran naturally or were triggered last week.  However, yesterday's storm will once again add stress to various weak layers in the upper snowpack (all zones) or deep in the snowpack (the shallow zone). With clear weather over the next two days, we'll gain a better sense of how those layers responded.  I'm hopeful we'll see less activity, but I'm still giving concerning terrain features a wide buffer until we can get more data and the snowpack has time to adjust.  The most concerning features are slopes with an unusually shallow snowpack or wind-drifted terrain, especially easterly-facing slopes. 

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