Bulletin d’avalanche

Northwest Mountains

Crested Butte Avalanche Center
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Danger d’avalanche

Mon
Alpin
4 · Fort
Limite forestière
4 · Fort
Sous la limite
3 · Marqué
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Points saillants

An AVALANCHE WARNING is in effect for the Northwest Mountains.  Winds are cranking, storm totals are nearly 3 feet, and we have a guilty weak layer near the base of the snowpack.  It's going to be a mess out there!  A large natural avalanche cycle is underway and human-triggered avalanches are very likely.  It is also easy to remotely trigger avalanches from flat terrain below or adjacent to slopes steeper than about 30 degrees.  We recommend avoiding avalanche terrain, including low-angle runout zones.   

Problèmes avalancheux

  • Persistent Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Very Likely
    Taille
    1.5–2.5

    Large and dangerous avalanches breaking into old weak layers will be easy to trigger and are running naturally today on northerly and easterly-facing terrain.   Slabs are typically 2 to 3 feet thick below treeline and grow larger and even more dangerous in wind-loaded terrain; a few could be up to D3 in size.   These avalanches have the potential to break wider than you expect, they can be triggered from flat slopes below steep terrain, and naturals could run well into low-angled runout zones.  Avoid travel on or below slopes steeper than 30 degrees in the terrain highlighted by the distribution rose above.      

    One of many large persistent slabs that failed naturally during last weekend's storm.  Similar, albeit larger, activity is expected this weekend.

  • Storm Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Very Likely
    Taille
    1–2

    Storm totals are approaching three feet in the Northwest Mountains.  Slabs of new and drifted snow were easy to trigger and failed naturally yesterday: the problem continues to grow larger today.  Slabs are larger and more sensitive in wind-loaded terrain.  Storm slabs are likely to step down and trigger a larger persistent slab where the problems overlap.  You can avoid this problem by staying off of slopes steeper than about 30 degrees.  

    A storm slab triggered yesterday below treeline in the Slate River drainage. 

Avalanche Discussion

As of 5 a.m., 3-day storm totals range from 1 to 3 feet, favoring the Northwest Mountains.  Storm water is 1.6" at Irwin, 1.3" at Schofield, .9" at Butte, and .6" at Upper Tayler.  Winds increased over the past 24 hours and are blowing 20-30 mph this morning, gusting beyond 60 in the alpine.  This illustrates how the most dangerous conditions are in the snow-favored and wind-loaded terrain of the Northwest Mountains, where slabs are the largest and most widespread.  We have documented a clear pattern of avalanches breaking on our early season weak layers with each storm, and our current storm is the most potent yet. Easily triggered avalanches and natural activity are expected across the entire forecast area, especially in windrifted terrain or on shaded aspects with more than about 18" of storm snow.  The avalanche cycle culminates today under peaking wind speeds and the final pulse of snowfall. Although natural activity will diminish tomorrow the threat of human-triggered slides will remain very concerning.  

We expanded the distribution of persistent slabs today to include a few more aspects (SE and W) harboring collapsible crusts at the start of the storm.  It's another persistent weak layer that could produce unusual behavior and remote triggering. That's in addition to our well-documented persistent weak layer that formed during the November dryspell on the northern half of the compass.  We have some uncertainty this morning on whether storm totals in the Southeast Mountains are enough to form reactive slabs below treeline. We errored on the side of caution with the danger rating there, but with careful snowpack evaluation, you may find some slopes are still lacking a slab.  Elsewhere, avalanche conditions are full on today, with high confidence.  

Régions couvertes