Bollettino valanghe

Northwest Mountains

Crested Butte Avalanche Center
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Pericolo valanghe

Fri
Alta quota
3 · Marcato
Limite del bosco
3 · Marcato
Sotto il limite
2 · Moderato
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In sintesi

Strong westerly winds and fresh snowfall have combined to create dangerous avalanche conditions on wind-loaded terrain. Fresh wind slabs are expected to be sensitive to human triggers and could release large enough to bury you. Very large avalanches that would release near the bottom of the snowpack are most suspect on the northern half of the compass where the snowpack depth is irregular or variable. For a reduced risk of triggering an avalanche, search for wind-sheltered terrain that has an even distribution of snow across the terrain. 

Problemi valanghivi

  • Deep Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    2.5–3.5

    It is possible to trigger a very large and destructive avalanche today in steep terrain or from flatter terrain where the snowpack connects to a start zone.  These widely propagating and long-running avalanches will break 6 to 10 feet thick or more on weak layers near the bottom of the snowpack.    Persistent slabs are most likely to be trigged on the northern half of the compass, where buried layers are most fragile.  Slabs have grown stubborn to human triggers because of how thick they are.  You are most likely to trigger a monster avalanche where slabs are thinner.  Thin areas are more common at higher elevations where the wind has redistributed snow into variable snow depths.   

    A recent deep persistent slab avalanche near Yule Pass.  

  • Wind Slabs

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

    Westerly winds have drifted snow at upper elevations for several days now. The recent new snow over the last 24hrs is expected to have created fresh wind slabs that will be sensitive to human triggers. These slabs will most likely release 1 to 2 feet deep, but they could further step down into old snow layers and produce a destructive avalanche. Look for thicker, denser lobes of drifted snow or blowing snow to identify the problem. Those areas will be most common below ridgelines and in cross-loaded terrain features at near and above treeline elevations. New slab formation will be more isolated in distribution at lower elevations and on aspects not highlighted by the problem rose above. 

    A small wind slab that ran naturally on the south face of Mt. Owen sometime around 1/2.

Avalanche Discussion

You may have noticed that it's been a bit breezy since yesterday. The wind has been gusting into the 30's through valley locations like Crested Butte, and into the 60's at 12,000ft. 24hr snow totals look to be in the 3 to 6" range on average, but those remote weather station numbers are going to be inaccurate with all the wind. The snow that accumulated yesterday morning was very light and low density. Yesterday afternoon the new snow was becoming heavier and rimed. We've also seen an increasing temperature trend over the last 24hrs. If we had bigger snow totals over the last 24hrs we would be setting up for a widespread storms slab avalanche problem. With new snow depths averaging less than 8", new slab formation is expected to be more specific to wind-loaded terrain. The wind forecast stays elevated through Thursday, before decreasing into Friday. The avalanche danger is expected to decrease on Friday if we see an end to the continued wind-loading by tomorrow morning. 

The second issue with new snowfall and additional wind-loading is the impact on deeply buried weak layers. The likelihood of triggering a Deep/Persistent Slab avalanche was raised to Possible on Wednesday, to account for uncertainty on leeward terrain features accumulating more wind-driven snow. Thick, hard slabs that formed over the holidays will likely tolerate substantial new load before failing, but this will be the first storm to test these structures.  Explosive testing at Irwin produced a sizeable avalanche on westerly-facing terrain that was shallower due to the wind stripping away a good portion of the slab from the holiday stormy period. Areas with a below-average snowpack or a variable snowpack depth would be the most suspect areas for humans triggering this avalanche problem. A triggered avalanche could then propagate widely into a much deeper snowpack that would produce a very large and destructive avalanche. 

Regioni coperte