Bulletin d’avalanche

Bitterroot

West Central Montana Avalanche Center
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Danger d’avalanche

Thu
Alpin
4 · Fort
Limite forestière
4 · Fort
Sous la limite
4 · Fort
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Périmé — ce bulletin a expiré 4 years ago. Vous consultez des conditions passées, pas le bulletin en cours.

Points saillants

New snow and strong winds have created dangerous avalanche conditions today. Large human triggered avalanches are certain, and natural avalanches are likely. 

Avoid all avalanche terrain, and do not travel under or through avalanche paths. 

Problèmes avalancheux

  • Storm Slabs

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

    Over a foot of snow has fallen in the past 24 hours across the whole forecast area. This snow has fallen onto a weak interface. Expect to encounter a sensitive storm slab 1-2 feet deep on all aspects and elevations. 

    This problem is combined with wind slab today. Strong transport winds have created touchy wind slabs, in areas up to 3 feet deep. For the past 24 hours, wind direction has shifted from southwest to northwest and will continue to shift to northeast today. This will effectively create wind slab on all aspects. 

    Any wind slab triggered today will release a storm slab. Storm slabs will also have elements of wind slabs. Treat these problems the same today. Where one exists, the other is likely present.

  • Persistent Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Likely
    Taille
    2–3

    Over an inch of water has fallen in the past 24 hours, increasing the stress on an already poor snowpack structure. The two layers of concern are facets deep in the snowpack and a layer of surface hoar buried 3 feet down.

    Human triggered avalanches are very likely. These avalanches could break up to 5 feet deep and be triggered from above, to the side, or below. You are most likely to trigger these around shallow areas of the slab, convexities, or buried rocks.

    Large natural avalanches are likely. Small avalanches may step down and release full depth avalanches.

     

Avalanche Discussion

Over a foot of new snow, strong winds, and bitter cold temperatures have created dangerous avalanche conditions. In the past 24 hours, all mountainous terrain between Seeley Lake and Lost Trail Pass received at least a foot of snow with at least an inch of SWE (snow water equivalent).  The central Bitterroot has received the most snow so far, with totals at Twin Lakes SNOTEL of 16 inches of snow and 1.7” SWE. Some areas may have received even more. 

Strong winds shifted around the compass as the storm progressed. Initially, southwest winds loaded northeast aspects, then northwest winds loaded southeast aspects, and now winds are shifting to the northeast and actively loading southwest aspects. In short, expect a sensitive wind slab on all ridges and exposed terrain. Yesterday, an observation from Bass Creek noted shooting cracks on the newly formed slab anytime they left the skin track. This is a clear indication that avalanches are likely in steeper terrain. 

High spatial variability of persistent weak layers exists both between zones and inside each individual forecast zone. Within the forecast area, we have multiple buried layers of surface hoar, depth hoar, facet/crust combinations, and buried near surface facets. The slab above these weak layers is now a foot deeper and stressed by the weight of an additional inch of water. 

Large (4-5 foot deep) natural avalanches are expected today. An avalanche of this size has the potential to run full track through all elevation bands. 

Temperatures for this storm may reach historic lows. Think about the consequences of your travel decisions. Even a small injury or equipment issue could prove deadly.

Careful route finding and attention to conditions are necessary to travel in the mountains today. Be aware of what is above you. Stay out of avalanche terrain. 

If you find yourself bundled up on the couch tonight and trying to stay warm, think about submitting an observation and let us know what you saw today! 

Ski and Ride safe!

Régions couvertes