Avalanche forecast

Bitterroot

West Central Montana Avalanche Center
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Avalanche danger

Sun
Alpine
3 · Considerable
Treeline
2 · Moderate
Below
1 · Low
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Out of date — this forecast expired 4 years ago. You’re viewing past conditions, not the current bulletin.

Highlights

Drastic differences in snowpack depth, coverage, and weak layer distribution mean varied hazards in the Bitterroot and necessitate careful attention to conditions. In shallower areas of the Bitterroot, human triggered avalanches larger enough to bury, injure or kill a person are likely in terrain over 30º, especially in northern aspects. Full depth avalanches are possible. These may be triggered from above, below, and to the side. Leave a wide margin of safety between you and avalanche slopes. 

Avalanche problems

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    1.5–2.5

    The snowpack across much of the Bitterroot zone has a poor structure. There are widespread layers of surface hoar and depth hoar buried 1.5 to 3 feet deep in the snowpack. These weak layers are getting more stubborn to trigger, but the consequences remain high. 

    Human triggered avalanches are likely on upper elevation slopes and possible at middle elevations. Northerly aspects are more likely to harbor instabilities.  Avoid terrain steeper than 30 degrees to reduce your risk. Avalanches can be remotely triggered from below, so give steep terrain a wide berth. Shallow areas and convexities are likely trigger spots. 

    Certain areas, such as around Lost Horse and Camas Lake in the central Bitterroot have a deeper snowpack, and more bonding has taken place, triggering avalanches is less likely. However, travel in these areas still requires careful assessment and terrain management. Recent avalanches, whumpfing, and cracking are all signs of unstable snow and should be weighted more heavily than snowpit stability test results. When in doubt, stick to lower angle terrain. 

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    1–1.5

    Winds have been westerly and moderate, with strong gusts. Easterly slopes have been loaded, and wind slab can be found at upper elevations below ridges and along the edges of cross loaded gullies. 

    Wind lips and cornices indicate wind direction and loading. Pay attention to the snow and avoid hollow sounding pillows, drifts, and wind loaded start zones over 35 degrees. 

    Wind slab may trigger deeper persistent weak layers. Avoid slopes over 30º where these are present.

     

Avalanche Discussion

In the past 48 hours, the mountains have received between 2” and 4” new snow. Winds have been moderate and westerly with strong gusts. Temperatures have been in the mid teens. Today a trace of snow is forecast. Winds will be moderate out of the southwest, and temperatures will be in the mid 20s.

The past week has been relatively mild compared to the week before. Some snow trickled in, a few inches here and a few inches there. Not enough to tip the balance on the weak layers present. The temperatures have been moderate, not warm enough to significantly bond the facets in the snowpack, and not cold enough to make them much worse. Conditions remain similar to the previous two forecasts, challenging and sketchy. 

Drastic differences in snowpack depth coverage and weak layer distribution mean varied hazard in each zone and necessitate careful attention to conditions. In many places in our forecast area, the snowpack structure is very poor; in these places, human triggered avalanches are likely. The problem is widespread enough to keep the hazard at considerable. Generally, areas where the snowpack is under 4 feet deep have touchier persistent weak layers, and the likelihood of triggering slides is higher.

The poor structure alone in these areas should be enough to keep you off any avalanche slopes. Collapsing and cracking are further signs of instability. These warning signs have decreased as the overlying slab has stiffened, but the potential for larger avalanches has increased. Stability tests are redundant when poor structure and signs of instability are present. If you use the extended column test to check stability, don’t trust a lack of propagation as a sign of stability, the current persistent problem can produce false stable results. Take any propagation that does happen as extra confirmation to stay out of avalanche terrain.

Certain areas, such as the southern Missions and the central Bitterroots around Lost Horse and Camas Lake, have a deeper snowpack, and weak layers have bonded. As a result, avalanches are less likely. If you are in doubt about stability, the easiest approach is to manage terrain and stick to slopes under 30º. 

Remember to check the forecast zone you plan to ride, sled or ski in, as there is information specific to each zone. 

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Ski and ride safe. 

 

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