Avalanche forecast

Rattlesnake

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Out of date — this forecast expired 4 years ago.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

In the Rattlesnake, skiers and riders are likely to trigger avalanches large enough to bury a person on slopes over 30º, especially on north aspects. These may be triggered from above, below, and to the side. Leave a wide margin of safety between you and avalanche slopes. Thin snowpack below 7000' increases the risk of injury from hazards like rocks and stumps.

Avalanche problems

  • Persistent Slabs

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

    I wish I could say that the persistent slab in the Rattlesnake has improved more than it has. Unfortunately, it may have improved just enough to lure skiers and riders into a false sense of security. Collapsing and cracking are now isolated and small compared to last week. However, the structure remains poor. On upper elevation northerly and shaded slopes, it is likely for skiers and riders to trigger avalanches large enough to bury, injure or kill a person. On south slopes, it may be harder to trigger slides, but the structure exists for large avalanches if you find a trigger point, such as thin areas around rollovers, rocks, and bushes. 

    Compounding the problem in the Rattlesnake, is a shallow snowpack below 7000’ with a wide variety of early season hazards that increase the risk of trauma if caught in an avalanche. The snowpack at mid elevation is mostly faceted and could result in higher avalanche hazard when a slab forms on top of it. 

    Avoid slopes over 30º in the Rallesnake to mitigate the avalanche hazard. Avalanches in this zone could be triggered remotely from above, below, and to the side. Leave a wide buffer between yourself and avalanche terrain. 

  • Wind Slabs

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

    Winds have been moderate, with strong gusts and westerly. 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. 

    Wind slab may trigger deeper persistent weak layers. Avoid slopes over 30º in this area.

     

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. However, careful assessment is still required. 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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