Lawinenlagebericht

Rattlesnake

West Central Montana Avalanche Center
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Lawinengefahr

Wed
Hochgebirge
2 · Mässig
Waldgrenze
1 · Gering
Unterhalb
1 · Gering
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Kernpunkte

Skiers and riders could trigger small wind slab avalanches on wind-loaded upper elevation slopes. It is also still possible to trigger persistent slab avalanches failing on buried surface hoar on upper elevation north and east aspects. Avoid steep wind-loaded slopes and convexities. 

Lawinenprobleme

  • Wind Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Possible
    Größe
    1–1.5

    Moderate west winds have built wind slabs on upper-elevation east aspects. Wind slabs will grow this afternoon through tonight as light to moderate snow falls with continued west winds. We can expect 3-8 inches of snow by tomorrow morning. Wind slabs are often found just below ridges and in cross-loaded gullies. They can be identified by their smooth and rounded appearance. Cornices, wind lips, and drifts all indicate wind direction and loading. Shooting cracks are a red flag that dangerous conditions exist, but you may not see them until you are on the slab and it is too late. Test small, safe, representative slopes before moving into bigger terrain. The best way to mitigate this problem is to avoid wind-loaded slopes over 35º. 

    (1/23/2023)

    Ski cuts are able to produce D1 wind slabs along ridgelines on northeast aspects.

    Photo: Mackenzie

  • Persistent Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Possible
    Größe
    1–2

    A layer of surface hoar is buried 8-24 inches deep throughout the region. The slab above the surface hoar is getting deeper and more cohesive due to recent snowfall and wind loading. The reactivity of the surface hoar has been patchy, but observers continue to report propagating test results and small avalanches in isolated areas. 

    You are most likely to trigger an avalanche on this layer on upper elevation north and east aspects that are either wind-loaded or steep, unsupported, convex rolls. The best way to manage the problem is to avoid slopes and convex rolls over 35º. If you choose to ski steeper terrain, carefully assess the snowpack for a lower-density stripe of buried surface hoar. Use small, safe, and representative convex rolls to test the reactivity of the layer. Avoid traveling in or above terrain traps. In most places, the surface hoar is laying flat and is not well preserved, so you probably won't get red flags like shooting cracks or whumpfing, but if you do, stick to low-angle slopes. 

Avalanche Discussion

After a calm, sunny day on Saturday, we’ve seen cloudy skies with light snow showers and moderate west winds. That pattern will continue today and through this week until a stronger storm impacts us Friday and Saturday. There will be a slight bump up in snow shower activity this afternoon through tonight, leading to 3-8 inches of snow by tomorrow morning. 

Avalanche conditions remained fairly consistent over the last week, except on Saturday when we experienced a natural wet loose avalanche cycle on steep south facing slopes. The snow has frozen back up, and crusts are now present under the fresh snow on south aspects and at lower elevations. 

The primary avalanche problems today are wind slabs on wind-loaded upper elevation slopes and persistent slabs failing on buried surface hoar. The layer of surface hoar is generally buried deeper as you go north. It is up to 24 inches deep in the Seeley Lake zone but only 8 inches deep in the Southern Bitterroot. 

The sensitivity of the surface hoar has been an ongoing race between the bonding and healing of the surface hoar crystals and the deepening and stiffening of the slab above due to settlement, snowfall, and wind loading. The result is that the reactivity of the surface hoar has been patchy. In some areas, the surface hoar is less reactive because it is toppled over and bonding. Observers reported non-propagating snowpit test results in the Rattlesnake over the weekend. However, the layer has remained sensitive in other areas. Yesterday, in the Southern Bitterroot, the slab above the surface hoar had stiffened enough to create very easily propagating snowpit test results. On Sunday, riders in the Seeley Lake zone near Lake Dinah were able to trigger small avalanches on the surface hoar on unsupported, convex rolls. 

Unfortunately, we can’t write off the surface hoar layer yet. The most likely trigger spots are wind-loaded areas and steep, unsupported, convex slopes at upper elevations. North and east aspects are the most suspect because they are the least affected by sunshine and wind scouring. We are generally not seeing obvious signs of instability like shooting cracks, so it feels stable despite the lurking weak layer. Before you decide to ski steeper terrain, carefully asses the snowpack for a lower-density stripe of buried surface hoar. Consider the consequences of an avalanche in the terrain you’re riding. Is there a clean runout? Or are there trees, cliffs, and other terrain traps? Avoiding slopes and convex rolls over 35º remains the best way to manage the problem. 

Ski and ride safe. 

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