Avalanche forecast

Rattlesnake

West Central Montana Avalanche Center
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Avalanche danger

Fri
Alpine
2 · Moderate
Treeline
1 · Low
Below
1 · Low
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Highlights

Wind slabs on upper elevation east aspects will grow today as a significant storm begins to impact our region. Skiers and riders could possibly trigger persistent slab avalanches on buried surface hoar on isolated upper elevation north and east aspects. Avoid steep wind-loaded slopes and convexities. Expect dangerous avalanche conditions to develop tonight and last into the weekend. 

Avalanche problems

  • Wind Slabs

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

    Wind slabs on upper elevation east aspects will grow today as a significant storm begins to impact our region. Winds increased to moderate to strong out of the west early this morning. The forecast is calling for 3-6 inches of snow by nightfall today and over 30 inches by Saturday. Stay alert for changing conditions. 

    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. 

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Unlikely
    Size
    1–2

    A layer of surface hoar is buried 10-24 inches deep throughout the region. The reactivity of the surface hoar is patchy, and triggering an avalanche on the layer is now unlikely. In most places, the surface hoar is lying flat and rounding, and is stubborn or unreactive. Over the weekend, observers reported propagating snowpit test results and small avalanches on unsupported convexities. Instabilities may still exist in isolated areas. 

    You could possibly 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 layer of buried surface hoar. It may be lying flat and hard to identify. Use snowpit stability tests and small, safe, representative convex rolls to test the reactivity of the layer. If you observe shooting cracks, whumpfing, or avalanching, stick to low-angle slopes. Be cautious traveling in or above terrain traps that can magnify the consequences of small avalanches. 

Avalanche Discussion

Since the last forecast, we received 3-5 inches of new snow, winds were light to moderate out of the west, and temperatures were near average. The avalanche danger trended towards LOW. This morning avalanches will only be possible in isolated areas. However, a significant storm is on our doorstep today. Winds increased to moderate to strong out of the west early this morning. There was snow available for transport yesterday, so wind slabs will already be growing. Snow showers will begin this afternoon and ramp up overnight through tomorrow. The Rattlesnake and Seeley Lake zones are favored for over 30 inches of snow by Saturday. The Bitterroot should receive over 18 inches. In the Rattlesnake and Seeley Lake zones, up to 6 inches of snow will fall by nightfall today, so look out for rapidly growing wind slabs in the afternoon and evening. 

Expect dangerous avalanche conditions to develop tonight and tomorrow. Wind slabs will build first and become large and sensitive in wind-loaded areas at middle and upper elevations. Winds will blow out of the west until Saturday morning, when they will switch to northeast. Snow-liquid ratios will decrease during the storm, and the snowfall should be right-side up, but storm slabs may still develop. In isolated areas, the added load may be enough to reactivate the buried surface hoar layer. Stay alert for changing conditions. 

Ski and ride safe. 

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