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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Out of date — this forecast expired 4 years ago. You’re viewing past conditions, not the current bulletin.

Highlights

Buried surface hoar makes it possible for skiers and riders to trigger a persistent slab avalanche up to 15" deep, above 7000'.  Avalanches on this layer are more likely in the east Rattlesnake. Reduce the possibility by avoiding convexities and slopes over 35º, especially on shaded north slopes. 

Avalanche problems

  • Persistent Slabs

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

    The primary concern in the Rattlesnake is a layer of buried surface hoar up to 15" below the surface. Wind drifted areas below ridgeline have the potential to be deeper and denser.

    The reactivity of this layer is rather patchy, with no alarming test results or slides noted in the West Rattlesnake. However, there is not yet a slab as the new snow is low-density. As the new snow consolidates, the chances of a slide will increase. 

    In the East rattlesnake, a skier-triggered slide was reported at Wisherd ridge on Sunday; the structure for skier-triggered avalanches already exists. With new snow avalanches will be deeper with higher consequences. Given the nature of buried surface hoar it will take some time for the problem to go away. However, we are not seeing obvious signs of instability like shooting cracks, so it feels stable despite the lurking weak layer.

    Buried surface hoar can be found throughout the zone. You are most likely to find it above 7000' on north and east aspects. The best way to manage it 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 to skier travel. Avoid traveling in or above terrain traps. Whumpfing or cracking are obvious signs to stay in low-angle terrain.

    (1/15/2023)

    Skier triggered avalanche on Wisherd Ridge

  • Wind Slabs

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

    Ridgetop winds out of the west have been strong enough to transport snow. Wind slab may be found today in upper elevations below ridgeline and along the edges of gullies on east slopes. Wind slab will be small but may sit on top of the buried surface hoar, making it both deeper and touchier. Avoid wind loaded pockets over 35º to manage this problem.

  • Loose Dry

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    1

    A public observation yesterday noted sluffing on Murphy that rapidly entrained more snow. If you choose to ski very steep terrain, greater than about 40º, practice sluff management by skiing out of the sluff and allowing it to run. Plan your descent carefully and avoid terrain traps like tight chutes where loose snow can become funneled and powerful.

    (1/18/2023)

    The debris from a loose dry avalanche is visible on the apron below Mt. Murphy.

    Photo: Kevin Bissett

Avalanche Discussion

Since the last forecast, the Rattlesnake has received up to 8" of new snow. Temperatures have been mild but below freezing, and winds have been light to moderate out of the west. These are good conditions for the snowpack that will help it gain strength. Basal facets that have been plaguing us for over a month have been removed from the problems in the Swans and Rattlesnake. They are gaining strength in the southern Bitterroots, but there is still too much uncertainty to remove them yet.   New snow from Monday, up to 10" in the northern Bitterroot, is bonding well to old snow surfaces and melt-freeze crusts. However, a buried surface hoar layer is causing issues throughout the forecast area. This one will linger for a bit and may become more reactive as new snow consolidates into a cohesive slab. 

Crusts extending to 7000' have been covered by new snow, and the riding has improved considerably since earlier in the week. The possibility of avalanches at mid-elevations remains low in the Bitterroot and Rattlesnake. However, exceptions exist, and avalanche conditions could be found. Stay alert to changing snow conditions and red flags as you travel.

The lower elevation snowpack has shrunk considerably, and many early-season hazards are re-exposed. In the central Bitterroot, slopes are almost bare to 6000' on the south aspects and 5300' on the north. Today temperatures and cloud cover should keep snow surfaces frozen. However, wet loose may be possible if temperatures warm enough in areas with fresh low-elevation snow. These would be small but could push a skier into hazards. 

The high-quality observations that continue to be submitted greatly help the backcountry community and forecasters. Thank you for submitting them. You can submit further observations here.

Ski and ride safe.

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