Avalanche forecast

Central Cascades

Central Oregon Avalanche Center
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Avalanche danger

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

On Sunday, small avalanches will be possible in open areas where the wind can deposit snow. Pay attention to the depth of new snow over a recent crust, and if the snow is deeper than the top of your boots, choose a lower angle slope. Use extra caution in the alpine, where a small avalanche could trigger a much larger slide. 

Avalanche problems

  • Wind Slabs

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

    Across most of the Cascades zone, wind slabs will be small on Sunday because we simply don't have enough new snow available for transport. On Saturday, Mt. Bachelor received an inch of new snow. This was enough to make the skiing on Tumalo transition from crusty and pretty bad to quite nice in open glades by mid-afternoon. I observed some very small wind slabs on test slopes that were 1-3" inches thick. This is not enough to spike the avalanche hazard, but with continued snowfall and sustained winds, it remains our primary avalanche concern. At mid-elevations, a crust formed on the snow surface on Friday. Use this crust to measure the depth of new snow, and if you measure any depths greater than your boot tops, back off to lower angle or less wind-loaded slopes. Note: Because of the crust, the low angle open glades are skiing the best!

    Keep in mind that snowfall amounts and wind at upper elevations will be higher, and there's a lot more open terrain to catch wind-deposited snow. Higher elevations may also lack Friday's crust, which will make it more challenging to assess the degree of wind-transported snow. 

    Ultimately, the wind slab problem will depend on the snow totals overnight and into Sunday. If we're on the higher end (up to 5 inches by Sunday afternoon), the wind slab hazard will be higher. If we only get an inch or two, it won't be much of a concern in most of our terrain. Winds will be predominantly from the southwest, but they could shift around, so keep an out for this problem anywhere that you notice snow drifts forming. 

  • Deep Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Unlikely
    Size
    2–3

    A deep persistent slab avalanche is unlikely, but if it were to occur, it would be large and difficult for a human to survive. The best way to avoid this problem to use extra caution at upper elevations, especially on slopes that could be wind-loaded or around cornices. The way to manage this hazard is by avoiding the terrain where it could be triggered. 

    The deep persistent slab problem relates to a layer of weak snow underneath a rain crust from a couple weeks ago. The COAC forecast team has been tracking this layer and has dug dozens of snow pits across the Cascades zone to assess it. Generally, we have not been finding information in the snowpack to indicate a persistent slab problem, such as faceted snow, sudden collapses, or sudden planar test results. However, on Thursday, 12/19, Mt. Bachelor ski patrol intentionally triggered a slab avalanche using explosives. This initial avalanche "stepped down" (i.e., triggered a deeper weak layer) and resulted in a large avalanche. This tells us that the deep persistent problem is there, but it is difficult to trigger. The potential place a human would trigger a deep persistent slab avalanche would be in the alpine, and they would do so by first triggering a wind slab or by a cornice fall. Such an event could deliver enough force to collapse a weak layer deeper in the snow pack. 

Avalanche Discussion

The key with our two current avalanche problems is the interplay between the wind slab and deep persistent slab problems. Although it would be difficult for a human to trigger a deep persistent slab, they would potentially do so in the alpine by first triggering a wind slab, which then steps down to the deeper layer. A video linked in this forecast provides another format to explain the two problems and how they relate. 

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