18-19

We found weak facets that formed in early December are buried 10" deep outside Cooke City. This layer produced unstable results in stability tests (ECTP 20) on a lightly wind loaded slope near Lulu Pass (12/20). This layer will be the likely place to see avalanches break in the snowpack on wind loaded slopes and when we get more snow. Photo: GNFAC


Forecast link: GNFAC Avalanche Forecast for Sat Dec 22, 2018GNFAC Avalanche Forecast for Sun Dec 23, 2018GNFAC Avalanche Forecast for Wed Dec 26, 2018GNFAC Avalanche Forecast for Thu Dec 27, 2018
Cooke City, 2018-12-20
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    • GNFAC Avalanche Forecast for Thu Dec 20, 2018

    Snowpack and Avalanche Discussion

    <p>The snowpack in the mountains around West Yellowstone is plagued by a foundation of rotten, sugary facets. This persistent weak layer has produced unstable results in stability tests and has caused recent avalanches on wind loaded slopes (<u><strong><a href="https://www.youtube.com/watch?v=rQ7fzq4X1-4&amp;index=2&amp;t=0s&amp;li…;, <strong><a href="https://www.mtavalanche.com/images/18/ernest-miller-avalanche">photo</a…;). The 6” of snow that fell Tuesday night did not create widespread avalanche activity - but it did put the snowpack on edge.</p>

    <p>Yesterday, my partner and I toured around Bacon Rind in the southern Madison Range and found a snowpack structure on the cusp of producing avalanches (<strong><a href="https://www.youtube.com/watch?v=3NMgoDNKUQg&amp;t=0s&amp;list=PLXu5151n…;). On non-wind loaded slopes, the snowpack was not quite cohesive enough to support widespread propagation. However, on any slope that had wind drifted snow we experienced cracking and collapsing. It is likely wind loaded slopes would have produced avalanches in steeper terrain.</p>

    <p>Although the snowpack has had time to adjust, I would still be cautious on any slope steeper than 30 degrees, especially those with wind drifted snow. Given the poor structure, human triggered avalanches are possible today and the avalanche danger is rated <strong>MODERATE</strong>.</p>

    <p>The mountains near Bozeman, Big Sky and Cooke City have a few weak layers to look out for. &nbsp;The main one being a layer of small grained facets buried 1-1.5’ deep (<strong><a href="https://www.youtube.com/watch?v=mJRtrBZkgtY&amp;list=PLXu5151nmAvRNl9ku…;). This layer has been the culprit of recent avalanche activity, mainly on wind loaded slopes (<strong><a href="https://www.mtavalanche.com/images/18/snowmobile-triggered-avalanche-bu…;, <strong><a href="https://www.mtavalanche.com/images/18/natural-avalanche-hourglass">phot…;). A skier triggered slide on Bridger Peak a few days ago is a great example of what’s possible under these conditions (<u><strong><a href="https://www.mtavalanche.com/images/18/bridger-peak-avalanche-1-17-dec">…;, <u><strong><a href="https://www.youtube.com/watch?v=QgnQq4PQMi8&amp;list=PLXu5151nmAvRNl9ku…;, <u><strong><a href="https://www.mtavalanche.com/node/19330">details</a></strong></u&gt;). &nbsp;Look for and assess this layer before committing to steeper slopes, especially those with drifted snow.</p>

    <p>Avalanches failing on weak snow near the ground are less likely, but something to consider. Yesterday, the Yellowstone Club Ski Patrol triggered a slide that broke 2-3’ deep on facets near the ground. This slide occurred in steep, wind loaded terrain. Another slide at the Yellowstone Club earlier in the week also failed on facets near the ground (<strong><a href="https://www.mtavalanche.com/images/18/explosive-triggered-avalanche-yc-…;). This problem is not widespread, but something to think about as incremental loading adds weight and stress to the snowpack.</p>

    <p>Today, buried weak layers make human triggered avalanches possible, mainly on wind loaded slopes. For this reason the avalanche danger is rated <strong>MODERATE</strong>. &nbsp;</p>

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    BOZEMAN

    Explosive triggered slides, YC and Big Sky

    NORTHERN MADISON RANGE
    Northern Madison
    Code
    SS-ABc-R2-D2-O
    Notes

    The Big Sky and YC Ski Patrols triggered numerous avalanches during control work. Most slides were fresh wind slabs created by the 4-6" of new snow and strong winds. Most stayed confined to the upper layers of the snowpack, but one slide at the YC broke on facets near the ground. This slide broke 2-3' deep and ran nearly full path. 

    Multiple Avalanches
    Number caught
    0
    Number buried
    0
    Avalanche Type
    Soft slab avalanche
    Trigger
    An explosive detonated above the snow surface (air blast)
    Trigger Modifier
    c-A controlled or intentional release by the indicated trigger
    R size
    2
    D size
    2
    Bed Surface
    O - Old snow
    Slab Thickness units
    centimeters
    Single / Multiple / Red Flag
    Multiple Avalanches
    Advisory Year
    18-19
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