Bollettino valanghe

Seeley Lake

West Central Montana Avalanche Center
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Pericolo valanghe

Wed
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3 · Marcato
Limite del bosco
2 · Moderato
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1 · Debole
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In sintesi

Triggering an avalanche on persistent weak layers is likely at upper elevations and possible at mid elevations.  Human triggered wind slabs are possible along ridgelines on all aspects. 

 Avoid slopes over 30 degrees, and be aware of what is above you, as remote triggering is possible. Collapsing, whumpfing and cracking are obvious signs of instability.

Problemi valanghivi

  • Wind Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Likely
    Dimensione
    1–2

    Wind slab is likely along ridgelines on North through East aspects from last week.  It is buried by 2-5 inches of new snow and may not be immediately visible.

    Over the weekend, easterly ridgetop winds have created isolated wind slabs on west aspects.

    In the Seeley Lake zone, a natural wind slab was observed off the Swan crest on a high elevation west aspect. 

    Avoid any pillows, obviously loaded areas, and hollow feeling snow. Be aware that wind slabs may be buried by new snow. 

    (12/12/2022)

    R1, D2 natural avalanche on Sunday Mountain

    Photo: Jeff Carty

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Likely
    Dimensione
    1–2

    The primary persistent weak layers in the Seeley Lake zone are buried facets. Depending on aspect and elevation, these may be basal facets on sheltered north facing slopes, or a faceted layer buried mid snowpack at mid elevations.

    There is a significant difference in snow totals and structure between the east and west zones. In the eastern part of the zone along the swan crest, avalanche hazard is higher with a weaker structure and more persistent weak layers. West of Seeley lake, the deeper snow is bonding better and does not have as weak of an interface below it.  

    With variable depths comes varying slab thickness and sensitivity. Observers report widespread collapsing and whumpfing in shallower areas with some collapses traveling a long distance. The only way to manage this problem is to avoid avalanche terrain. Avoid slopes over 30 degrees in the Swan range. Avalanches are unlikely on slopes under 35 degrees in the southern Mission range.

Avalanche Discussion

East winds and warmer temperatures have been the story for the past two days. The low pressure system passing below us through Idaho has given us light ridgetop winds out of the east. With this came an increase in temperatures and low clouds. Cloud cover decreases as you move north through the zone with Seeley lake having only a few lingering clouds yesterday. This system did not bring significant snowfall, with totals of only a few inches of snow and 0.1" to 0.2" SWE (snow water equivalent) since Saturday. 

This has done little to help with our snowpack, and we are continuing to deal with sensitive persistent weak layers. The two layers of concern continue to be the layer of buried surface hoar from late November on all aspects and depth hoar on northerly aspects. The natural avalanche cycle on these layers has paused for now, although any significant load will bring with it another natural cycle. 

Snow depths  are highly spatially variable. On Rice Ridge on the east side of the zone, depths were measured at 70cm (28 inches) on east aspects. Compare that to depths of 140cm (55 inches) just across the zone in the southern Missions at the same elevation. At Morrel mountain, depths are 90cm (35 inches) at 7100'. 

The buried layer of surface hoar found in the Rattlesnake and Bitterroot is not as widespread in the Seeley Lake zone. Do not totally discount it, however, as areas of buried surface hoar are still likely on sheltered slopes at mid elevation. 

Snowpit scores on these layers are increasing. Extended column tests (ECT) are not propagating across the column as readily, but propagation saw tests (PST) are still consistently failing to the end. Observers are reporting widespread collapsing and whumpfing. This highlights why putting too much faith in snowpit tests alone in light of obvious instability is a dangerous decision making trap. 

How much snow was available for transport by the east winds is highly variable depending on elevation and surface conditions. Wind slabs exist on north through east aspects from previous winds and, in some areas, may be hidden by new snow. In the Seeley lake area, new snow totals have been higher the past two days, and wind slab has formed on west aspects. The bottom line for the wind slab is to stay off any hollow feeling snow along ridgelines and avoid any visibly loaded features. 

Between 4000 and 5500’, avalanche danger is low, with the exception of the terrain around Lolo Pass, where pockets of increased hazard may exist in sheltered areas.

Conservative decisions are still necessary in the backcountry. As the slab above the persistent weak layers gains strength and avalanches become harder to trigger, it will become tempting to move out onto steeper terrain. Do not let a lack of unstable results in snowpits convince you to travel in avalanche terrain when the snowpack structure is still so poor. Any avalanche that does break will be large and destructive. 

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Ski and ride safe. 

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