Bollettino valanghe

Central Sierra Nevada

Sierra Avalanche Center
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Pericolo valanghe

Alta quota
1 · Debole
Limite del bosco
2 · Moderato
Sotto il limite
2 · Moderato
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In sintesi

Unusual avalanche conditions exist with pockets of MODERATE avalanche danger in open areas at and below treeline on N-NE-E-SE aspects on slopes 35 degrees and steeper. This is due to persistent slabs associated with the Jan 5 buried surface hoar layer. For all other areas, LOW avalanche danger exists. Avoid steep terrain adjacent to areas of recent avalanche activity, even if the crown and debis are covered under a couple inches of new snow and now look old.

Problemi valanghivi

  • Deep Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    2

    The Jan 5 surface hoar layer now buried 1 to 1.5 feet deep in the snowpack became very reactive on Tuesday and continued to contribute to skier/snowboarder triggered avalanches on Thursday and Friday on Andesite Ridge (Donner Summit area) and on Red Lake Peak (Carson Pass area). This is a very unusual avalanche problem for this region. Currently, the areas where avalanches are most likely to be triggered are not in the traditional wind loaded zones along the ridgelines, but in steep open areas at treeline and below treeline on N-NE-E aspects and possibly SE aspects. This layer has been very slow to stabilize with recent snowpit data pointing to ongoing problems. This weak layer is certainly not abiding by any mythical "24 hour rule".

    Making travel decisions more difficult is the fact that this weak layer does not exist or is not reactive in all areas. Don't let a lack of evidence of instability in one portion of the forecast area affect how decisions are made when moving to another part of the forecast area. Continue to search out signs of instability including recent avalanche activity, even if it "looks old" buried under an inch or two of recent snowfall. Snowpit data is very helpful, but it has to come from the right locations, and it never proves stability. Have a conversation with your travel partner(s) and make sure everyone understands that this is an at treeline and below treeline problem. Shift the focus from the traditional wind loaded avalanche start zones up high to the steep wind protected open areas lower down on the mountain. If in doubt, stick to slopes less than 32 degrees in steepness.

Avalanche Discussion

Skier and snowboarder triggered avalanches were reported on Thursday on Andesite Ridge (Donner Summit area) and on Thursday and Friday on Red Lake Peak (Carson Pass area). In both cases avalanches occurred in steep open terrain near treeline to below treeline on NE to E aspects. Snowpack failure occurred 1 to 1.5 below the snow surface at the base of the recent storm snow accumulated since Jan 5. Snowpit data collected over the last two days in the Donner Summit area is indicating that where Jan 5 surface hoar crystals are 6 mm to 10 mm or larger they are remaining reactive and where they are 5 mm and smaller they are becoming less reactive. Several testpits dug yesterday in likely buried surface hoar terrain around Barker Peak and Barker Pass (Blackwood Canyon area) showed an absence of the Jan 5 buried surface hoar layer and no formal or informal signs of instability. No signs of the Jan 5 surface hoar layer have been reported this past week from the Mount Rose area.

Ridgetop winds did not materialize as forecast yesterday. This created little to no new wind slab formation over the past 24 hours in the vast majority of areas. Any isolated wind slabs that did form yesterday are expected to remain small. Isolated cracking is possible today in above treeline areas that actually received some wind loading. Actual natural or human triggered wind slab avalanches are unlikely today.

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