Boletín de aludes

Central Sierra Nevada

Sierra Avalanche Center
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Peligro de aludes

Alta montaña
2 · Moderado
Límite del bosque
1 · Débil
Bajo el límite
1 · Débil
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Desactualizado: este boletín caducó 10 years ago. Estás viendo condiciones pasadas, no el boletín vigente.

Lo esencial

MODERATE avalanche danger may exist on NW-W-SW-S-SE aspects in above treeline terrain due to possible wind slabs created by forecasted NE and E winds. These new wind slabs should remain small but could knock a person off their feet. Triggering a deep slab avalanche on NW-N-NE aspects above 9000 ft. has become unlikely but not impossible. At elevations below 9000 ft. less avalanche danger exists, but firm icy slopes with a dusting of snow may exist on all aspects.

Problemas de aludes

  • Wind Slabs

    • Alta montaña
    • Límite del bosque
    • Bajo el límite del bosque
    Probabilidad
    Possible
    Tamaño
    1

    Light to moderate east and northeast winds may continue to transport small amounts of snow today. This wind loading may create very small wind slabs on some of the NW-W-SW-S-SE aspects in above treeline terrain. These small isolated wind slabs should not extend very far down-slope and should be limited to upper elevation areas with available snow where the wind may be strong enough to transport snow. While some of these may be sensitive to human triggering today they should remain too small to pose much threat to backcountry travelers except in areas where a stumble or getting knocked off balance could have consequences. Triggering the older larger wind slabs that formed during the 12/15 storm wind-loaded on N-NE-E aspects and cross-loaded NW and SE aspects has become unlikely. The cornices that exist above many of these wind loaded slopes may still remain fragile today and could still break well away from their edges.

  • Storm Slabs

    • Alta montaña
    • Límite del bosque
    • Bajo el límite del bosque
    Probabilidad
    Unlikely
    Tamaño
    3

    Triggering a deep slab avalanche has become unlikely due to the strength of the snow above the deeply buried persistent weak layers that still exist on NW-N-NE aspects above 9000 ft. However, in the unlikely event that this layer does break, fractures can still travel along the layer and the resulting avalanche would be large, destructive, and likely unsurvivable. While triggering these deeps slabs has become unlikely, it is not impossible for some very large trigger or a trigger on just the right trigger point to break the PWL and release a deep slab on some isolated terrain. 

    Due to the consequences associated with the large destructive avalanches that could occur if the persistent weak layer fails and the high degree of uncertainty associated with it, this unlikely but not impossible avalanche problem still warrants extra caution. 

Avalanche Discussion

Yesterday observations from Donner Summit, Carson Pass, and the Mt. Rose backcountry all showed some amount of cold unconsolidated snow sitting on top of the 12/15 rain crust in the upper snowpack in sheltered areas. At the higher elevations in the Mt. Rose area 10 to 18 inches of this colder snow existed while only about 4 to 6 inches existed along the Sierra Crest. In all these areas, the top 4 to 12 inches of the snowpack showed signs of becoming looser and weaker (facetting). Snowpit data from Meiss Headwaters on at 8700 ft. and snowpit data from Andesite Peak at 7400 ft. on E aspects both showed some weakness forming around the 12/15 rain crust buried about 4 to 6 inches below the surface. These weaknesses do not have much snow above them at the moment but will need monitoring to determine how they develop and how they could respond to future loading. Below this soft snow and the 12/15 rain crust a slowly refreezing snowpack existed along the Sierra Crest below 9000 ft.  

In the Mt. Rose backcountry above 9000 ft, a much deeper and colder snowpack exists. Recent snowpit data from the Mt. Rose backcountry including snowpit data from Tamarack Peak and Incline Lake Peak yesterday showed good bonding and consolidation in the upper snowpack, but observations also still show a slowly consolidating layer of weak snow near the base of the snowpack on northerly aspects in the Mt. Rose backcountry. This persistent weak layer exists between 120 and 160 cm below the surface and below all the strong snow. Tests targeting this layer indicated that actually getting enough force through the snowpack to break this layer has become unlikely without a very large trigger or some exceptional circumstances. However, tests also indicated that if the layer does break, fractures could still travel along this persistent weak layer.  

In addition to the snowpack data from yesterday, two small avalanches were reported, one observed on the E face of Mt. Judah and one on the NE face of Incline Lake Peak. Both of these were observed from a distance and likely occurred on 12/16 based on some snow covering up the crowns. In more exposed above treeline areas at Carson Pass and on Tamarack Peak scoured icy surfaces existed on windward aspects. 

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