Bulletin d’avalanche

Central Sierra Nevada

Sierra Avalanche Center
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Danger d’avalanche

Alpin
1 · Faible
Limite forestière
2 · Modéré
Sous la limite
2 · Modéré
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Périmé — ce bulletin a expiré 13 years ago. Vous consultez des conditions passées, pas le bulletin en cours.

Points saillants

MODERATE avalanche danger will continue on the near and below treeline NW-N-NE aspects on slopes 32 degrees and steeper due to lingering persistent slabs.

Human triggered avalanches remain possible. Due to the shallow snowpack any avalanches that do occur would drag people through and into obstacles like rocks, stumps, and other hard immobile obstacles.

Problèmes avalancheux

  • Deep Persistent Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Possible
    Taille
    2

    Persistent slabs still exist on the near and below treeline NW-N-NE aspects. A widespread persistent weak layer (the Dec. 7th facets) with a small layer of cohesive snow above it provides the ingredients needed to keep this problem active: a weak layer and a slab. The additional snowfall expected today should not add enough weight to the snowpack to cause natural avalanche activity. However, it will keep this already unstable snowpack teetering on the edge just waiting for some kind of trigger to break it. Human triggered avalanche activity remains possible on these slopes. People can trigger these kind of persistent slabs remotely while standing in low angle terrain or on ridgelines or anywhere that the snowpack under their feet connects to steeper and/or deeper terrain because fractures can travel through the persistent weak layer. 

    Any of these avalanches that do occur would drag a person through rocks, stumps, trees, and other obstacles because of the shallow snowpack and the fact that the near and below treeline terrain holds the most unstable snow.

  • Wind Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Unlikely
    Taille
    1

    The small amount of new snow and the strong NE winds could form some small, isolated, and mostly insignificant wind slabs on the above treeline wind-loaded W-SW-S aspects and a few of the cross loaded NW and SE aspects. In most areas these slabs will build on top of bare ground. Some cracking or other signs of instability may occur where these small slabs exist, but significant avalanche activity is not expected from these wind slabs. Still it only takes a small amount of moving snow to knock a person over. Falling onto the rocks and other hard obstacles that remain exposed on these aspects could take a toll on people and equipment.   

Avalanche Discussion

Yesterday on Castle Peak (snowpit, more info), observations and data showed a well consolidated layer of snow resting on top of a thin melt freeze crust above the weak sugary facets. No human triggered whumpfing or collapsing occurred on the NW aspects in the areas I traveled through. Snowpit data still indicated that the sugary snow grains that comprise the the Dec. 7th facets remain weak, and that they are easy to break. However, those tests also indicated that the resulting fractures did not travel very far through the snowpack. On every other day since 12/7 some significant sign of instability has occurred (whumpfing, collapsing, small avalanches, unstable snowpit test results, etc). Yesterday's data represents the first and only instance of a day without some of those significant signs of instability since 12/7. Until more data supporting a strengthening snowpack exists, consider this an outlier. I for one am not willing to put much faith in this data due to the overwhelming number observations pointing towards persistent instability.

On the more exposed above treeline northerly aspects, large areas of bare ground remain exposed due to wind scouring by strong E-NE winds from earlier this fall. Clear sunny weather and warmer temperatures have melted most of the snow off of the aspects facing the southerly half of the compass and only few small areas of snow remain on some of them.

Barely covered rocks, stumps, logs, and other hard, immobile, season-ending, equipment-breaking objects still exist on all slopes. The shallow snowpack does not have enough structure or strength to keep a person above all these things. It only camouflages their existence.

Régions couvertes