Bulletin d’avalanche

Central Sierra Nevada

Sierra Avalanche Center
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Danger d’avalanche

Alpin
2 · Modéré
Limite forestière
3 · Marqué
Sous la limite
3 · Marqué
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Périmé — ce bulletin a expiré 7 years ago. Vous consultez des conditions passées, pas le bulletin en cours.

Points saillants

A worrisome persistent slab avalanche problem exists below treeline and near treeline. Lingering unstable slabs of wind drifted snow may exist above treeline and near treeline. Warming air temperatures today may also trigger some loose wet snow instability. CONSIDERABLE avalanche danger continues below treeline and near treeline with MODERATE avalanche danger above treeline.

Problèmes avalancheux

  • Wind Slabs

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

    Lingering unstable slabs of wind drifted snow remain possible today in above treeline and near treeline lee areas mainly on NW-N-NE-E-SE aspects. Avalanche size up to D2 is expected.

    The wind direction has not changed so use the clues of cornice formations, wind pillows, snow surface cracking, and any signs of recent avalanches to determine where unstable wind slabs are most likely to be encountered. Make a plan and discuss it within your group for how to travel around these specific areas with caution.

  • Deep Persistent Slabs

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

    Ongoing instability associated with the buried Jan 4 crust/facet weak layer has now transitioned from a storm slab problem to a persistent slab problem. The Jan 4 crust/facet weak layer exists near treeline and below treeline on NW-N-NE-E aspects. In most below treeline locations, it is found about 2 feet below the snow surface. In some near treeline locations, it is found deeper under older wind drifted snow.

    While the likelihood of triggering an avalanche on this weak layer has decreased since the peak of the storm, it is still possible and the consequences of being caught in a below treeline avalanche of this type may not be survivable. Unless there is an extremely clean runout, you will almost certainly have a high force impact with trees. Avalanche size is expected at D2, but do not underestimate how far the avalanche may travel.

    If you encounter signs of recent avalanches near to below treeline, evidence of snow surface cracking, snowpack collapse (possible whumpf sound), or unstable snowpit test results, avoid travel on or below steep slopes in the area. An avalanche of this type could easily fracture above and behind you. Previous tracks are not an indication of stability.

    If you want to avoid this avalanche problem altogether, select terrain with a maximum slope angle under 30 degrees and without steeper terrain above or to the side. The quality of the snow is so good right now that these lower angle slopes are providing excellent skiing, snowmobiling, and snowboarding experiences.

  • Loose Wet

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

    Air temperatures rising well above freezing today may create some minor snow surface melt and loose wet snow instability. Rollerballs occurring on steep slopes is a clue to the presence of this avalanche problem. Avalanche size is expected at D1. Most consequences would occur from being pushed over cliffs, into rocks, trees, or terrain traps below.

Avalanche Discussion

* A recent natural avalanche was reported yesterday on Angora Peak (Echo Summit/Fallen Leaf Lake area), below treeline on a N aspect at approximately 7,300'. Snowpack collapse with whumpfing and shooting cracks was reported in the adjacent terrain. Other details are unknown, but this has the signature of the current persistent slab avalanche problem.

* The Jan 4 crust/facet weak layer is showing continued instability in many areas. Skier triggered snowpack collapse with whumpfing was reported yesterday from the Pole Creek, Deep Creek, and Echo Summit/Fallen Leaf Lake areas. Unstable snowpit tests indicating continued likely propagation along this layer were reported from the Independence Lake, Donner Summit, Luther Pass, and Carson Pass areas.

* The Jan 4 crust/facet weak layer is most prevalent in the 7,000' to 8,500' elevation range but has been observed with concerning weakness in more isolated distribution at higher elevations.

* Ongoing wind slab instability was inconsistent around the forecast area yesterday with continued unstable wind slabs in some areas and limited evidence of unstable wind slabs in other areas.

Régions couvertes