Bulletin d’avalanche

Central Sierra Nevada

Sierra Avalanche Center
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Danger d’avalanche

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

Points saillants

A weak and problematic snowpack exists in portions of the forecast area.  Evidence of snowpack instability at this time is often not obvious. Thoroughly assess the snowpack before committing to avalanche terrain on shaded slopes. Opt for lower angle, non-avalanche terrain if any indications of instability are present.

Problèmes avalancheux

  • Persistent Slabs

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

    Faceted, weak sugary layers of snow mostly within the lower half of the snowpack remain problematic. The snowpack structure changes with elevation, with a high level of variability in the degree of layer weakness and overlying slab strength depending on location. Currently, the areas with the most frequent signs of instability are found on NW-N-NE aspects above about 8,000' to 8,300'. Snowpit tests continue to show weakness and unstable results on these well developed weak layers.  Any avalanche involvement with this persistent slab problem could have high consequences from trauma due to our rocky, shallow snowpack.

    Unless you are specifically looking, signs of instability associated with the current avalanche problem are often not obvious. While there are plenty of tracks in suspect terrain with a reported close call with a slope scale collapse, no one has reported triggering a persistent slab avalanche just yet.  Currently, signs of unstable snow are most likely to be observed in snowpit tests, rather than from more obvious signs such as shooting cracks, collapsing, or whumpfing sounds.  Utilizing non-avalanche terrain under 30 degrees in steepness without steeper connected terrain above is a great strategy to minimize exposure to this persistent slab avalanche problem. This strategy conveniently avoids the need to dig snowpits in meaningful locations and correctly interpret the snowpack structure and test results.

Avalanche Discussion

A weak faceted (sugary) snowpack exists on most NW-N-NE aspects above about 7,400'.  Near and below around 8,500', faceted snow exists around and in between rain crusts.  Gaining elevation above 8,500', rain crusts gradually disappear or decompose and the lower half of the snowpack becomes nearly all weak faceted snow. Generally speaking, the higher up you go on wind protected NW-N-NE aspects, the weaker the snowpack. These weak layers are well developed, with depth hoar found in many areas at the base of the snowpack.  Early season NE wind events previously scoured some exposed slopes down to dirt, rock, or ice-leaving weak faceted snow mid-slope and below.

Cold temperatures, clear skies, low sun angles, and long nights will continue the faceting process in our shallow snowpack, further weakening the snowpack structure in some areas.

There is still the potential to hit many obstacles (rocks, logs, open water, bushes, etc.) that are barely covered (or not) by the shallow snowpack. This is especially true in areas below 7,500' and in very rocky areas up to at least 8,000'. Anticipate encountering a different snowpack structure and a different set of travel conditions if you are moving from one place to another around the forecast area.

Régions couvertes