Bollettino valanghe

Central Sierra Nevada

Sierra Avalanche Center
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Pericolo valanghe

Alta quota
3 · Marcato
Limite del bosco
3 · Marcato
Sotto il limite
3 · Marcato
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In sintesi

CONSIDERABLE avalanche danger exists at all elevations in areas that held snow cover prior to last night's snowfall. A combination of wind slab, storm slab, and persistent slab avalanche problems exist. Very large avalanches could occur below treeline. Complex and dangerous avalanche conditions exist. Powder fever could easily induce poor decision making today.

Problemi valanghivi

  • Wind Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Likely
    Dimensione
    1

    New snow last night combined with SW winds is expected to have created new wind slabs in near treeline and above treeline terrain on NW-N-NE-E-SE aspects. Avalanche size to D2 is possible. Look for and avoid areas of recently drifted snow. Uses clues such as cornice features, wind pillows, snow surface sculpting, and snow surface cracking to identify likely areas of unstable wind slab.

  • Loose Dry

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    1

    New storm slabs may exist in wind protected areas near treeline or below treeline on any aspect that held previous snow cover. Avalanche size to D2 is possible. Snowpack failure could occur at the old/new snow interface or within the new snow. Use clues such as snow surface cracking while breaking trail to identify areas of potential storm slab instability.

  • Deep Persistent Slabs

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

    New snow load will add significant stress to the loose grain (sugary) mid snowpack facet layer that is the weak layer of concern for the persistent slab avalanche problem. This weak layer exists in isolated areas near treeline and below treeline on NW-N-NE aspects. Avalanche size to D3 is possible. Snowpit tests performed yesterday in the Carson Pass and Mount Rose areas indicated that this weak layer was in poor condition to handle new snow loading in isolated areas.

    This avalanche problem is often found in the lower to middle portions of a slope and not at the top. Human triggering may occur while either approaching a slope from the bottom or part way down on the descent. This will be quite different from triggering a wind slab avalanche near the top of a slope. The snowpack may fail 1 to 2+ feet deep into the old snow, creating an avalanche that could be very difficult to survive.

    Avoid avalanche terrain in areas where snowpack collapse with a possible whumpfing sound is occurring. Snowpits dug near the top of the avalanche start zone may not give indications of problematic instability mid slope. Significant spatial variability exists. Adjust thinking and travel planning to anticipate the possibility of high consequence avalanches in less common areas near and below treeline.

Avalanche Discussion

* Snowpit test results targeting mid pack faceted snow associated with the persistent slab avalanche problem vary between unstable and stable around the forecast area. There is no clear explaination for these variations in stability between similar terrain at different locations.

* Old snow surfaces beneath the new storm snow range from crust to near suface facets, largly dependent on elevation and aspect.

* NW-N-NE aspects above 8,000' in the Mt. Rose area and along the Sierra Crest north of Emerald Bay hold the best coverage at 2 to 4+ feet. Overall less snow cover exists south of Emerald Bay. Areas of decent coverage exist above 8,300' on NW-N-NE aspects in the Carson Pass area.  Coverage decreases dramatically on all other aspects.

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