Lawinenlagebericht

Central Sierra Nevada

Sierra Avalanche Center
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Lawinengefahr

Hochgebirge
2 · Mässig
Waldgrenze
2 · Mässig
Unterhalb
2 · Mässig
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Kernpunkte

Avalanche danger is MODERATE for all elevations. In isolated areas, the snowpack has weakened to the point that human triggered avalanches are now possible. Have a group discussion highlighting how managing a persistent slab avalanche problem is different from managing a wind slab avalanche problem. Adjust terrain selection and travel tactics accordingly.

Lawinenprobleme

  • Deep Persistent Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Possible
    Größe
    1

    Faceting, faceting, and more faceting has weakened the existing snowpack during the current dry spell. This has created isolated areas of persistent slab instability that are reactive to the weight of a person on the snowpack. It doesn't matter that it hasn't snowed since Dec 20th, metamorphic change of snow crystals on the ground is making a loose sugary weak layer get weaker by the day. NW-N-NE aspects near and above treeline hold isolated hard slabs over this weak layer. NW-N-NE aspects below treeline hold isolated soft slabs over this weak layer.

    Faceted snow is widespread on NW-N-NE aspects around the forecast area, but the reactive combination of slab over faceted weak layer is isolated. This distribution makes human triggered avalanches possible in isolated areas while avalanches remain unlikely in the majority of areas on a regional scale.

    Isolated persistent slabs are challenging to manage from a travel standpoint, especially when signs of instability to aid in decision making are not readily available. Previous tracks are not an indication of stability, especially when the weak layer is becoming progressively weaker. Use best practice travel techniques for minimizing exposure in and around avalanche terrain. Keep in mind that persistent slabs are often triggered much lower in the start zone than wind slabs. This tends to increase consequences, even from very small avalanches.

Avalanche Discussion

* Signs of persistent slab instability on NW-N-NE aspects including snow surface cracking, whumpfing, and/or unstable snowpit test results have occurred during the past several days in the Carson Pass, Desolation Wilderness, West Shore Tahoe,  Deep Creek, and Pole Creek areas. In all cases failure occurred on faceted snow beneath either a hard or soft slab.

* Mt. Rose Ski Patrol reported unstable snowpit test results on a similar slab over facet weak layer in closed, uncontrolled terrain (natural state snowpack that is analogous to a backcountry snowpack).

* Near surface facets without an overlying slab are fairly widespread on northerly aspects in wind protected areas below treeline.

* Snow surface conditions range from wind scoured snow and firm ice near and above treeline to lingering areas of soft unconsolidated snow on sheltered northerly aspects below treeline.

* Northerly aspects above 8000 ft in the Mt. Rose area and along the Sierra Crest north of Emerald Bay hold the best snow coverage at 2 to 3+ feet. Overall less snow cover exists south of Emerald Bay, but decent coverage exists above 8,500 ft. on northerly aspects. Coverage becomes patchy on other aspects and large areas of bare ground exist on many southerly aspects at all elevations.

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