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 remains MODERATE on all aspects at all elevations on slopes 35 degrees and steeper due to the presence of established wind slabs and the possibility of loose wet avalanches. Avalanche danger will increase late tonight into tomorrow as the next storm system impacts the region.

Lawinenprobleme

  • Wind Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Possible
    Größe
    2

    Lingering wind slab instability is expected to continue today near treeline and above treeline on NW-N-NE-E aspects on slopes 35 degrees and steeper. Human triggered avalanche activity has been observed over the past two days. Recent wind loading has allowed for wind slabs up to 2+ feet thick to form in lee areas. Time has allowed for some improvement in stability, but difficult to trigger human triggered avalanches remain possible today in recently wind loaded areas. In the more heavily wind loaded areas, avalanche size will easily be large enough to bury or injure a person.

  • Loose Wet

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

    During any cloud cover breaks today that allow for periods of sunshine, rapid warming will occur. Natural and human triggered loose wet avalanches will become possible in sun exposed areas on NE-E-SE-S-SW-W-NW aspects. Roller balls initiating from snow falling off of rocks and trees or from human activity could progress to a loose wet avalanche. Involvement of terrain features such as cliffs and terrain traps could greatly increase the consequences of becoming caught in a small loose wet avalanche.

Avalanche Discussion

A skier triggered wind slab avalanche occurred early yesterday morning below Grouse Rocks in the Ward Canyon area. The avalanche occurred on a wind loaded convex slope in near treeline terrain on a NE aspect at 7,900'. The avalanche was approximately 50' wide and ran 150' down slope. The crown was estimated at around 1 foot and the slope angle estimated at 35 degrees. The party had identified this terrain as an area of concern. The caught individual was partially buried and uninjured.

Observations made yesterday on Meiss Ridge (Carson Pass area) and on the East Ridge of Tamarack Peak (Mount Rose backcountry) revealed difficult to trigger but still unstable wind slabs near and above treeline on NW-N-NE aspects. Informal observations revealed skier triggered cracking and recent small avalanches from natural cornice collapse. Formal snowpit tests continued to indicate the possibility of human triggered avalanches failing on lower density layers of recent storm snow below the surface wind slabs.

Observations made yesterday on Rubicon Peak (West Shore Tahoe area) revealed minimal instability associated with wind slabs and storm slabs in the immediate area, but a much more significant potential developing persistent slab problem. Formal snowpit propagation tests targeting the layer of near crust facets just below the crust at the old snow/storm snow interface showed that propagation is likely once the faceted weak layer is collapsed. At this time the overlying snow load is insufficient to overburden this weak layer.

Heads up - We are currently tracking a potential developing persistent slab problem. Near crust facets found just below (and sometimes on top of) the melt-freeze crust at the base of the recent storm snow are the weak layer of concern. This layer has been observed to be most well developed and weakest near treeline and below treeline on NW-N-NE aspects. In some areas the associated crust is strong, in other areas it is weak and eroding. The lower elevation limit for this weak layer remains unclear at this time, but thus far it has been noted between 8,500' and 10,000' and may exist as low as 7,500'. In some areas, propagation tests are indicating that fractures are likely to travel through the snowpack once the weak layer is collapsed. At this time there is insufficient snow load to overburden this weak layer. With more storms in the forecast, additional snow loading will further stress this weak layer. Once critical mass is reached, avalanches failing on this weak layer will involve all of the recent storm snow accumulated since March 25th.

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