Lawinenlagebericht

Eastside Region

Eastern Sierra Avalanche Center
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Lawinengefahr

Thu
Hochgebirge
3 · Erheblich
Waldgrenze
3 · Erheblich
Unterhalb
2 · Mässig
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Kernpunkte

3-9” of new snowfall today combined with moderate SW winds will lead to increasing avalanche danger in the form of fresh sensitive Wind Slabs. Danger is expected to rise to CONSIDERABLE at mid and upper elevations and MODERATE at lower elevations as Wind Slabs build. Concern lingers for the unlikely but potentially very dangerous human triggering of a Deep Persistent Slab avalanche.

Lawinenprobleme

  • Wind Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Likely
    Größe
    1–2

    Moderate SW winds gusting into the 50mph range over ridgetops will transport soft new snow falling today into new sensitive wind slabs. These will develop on the leeward side of ridgelines, gully sidewalls and cross-slope areas of deposition on NW-N-E-SW facing terrain. Blowing snow, cornice formation, new denser snow and shooting cracks are all indications that sensitive wind slabs may be lurking. These slabs will start small but will build throughout the day.  Human triggering will become likely and natural triggering possible.

    Wind slabs formed by extreme winds two days ago have likely settled for the most part, but some places could remain where they could be triggered stubbornly such as a steep convexity or an unsupported slope above a drop-off.

    The extreme winds two days ago swept most windward fetches either clear to the ground, or left them with very firm wind board even at low elevations. Without the addition of today’s new snow, today’s winds won’t have much to build with.

  • Deep Persistent Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Unlikely
    Größe
    2–3.5

    An impressive 6-10”+ of water content and many feet of snow have been added to our snowpack since last Wednesday the 22nd. While it’s our hope that all this intense loading either caused large avalanches that cleaned out the weak faceted snow near the base of our snowpack, or buried it so well that it is no longer sensitive to a surface trigger, we can’t be 100% positive. 

    Mammoth ski patrol triggered very large Deep Persistent Slab avalanches failing in weak facets near the ground on Sunday with explosives. Recent reports of some very large natural avalanches in the backcountry came in with Sunday morning’s clearing as well. While it’s not certain if these natural avalanches failed in old deeply buried weak layers, what is certain is that all these avalanches (natural and patrol triggered) required very intense new snow loading to release.

    With the additional time passed now since the last heavy loading, we feel confident saying that it is unlikely for a human to trigger a Deep Persistent Slab avalanche, but we can’t say it’s impossible. A rider hitting just the right 1/100 shallow sensitive spot or the force of a smaller Wind Slab avalanche could be what it takes to trigger a failure in deeply buried weak layers near the ground resulting in a very dangerous avalanche. Steep slopes with snow cover prior to December are where this concern lies (>~9,200’, W-N-E-SE facing). The conservative call is to stay off of steep rocky terrain where a shallow spot may be more easily found.

Avalanche Discussion

The trend continues where the southern reaches of the forecast zone outside of Bishop is forecasted to receive less new snow than the rest.  As such, avalanche danger may not rise as much down there as areas north that receive more new snow.

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