Lawinenlagebericht

Central Sierra Nevada

Sierra Avalanche Center
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Lawinengefahr

Mon
Hochgebirge
2 · Mässig
Waldgrenze
2 · Mässig
Unterhalb
2 · Mässig
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Veraltet — dieser Lagebericht ist 3 years ago abgelaufen. Du siehst vergangene Verhältnisse, nicht den aktuellen Bericht.

Kernpunkte

Old, weak snow still lurks below the surface in many parts of the forecast area. Signs that unstable snow exists may not be obvious, so people should thoroughly assess the snowpack before committing to slopes steep enough for snow to slide down, especially on colder shaded slopes. If any indications of instability exist, opt for lower-angle, non-avalanche terrain.

Lawinenprobleme

  • Persistent Slabs

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

    Buried weak layers of rotten, sugary snow with more cohesive snow (aka slab layers) above them provide the ingredients for a persistent slab avalanche problem. The specific snowpack structure and weak layers vary with elevation and location from one spot to another around the forecast area. Data and observations have shown that NW-N-NE aspects above 8000 to 8300 ft hold the weakest snowpack. Triggering a persistent slab avalanche remains possible, and getting caught in such an avalanche could have higher consequences due to the shallow rock-infested snowpack.

    Persistent slab problems can be tricky since signs of unstable snow may not be obvious. Sometimes, triggering an avalanche represents the first sign of an unstable slope. Using terrain less steep than 30 ° not connected to steeper areas can help minimize your exposure to this avalanche problem. As an added benefit, lower angle slopes make controlling your speed and avoiding the barely covered obstacles easier.

     

Avalanche Discussion

A weak faceted (sugary) snowpack exists on most NW-N-NE aspects, especially in areas above 8000 ft, with a few spots holding weak snow down to about 7,400'.  Near and below about 8,500', faceted snow exists near buried rain crusts. Above 8,500', the rain crusts gradually disappear, and almost the entire lower half of the snowpack becomes weak faceted snow. Generally speaking, the higher you go on wind-protected NW-N-NE aspects, the weaker the snowpack. These weak layers are well developed, with depth hoar found at the base of the snowpack in many areas. Low sun angles that leave the northerly aspects shaded during the day and long, clear nights allow the weak snow to persist and even weaken.

While no avalanches have been reported on these weak layers, people did trigger collapses that propagated across entire slopes after each of the last two storms. One of these occurred in the Hourglass and one in Broken Glass. The upcoming storms will add more load than either of those previous storms. They will add slabs of heavy, dense snow above these weak layers. The high snow levels will also push people to recreate where the weakest snow exists.  This combination of heavy slabs on top of old, weak snow and increased use could be a recipe for problems.

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 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.

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