Bulletin d’avalanche

Central Sierra Nevada

Sierra Avalanche Center
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Danger d’avalanche

Alpin
2 · Modéré
Limite forestière
2 · Modéré
Sous la limite
2 · Modéré
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Points saillants

Avalanche danger has been elevated from yesterday's LOW danger back up to MODERATE danger for today at all elevations due to uncertainty associated with a lingering persistent slab avalanche problem. Large destructive avalanches are possible again today, mainly in response to large triggers.

Problèmes avalancheux

  • Deep Persistent Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Possible
    Taille
    2

    A lingering persistent slab avalanche problem exists above and below treeline on NW-N-NE-E aspects. Getting force down to the buried weak layers of faceted snow is difficult, generally requiring large triggers such as cornice collapse. The slope on the north side of Castle Peak where a large avalanche occurred yesterday had numerous tracks on it prior to avalanching in response to a large cornice collapse. Any persistent slab avalanches that occur today will be large and possibly destructive at size D2 to D3 (large enough to bury or injure a person up to large enough to bury and destroy a car, damage a truck, destroy small buildings, or break a few trees).

    From a travel advice standpoint, this avalanche problem is very challenging. Finding evidence of instability on a specific slope prior to an avalanche occurring is not easy to gather. Previous tracks are not an indication of stability, rather just that the trigger was either not large enough or not in the exact right spot on the slope to create an avalanche. Snowpit tests can produce false stable results with the current setup of deep weak layers and dense overlying snow. Choosing simple terrain without rocks and cliffs that is not capable of producing large avalanches and giving yourself a wide margin for error is a good choice for today.

     

Avalanche Discussion

* A large persistent slab avalanche was triggered yesterday on the north side of Castle Peak at 8,600' by human triggered collapse of a small car-sized cornice. There were numerous ski, snowboard, and snowmobile tracks all over the area, including on the specific slope before the avalanche occurred.

* Reports were received of other large human triggered cornice collapses that did not trigger avalanches on NW aspect terrain from the Broncho Chutes on Relay Peak (Mount Rose area) and from NE aspect terrain on Little Alaska near Twin Peaks (Ward Canyon area).

* Recent snowpit data from along the Sierra Crest has indicated improving stability on the weak layers of faceted snow deeply buried below the Jan 5-7 storm snow. All available data indicates this problem is difficult to trigger (stubborn) and requires large triggers to create avalanches.

* A small (size D1) wind slab avalanche with a 6 inch crown was triggered by a snowboarder yesterday on the north side of Castle Peak.

* Snowpit data and other observations have indicated an absence of the persistent slab problem in the Mount Rose area.

SAC forecasters use the Conceptual Model of Avalanche Hazard (Statham et al, 2017) as guidance for selecting the danger rating. "Likelihood of triggering" combined with the avalanche problem leads to the danger rating. Likelihood of triggering is derived from a combination of "sensitivity to triggering" (unreactive, stubborn, reactive, touchy) and "spatial distribution" in terrain (isolated, specific, widespread). In the current case of our lingering persistent slab problem, sensitivity to triggering is clearly stubborn. Uncertainty exists for spatial distribution whether it is isolated or specific terrain at this time. Stubborn with isolated leads to unlikely triggering in the model. Stubborn with specific terrain leads to possible triggering in the model. This puts the current regional scale avalanche danger somewhere in the LOW to MODERATE range, even though the large avalanche size creates larger consequences when an avalanche does occur. On a regional scale across the forecast area, the number of avalanches occurring in relation to the number of people traveling in avalanche terrain over the past few days is minimal.

Régions couvertes