Avalanche forecast

Eastside Region

Eastern Sierra Avalanche Center
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Avalanche danger

Fri
Alpine
3 · Considerable
Treeline
3 · Considerable
Below
2 · Moderate
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Highlights

Large and sensitive wind slabs are the main concern today. Use wide safety margins around leeward avalanche paths at mid and upper elevations. 

Avalanche problems

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    1–2.5

    Yesterday both natural and human-triggered large wind slab avalanches were reported. Abundant soft snow,  and continued moderate to strong ridgetop winds will maintain the wind slab problem as the number one hazard in the mountains today. Wind slabs are commonly found on the leeward side of terrain features that block or re-direct winds. Today these areas are most likely just below ridge lines and in the leeward start zones of defined avalanche paths near and above treeline that collect wind-drifted snow.  The hardness of wind-deposited snow means that wind slabs may be triggered from low on the slope or from areas you might consider "safe spots". If triggered expect these avalanches to propagate across start zones. Snow plumes along ridgelines and blowing snow along the snow surface are good indications that new wind slabs are building in your area. 

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Unlikely
    Size
    2–3

    No new avalanches have been associated with the 12/01 PWL in over two weeks. During that time the snowpack has gained strength and the problem became more isolated. Our latest storm was another good test of layers in the lower snowpack, however, none of the avalanches reported yesterday seem to have been related to the persistent slab problem.  It would likely take a large trigger to initiate a failure at the base of the snowpack. This type of trigger could come from a wind slab avalanche stepping down, or a large cornice fall. The problem might be found in complex terrain near and above treeline, on steep and unsupported slopes, and in areas with a generally thinner snowpack. The problem is now less likely to exist at lower elevations, with wetting of the snowpack followed by a refreeze in the past 48 hours.

Avalanche Discussion

Uncertainty surrounding the persistent slab problem has decreased following a good round of observations yesterday. The distribution and likelihood have decreased accordingly. There are three main reasons for the lower persistent slab hazard. (1) Lower-elevation surface snow became wet prior to, and during the last storm, and has subsequently refrozen causing it to gain strength. (2) There were no reported persistent slab avalanches in the ten days before the storm, and none were identified yesterday when the storm cleared, despite good views of start zones up and down highway 395. (3) The weak layer is now buried more deeply, and therefore harder for humans to trigger. The poor snowpack structure still exists so there is the potential for the problem to re-emerge as a series of atmospheric rivers impact our area this weekend.

The next atmospheric river event arrives this evening. All of the weather red flags for rising avalanche hazard will be in effect tomorrow: wind loading, snow loading, and rain loading / rapid warming.  Avalanche danger will likely be High again tomorrow.

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