Avalanche forecast

Central Sierra Nevada

Sierra Avalanche Center
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Avalanche danger

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

Numerous human-triggered avalanches occurred yesterday. Dangerous avalanche conditions will exist in the backcountry again today. Watch out for fresh slabs of wind drifted snow near ridgelines, in gullies, and on open slopes exposed to the wind. In more sheltered areas, slabs of storm snow may exist. Human-triggered avalanches with serious consequences are likely on wind-loaded slopes and possible on steep sheltered slopes today. CONSIDERABLE avalanche danger exists in near and above treeline terrain and MODERATE danger exists below treeline.

Avalanche problems

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    1–2

    Several human-triggered wind slab avalanches occurred yesterday. Expect more fragile wind slabs to exist in the backcountry today as additional snow and wind build new wind slabs on top of yesterday's wind slabs. These slabs of wind drifted snow will exist on leeward slopes near ridgelines, in gullies, and on open slopes exposed to the wind. Some of them may form in unusual places due to the wind shifting between the SW and W. Some wind slabs may extend into lower elevation areas as the mid-slope winds gain strength today. They have not had time to form strong bonds with the underlying snow, and a person's weight would likely trigger wind slab avalanches again today. Some natural wind slabs are possible. These wind slabs could measure several feet in depth and could easily involve enough snow to bury or injure a person.

    Clues like recent wind slab avalanches, cornices above a slope, blowing snow, snowdrifts, ripples on the snow surface, and other wind created features can help determine where wind slabs exist. Wind slabs will be fragile today. Give wind-loaded areas and the cornices above them a wide berth to avoid this avalanche problem.

    (2/12/2021) Skier-triggered wind slab avalanche on Mt. Judah

  • Storm Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    1–2

    People triggered numerous storm slab avalanches on steep slopes sheltered from the wind yesterday as well. More snow today means that human-triggered storm slab avalanches will be possible again on steep slopes. Some of these may only involve today's storm snow and some may still fail at crust below Thursday night's storm snow. Storm slab avalanches could involve enough snow to bury or injure a person. This avalanche problem can also exist in places that people gravitate towards on storm days like steep tree-covered slopes. In these cases, avalanches could have more severe consequences if they push people into trees or other obstacles. Terrain traps like narrow gullies can also magnify the consequences of a storm slab avalanche.   

    Recent avalanche activity, shooting cracks, or punchy (dense over soft) feeling snow, can indicate storm slab instability. Identify where storm slabs may exist. If there is any doubt about the stability of a slope change plans and move to lower angle terrain where the snow will still be just as soft.

    (2/12/2021) Skier triggered avalanche on Flagpole Peak. This skier was carried for some distance and then able to self-arrest.

Avalanche Discussion

Numerous human-triggered avalanches occurred during the day yesterday. The avalanches failed just above a crust at the base of Thursday night's snow. Some of these were wind slabs (Mt Rose, Mt. Judah, Little Truckee Summit area, Blue Lakes area) and some were storm slabs (Silver Peak, Tamarack Peak, Mt. Tallac, Flagpole Peak, Slide Mtn). Some were likely a mix of the two types. Crown heights measured ~1 ft for most of the storm slabs and ~ 2 ft for the reported wind slabs. These avalanches occurred on steep slopes on N-NE-E-SE-S aspects. Sun and warming during the day appeared to have contributed to the reactivity on some slopes.

We know of at least two cases where the person who triggered the avalanche was also caught and carried by it. One on Flagpole Peak (reported as a storm slab) where the person lost a ski but was otherwise unharmed, and one in the Blue Lakes area (likely a wind slab) where the person was fully buried and suffered serious injuries.

Several natural avalanches were also reported that likely occurred during the storm on Thursday night (Frog Lake Cliffs, Donner Summit, Castle Peak, Mt. Tallac, Incline Lake Peak, Desolation Wilderness). 

No avalanches were reported on the old weak layers like the Jan 22 crust/facet combo or the Dec 11 facets. These layers are buried under strong snow and have been gaining strength. The vast majority of observations and data collected across the region since the major January storm indicate that these old deep weak layers have gone dormant. Prior to this storm only a few snowpit tests still yielded some mixed (stable and unstable) results in a few isolated areas with a very weak and shallow snowpack. It is unlikely that these storms have added enough new weight to reactivate those deeply buried weak layers.

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