Avalanche forecast

Central Sierra Nevada

Sierra Avalanche Center
View on map

Avalanche danger

Alpine
3 · Considerable
Treeline
3 · Considerable
Below
3 · Considerable
Open full map
Out of date — this forecast expired 9 years ago. You’re viewing past conditions, not the current bulletin.

Highlights

As more snow, rain, and wind impact the forecast area over the next 24 hours, human-triggered avalanches will be likely and natural avalanches will be possible especially later today and during the night. Avalanche problems include persistent slabs, loose wet avalanches, and wind slabs. CONSIDERABLE avalanche danger will exist at all elevations. Avalanches could involve enough snow to bury a person. Dangerous avalanche conditions will exist across the forecast area.

Avalanche problems

  • Deep Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    1

    Loose weak snow (facets) still exists below the surface on many northerly aspects across the forecast area. In some areas, the recent rain has soaked this layer and started to change it. In those areas while this layer of loose snow remains wet, it remains weak. Once it refreezes it will gain strength, but refreezing will not happen until after this storm has a chance to load that loose weak layer. In areas where the facets remain dry, they also remain weak. Regardless of whether this layer is wet or dry it is barely able to support the snowpack currently resting above it. As more precipitation adds more weight to the snowpack over the next 36 hours expect persistent slab avalanches to become more widespread. Human-triggered persistent slabs will become likely today and some natural avalanches will be possible especially later in the day and during the night. In areas where rain falls on the snowpack, the rain could make this avalanche problem easier to trigger and those persistent slabs could entrain more wet snow and have some wet slab characteristics. The persistent slab problem should grow in size as more precipitation accumulates and avalanches could involve enough snow to bury a person.

    Avoid steeper slopes in areas where snow surface cracking, whumpfing, and/or unstable snowpit test results are occurring.  Quick hand pits remain an effective tool for seeing if a slab over weak loose snow exists. These kinds of avalanches can be triggered from connected slopes or may not release until a person has fully committed to the slope. 

  • Loose Wet

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    1

    1 to 2 inches of rain falling on the snowpack in the next 24 hours will cause loose wet instabilities to become widespread. Expect roller balls, pinwheels, and wet point release avalanches during the next 24 hours in any areas that receive rain. Some of these could entrain enough snow to bury a person especially on northerly aspects where the snow has not seen rain before. Rain could also exacerbate today's other avalanche problems.

    Avoiding steep slopes where rain is falling or where wet snow exists represents the easiest way to avoid loose wet avalanches.

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    1

    Winds strong enough to transport snow and new snow will allow new wind slabs to form on wind-loaded slopes. These wind slabs should remain limited to areas above 8500 ft. today, but they should start to become more widespread overnight as snow levels start to fall. In isolated areas, wind slabs may be on top of old near surface facets on NW-N-NE aspects.

    Look for cornice formation, blowing snow, and signs of previous blowing snow, especially along ridgelines and wind exposed areas. Use these clues to identify where wind slabs may exist and avoid those steep wind-loaded slopes. 

Avalanche Discussion

* On Tamarack Peak, a small human-triggered avalanche was reported on Prolateriate yesterday, and a similar small natural avalanche was reported in Broken Glass.

* Human-triggered cracking, collapsing, and whumpfing occurred on Red Lake Peak and near Woods Lake on Carson Pass. The weak loose facetted snow below the new snow represented the failure layer.

* In the Bear Valley area, on Carson Pass, and on Red Lake Peak, snowpit tests targeting the facet layer yielded unstable test results.

* Observations showed moist facets in some areas yesterday and dry facets in other areas. Whether they were dry or moist depended on elevation and on whether or not a slab existed above the facets. Regardless of whether they were moist or dry, they remained loose and weak.

* NW-N-NE aspects above 8,000' in the Mt. Rose area and along the Sierra Crest north of Emerald Bay hold the best snow coverage at 2 to 3+ feet. Overall less snow cover exists south of Emerald Bay. Areas of decent coverage exist above 8,500' on NW-N-NE aspects in the Carson Pass area.  Coverage decreases on all other aspects. Large areas of bare ground exist on the vast majority of southerly aspects at all elevations.

Regions covered