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Out of date — this forecast expired 11 years ago. You’re viewing past conditions, not the current bulletin.

Highlights

Human triggered avalanches will become likely today and natural avalanches will become possible as a winter storm impacts the forecast area. On slopes 35 degrees and steeper, CONSIDERABLE avalanche danger will form on NW-N-NE-E-SE aspects in near and above treeline terrain with MODERATE danger in all other areas. Wind slabs and loose dry avalanches represent the main avalanche problems today with deep slabs on NW-N-NE aspects representing a third unlikely but not impossible problem. 

Avalanche problems

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    3

    New snow and wind will create new wind slabs on the leeward slopes today. These wind slabs will accumulate on top of icy crusts and firm snow surfaces. They will likely form faster than they can bond to these old snow surfaces. Human triggered wind slab avalanches will be likely today and natural wind slab avalanches will be possible. Wind loaded N-NE-E and cross loaded NW and SE aspects in exposed near and above treeline terrain will represent the best places to find these unstable wind slabs today. While these wind slabs may start out small this morning, they will quickly grow in size and extend as the storm continues. They will quickly become large enough to bury a person. By the end of the day wind slabs could measure more than 3-4 ft in depth in some areas or more if snow totals reach the high end of the forecasted amounts. If these wind slabs form in areas where the persistent weak layers mentioned below exist, the unlikely but not impossible scenario of wind slab avalanches stepping down into older snow layers exists. 

  • Wet Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    1

    Cold new snow accumulating on top of frozen icy crusts means that human triggered loose dry avalanches/sluffs will be likely today, and some natural sluffing could occur. These sluffs would only involve the new snow. While they may not involve enough snow to bury a person, they could push people into areas with other consequences like hitting trees or going over cliffs. These sluffs will mostly occur on sheltered terrain on steep slopes with new snow on them. 

  • Storm Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Unlikely
    Size
    3

    Snowpit data still indicates that persistent weak layers do exist deep in the snowpack. In the Mt. Rose backcountry a layer of old facets still lurks near the base of the snowpack underneath the 12/10 rain crust, and some areas along the Sierra Crest still have a buried surface hoar layer hiding under the 12/13 rain crust. Tests and observations have indicated that NW-N-NE aspects in more sheltered terrain represent the best places to find these layers and that even in this kind of terrain they don't exist on every slope. Even in the places that they do exist, they may have gone dormant in some spots. These layers have 3-6 ft of strong, well-bonded snow above them and actually getting enough force through that strong upper snowpack to break these layers would be very difficult and require a very large trigger like a large cornice collapse, another avalanche, or several people on a slope at once. Still, if these layers do break, huge avalanches would result. Even though these kind of avalanches are unlikely, the consequences associated with them are dire, and this problem still warrants attention and extra caution. 

Avalanche Discussion

Yesterday observations from Becker Ridge showed variable surface conditions ranging from breakable rain crusts to firm icy surfaces to some areas with 1 inch of light cold snow on top of firm icy crusts. Farther south on Red Lake Peak more consistent surface conditions of hard, frozen, ice crusts existed on all aspects in more exposed terrain. On Tamarack Peak in the Mt. Rose backcountry, snow surfaces consisted of firm wind packed snow with some areas of 2 inches of softer snow above this firm snow surface.

On Becker Peak cornices remained sentive to human triggering. Snowpit data from an exposed NE facing slope on Red Lake Peak did not reveal signs of instability or buried persistent weak layers. Farther north on Tamarack Peak probing and observations on exposed above treeline N-NE aspects also did not find evidence of buried weak layers; however, snowpit data in more sheltered near and below treeline terrain did reveal a lingering persistent weak layer near the base of the snowpack. Tests on this layer did indicate that actually trigggering a break in this layer would be very difficult and require a very large trigger, but the tests also showed that if this layer were to break, fractures could travel through it. In all three areas the recent snow has bonded well to itself and to the old snow surfaces below it and observers reported signs of settlement and consolidation. 

Regions covered