Avalanche forecast

Central Sierra Nevada

Sierra Avalanche Center
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Avalanche danger

Alpine
3 · Considerable
Treeline
3 · Considerable
Below
3 · Considerable
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Highlights

Human triggered wind slab avalanches remain likely today and storm slab avalanche problems remain possible. In addition to these issues, the possiblity of an isolated deep slab avalanche cannot be ruled out. For these reasons, CONSIDERABLE avalanche danger exists at all elevations today. Some avalanches that occur today could be very large and destructive and have very serious consequences.

Avalanche problems

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    3

    Large unstable wind slabs exist on wind-loaded N-NE-E aspects and cross-loaded NW and SE aspects in near and above treeline terrain. Human-triggered wind slab avalanches large enough to bury a car or even a train (D4) will remain likely today. Some wind slabs could lurk in more traditionally sheltered spots or extend into below treeline areas. Remote triggering and avalanches that start as smaller slides then step down into deeper weak layers remain possible today.

    Recent avalanches, blowing snow, cornices, wind pillows, and other signs of wind-loading can help identify where wind slabs may exist.  Avoid steep wind-loaded terrain and run out zones below corniced or wind-loaded slopes in favor of more sheltered less steep terrain.

  • Loose Dry

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    2

    Human-triggered storm slab avalanches large enough to bury or injure a person (D2) and or even a car (D3) remain possible today due to lingering storm snow weaknesses. Like the wind slabs mentioned above, some of these storm slabs could step down to deeper layer after they release. In areas where the surface snow has less cohesion, some loose dry sluffs could also occur on steep slopes.

    Recent avalanche activity and shooting cracks can provide clues as to where storm slab problems may exist. Snowpits or hand pits to feel for weak layers and slab layers in the new snow can also help identify potential storm slab problems. This avalanche problem could exist in areas traditionally thought of as safe like tree-covered slopes.

  • Storm Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    4

    Most observations and data targetting the persistent weak layer and the slab above it have indicated that they may have gained enough strength to handle the new load. However, a few pieces of data (some of which were collected on Thursday) still indicated that with enough loading or a large enough trigger (like the other avalanches mentioned above) this layer could possibly reactivate. Due to the lack of data pertaining to this problem from yesterday and the variability of previous data, a significant amount of uncertainty exists as to whether or not deep slabs still represent a problem for backcountry travelers. In many areas, this layer should be able to support the new load, but it is possible that in some isolated places it cannot. If it does break the resulting deep slab avalanches would be huge, unsurvivable, and could run very long distances (size D4 to D5).

    Due to the combination of uncertainty, variability in the data, and the catastrophic consequences of such an avalanche, deep slabs continue to warrant inclusion as an avalanche problem and avoiding slopes where even a remote possibility of a deep slab may exist is the best choice for today.  

Avalanche Discussion

* Ski cuts and ski kicks on low angle terrain near or above wind-loaded slopes on Andesite Peak remotely triggered numerous wind slab avalanches yesterday. Most of these measured 8 to 16 inches deep. One of these smaller wind slabs released a secondary avalanche on the slope with a crown of 3 to 4 ft in depth that likely failed on a lower storm snow weakness. All of these wind slabs propagated across entire slopes (up to several hundred feet across) and wrapped around features on the slope.

* Ski cuts on a low angle slope in below treeline terrain on Andesite Peak remotely triggered a storm slab avalanche that started as a 6 to 8 inch crown and stepped down to a lower storm snow weakness resulting in an 18 to 24 inch crown. Other ski cuts on more sheltered below treeline test slopes caused some slopes to crack but as they slid downhill the softer storm slabs quickly broke apart. Observers on Powderhouse Peak also reported some minor storm slab cracking in below treeline terrain. 

* Numerous skier trigger loose dry sluffs occurred in below treeline terrain on Powderhouse Peak, Andesite Peak, and on road cuts off Pole Creek Road. 

* While most data and evidence has indicated that the old persistent weak layer buried deep below the surface has gained strength, snowpit tests and data in the Elephant's Hump area and in a backcountry area near Squaw Creek on Thursday indicated that this layer could reactivate with enough new loading. No new deep slab data came in yesterday and there is still a great deal of uncertainty about the state of the possible deep slab problem.

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