THIS AVALANCHE ADVISORY EXPIRED ON December 13, 2013 @ 6:49 am
Avalanche Advisory published on December 12, 2013 @ 6:49 am
Issued by Andy Anderson - Tahoe National Forest

On slopes steeper than 32 degrees, MODERATE avalanche danger exists on near and below treeline NW-N-NE aspects due to lingering persistent slabs. Some isolated areas of unstable snow may still remain on above treeline slopes as well. Human triggered persistent slabs remain possible in near and below treeline terrain. Remotely triggered avalanches can occur under these conditions.

In addition to the avalanche danger a myriad of season-ending obstacles still lurks just below the surface. Conservative terrain selection, careful snowpack evaluation, and conservative decision-making are essential.

1. Low

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Above Treeline
Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.

2. Moderate

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Near Treeline
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.

2. Moderate

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Below Treeline
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
    Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
Avalanche Problem 1: Persistent Slab
  • Type ?
  • Aspect/Elevation ?
  • Likelihood ?
    Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size ?
    Historic
    Very Large
    Large
    Small

Human triggered avalanches remain possible in near and below treeline terrain on NW-N-NE aspects where the Dec. 7th facet layer exists. While the vast majority of these persistent slabs exist in near and below treeline terrain, the possibility remains that some isolated areas above treeline where snow did not get scoured away earlier this season could pose a problem. Currently the snow that fell on Dec. 7th represents a soft slab sitting above the older layers of weak sugary snow. Even though some data indicates that the snowpack has become less fragile, the addtional weight of a person on a slope where this persistent weak layer/soft slab combination exists can still cause the slope to fracture. Unstable conditions like these take a long time to change and can easily catch people off-guard.

Since fractures can travel through the persistent weak layer, people can trigger persistent slabs remotely while standing in low angle terrain or on ridgelines or anywhere that the snowpack under their feet connects to steeper and/or deeper terrain. Wide propagation and avalanche activity on lower angle slopes to around 32 degrees remains possible below treeline.

recent observations

Yesterday some collapsing and whumpfing continued to occur on Donner Peak. As in other areas these collapses happened when the weak sugary snow (Dec. 7th facets) below the recent new snow broke and a fracture propagated along the weak layer. These collapses were less consistent and smaller than those that occurred in other areas over the weekend and earlier this week. Snowpit tests did indicate that if this layer does break the resulting fracture can travel along this persistent weak layer. Observations and snowpit data seem to indicate that while the Dec. 7th facets remain weak they have started to adjust VERY SLOWLY to the load above them. Data also indicates that the snow layer above the Dec. 7th facets still represents a soft slab.

Barely covered rocks, stumps, logs, and other hard, immobile, season-ending, equipment-breaking objects still exist on all slopes. The shallow snowpack does not have enough structure or strength to keep a person above all these things. It only camouflages their existence.

Weather and CURRENT CONDITIONS
weather summary

The winds have shifted to the SW and increased ahead of a weak low pressure trough. This low pressure should move through region tonight and tomorrow morning bringing partly cloudy skies, cooler temperatures, and slightly increased winds. By tomorrow afternoon the winds should shift to the NE as the low exits the region. For today expect another day of clear dry weather around the region with daytime highs in the upper 30's and low 40's above 7000 ft. Tomorrow should see highs only reach the upper 20's and low 30's above 7000 ft.  

Weather observations from along the Sierra Crest between 8200 ft. and 8800 ft.
0600 temperature: 24 to 33 deg. F.
Max. temperature in the last 24 hours: 36 to 42 deg. F.
Average wind direction during the last 24 hours: E shifting to the SW yesterday afternoon
Average wind speed during the last 24 hours: 10-15 mph
Maximum wind gust in the last 24 hours: 31 mph
New snowfall in the last 24 hours: 0 inches
Total snow depth: 14-24 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Thursday Thursday Night Friday
Weather: Sunny Partly Cloudy Partly Cloudy
Temperatures: 40-45 deg. F. 23-38 deg. F. 29-33 deg. F.
Wind Direction: SW W NW shifting to the NE in the afternoon
Wind Speed: up to 10 mph in the morning, increasing to 10-15 mph in the afternoon 10-15 mph with gusts to 25 mph 10-15 mph with gusts to 25 mph
Expected snowfall: 0 in. 0 in. 0 in.
For 8000 ft. to 9000 ft.
Thursday Thursday Night Friday
Weather: Sunny Partly Cloudy Partly Cloudy
Temperatures: 34-40 deg. F. 16-23 deg. F. 23-28 deg. F.
Wind Direction: SW W NW shifting to the NE in the afternoon
Wind Speed: 10-20 mph with gusts to 35 mph 15-25 mph with gusts to 35 mph 15-25 mph with gusts to 45 mph
Expected snowfall: 0 in. 0 in. 0 in.
Disclaimer

This avalanche advisory is provided through a partnership between the Tahoe National Forest and the Sierra Avalanche Center. This advisory covers the Central Sierra Nevada Mountains between Yuba Pass on the north and Ebbetts Pass on the south. Click here for a map of the forecast area. This advisory applies only to backcountry areas outside established ski area boundaries. This advisory describes general avalanche conditions and local variations always occur. This advisory expires 24 hours after the posted time unless otherwise noted. The information in this advisory is provided by the USDA Forest Service who is solely responsible for its content.