THIS AVALANCHE ADVISORY EXPIRED ON December 12, 2013 @ 6:54 am
Avalanche Advisory published on December 10, 2013 @ 6:54 am
Issued by Andy Anderson - Tahoe National Forest

On slopes steeper than 30 degrees, MODERATE avalanche danger exists at all elevations on NW-N-NE aspects an on the near and above treeline E-SE aspects due to a combination of widespread persistent slabs and lingering wind slabs. 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.

I will update the advisory next on Thurday morning at 7am. 

2. Moderate

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

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
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Human triggered avalanches remain possible in near and below treeline terrain on NW-N-NE aspects where the recent snow rests on top of the Dec. 7th facet layer. Any areas that had snow cover prior to this storm remain suspect. The recent snow currently represents a soft slab. Depending on how fast it consolidates or how fast it weakens the new snow may become more (faster consolidation) or less (faster weakening) slab-like. A more slab-like layer would make these persistent slabs easier to trigger.

Since fractures can travel so easily 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 30 degrees remains possible below treeline.

Avalanche Problem 2: Wind Slab
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Some fragile wind slabs that formed during the storm may still linger in isolated areas on the near and above treeline NW-N-NE-E-SE aspects. Most of these wind slabs should have become more difficult to trigger, smaller, and more shallow over the last few days. However, some of these wind slabs could sit on top of the persistent weak layers mentioned above on the near treeline NW-N-NE aspects. Other larger and more fragile wind slabs may also linger in more in complex or extreme terrain (unsupported slopes, couloirs , cliffy areas, etc). Human triggered avalanche activity resulting from the failure of these wind slabs remains possible.

recent observations

More whumpfing and cracking occurred on Rubicon Peak (more info, snowpit) yesterday in N-NE facing near and below treeline terrain. 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. The collapses in this area were less widespread, smaller, and did not travel as far as the failures that occurred in other areas over the last 2 days. Observations and snowpit data showed that while the Dec. 7th facets remain weak they have started to adjust VERY SLOWLY to the recent snow above them. Data also indicates that the recent snow layer still represents a soft slab. This recent snow has also started to weaken due to the cold clear weather across the region.

In the more wind exposed terrain near the top of Rubicon, some small wind slabs remained on the N-NE aspects. Stepping onto small wind loaded test slopes in this area did cause some small shooting cracks 3-5 ft in length. Very little evidence of east wind scouring existed in this area.

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

A high pressure ridge over the forecast area should allow the clear, dry weather to continue through the end of the week. In the mountains temperatures have started to warm with many of the remote sensors reporting temperatures in the low 30's already this morning. Expect daytime highs to climb into the mid to upper 30's each of the next three days. Cold air remains trapped in the valleys causing temperatures at the lower elevation to stay colder than temperatures in the mountains. This inversion should continue until stronger winds arrive over the region. The east winds decreased throughout the day yesterday and remain light this morning. The forecast calls for light winds to continue through tomorrow and increase slightly on Thursday.

Weather observations from along the Sierra Crest between 8200 ft. and 8800 ft.
0600 temperature: 20 to 32 deg. F.
Max. temperature in the last 24 hours: 26 to 32 deg. F.
Average wind direction during the last 24 hours: E shifting to the NW after midnight
Average wind speed during the last 24 hours: 5 to 20 mph
Maximum wind gust in the last 24 hours: 61 mph
New snowfall in the last 24 hours: 0 inches
Total snow depth: 16 to 27 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Tuesday Tuesday Night Wednesday
Weather: Sunny Mostly clear with a few scattered clouds before midnight Sunny
Temperatures: 36-41 deg. F. 20-26 deg. F. 37-42 deg. F.
Wind Direction: Variable East East
Wind Speed: Light Light Light
Expected snowfall: 0 in. 0 in. 0 in.
For 8000 ft. to 9000 ft.
Tuesday Tuesday Night Wednesday
Weather: Sunny Mostly clear with a few scattered clouds before midnight Sunny
Temperatures: 32-38 deg. F. 26-32 deg. F. 36-41 deg. F.
Wind Direction: North Northeast East
Wind Speed: 10-15 mph 10-15 mph 10-15 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.