THIS AVALANCHE ADVISORY EXPIRED ON December 27, 2015 @ 7:00 am
Avalanche Advisory published on December 26, 2015 @ 7:00 am
Issued by Andy Anderson - Tahoe National Forest

Human triggered avalanches will remain possible today on slopes 35 degrees and steeper on all aspects above treeline and on NW-N-NE-E-SE aspects and in near treeline terrain. MODERATE avalanche danger exists in those areas. Shifting winds mean that wind slabs could exist in any of these areas. Evaluate snow and terrain carefully before committing to a slope.

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.

1. Low

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Below Treeline
Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.
    Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
Avalanche Problem 1: Wind Slab
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    Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size ?
    Historic
    Very Large
    Large
    Small

Strong east winds will create new wind slabs on the SW-W-NW aspects and could cross load the S and N aspects as well. These new wind slabs should remain confined to areas along wind loaded ridges in above treeline terrain and not extend very far down slope, but they could still involve enough snow to cause problems for backountry travelers. These new wind slabs will be sensitive to human triggering today.

The scouring east winds will remove yesterday's wind slabs from some slopes, diminish them on others, and turn them into hard slabs on still others. Also the east winds may not impact all of the slopes where previous wind slabs existed. Some of yesterday's wind slabs could remain on the near treeline NW-N-NE-E-SE aspects as well as some of the above treeline slopes that face that direction but have some protection from the strong east winds. In areas where these older wind slabs remain human triggered avalanches remain possible as well. Couliors, steep cliffy areas, and other complex or extreme terrain may be the best places to find older wind slabs that remain unstable, but they could also still exist on any previously wind loaded slope that has not been fully scoured. These slabs could take the form of hard wind slabs that sound hollow below them on some slightly scoured slopes or may remain softer on the slopes protected from the east winds. 

advisory discussion

As for as the previous deep slab issues, snowpit data and observations from Drifter Bowl, Powderhouse Peak, Red Lake Peak, Hidden Peak, and Becker Peak have all indicated that the persistent weak layer issues along the Sierra Crest have subsided. In the Mt. Rose backcountry tests conducted on 12/23 still showed a layer of old facets lurking near the base of the snowpack underneath the 12/10 rain crust on some NW-N-NE aspects in more sheltered terrain but not in other similar terrain. Those tests also indicated that actually getting enough force through that strong upper snowpack to break this layer would be extremely difficult and require a huge trigger like a giant cornice collapse, another very large avalanche, or maybe several people on a slope at once. These kind of large avalanches have become very unlikely and are becoming more and more unlikely with each passing day, but until we have more data on the changes in this layer, paying attention to where it may possibly exist and whether or not it may still have potential to let fractures travel through it still represents a prudent course of action.

recent observations

Yesterday observations on N-NE facing wind loaded test slopes along Donner Ridge between Sunrise Bowl and Drifter Bowl showed fragile cornices and wind slabs that would crack under the weight of a skier. Snowpit tests and probing in more sheltered areas did not reveal any signs of the mid December persistent weak layers. On Powderhouse Peak off of Luther Pass snowpit data and observations also did not reveal any signs of lingering persistent weak layers. In both of these areas the snowpit data as well as general observations indicated that the new snow has started to slowly consolidate and bond to itself and the old snow surfaces below it. 

Weather and CURRENT CONDITIONS
weather summary

Winds shifted to the east yesterday afternoon and evening and increased dramatically along the Sierra Crest as a high pressure ridge started building over the region. Sensors along the ridgetops indicated east winds at 45 to 70 mph with gusts to 90 mph this morning. Air temperatures dipped into the single digits overnight in most areas. As the ridge establishes itself over the forecast area today, expect clear skies, continued east winds, and daytime highs in the 20's. Once the ridge is in place, the east winds should start to diminish tonight leading to a calm, clear, and cold night. The forecast calls for slightly warmer temperatures tomorrow. The wind should shift back to the southwest tomorrow ahead of a small low pressure system projected to arrive Sunday night / Monday morning.

Weather observations from along the Sierra Crest between 8200 ft. and 8800 ft.
0600 temperature: 8 to 11 deg. F.
Max. temperature in the last 24 hours: 15 to 24 deg. F.
Average wind direction during the last 24 hours: Southwest until noon yesterday since then east
Average wind speed during the last 24 hours: 30 to 40 mph
Maximum wind gust in the last 24 hours: 90 mph
New snowfall in the last 24 hours: trace inches
Total snow depth: 53 to 66 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Saturday Saturday Night Sunday
Weather: Partly cloudy then becoming sunny Clear Partly cloudy
Temperatures: 20 to 27 deg. F. 10 to 18 deg. F. 26 to 33 deg. F.
Wind Direction: East East Variable in the morning becoming southwest in the afternoon
Wind Speed: 20 to 30 mph with gusts to 45 mph 10 to 15 mph with gusts to 30 mph in the evening becoming light overnight Light in the morning increasing to 10 to 15 mph in the afternoon
Expected snowfall: 0 in. 0 in. 0 in.
For 8000 ft. to 9000 ft.
Saturday Saturday Night Sunday
Weather: Partly cloudy then becoming sunny Clear Partly cloudy
Temperatures: 20 to 27 deg. F. 16 to 22 deg. F. 26 to 33 deg. F.
Wind Direction: East East Southwest
Wind Speed: 35 to 45 mph with gusts to 65 mph 25 to 30 mph with gusts to 45 mph in the evening becoming light overnight 15 to 20 mph with gusts to 30 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.