THIS AVALANCHE ADVISORY EXPIRED ON December 12, 2015 @ 6:58 am
Avalanche Advisory published on December 11, 2015 @ 6:58 am
Issued by Andy Anderson - Tahoe National Forest

CONSIDERABLE avalanche danger exists on slopes steeper than 32 degrees on NW-N-NE-E-SE aspects in near and above treeline terrain. MODERATE avalanche danger exists on these aspects in below treeline terrain. Wind slabs will exist on wind loaded slopes, and persistent slabs may exist on sheltered northerly aspects toady. Careful snowpack evaluation, cautious route-finding, and conservative decision-making are essential. Human triggered avalanches will be likely today.

3. Considerable

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Above Treeline
Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.

3. Considerable

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Near Treeline
Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.

2. Moderate

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Below Treeline
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
    Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.
Avalanche Problem 1: Wind Slab
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10 to 20 inches of new snow in the last 24 hours combined with strong southwest winds means that human triggered avalanches involving wind slabs will remain likely today. These wind slabs will exist on wind loaded N-NE-E aspects and cross loaded NW and SE aspects. Clues like blowing snow, cornices above a slope, pillows of drifted snow, and other wind created textures can help identify where these fragile wind slabs exist. Open, exposed, near and above treeline terrain will represent the most common locations for these new wind slabs, but some wind slabs could also exist on open slopes in below treeline terrain due to the strength of the winds at all elevations during this storm. Wind slabs could easily measure more than 4 ft in depth in heavily wind loaded areas and more than 3 ft in depth in almost any wind loaded area. Avalanches involving these wind slabs could easily involve enough snow to bury a person. If these wind slabs release on slopes where the persistent weak layers mentioned below exist, they could overload that deeper layer and involve even more snow. 

Avalanche Problem 2: Persistent Slab
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A persistent weak layer exists near the bottom of the snowpack on NW-N-NE aspects. So far observations and data have shown this layer is weakest on the north aspects. The additional load of new snow could be enough to push this persistent weak layer so close to the breaking point that the weight of a person on the slope could trigger a persistent slab avalanche in some areas. In many areas these persistent slabs remain confined by anchors poking through the buried weak layers that should prevent these avalanches from fracturing over large areas. However, these persistent slab avalanches could fracture across wider areas and involve much more snow on slopes with smooth ground cover and less anchors. Cracking, collapsing, whumphing, and recent avalanche activity can all provide clues as to where these persistent slabs may be an issue, but they could still occur on slopes without those clues. Digging into the snowpack to find this persistent weak layer and avoiding slopes where it exists, traveling on lower angle terrain, or traveling on well anchored terrain represents the best way to avoid this avalanche problem.

recent observations

Yesterday observations from lower elevations on Waterhouse Peak showed wet snow under the new snow that collapsed under the weight of a splitboarder. At upper elevations the snowpack showed a structure of softer snow sandwiched between layers of melt-freeze crusts. Snowpit tests targeting the buried weak layers of sugary snow near the base of the snowpack showed inconsistent results in this area. On Tamarack Peak in the Mt. Rose backcountry, a similar snowpack structure existed. However, several skier triggered whumphs occurred on N facing slopes as the weak layers near the base of the snowpack collapsed. Snowpit tests targeting this persistent weak layer also yielded unstable results. Observations in this area showed that some wind slabs had formed on leeward aspects, but that they had not grown very large by yesterday afternoon. Ski cuts and weighting wind loaded test slopes did not produce any signs of instability in the Tamarack Peak area yesterday. 

Weather and CURRENT CONDITIONS
weather summary

Yesterday less snow fell than expected due to less moisture with the storm and the fact that more of the precip fell while snow levels were higher. Still, areas above 8000 ft. received 4 to 8 inches of new snow in areas north of Emerald Bay and 2 to 6 inches in areas south of Emerald Bay. Overnight another 8 to 12 inches of snow fell across the forecast area at most elevations since snow levels dropped below 5000 ft. Snow showers may continue this morning with some areas seeing an additional 1 to 4 inches of accumulation. These snow showers should taper off today as the low pressure system moves eastward. The strong southwest winds started to decrease yesterday afternoon and last night and they should continue to diminish during the day today. The forecast calls for colder temperatures to remain over the forecast area today and tonight. A short lived high pressure ridge should bring warmer temperatures and less cloud cover to the region tomorrow, but the winds should start to increase again ahead of another storm projected to reach the forecast area on Sunday. 

Weather observations from along the Sierra Crest between 8200 ft. and 8800 ft.
0600 temperature: 18 to 22 deg. F.
Max. temperature in the last 24 hours: 34 to 36 deg. F.
Average wind direction during the last 24 hours: Southwest
Average wind speed during the last 24 hours: 30 to 50 mph
Maximum wind gust in the last 24 hours: 130 mph
New snowfall in the last 24 hours: North of Emerald Bay: 12 to 20 inches | South of Emerald Bay: 10 to 18 inches
Total snow depth: 20 to 40 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Friday Friday Night Saturday
Weather: Cloudy becoming mostly cloudy. Snow showers in the morning tapering off during the day. Mostly cloudy with isolated snow showers in the evening Mostly cloudy with clouds increasing during the day. 25% chance of snow showers.
Temperatures: 24 to 29 deg. F. 15 to 21 deg. F. 30 to 35 deg. F.
Wind Direction: West West Southwest
Wind Speed: 10 to 20 mph with gusts to 30 mph 10 to 15 mph increasing to 10 to 20 mph with gusts to 35 mph after midnight 10 to 20 mph with gusts to 30 mph
Expected snowfall: up to 3 in. 0 in. 0 in.
For 8000 ft. to 9000 ft.
Friday Friday Night Saturday
Weather: Cloudy becoming mostly cloudy. Snow showers in the morning tapering off during the day. Mostly cloudy with isolated snow showers in the evening Mostly cloudy with clouds increasing during the day. 25% chance of snow showers.
Temperatures: 20 to 25 deg. F. 14 to 19 deg. F. 25 to 32 deg. F.
Wind Direction: West Northwest West
Wind Speed: 30 to 45 mph with gusts to 65 mph decreasing to 20 to 30 mph with gusts to 50 mph in the afternoon 20 to 25 mph with gusts to 35 mph increasing to 30 to 35 mph with gusts to 50 mph after midnight 35 to 45 mph with gusts to 75 mph
Expected snowfall: 1 to 4 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.