THIS AVALANCHE ADVISORY EXPIRED ON November 26, 2015 @ 7:00 am
Avalanche Advisory published on November 25, 2015 @ 7:00 am
Issued by Andy Anderson - Tahoe National Forest

Human triggered avalanches remain likely today. CONSIDERABLE avalanche danger exists on near and above treeline NW-N-NE-E-SE aspects 35 degrees and steeper with MODERATE avalanche danger on the other aspects and elevations. Newly formed wind slabs on the wind loaded NW-N-NE-E-SE aspects in near and above treeline terrain represent the primary avalanche concern today but loose dry instabilities and isolated storm slabs may also exist in non wind loaded areas. 

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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The wind slabs that have formed on leeward aspects since noon yesterday will remain fragile today. Human triggering of these wind slabs will remain likely. Most avalanches that result from the failure of these wind slabs should fail at or above the old snow surface; however, some of them could step down into older snow layers in areas where the new wind slabs exist on top of old hard slabs. The most sensitive wind slabs will exist on wind loaded N-NE-E aspects and cross loaded NW and SE aspects in near and above treeline terrain. These winds slabs could easily measure 2-3 ft in thickness or more in heavily wind loaded areas, and avalanches involving them could entrain enough snow to bury a person.

Cornices above a slope, wind ripples, drifts, current wind loading, and other signs of wind loading can all serve as clues to help recognized slopes where wind slabs may exist. If a near or above treeline area looks nicely filled in and smooth and rounded, wind slabs likely exist in that area. Today is a good day to exercise some patience and avoid slopes where wind slabs may exist instead of letting the excitement of early season snow lead to poor decision making. 

Avalanche Problem 2: Loose Dry
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Loose dry snow sluffs will remain possible today. These sluffs could entrain all of the new snow and slide on the old snow surfaces or only involve a portion of the new snow and fail within the new snow. Steep slopes in near and below treeline terrain on NW-N-NE-E aspects sheltered from the winds hold the best potential for these kind of instabilities since they had more previous snow coverage. These instabilities could also exist on other near and below treeline aspects, but they will be more isolated and smaller on the SE-S-SW-W aspects due to the fact that most of those aspects did not have snow cover prior to this storm.

Avalanche Problem 3: Storm Slab
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While loose dry sluffs will represent most of the non wind slab instabilities today, small and isolated storm slabs may exist on the near and below treeline NW-N-NE-E aspects in areas where the most new snow fell and where some consolidation has taken place in the new snow. Some uncertainty remains about how widespread these small storm slabs may be, but human triggering of these storm slabs will be possible today if one finds an area where they exist. These small slabs could fail on the old snow surfaces below them or within the new snow. 

recent observations

Observations from Tamarack Peak yesterday indicated that poor bonding existed between the new snow and the old snow surfaces. In non wind affected areas the new snow exhibited very few slab characteristics and ski cuts on steep test slopes only triggered small loose dry sluffs. On the more exposed NW-N-NE-E aspects, wind loading had created wind slabs up to 12 inches in depth by 2 pm. Snowpit tests on these wind slabs indicated that the additional weight of a person would likely cause them to fail. Ski cuts and kicks on small wind loaded test slopes confirmed this data and caused small wind slab failures as well as shooting cracks. On one test slope where a previous hard slab existed a ski cut triggered the wind slab and the failure stepped down to the older snow (facets) below the hard slab.

Weather and CURRENT CONDITIONS
weather summary

Yesterday 5 to 10 inches of new snow accumulated across the forecast area by 6 pm and another 1 to 2 inches fell overnight. Snow showers and unsettled weather should continue today as small waves of weather spin around the main low pressure that is stalled over northern NV. The forecast calls for another 1 to 4 inches of accumulation today in most areas. Areas east of Lake Tahoe may see greater snowfall amounts and intensity due to the possibility of lake effect snow. Snow showers should taper off by tomorrow. Wind speeds have started to decrease and should shift to the west today. They should continue to decrease over the next 24 hours and the forecast calls for them to shift to the north and east tonight and tomorrow. Temperatures will remain cold with daytime highs hovering in the teens and overnight lows in the single digits above 7000 ft.

Weather observations from along the Sierra Crest between 8200 ft. and 8800 ft.
0600 temperature: 13 to 19 deg. F.
Max. temperature in the last 24 hours: 30 to 35 deg. F.
Average wind direction during the last 24 hours: Southwest to West
Average wind speed during the last 24 hours: 35 to 45 mph
Maximum wind gust in the last 24 hours: 100 mph
New snowfall in the last 24 hours: 6 to 12 inches
Total snow depth: 15 to 25 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Wednesday Wednesday Night Thursday
Weather: Cloudy with snow showers Mostly cloudy with scattered snow showers Mostly cloudy with a slight chance of snow showers
Temperatures: 14 to 21 deg. F. 9 to 15 deg. F. 14 to 21 deg. F.
Wind Direction: West West Variable
Wind Speed: 15 to 20 mph with gusts to 30 mph 10 to 15 mph in the evening becoming light during the night Light
Expected snowfall: 1 to 4 in. up to 1 in. 0 in.
For 8000 ft. to 9000 ft.
Wednesday Wednesday Night Thursday
Weather: Cloudy with snow showers Mostly cloudy with scattered snow showers Mostly cloudy with a slight chance of snow showers
Temperatures: 10 to 17 deg. F. 4 to 11 deg. F. 11 to 18 deg. F.
Wind Direction: West North East
Wind Speed: 15 to 25 mph with gusts to 40 mph 15 to 20 mph with gusts to 30 mph 10 to 15 mph
Expected snowfall: 1 to 4 in. up to 1 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.