THIS AVALANCHE ADVISORY EXPIRED ON December 22, 2017 @ 6:58 am
Avalanche Advisory published on December 21, 2017 @ 6:58 am
Issued by Andy Anderson - Tahoe National Forest

MODERATE avalanche danger exists in near and above treeline terrain due to yesterday's lingering wind slabs and today's newly formed wind slabs. Small loose dry avalanches could also occur on steep slopes where loose unconsolidated surface snow exists in more sheltered terrain. Human triggered avalanches remain possible today. Evaluate snow and terrain carefully; identify areas where avalanche problems exist; and avoid those areas.

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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Strong NE winds will transport snow away from previously wind-loaded N-NE-E aspects and create new wind slabs on the S-SW-W aspects and cross-loaded NW and SE aspects. These new wind slabs may be sensitive to human-triggering today and human-triggered wind slab avalanches will be possible. In addition to these newly formed wind slabs, some of yesterday's wind slabs may still linger on wind-loaded N-NE-E aspects and cross-loaded NW and SE aspects in areas that have not been scoured by the NE winds. In either case, these wind slabs exist on top of smooth icy surfaces and weak old snow. While many of these wind slabs should remain small and not extend very far away from ridgelines, some larger and deeper wind slabs could exist in the most heavily wind loaded areas or in areas that received the most new snow. Avalanches resulting from these wind slabs could involve enough snow to bury a person. The wind slabs should be denser and harder than yesterday's wind slabs. They could break above the person who triggers them or not fail until that person has already committed to the slope.

Use clues like blowing snow, hollow sounding snow, cornices above slopes, drifted snow, and other wind created textures to identify where wind slabs may exist and avoid traveling on or under steep slopes in those areas in favor of less exposed, less steep, more sheltered slopes where softer snow may reside.

Avalanche Problem 2: Loose Dry
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A cold clear night should have caused yesterday's snow to weaken and become less cohesive in areas sheltered from the winds. While weaker surface snow means slab avalanches should be unlikely in sheltered near and below treeline areas, loose dry sluffs could occur today. Steep slopes where the new snow rests on top of smooth icy old snow surfaces or on top of loose weak old snow represent the most likely terrain to trigger a loose dry avalanche. These sluffs could involve enough snow to knock a person off balance but may not involve enough snow to bury a person. With the current early season conditions, getting knocked off balance could result in getting pushed into a rock, tree, stump or other hard immovable objects. These sluffs would could also have more serious consequences in areas near cliffs or other terrain traps

recent observations

* Small but reactive wind slabs existed on Castle Peak and Chickadee Ridge yesterday. Ski kicks caused shooting cracks and small 6 to 14 inch deep wind slab failures on N-NE-E facing test slopes. Variable old snow exists under these wind slabs ranging from firm crusts to old hard slabs

* Snow totals averaged about 4 inches across the forecast area yesterday.

* Weak snow still exists under the new snow in sheltered areas, but the new snow did not have enough slab characteristics to form a storm slab problem.

* The most snow coverage still exists in areas north of Emerald Bay above 8,000'. Below 8,000' snow coverage remains patchy and shallow. South of Emerald Bay, less snow exists with areas below 8,500' holding very little snow. 

Weather and CURRENT CONDITIONS
weather summary

After a short burst of snowfall that totaled about 4 inches in most places yesterday morning, the cold front departed the area leaving cold dry weather behind. The wind shifted to the NE yesterday evening and has been steadily increasing since then. The Ward Peak sensor along the Sierra Crest registered sustained NE winds in the 50 to 70 mph range with gusts as high as 92 mph this morning. These NE winds should peak by midday and then start to decrease this afternoon. By tomorrow the forecast calls for light winds and warming temperatures at the upper elevations. Some thin high clouds may also develop over the area tonight and tomorrow. 

Weather observations from along the Sierra Crest between 8200 ft. and 8800 ft.
0600 temperature: 5 to 12 deg. F.
Max. temperature in the last 24 hours: 22 to 26 deg. F.
Average wind direction during the last 24 hours: SW shifting to NE after 4 pm yesterday
Average wind speed during the last 24 hours: Before 4 pm yesterday: 10 to 20 mph | After 4 pm yesterday: 20 to 40 mph
Maximum wind gust in the last 24 hours: 92 mph
New snowfall in the last 24 hours: 0 to 2 inches
Total snow depth: 17 to 34 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 Increasing high clouds Thickening high clouds
Temperatures: 31 to 35 deg. F. 15 to 21 deg. F. 40 to 45 deg. F.
Wind Direction: Northeast Variable Southwest
Wind Speed: 10 to 20 mph Light Light increasing to 10 mph
Expected snowfall: 0 in. 0 in. 0 in.
For 8000 ft. to 9000 ft.
Thursday Thursday Night Friday
Weather: Sunny Increasing high clouds High clouds thickening
Temperatures: 28 to 33 deg. F. 26 to 31 deg. F. 35 to 40 deg. F.
Wind Direction: Northeast Northeast shifting to northwest after midnight West
Wind Speed: 25 to 45 mph with gusts to 75 mph 15 to 25 mph with gusts to 55 mph decreasing to 10 to 20 mph after midnight 15 to 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.

For a recorded version of the Avalanche Advisory call (530) 587-3558 x258