THIS AVALANCHE ADVISORY EXPIRED ON December 25, 2016 @ 7:00 am
Avalanche Advisory published on December 24, 2016 @ 7:00 am
Issued by Andy Anderson - Tahoe National Forest

This advisory is valid until 12/26 at 7am.

Wind slabs in wind-loaded near and above treeline areas mean that human-triggered avalanches remain likely today and will remain possible tomorrow. In near and below treeline areas, human triggered storm slab avalanches will be possible in some areas both today and tomorrow. Evaluate the snowpack and terrain carefully and use this data to inform your route planning. Cautious and conservative decision-making represents the best plan for recreating in the backcountry. 

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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Strong winds and new snow have allowed wind slabs to form on the wind-loaded N-NE-E and cross-loaded NW and SE aspects on slopes steeper than 34 degrees. In addition to the wind slabs that formed during the 12/23 storm, shifting winds on 12/24 and 12/25 could cause some smaller wind slabs to form on additional aspects including S facing slopes. All of these wind slabs rest on old snow surfaces consisting of loose weak snow (near surface and near crust facets) and some more firm wind scoured surfaces. Human triggered wind slab avalanches will be likely today and will remain possible on 12/25. The best window for natural avalanche activity involving these wind slabs occurred yesterday evening and into the night, but some natural avalanches involving these wind slabs may remain possible this morning. The potential for natural avalanches involving these wind slabs should continue to decrease during the next 48 hours. In many areas, wind slabs could measure 1.5 to 3 ft in depth with even larger wind slabs on the most wind loaded slopes. They could propagate across a slope and could break above the person who triggers them.  

Use clues like cornices, blowing snow, snow drifts, wind pillows, scoured surfaces, ripples in the snow, and other wind created textures to help determine where wind slabs may exist. Use this knowledge to find terrain without wind slabs to recreate on. 

Avalanche Problem 2: Storm Slab
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In sheltered near and below treeline areas, the new snow has fallen on top of loose, weak, old snow (near surface and near crust facets) that exists above the 12/15 rain crust. Tests and observations yesterday at the start of the storm and before the storm indicated that the new snow may not bond well to this loose, weak, old snow. In some areas, this new snow may have enough cohesion to behave as a slab layer and a storm slab avalanche problem may now exist. In these areas, human triggered storm slab avalanches will be possible today and tomorrow. While most of the possible storm slab avalanches should break at the old snow surface and involve all of the new snow, some of them could break just above the 12/15 rain crust and potentially involve the new snow and some of the old snow. They may measure 18 inches or deeper in some areas if they do break below the old snow surface. Other areas may not have received quite enough new snow or the new snow may not be quite slabby enough for this problem to exist. Storm slabs could exist on any aspects on slopes steeper than 34 degrees. 

Clues like other avalanches, shooting cracks, collapsing, and an upside down snowpack can all help in figuring out where storm slabs may exist. Avoiding slopes with potential storm slabs by sticking to lower angle slopes or well-anchored slopes can allow for a more fun and safer day recreating on the snow. 

recent observations

Observations made at the beginning of the storm on 12/23 on Tamarack Peak in the Mt. Rose backcountry, showed significant wind transport and wind slab formation on exposed near and above treeline wind loaded slopes. Even though only 4 inches of new snow had fallen by the early afternoon, these wind slabs already measured more than a foot in depth in some areas. Ski cuts and kicks on wind loaded test slopes caused shooting cracks and wind slab failures. In sheltered areas, new snow accumulation had not yet reached a point where storm slabs had become widespread, but observations did indicate that the new snow was not bonding very well to the old snow surfaces. In these areas, soft new snow rested on a more dense layer of new snow that all rested on top of the old loose weak snow (near surface facets). 

Weather and CURRENT CONDITIONS
weather summary

Snowfall tapered off around midnight last night and left 9 to 14 inches of new snow in the mountains above 7000 ft. Temperatures also continued to fall during the night and have dropped down into the low teens and high single digits at the upper elevations this morning. While the main part of the storm has ended, another small wave of unsettled weather could pass through the area today bringing a chance for an additional 1 to 3 inches of snow above 7000 ft. By the end of the day, cloud cover should begin to decrease with skies becoming clear overnight. The winds may shift toward the NW this evening before trending back to the west for tomorrow. The forecast calls for mostly clear, cold and dry weather for Sunday and Monday. This cold dry weather should continue through mid-week.

Weather observations from along the Sierra Crest between 8200 ft. and 8800 ft.
0600 temperature: 11 to 18 deg. F.
Max. temperature in the last 24 hours: 25 to 33 deg. F.
Average wind direction during the last 24 hours: Southwest
Average wind speed during the last 24 hours: 30 mph
Maximum wind gust in the last 24 hours: 80 mph
New snowfall in the last 24 hours: 9 to 14 inches
Total snow depth: Along the Sierra Crest: 28 to 45 inches | In the Mt. Rose area: 63 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Saturday Saturday Night Sunday
Weather: Cloudy with some snow showers. Clouds decreasing during the day. Mostly cloudy with scattered snow showers becoming clear overnight. Sunny becoming partly cloudy in the afternoon.
Temperatures: 20 to 25 deg. F. 3 to 11 deg. F. 23 to 28 deg. F.
Wind Direction: Southwest Variable Variable
Wind Speed: 10 to 15 mph decreasing in the afternoon Light with gusts up to 25 mph in the evening. Light
Expected snowfall: 1 to 3 in. up to 1 in. 0 in.
For 8000 ft. to 9000 ft.
Saturday Saturday Night Sunday
Weather: Cloudy with some snow showers. Clouds decreasing during the day. Mostly cloudy with scattered snow showers becoming clear overnight. Sunny becoming partly cloudy in the afternoon.
Temperatures: 17 to 23 deg. F. 2 to 8 deg. F. 20 to 26 deg. F.
Wind Direction: Southwest to west Northwest West
Wind Speed: 15 to 25 mph with gusts to 50 mph decreasing to 30 mph in the afternoon 10 to 15 mph with gusts to 35 mph 10 to 15 mph with gusts to 30 mph
Expected snowfall: 1 to 3 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.

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