THIS AVALANCHE ADVISORY EXPIRED ON December 13, 2014 @ 7:00 am
Avalanche Advisory published on December 12, 2014 @ 7:00 am
Issued by Andy Anderson - Tahoe National Forest

Human-triggered avalanches are likely (CONSIDERABLE danger) on NW-N-NE aspects steeper than 32 degrees due to wind slabs (all elevations) and deep persistent slabs (above 9000 ft). In other wind loaded areas on NW-N-NE-E-SE aspects human triggered avalanches will be possible (MODERATE danger) on slopes 35 degrees and steeper. Strong winds may mean wind slabs formed at lower elevations and in atypical areas. In areas east of Lake Tahoe where less snow fell, less avalanche danger will exist. 

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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New snow and wind during this storm have formed wind slabs on wind-loaded NW-N-NE-E-SE aspects. Due to the strength of the winds associated with this storm wind slabs likely exist in some areas that typically remain sheltered from the winds. The largest wind slabs will exist in near and above treeline terrain, but some may have formed in below treeline terrain as well. Additional snow and wind today will increase the size and extent of these wind slabs. Human triggered wind slab avalanches are possible today anywhere wind slabs exist and are likely on the most heavily wind loaded slopes along the Sierra Crest especially if these wind slabs have formed above the persistent slabs mentioned below. Wind slab avalanches deeper than 3 ft are possible today in heavily wind loaded areas along the Sierra Crest and could break to the ground if they trigger the persistent slabs below them.

Avalanche Problem 2: Deep Slab
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Even thought snowfall amounts ended up less than forecasted, the additional weight will push the persistent weak layers near the base of the snowpack on some NW-N-NE aspects above 9000-9500 ft. close to their limit. Triggering a deep persistent slab avalanche will be possible today where these layers exist and are continuous. In areas where wind slabs sit on top of the persistent slabs, human triggered deep persistent slab avalanches are likely. These avalanches would break near the ground, involve the entire snowpack, can release on less steep slopes, can be triggered from a distance, and have very serious consequences. For these reasons any terrain that has the potential for deep persistent slabs is suspect and deserves extra caution or better yet avoidance, even though some uncertainty still remains concerning how continuous and widespread these layers are.

advisory discussion

Snow covered mountains, improving weather, and fresh tracks will make it easy to forget about today's avalanche problems. For some advice on dealing with various avalanche problems check out this page.

recent observations

Yesterday above Red lake on Carson Pass small wind slabs had already started to form with only about an inch of new snow accumulation. These wind slabs formed on top of a mix of thin crusts and some softer snow in the areas below 8500 ft. Recent data from other places around the forecast area continue to reveal the presense of weak sugary snow layers near the base of the snowpack on northerly aspects above 9000 to 9500ft. Tests on these persistent weak layers keep indicating that if these layers break the resulting fracture could travel through the persistent weak layers.

Weather and CURRENT CONDITIONS
weather summary

The SNOTEL remote sensor system went down around 1 am last night, so snow totals are an estimate pieced together from webcams, ski area reports, and the few sensors still operating. It is safe to say that snow totals ended up less than forecasted with the Sierra Crest areas receiving around 8 to 16 inches of new snow with the highest amounts north of Lake Tahoe and west of Lake Tahoe along the Sierra Crest. Areas east of Lake Tahoe received significantly less snow (less than 6 inches). Temperatures fell more slowly overnight than anticipated and did not drop below 7000 ft. until after midnight. Therefore, the lower elevations also received less snow than predicted. Today, the forecast calls for continued snow this morning and for the snow to start tapering off during the day. Another 4 to 8 inches could accumulate above 8000 ft. with 3 to 6 possible in the 7000 to 8000 ft. range and a few inches below 7000 ft. The winds started to decrease last night and should continue to diminish over the next 24 hrs. For tomorrow expect cool temperatures, light winds, and mostly to partly cloudy skies.

Weather observations from along the Sierra Crest between 8200 ft. and 8800 ft.
0600 temperature: 22 to 30 deg. F.
Max. temperature in the last 24 hours: 39 to 42 deg. F.
Average wind direction during the last 24 hours: Southwest
Average wind speed during the last 24 hours: 40 to 45 mph
Maximum wind gust in the last 24 hours: 113 mph
New snowfall in the last 24 hours: 8 to 16 inches
Total snow depth: 20 to 35 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 with snow in the morning becoming snow showers in the afternoon Cloudy with scattered snow showers in the evening and isolated snow showers after midnight Mostly cloudy becoming partly cloudy
Temperatures: 25 to 33 deg. F. 15 to 22 deg. F. 28 to 35 deg. F.
Wind Direction: Southwest West Variable
Wind Speed: 15 to 25 mph with gusts to 35 mph 10 to 15 mph with gusts to 25 mph decreasing after midnight Light
Expected snowfall: 3 to 6 in. up to 2 in. 0 in.
For 8000 ft. to 9000 ft.
Friday Friday Night Saturday
Weather: Cloudy with snow in the morning becoming snow showers in the afternoon Cloudy with scattered snow showers in the evening and isolated snow showers after midnight Mostly cloudy becoming partly cloudy
Temperatures: 21 to 27 deg. F. 13 to 22 deg. F. 22 to 29 deg. F.
Wind Direction: Southwest shifting to the west in the afternoon West Variable
Wind Speed: 40 to 45 mph with gusts to 65 mph decreasing to 20 to 25 mph with gusts to 40 mph in the afternoon 20 to 25 mph with gusts to 35 mph decreasing during the night Light
Expected snowfall: 4 to 8 in. up to 2 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.