THIS AVALANCHE ADVISORY EXPIRED ON December 12, 2016 @ 7:00 am
Avalanche Advisory published on December 11, 2016 @ 7:00 am
Issued by Andy Anderson - Tahoe National Forest

MODERATE avalanche danger exists due to wind slabs on near and above treeline NW-N-NE-E-SE aspects steeper than 35 degrees and possible persistent slabs on high elevation N aspects and isolated NE aspects on slopes steeper than 32 degrees. While avalanches may have become slightly harder to trigger today than they were yesterday, large destructive human-triggered avalanches remain possible today. Dangerous avalanche conditions could exist today.

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.

2. Moderate

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Below Treeline
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
    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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The wind slabs that formed yesterday will remain on the near and above treeline wind loaded N-NE-E aspects and some of the cross-loaded NW and SE aspects. These wind slabs should have become slightly more difficult to trigger today due to the end of new precipitation, decreasing winds, and cooling temperatures. However, human-triggered wind slab avalanches will remain possible where wind slabs exist. The largest and most fragile wind slabs will exist above yesterday's snow line where most of the precipitation fell as snow like the Mt. Rose area. In some of these areas the wind slabs could measure several feet in depth. In areas where most of the precipitation fell as rain, wind slabs should remain smaller and more isolated. In the event that a wind slab avalanche occurs it could step down to the persistent weak layer mentioned below especially on upper elevation north aspects where the PWL is most likely to exist. 

Clues like cornices, blowing snow, wind drifted snow, wind pillows, snow surface scouring, ripples, and other wind created textures can help determine where wind slabs may exist.

Avalanche Problem 2: Persistent Slab
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Like the wind slabs mentioned above, persistent slab avalanches should have become more difficult to trigger today, but human-triggering of these large destructive avalanches will remain possible. Any persistent slab avalanches that occur would have very serious consequences and would likely be unsurvivable. The persistent weak layer now has anywhere from 2 to 4 ft. of snow on top of it that can act as a slab layer. In some areas where the warm temperatures and rain may have penetrated to this layer, the PWL will gain strength once the snowpack refreezes. That refreeze will take a few days and the PWL could remain weak during that time period. Data and observations have shown this layer to be most prevalent on N aspects above 9000 ft. in the Mt. Rose area and above 8300 ft. along the Sierra Crest with some isolated observations showing it on a few NE aspects as well. This layer could have changed dramatically during the storm and a significant amount of uncertainty exists as to where it may have gained enough strength to support the snow above it and where it will remain weak.

Due to the consequences associated with the large destructive avalanches that could occur if the persistent weak layer fails and the high degree of uncertainty associated with it, avoiding areas where this layer might exist is recommended. Larger triggers like multiple people on a slope, other avalanches, or cornice failures could easily cause a failure on the persistent weak layer. Areas where a shallower snowpack exists or where trigger points are closer to the snow surface are especially suspect.  These avalanches could connect multiple start zones, could be triggered remotely, and could run farther than expected.  Avoid North aspects above 32 degrees in steepness.  Have a travel plan.  Pay close attention to run out zones, up tracks, and regrouping areas. 

recent observations

Yesterday morning two skiers traversed/hiked into a CLOSED area at Mt. Rose Ski Tahoe and triggered a large avalanche in this CLOSED area. This area had been closed, had not seen skier traffic, and had not been controlled yet this winter. Due to these facts, the snowpack more closely resembled a backcountry snowpack. The avalanche likely broke as a wind slab at first and then stepped down into the persistent weak layer of facets near the base of the snowpack. It started on a N to NNE aspect at around 9000-9300 ft. and broke across several start zones. Slope angles in these starting zones ranged from 40 to 50 degrees. The slide measured between 350 and 550 ft. wide and ran downslope for 600 to 1000 ft. It buried the skier who triggered it in debris that averaged 5 ft. in depth with some areas covered in more than 10 ft. of debris. The search for the buried skier will continue this morning.  For more details and photos click here. Observations later in the day showed skier triggered shooting cracks still occurred on wind loaded test slopes in that area. Across the road on Tamarack Peak, skiers saw similar snow conditions with a slab layer resting on softer older weak snow. One party reported a smaller avalanche on a N aspect to the skiers right of Hourglass Bowl. This party also reported widespread whumfing in the area. 

Weather and CURRENT CONDITIONS
weather summary

Precipitation tapered off last night after this atmospheric river dropped 3 to 7 inches of water onto the forecast area according to the remote sensors. Areas north of Echo Summit along the Sierra Crest and the Mt. Rose area received the most precipitation with storm totals nearing the 7-inch mark in some places. The southern portion of the forecast area received less overall precipitation. Much of this precipitation came between early yesterday morning and last night. Snow levels remained above 8000 ft. for much of the storm. Areas east of Lake Tahoe in the Mt. Rose area and in Incline Village saw much lower snow levels (down to around 7000 ft.) at times.  Snow totals ranged from 6 to 12 inches along the Sierra Crest above 8000 ft. to up to 27 inches in the Mt. Rose area. As the precip started to taper off last night temperatures also fell and many areas below 8000 ft. did end up with some light snow accumulation. 

The forecast calls for a break in the weather today and tomorrow as dry air pushes into the region behind the atmospheric river. While some clouds may linger over the area, expect clearer skies cooler temperatures, and decreased southwest winds for today and tomorrow before another storm pushes into the region on Monday night. 

Weather observations from along the Sierra Crest between 8200 ft. and 8800 ft.
0600 temperature: 23 to 31 deg. F.
Max. temperature in the last 24 hours: 36 to 38 deg. F.
Average wind direction during the last 24 hours: Southwest
Average wind speed during the last 24 hours: 45 to 50 mph
Maximum wind gust in the last 24 hours: 90 mph
New snowfall in the last 24 hours: Along the Sierra Crest: 6 to 12 inches | In the Mt. Rose area: 27 inches
Total snow depth: Along the Sierra Crest: 18 to 24 inches | In the Mt. Rose area: 50 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Sunday Sunday Night Monday
Weather: Partly cloudy becoming mostly cloudy Mostly cloudy Mostly cloudy
Temperatures: 36 to 42 deg. F. 22 to 27 deg. F. 37 to 42 deg. F.
Wind Direction: Southwest Southwest Southwest
Wind Speed: 15 to 20 mph with gusts to 45 mph 15 to 20 mph with gusts to 40 mph 15 to 25 mph with gusts to 45 mph
Expected snowfall: 0 in. 0 in. 0 in.
For 8000 ft. to 9000 ft.
Sunday Sunday Night Monday
Weather: Partly cloudy becoming mostly cloudy Mostly cloudy Mostly cloudy
Temperatures: 33 to 39 deg. F. 22 to 27 deg. F. 34 to 40 deg. F.
Wind Direction: Southwest Southwest Southwest
Wind Speed: 25 to 40 mph with gusts to 80 mph 30 to 45 mph with gusts to 80 mph 25 to 35 mph with gusts to 75 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