THIS AVALANCHE ADVISORY EXPIRED ON December 6, 2014 @ 7:00 am
Avalanche Advisory published on December 5, 2014 @ 7:00 am
Issued by Andy Anderson - Tahoe National Forest

Above 9000 ft. on slopes 32 degrees and steeper, human-triggered avalanches remain possible on NW-N-NE aspects due to possible persistent slabs. Near and above treeline wind-loaded NW-N-NE-E-SE aspects above 8000 ft. steeper than 35 degrees may still hold human-triggerable wind slabs especially on unsupported slopes, couliors and gullies, and in complex and/or extreme terrain. The avalanche danger rating for these areas remains MODERATE.

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: Persistent Slab
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Warmer weather and wet snow/rain has allowed the Dec. 2nd facets to bond together and gain strength in terrain below 9000 ft. However, this persistent weak layer exists and remains weak on NW-N-NE aspects above 9000 ft. Recent data indicates that as elevation increases the Dec. 2nd facets weaken and that fractures can still travel through this persistent weak layer. Even though the snow above the Dec. 2 facets seems strong and consolidated, one trigger point is all it takes for a person to start a fracture in the buried weak layer that could result in a large dangerous avalanche. Observations like slope cuts and previous tracks are not reliable indications of stability when dealing with persistent slabs. Avoiding areas where these layers might exist or sticking to lower angle slopes without steeper terrain above them are the only mitigation options for persistent slabs. Any other technique is just rolling the dice.

Avalanche Problem 2: Wind Slab
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Wind slabs and fragile cornices still exist on wind-loaded N-NE-E aspects and on the cross-loaded NW and SE aspects in near and above treeline terrain. While they have become more difficult to trigger due to settlement and consolidation in the new snow, human triggered wind slab avalanches large enough to bury or injure a person may remain possible today. Wind-loaded slopes above cliffs, in gullies or couloirs, in other complex or extreme terrain, or in heavily wind loaded areas at the higher elevations represent the most likely places to find lingering unstable wind slabs. Wind slabs avalanches on slopes above 9000 ft. where the Dec. 2nd facet layer still exists could step down into those layers as well.

recent observations

Observations from Carson Pass on Red Lake Peak and near Frog Lake yesterday found consolidated new snow on top of strengthening weak layers on NW-N-NE aspects below 9000 ft. Snowpack depths between 8000 and 9000 ft. ranged from 40 to 80 cm.

Above 9000 ft. on Red Lake Peak the story changed. At these higher elevations where temperatures remained colder during the storm the snowpack measured 100 to 140 cm in non-wind-loaded areas and the old persistent weak layers (now known as the Dec. 2 facet layers since they were buried on Dec. 2) at the base of the snow remained loose, dry, and weak. Snowpit data from a NE facing slope at 9700 ft. where the Dec. 2nd facets existed indicated that if they start to break, the resulting fracture can travel through that layer (video, snowpit). Hand pits, probing, and quick shovel pits only revealed the old facet layers on the NW-N-NE aspects (places that held snow during the fall). At these upper elevations, snowpit tests also indicated that fractures could travel along a graupel layer buried about one foot deep.

On Red Lake Peak cornices above wind-loaded test slopes broke easily when weighted, but those wind-loaded test slopes did not fracture when the oven-sized cornice pieces fell onto them. Ski cuts also did not produce any results on wind-loaded test slopes between 8700 and 9700 ft. even though snowpit data indicated possible instabilities concerning the winds slabs resting on top of a thin graupel layer.

Below 8000 ft. snow cover remains limited.

Weather and CURRENT CONDITIONS
weather summary

Over the last 24 hrs wind, cloud cover, and precipitation all decreased as the storm departed the area. Expect partly cloudy skies, light winds, and mild temperatures this this morning. The winds and cloud cover should begin to increase during the day as another small system approaches the area. Some showers could also occur this afternoon. By tonight rain and snow should again impact the forecast area. Snow accumulation amounts up to 5 inches by mid morning on Saturday represent the most likely scenario, but a chance for more accumulation does exist. Snow levels should remain between 7000 and 7500 ft through tonight before temperatures begin to rise tomorrow during the day. The forecast calls for increased south and southwest winds that average ~ 40 mph and gust to 60 mph along the ridgetops tonight and tomorrow morning.

Weather observations from along the Sierra Crest between 8200 ft. and 8800 ft.
0600 temperature: 29 to 35 deg. F.
Max. temperature in the last 24 hours: 34 to 40 deg. F.
Average wind direction during the last 24 hours: Southwest
Average wind speed during the last 24 hours: 15 to 25 mph
Maximum wind gust in the last 24 hours: 36 mph
New snowfall in the last 24 hours: 0 to 2 inches
Total snow depth: 14 to 29 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Friday Friday Night Saturday
Weather: Mostly cloudy with some isolated showers in the afternoon. Snow levels between 7000 and 7500 ft. Cloudy with a chance of rain and snow. Rain and snow increasing after midnight. Snow levels between 7000 and 7500 ft. Partly cloudy with a chance of showers. Snow levels between 7500 and 8000 ft.
Temperatures: 37 to 45 deg. F. 24 to 31 deg. F. 37 to 45 deg. F.
Wind Direction: South South Southwest
Wind Speed: Light increasing to 10 to 20 mph with gusts to 30 mph this afternoon. 10 to 20 mph with gusts to 30 mph 15 to 20 mph with gusts to 30 mph
Expected snowfall: 0 in. with a slight chance of up to 1 in. up to 4 in. up to 2 in.
For 8000 ft. to 9000 ft.
Friday Friday Night Saturday
Weather: Mostly cloudy with some isolated snow showers in the afternoon. Cloudy with a chance of snow. Snow increasing after midnight. Partly cloudy with a chance of snow showers.
Temperatures: 32 to 38 deg. F. 22 to 29 deg. F. 32 to 38 deg. F.
Wind Direction: South South Southwest
Wind Speed: 10 to 15 mph with gusts to 25 mph increasing to 25 to 30 mph with gusts to 40 mph 30 to 40 mph with gusts to 60 mph 35 to 40 mph with gusts to 60 mph decreasing to 25 to 30 mph with gusts to 45 mph in the afternoon
Expected snowfall: 0 in. with a slight chance of up to 2 in. 2 to 6 in. up to 3 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.