THIS AVALANCHE ADVISORY EXPIRED ON December 17, 2015 @ 7:00 am
Avalanche Advisory published on December 16, 2015 @ 7:00 am
Issued by Steve Reynaud - Tahoe National Forest

Isolated pockets of moderate avalanche danger will exist throughout the forecast area today.  Wind slabs and persistent slabs remain a lingering concern on all aspects above treeline and NW-N-NE aspects near treeline and below treeline.  Low danger exists for all other areas.  Human triggered avalanches will remain possible in isolated terrain.  Evaluate snow and terrain carefully and use proper travel techniques to help mitigate risk.

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 moderate to strong NE winds have been creating reactive wind slabs throughout the forecast area above treeline.  We had 2 reports of wind slab avalanches from yesterday in near and above treeline terrain that was actively being wind loaded.  The last few days of moderate to strong NE winds have redistributed snow back to SE-S-SW-W-NW aspects.  In complex terrain, many features have been top loaded and cross loaded making for potential wind slabs on all aspects near and above treeline.  The winds will shift today from the NE to the NW and decrease bringing an end to the wind slab development.  Below average cold temperatures could slow the bonding process of these wind slabs making them a concern for today.  Look for cornice formation, snow surface scouring, and wind pillows for clues as to where wind slabs may exist.

 

Avalanche Problem 2: Persistent Slab
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Near crust facets that have developed near the 12/10 and 12/13 rain crusts are still showing unstable results during snowpack tests.  The 12/10 rain crust has shown weakness mainly in the southern half of the forecast region and the 12/13 rain crust has been more widespread.  While it remains unlikely to trigger an avalanche on this layer if a user could find the right combination of weak layer with slab on top it remains possible.  On many exposed areas on N-NE-E aspects near and above treeline, the surface snow has been scoured by the recent winds.

The buried surface hoar layer that was responsible for avalanches this past Sunday is now thought to be unreactive in most areas.  The last few days of observation work show the distribution of this weak layer throughout the forecast area appears to be limited.  Where is has been found, it has collapsed causing less concern as a weak layer

recent observations

An avalanche was reported from Incline Lake Peak yesterday.  This avalanche occurred on a 40 degree slope angle with an E/NE aspect.  It was 50' wide and ran downslope for 400'.  There were strong N winds in the morning hours that were cross loading this gully feature that was thought to create the wind slab that avalanched.  The trigger is unknown but with no signs of tracks in the area this could be a natural avalanche.  This avalanche was large enough to injure, bury or kill a backcountry user.

A smaller wind slab avalanche was reported on the SW upper face of Castle Peak.  This was skier triggered and this slope was being actively wind loaded by the moderate to strong NE winds yesterday.

Observations from Grouse Rocks (Ward Canyon) and Andesite Peak (Donner Summit) revealed weakness associated with the 12/13 rain crust.  Snowpack tests showed propagation was possible on this layer but no other signs of instability were noted.  Protected terrain near treeline and below still have unconsolidated surface snow with exposed areas above treeline being wind scoured.    

Weather and CURRENT CONDITIONS
weather summary

Another cold day across the forecast area before a warm front moves into the region Thursday morning.  The NE winds we have been experiencing the last few days will shift over to the NW and diminish in speed to the light to moderate range.  Temperatures will warm as cloud cover increases the next couple days and there is a slight chance of snow tonight with no accumulation expected.

Weather observations from along the Sierra Crest between 8200 ft. and 8800 ft.
0600 temperature: 16 deg. F.
Max. temperature in the last 24 hours: 20 deg. F.
Average wind direction during the last 24 hours: NE
Average wind speed during the last 24 hours: 30 to 40 mph
Maximum wind gust in the last 24 hours: 66 mph
New snowfall in the last 24 hours: 0 inches
Total snow depth: 32 to 40 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Wednesday Wednesday Night Thursday
Weather: Partly cloudy. Mostly cloudy, slight chance of snow after midnight. Cloudy, then becoming mostly cloudy. Slight chance of snow.
Temperatures: 30 to 35 deg. F. 19 to 26 deg. F. 35 to 40 deg. F.
Wind Direction: Variable W SW
Wind Speed: 5 to 15mph 5 to 15mph 10 to 15mph with gusts to 25mph.
Expected snowfall: 0 in. 0 in. 0 in.
For 8000 ft. to 9000 ft.
Wednesday Wednesday Night Thursday
Weather: Partly cloudy. Mostly cloudy, slight chance of snow after midnight. Mostly cloudy, slight chance of snow.
Temperatures: 24 to 31 deg. F. 19 to 24 deg. F. 30 to 35 deg. F.
Wind Direction: NW W W
Wind Speed: 15 to 25mph with gusts to 40. 20 to 30mph with gusts to 45mph. 20 to 30mph with gusts to 45mph.
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.