THIS AVALANCHE ADVISORY EXPIRED ON December 24, 2015 @ 7:02 am
Avalanche Advisory published on December 23, 2015 @ 7:02 am
Issued by Steve Reynaud - Tahoe National Forest

Avalanche danger is moderate on all aspects and elevations on slopes steeper than 35 degrees due to wind slabs and deep persistent slabs.  Our snowpack has doubled in size in most areas with a heavy, wet, dense layer of new snow.  Human triggered avalanches are possible today with large destructive avalanches possible in some areas.  Evaluate snow and terrain carefully.

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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Large wind slabs were formed by the Monday/Tuesday storm up to 4-5' in depth.  These wind slabs were weak enough to cause avalanches yesterday with small cornice drops in the Donner Summit area and the problem will continue through today until these wind slabs can gain strength. Continued active windloading was reported throughout the day yesterday with SW winds strong enough to move snow.  SW/W winds today are in the 40-50mph range with gusts to 70mph along the Sierra Crest.  The amount of available snow for transport is decreasing and we should have limited additional wind slab development today.  There is the possibility that triggering a wind slab could step down to a deep persistent weak layer, the resulting avalanche would be large and destructive.

Avalanche Problem 2: Deep Slab
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The persistent slab problems we had before this last storm system are now deep persistent slab problems and are buried anywhere from 3' to 8' deep in the snowpack.  These deep persistent slabs exist on NW-N-NE aspects at all elevations and include the 12/13 rain crust and 12/10 rain crust.  Deep persistent slabs are unpredictable in behavior, they could be remotely triggered, and most of our tests and observations are not reliable due to the depth of these weak layers.  Large triggers (cornice failure, large group, etc.) may be required to cause an avalanche on these layers, or a wind slab avalanche could step down into these weaker layers.  An avalanche involving any of these deeper layers would be large, destructive and have severe consequences. 

recent observations

Observations from Frog Lake Cliffs (Carson Pass area) showed many natural avalanches which appeared to be failing on the collapsed surface hoar layer below the 12/13 rain crust with crown lines greater than 3' deep.  Widespread cracking and active windloading occurred throughout the day.

On Andesite Ridge (Donner Summit Area) storm snow instabilities did exist with large density changes within the new snow.  Wind slabs failed with cornice drops in the 1-2' crown height range.  Rain was observed up to at least 8000' in the morning hours, much higher than forecasted for the day. 

Weather and CURRENT CONDITIONS
weather summary

We have a break in the weather today before our next storm impacts the forecast area on Thursday.  Cooling temperatures today with some lingering isolated snow showers over the Sierra Crest.  Gusty winds will be SW/W at 40 to 50mph with gusts up to 70mph at the higher elevations.  A winter weather advisory is in effect from 1am to 10pm Thursday with a colder storm forecasted.

Weather observations from along the Sierra Crest between 8200 ft. and 8800 ft.
0600 temperature: 20 to 22 deg. F.
Max. temperature in the last 24 hours: 32 deg. F.
Average wind direction during the last 24 hours: SW
Average wind speed during the last 24 hours: 35 to 50 mph
Maximum wind gust in the last 24 hours: 99 mph
New snowfall in the last 24 hours: 2 to 4 inches
Total snow depth: 48 to 62 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Wednesday Wednesday Night Thursday
Weather: Mostly cloudy then becoming partly cloudy. Isolated snow showers in the morning. Mostly cloudy then slight chance of snow in the evening. Then chance of snow after midnight. Cloudy. Snow.
Temperatures: 22 to 29 deg. F. 17 to 23 deg. F. 16 to 23 deg. F.
Wind Direction: W W W
Wind Speed: 15 to 20mph with gusts to 30mph. 15 to 20mph with gusts to 30mph. 30 to 40mph with gusts to 60mph.
Expected snowfall: 0 in. Up to 1 in. 5 to 10 in.
For 8000 ft. to 9000 ft.
Wednesday Wednesday Night Thursday
Weather: Mostly cloudy becoming partly cloudy. Mostly cloudy then slight chance of snow in the evening. Then chance of snow after midnight. Cloudy. Snow
Temperatures: 17 to 24 deg. F. 14 to 20 deg. F. 12 to 19 deg. F.
Wind Direction: NW W SW
Wind Speed: 40 to 50mph with gusts to 75mph. 30 to 35mph with gusts to 55mph. 55 to 65mph with gusts to 95mph.
Expected snowfall: 0 in. Up to 1 in. 5 to 10 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.