THIS AVALANCHE ADVISORY EXPIRED ON December 16, 2016 @ 6:57 am
Avalanche Advisory published on December 15, 2016 @ 6:57 am
Issued by Steve Reynaud - Tahoe National Forest

Considerable avalanche danger will exist as the next storm impacts our forecast area today and tonight.  Wind slabs will be likely above snow level on NW-N-NE-E-SE aspects near and above treeline.  Deep persistent slabs will be possible on NW-N-NE aspects above 9000' below treeline and above treeline.  Natural avalanches will be possible and human triggered avalanches will be likely.  Large destructive avalanches are possible today. 

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.

3. Considerable

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Below Treeline
Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.
    Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.
Avalanche Problem 1: Wind Slab
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As snowfall begins to intensify today at the higher elevations above snow level, wind slabs will quickly form and exist on NW-N-NE-E-SE aspects near treeline and above treeline.  Gale force winds gusting over 100mph above 8000' will have new snow available for transport.  Wind slab avalanches are expected to be likely today on slopes steeper than 35 degrees.  Wind slab avalanches on NW-N-NE aspects above 9000' will have the possibility of stepping down to the deep persistent slab weak layer and causing a very large and destructive avalanche.  Look for blowing snow, cornice formation, wind pillows and snow surface clues as to where wind slabs are forming.  Avoid terrain where active wind loading is occurring.

Avalanche Problem 2: Deep Slab
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The persistent slab avalanche problem we have been dealing with this season is now a deep persistent slab and buried anywhere from 3 to 5 feet deep in the snowpack.  This avalanche problem exists on NW-N-NE aspects above 9000' throughout the forecast area.  Recent snowpack tests continue to indicate that failure on this layer is possible.  Today and into tonight, we will see significant loading of heavy snow and rain to the snowpack and this deep persistent weak layer.  These deep persistent slabs could also be triggered by smaller wind slab avalanches that step down and cause failure on the persistent weak layer.  Large destructive and unsurvivable avalanches are possible today.

Deep persistent slabs have unpredictable avalanche behavior.  These avalanches can be remotely triggered from any connected terrain.  Avalanche avoidance, cautious route finding and conservative decision making are critical today.  Avoid slopes 32 degrees and steeper and give runout zones a wider berth than normal as any avalanche may exceed path boundaries.

recent observations

Observations were made and received from Andesite Peak (Donner Summit Area) and Tamarack Peak (Mt. Rose Area).  On Andesite Peak, 8200', rain soaked surface snow existed on top of a 8-10'' firm and strong rain crust.  Snowpack and informal observations showed a consolidated snowpack with no signs of instabilities.  On Tamarack Peak, 9900', strong SW winds were blowing snow and forming small wind slabs into the mid afternoon.  Snow surface conditions were a mix of wind effected snow, wind scoured areas, and wind crusts.  Ski cuts in this area did not show signs of instability.   

Weather and CURRENT CONDITIONS
weather summary

A moderate to strong atmospheric river will produce a period of heavy rain and high elevation snow through today and into late tonight.  Snow levels are forecasted to be at or above 8000' for most of the storm until they quickly drop as the low passes with snow levels possible down to 6000' early Friday morning with little accumulation expected at Lake Level.  Snow levels are hard to pinpoint and fluctuations are expected.  2 to 5'' of rain is possible along the Sierra Crest with up to 3 feet of snow possible at or above 8000'.  Winds are expected to be gusting in excess of 100mph over the higher ridges and peaks.  Much colder conditions are expected Friday with lingering snow showers and cold and dry conditions prevailing through the weekend.

Weather observations from along the Sierra Crest between 8200 ft. and 8800 ft.
0600 temperature: 30 to 38 deg. F.
Max. temperature in the last 24 hours: 41 deg. F.
Average wind direction during the last 24 hours: SW
Average wind speed during the last 24 hours: 50 to 60 mph
Maximum wind gust in the last 24 hours: 93 mph
New snowfall in the last 24 hours: 0 to trace inches
Total snow depth: Along the Sierra Crest: 14 to 24 inches | In the Mt. Rose area 42 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Thursday Thursday Night Friday
Weather: Mostly cloudy then becoming cloudy. Chance of rain and snow in the morning, then rain and snow in the afternoon. Cloudy. Rain and snow. Mostly cloudy Scattered snow showers.
Temperatures: 43 to 49 deg. F. 21 to 26 deg. F. 22 to 27 deg. F.
Wind Direction: SW SW W
Wind Speed: 25 to 45mph. Gusts of 75mph increasing to 90mph in the afternoon. 20 to 40mph. Gusts up to 75mph decreasing to 65mph after midnight. 15 to 30mph. Gusts up to 60mph decreasing to 40mph in the afternoon.
Expected snowfall: Up to 3 in. 3 to 8 in. 1 to 2 in.
For 8000 ft. to 9000 ft.
Thursday Thursday Night Friday
Weather: Mostly cloudy then becoming cloudy. Chance of rain and snow in the morning, then rain and snow in the afternoon. Cloudy. Rain and snow. Mostly cloudy. Scattered snow showers.
Temperatures: 38 to 46 deg. F. 17 to 22 deg. F. 18 to 23 deg. F.
Wind Direction: SW SW SW shifting to W
Wind Speed: 40 to 65mph. Gusts of 115mph increasing to 125mph in the afternoon. 40 to 65mph with gusts to 125mph decreasing to 30 to 50mph with gusts to 110mph. 25 to 45mph with gusts to 95mph shifting to the W at 20 to 30mph with gusts to 75mph.
Expected snowfall: 6 to 12 in. 12 to 20 in. 1 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.

For a recorded version of the Avalanche Advisory call (530) 587-3558 x258