THIS AVALANCHE ADVISORY EXPIRED ON December 11, 2016 @ 6:50 am
Avalanche Advisory published on December 10, 2016 @ 6:50 am
Issued by Steve Reynaud - Tahoe National Forest

Considerable avalanche danger could exist today due to wind slabs and persistent slab avalanchesWind slabs could form on NW-N-NE-E-SE aspects near and above treeline at snow level and above.  Persistent slabs are possible on North aspects along the Sierra Crest and in the Mt. Rose area.  All other areas will have Moderate avalanche danger.  Dangerous avalanche conditions could exist today-large avalanches are possible in specific terrain areas.

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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Winds slabs will become likely on NW-N-NE-E-SE aspects near and above treeline.  These wind slabs will only be possible at the snow level and above where new snow is accumulating.  Below this level, rain soaked snow and rain crusts will prevent most blowing snow from occurring and forming wind slabs.  Snow level could be anywhere from 8000-9500' or above today.  It is expected that late afternoon and into the evening for snow levels to drop.  Wind slabs will become possible at lower elevations if this occurs.

Look for blowing snow, cornice formation, snow surface scouring, and wind pillows as clues to where wind slabs are forming.

Avalanche Problem 2: Persistent Slab
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Persistent slab avalanches are possible today on North aspects along the Sierra Crest above 8300' and in the Mt. Rose area above 9300' on slopes steeper than 32 degrees.  The faceted weak layer is buried anywhere from 1 to 3 feet deep near the base of the snowpack.  Recent observations targeting this faceted layer have showed mixed results through the forecast region.  Some observations have seen the facet layer gain strength with the recent warm temperatures and rain.  Other areas still show the potential for propagation to occur with additional load.  Today will bring a significant amount of additional weight on the snowpack with the possibility of 2 to 4'' of rain or up to several feet of wet heavy snow at the highest elevations.  There is some uncertainty as to what could occur with this persistent weak layer today.

Large destructive avalanches are possible.  These avalanches could be natural or human triggered.  Any avalanches associated with this persistent slab would have large consequences.  Avalanche avoidance is recommended.  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

Observations were made and received yesterday from Grouse Rocks (Ward Canyon area), Stevens Peak (Carson Pass area), and Tamarack Peak (Mt. Rose area).  In the Grouse Rock area, the recent rain is slowly percolating down and consolidating the snowpack.  Snow level was around 8400' with a sticky and wet upper snowpack.  No signs of instabilities or any blowing snow observed with the rain soaked surface snow. 

At Stevens Peak and Tamarack Peak, the rain has had less effect on the lower snowpack with some of these layers still dry snow.  Snowpack tests targeting faceted layers near the ground showed weakness in some areas that are consistent with previous observations.  On the East Ridge of Tamarack Peak, 9500', there was evidence of the facets gaining some strength and not being as reactive as before.  Evidence of rain was up to at least 9600' in the Tamarack Peak area and close to the top of Stevens Peak.  A breakable surface rain crust was encountered at both locations.  

Weather and CURRENT CONDITIONS
weather summary

Heavy rain and gale force winds are expected throughout our region as this moderate to strong atmospheric river impacts our area today.  Snow levels are expected to remain very high at 8500-9500' before dropping towards the end of the storm sometime later this afternoon or tonight.  2 to 4'' of rain is expected through today with 1 to 3 feet of heavy wet snow possible at the highest elevations.  A river flood warning is in effect through today.  We should have a break in storm activity Sunday and Monday before the next atmospheric river impacts the Sierra Tuesday morning.  Stormy conditions look to stay with us for most of next week with the glimmer of hope that colder temperatures are coming.

Weather observations from along the Sierra Crest between 8200 ft. and 8800 ft.
0600 temperature: 31 to 32 deg. F.
Max. temperature in the last 24 hours: 34 deg. F.
Average wind direction during the last 24 hours: SW
Average wind speed during the last 24 hours: 45 to 55 mph
Maximum wind gust in the last 24 hours: 108 mph
New snowfall in the last 24 hours: .40 to 1.80'' rain inches
Total snow depth: 10 to 23 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Saturday Saturday Night Sunday
Weather: Cloudy. Rain through the day. Snow in the afternoon. Cloudy. Rain and snow in the evening. Then chance of rain and snow after midnight. Mostly cloudy. Slight chance of snow in the morning.
Temperatures: 41 to 46 deg. F. 24 to 29 deg. F. 36 to 41 deg. F.
Wind Direction: SW SW SW
Wind Speed: 20 to 35mph with gusts to 65mph. 20 to 30 mph. Gusts to 60mph decreasing to 45mph after midnight. 15 to 20mph with gusts to 45mph.
Expected snowfall: 0 in. Up to 5 in. 0 in.
For 8000 ft. to 9000 ft.
Saturday Saturday Night Sunday
Weather: Cloudy. Rain and snow. Cloudy. Snow and rain in the evening. Then chance of snow and rain after midnight. Mostly cloudy. Slight chance of snow in the morning.
Temperatures: 38 to 44 deg. F. 23 to 28 deg. F. 33 to 39 deg. F.
Wind Direction: SW SW W
Wind Speed: 45 to 55mph with gusts to 115mph. 30 to 45mph with gusts to 100mph decreasing to 25 to 35mph with gusts to 80mph after midnight. 25 to 40mph with gusts to 80mph.
Expected snowfall: 12 to 24 in. 2 to 6 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