THIS AVALANCHE ADVISORY EXPIRED ON December 25, 2015 @ 7:01 am
Avalanche Advisory published on December 24, 2015 @ 7:01 am
Issued by Andy Anderson - Tahoe National Forest

Human triggered avalanches will become likely today and natural avalanches will become possible as a winter storm impacts the forecast area. On slopes 35 degrees and steeper, CONSIDERABLE avalanche danger will form on NW-N-NE-E-SE aspects in near and above treeline terrain with MODERATE danger in all other areas. Wind slabs and loose dry avalanches represent the main avalanche problems today with deep slabs on NW-N-NE aspects representing a third unlikely but not impossible problem. 

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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New snow and wind will create new wind slabs on the leeward slopes today. These wind slabs will accumulate on top of icy crusts and firm snow surfaces. They will likely form faster than they can bond to these old snow surfaces. Human triggered wind slab avalanches will be likely today and natural wind slab avalanches will be possible. Wind loaded N-NE-E and cross loaded NW and SE aspects in exposed near and above treeline terrain will represent the best places to find these unstable wind slabs today. While these wind slabs may start out small this morning, they will quickly grow in size and extend as the storm continues. They will quickly become large enough to bury a person. By the end of the day wind slabs could measure more than 3-4 ft in depth in some areas or more if snow totals reach the high end of the forecasted amounts. If these wind slabs form in areas where the persistent weak layers mentioned below exist, the unlikely but not impossible scenario of wind slab avalanches stepping down into older snow layers exists. 

Avalanche Problem 2: Loose Dry
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Cold new snow accumulating on top of frozen icy crusts means that human triggered loose dry avalanches/sluffs will be likely today, and some natural sluffing could occur. These sluffs would only involve the new snow. While they may not involve enough snow to bury a person, they could push people into areas with other consequences like hitting trees or going over cliffs. These sluffs will mostly occur on sheltered terrain on steep slopes with new snow on them. 

Avalanche Problem 3: Deep Slab
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Snowpit data still indicates that persistent weak layers do exist deep in the snowpack. In the Mt. Rose backcountry a layer of old facets still lurks near the base of the snowpack underneath the 12/10 rain crust, and some areas along the Sierra Crest still have a buried surface hoar layer hiding under the 12/13 rain crust. Tests and observations have indicated that NW-N-NE aspects in more sheltered terrain represent the best places to find these layers and that even in this kind of terrain they don't exist on every slope. Even in the places that they do exist, they may have gone dormant in some spots. These layers have 3-6 ft of strong, well-bonded snow above them and actually getting enough force through that strong upper snowpack to break these layers would be very difficult and require a very large trigger like a large cornice collapse, another avalanche, or several people on a slope at once. Still, if these layers do break, huge avalanches would result. Even though these kind of avalanches are unlikely, the consequences associated with them are dire, and this problem still warrants attention and extra caution. 

recent observations

Yesterday observations from Becker Ridge showed variable surface conditions ranging from breakable rain crusts to firm icy surfaces to some areas with 1 inch of light cold snow on top of firm icy crusts. Farther south on Red Lake Peak more consistent surface conditions of hard, frozen, ice crusts existed on all aspects in more exposed terrain. On Tamarack Peak in the Mt. Rose backcountry, snow surfaces consisted of firm wind packed snow with some areas of 2 inches of softer snow above this firm snow surface.

On Becker Peak cornices remained sentive to human triggering. Snowpit data from an exposed NE facing slope on Red Lake Peak did not reveal signs of instability or buried persistent weak layers. Farther north on Tamarack Peak probing and observations on exposed above treeline N-NE aspects also did not find evidence of buried weak layers; however, snowpit data in more sheltered near and below treeline terrain did reveal a lingering persistent weak layer near the base of the snowpack. Tests on this layer did indicate that actually trigggering a break in this layer would be very difficult and require a very large trigger, but the tests also showed that if this layer were to break, fractures could travel through it. In all three areas the recent snow has bonded well to itself and to the old snow surfaces below it and observers reported signs of settlement and consolidation. 

Weather and CURRENT CONDITIONS
weather summary

Southwest winds increased overnight as a cold, fast-moving low pressure system approached the area. As this system arrived this morning snowfall started around 4 am in the northern part of the forecast area. Since then 2 to 4 inches have already accumulated. Snow should quickly spread south and should continue through this evening. The forecast calls for 10 to 18 inches of additional snow by this evening with another 2 to 5 inches possible tonight. Snow showers should begin to taper off tonight as this system moves out of the forecast area. The cold air associated with this system will keep temperatures cold and snow levels low. The forecast calls for daytime highs today and tomorrow in the teens above 7000 ft and overnight lows in the single digits. Southwest winds will remain strong today before they begin shifting to the north and decreasing as the storm exists the area tonight and tomorrow. 

Weather observations from along the Sierra Crest between 8200 ft. and 8800 ft.
0600 temperature: 15 to 20 deg. F.
Max. temperature in the last 24 hours: 23 to 30 deg. F.
Average wind direction during the last 24 hours: Southwest
Average wind speed during the last 24 hours: Yesterday: 10 to 20 mph | Since midnight: 35 to 40 mph
Maximum wind gust in the last 24 hours: 71 mph
New snowfall in the last 24 hours: 2 to 4 inches
Total snow depth: 38 to 57 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Thursday Thursday Night Friday
Weather: Snow Snow showers in the evening. Snow will start to taper off to scattered snow showers overnight. Mostly cloudy with a chance of snow showers in the morning. Partly cloudy with a slight chance of snow showers in the afternoon
Temperatures: 14 to 21 deg. F. 5 to 11 deg. F. 12 to 19 deg. F.
Wind Direction: Southwest Southwest shifting to west after midnight Northwest
Wind Speed: 20 to 30 mph with gusts to 45 mph 15 to 20 mph with gusts to 30 mph decreasing to 10 mph after midnight 10 to 15 mph
Expected snowfall: 10 to 18 in. 2 to 5 in. 0 in.
For 8000 ft. to 9000 ft.
Thursday Thursday Night Friday
Weather: Snow Snow showers in the evening. Snow will start to taper off to scattered snow showers overnight. Mostly cloudy with a chance of snow showers in the morning. Partly cloudy with a slight chance of snow showers in the afternoon
Temperatures: 9 to 16 deg. F. -1 to 6 deg. F. 10 to 16 deg. F.
Wind Direction: Southwest West Northwest
Wind Speed: 55 to 65 mph with gusts to 85 mph decreasing to 75 mph 45 to 50 mph decreasing to 20 to 25 mph with gusts to 40 mph after midnight 20 to 30 mph with gusts to 45 mph
Expected snowfall: 10 to 18 in. 2 to 5 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.