THIS AVALANCHE ADVISORY EXPIRED ON December 20, 2015 @ 7:00 am
Avalanche Advisory published on December 19, 2015 @ 7:00 am
Issued by Andy Anderson - Tahoe National Forest

MODERATE avalanche danger exists at all elevations on NW-N-NE aspects and in near and above treeline terrain on E-SE aspects on slopes 32 degrees and steeper due to the combination of wind slabs and persistent slabs. Where persistent slabs exist avalanches could occur on lower angle slopes as shallow as 25 degrees. Evaluate snow and terrain carefully before committing to a slope. If more snow falls than forecast, the avalanche danger could increase. Human triggered avalanches are possible today. 

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: Persistent Slab
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Even though the forecast calls for storm snow totals of 4 to 10 inches, this amount could add enough weight to the snowpack to reactivate the persistent weak layer of surface hoar buried under a rain crust in the upper snowpack. This new snow may also bury a new surface hoar layer that existed on open northerly facing slopes in some areas. While the new snow alone may not be enough to push these layers to the breaking point, adding a person on top of a slope where either of these layers exists could cause them to break. Human triggered persistent slab avalanches will be possible today and may be likely in areas where the storm snow buried surface hoar that existed on the snow surface yesterday. These persistent slabs can occur on lower angle slopes and the fractures that cause them can propagate across large areas. People can also trigger them from a distance. If an avalanche occurs due to the newly buried surface hoar layer failing, it could easily step down to the previously buried surface hoar layer as well. Recent avalanche activity, collapsing, cracking, and whumfing can all help identify where this problem exists, but this problem could exist on slopes without those clues since the new snow alone may not be enough break these layers. Open NW-N-NE aspects that are sheltered from the wind along the Sierra Crest represent the most likely places to find this problem. If less snow falls than forecasted, this problem will be less of an issue. If more snow falls than forecasted, this problem will be more of an issue. 

Avalanche Problem 2: Wind Slab
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New snow and wind started forming a new round of wind slabs yesterday afternoon. These wind slabs will continue to grow in size and extent as more snow and wind impact the forecast area today. Human triggered wind slabs will be possible today on wind loaded N-NE-E aspects and cross loaded NW and SE aspects. Avalanches resulting from these wind slabs could involve enough snow to bury a person especially in heavily wind loaded areas, in complex or extreme terrain, or in areas where terrain traps will magnify the consequences of an avalanche. Use clues like blowing snow, cornices, drifted snow, and other wind created textures and surfaces to help identify where these wind slabs exist.

recent observations

Yesterday observations from Folger Peak near Ebbetts Pass showed a weak layer of buried surface hoar below the uppermost rain crust. Observations from Indian Valley, Talking Mountain, Jake's Peak, Rubicon Peak, Grouse Rocks, and Andesite Ridge since 12/15 have all found this layer combination. In these areas most observations have indicated that this layer collapsed and went dormant after the last storm, but snowpit tests keep producing a mix of unstable results and results that do not show signs of instability. On Folger Peak yesterday snowpit tests yielded more consistent results that showed that if this layer breaks the resulting fracture could travel along the weak layer. So far observations have only revealed this layer along the Sierra Crest. Observations between Carson Pass and Ebbetts Pass yesterday also found a new layer of surface hoar on the snow surface on slopes sheltered from the wind and sun.

Across the lake on the Fireplug (Mt. Rose backcountry) yesterday, observations did not reveal any signs of this buried crust/surface hoar layer, nor did they point to any signs of current instabilities. Data from Tamarack Peak and Incline Lake Peak since 12/15 has shown similar conditions. 

Strong southwest winds existed along the Crest and in the Mt. Rose area yesterday and some snow (and crust) transport started to occur. Observers noted some small wind slabs already starting to form on leeward aspects in near and above treeline terrain by yesterday afternoon on both sides of the Lake. 

Weather and CURRENT CONDITIONS
weather summary

2 to 4 inches of snow has fallen in the mountains above 7000 ft. since midnight. The forecast calls for another 2 to 5 inches today with most of the accumulation occurring this morning. The strong southwest winds should also persist through this morning. By this afternoon this strom should start of wind down with snow and winds decreasing. The cloud cover may also decrease some this afternoon and evening. Cloud cover and southwest winds should start to increase again tomorrow as another small storm approaches the region for early next week. Check in with the Reno NWS for more details.

Weather observations from along the Sierra Crest between 8200 ft. and 8800 ft.
0600 temperature: 23 to 28 deg. F.
Max. temperature in the last 24 hours: 31 to 40 deg. F.
Average wind direction during the last 24 hours: Southwest
Average wind speed during the last 24 hours: 40 to 50 mph
Maximum wind gust in the last 24 hours: 88 mph
New snowfall in the last 24 hours: 2 to 4 inches
Total snow depth: 27 to 44 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 with snow in the morning snow decreasing to snow showers in the afternoon Mostly cloudy with scattered snow showers in the evening Cloudy with a chance of snow showers
Temperatures: 26 to 33 deg. F. 10 to 18 deg. F. 27 to 34 deg. F.
Wind Direction: Southwest Southwest Southwest
Wind Speed: 10 to 20 mph with gusts to 30 mph 10 to 15 mph with gusts to 25 mph 20 to 25 mph with gusts to 35 mph increasing to 30 to 35 mph with gusts to 50 mph in the afternoon
Expected snowfall: 2 to 5 in. trace to 0 in. up to 1 in.
For 8000 ft. to 9000 ft.
Saturday Saturday Night Sunday
Weather: Cloudy with snow in the morning snow decreasing to snow showers in the afternoon Mostly cloudy with scattered snow showers in the evening Cloudy with a chance of snow showers
Temperatures: 22 to 29 deg. F. 7 to 14 deg. F. 24 to 31 deg. F.
Wind Direction: Southwest West West shifting to southwest in the afternoon
Wind Speed: 45 to 50 mph with gusts to 80 mph decreasing to 35 to 40 mph with gusts to 60 mph in the afternoon 20 to 30 mph with gusts to 45 mph decreasing to 35 mph after midnight 30 to 35 mph with gusts to 55 mph increasing to 55 to 60 mph with gusts to 95 mph in the afternoon
Expected snowfall: 2 to 5 in. trace to 0 in. up to 1 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.