THIS AVALANCHE ADVISORY EXPIRED ON December 8, 2017 @ 6:48 am
Avalanche Advisory published on December 7, 2017 @ 6:48 am
Issued by Andy Anderson - Tahoe National Forest

MODERATE danger continues in near and above treeline terrain where hard wind slabs exist above older weaker snow. While these hard slabs may be difficult to trigger, human-triggered avalanches remain possible. If these slabs do break, they would likely fail above the trigger or after that person committed to the slope. Identify where wind slabs exist and avoid them. Firm or icy surfaces where arresting a fall would be challenging also exist on some exposed near and above treeline terrain.

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.

1. Low

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Below Treeline
Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.
    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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    Certain
    Very Likely
    Likely
    Possible
    Unlikely
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    Historic
    Very Large
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Stubborn, hard wind slabs exist above a layer of loose weak snow (facets) on some wind-loaded NW aspects. These conditions may also exist on some wind-loaded N-NE aspects where the NE winds have not scoured away the wind slabs. These wind slabs exist in specific areas in near and above treeline terrain near ridgelines and have grown up to 18 inches in depth. Triggering these wind slabs may be difficult due to the strength of the slab, but it is possible for larger triggers like large cornices or multiple people on a slope or even the right trigger in the right place to cause an avalanche. If they do break, these hard wind slabs would likely break above the person who triggers them or break after the person on the slab has already committed to the slope. Areas where a shallower wind slab exists, places near exposed rocks, convex rollovers, and unsupported slopes could represent some of the trigger points where breaking one of these hard slabs may be easier.

Use clues like blowing snow, hollow sounding snow, cornices above slopes, drifted snow, ripples in the snow surface, and other wind created textures to identify where wind slabs may exist and avoid those areas. Travelling in sheltered non-wind-affected terrain where softer snow still exists should provide safer and more enjoyable conditions today. 

recent observations

* Hard wind slabs resting above a weaker layer of older snow (facets) exist on wind-loaded NW aspects of Andesite Peak and Castle Peak. Tests targetting these stubborn, hard wind slabs yielded unstable results in both places on Tuesday and Wednesday (ECTP's and 2m ECTP's).

* Whumphing and cracking occurred yesterday in Munchkins east of Alpine Meadows on N-NE aspects near treeline where wind slabs existed.  

* Widespread firm, icy, scoured surfaces or hard wind packed surfaces exist on exposed N-NE-E aspects.

* Some areas of soft, unconsolidated snow still exist on near and below treeline northerly aspects sheltered from the E and NE winds, but the snow surface conditions are becoming more variable.

Snow coverage is deepest in areas above 8000 ft. in the northern half of the forecast area (north of Emerald Bay). Even at these elevations, snow coverage varies by location. Some areas at the upper elevations south of Emerald Bay still have marginal snow coverage. Below 8000 ft. most areas still have patchy snow coverage with many exposed obstacles and areas of bare ground. 

Weather and CURRENT CONDITIONS
weather summary

East and northeast winds have remained strong enough to scour surfaces and transport snow since Sunday night. Last night they increased again as a small disturbance passed the forecast area. These winds should begin to diminish today and the forecast calls for light winds at all elevations by tomorrow. Temperatures should continue to warm up at the higher elevations with daytime highs climbing into the low to mid 40's above 7000 ft. today and possibly into the upper 40's tomorrow. Many of the sensors above 8000 ft. already register near or above freezing temperatures this morning. Lower elevation sensors are reporting colder temperatures due to cold air pooling in the valleys overnight. The forecast calls for clear, dry weather as long as the high-pressure ridge remains in place over the region.

Weather observations from along the Sierra Crest between 8200 ft. and 8800 ft.
0600 temperature: 28 to 35 deg. F.
Max. temperature in the last 24 hours: 37 to 41 deg. F.
Average wind direction during the last 24 hours: East and Northeast
Average wind speed during the last 24 hours: 40 mph
Maximum wind gust in the last 24 hours: 78 mph
New snowfall in the last 24 hours: 0 inches
Total snow depth: 15 to 33 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Thursday Thursday Night Friday
Weather: Sunny Clear Sunny
Temperatures: 41 to 46 deg. F. 18 to 24 deg. F. 44 to 49 deg. F.
Wind Direction: Variable Variable Variable
Wind Speed: Light Light Light
Expected snowfall: 0 in. 0 in. 0 in.
For 8000 ft. to 9000 ft.
Thursday Thursday Night Friday
Weather: Sunny Clear Sunny
Temperatures: 39 to 44 deg. F. 21 to 26 deg. F. 41 to 46 deg. F.
Wind Direction: East East Variable
Wind Speed: 15 to 25 mph with gusts to 45 mph decreasing to 35 mph in the afternoon 10 to 15 mph with gusts to 25 mph in the afternoon Light
Expected snowfall: 0 in. 0 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