THIS AVALANCHE ADVISORY EXPIRED ON December 9, 2017 @ 6:46 am
Avalanche Advisory published on December 8, 2017 @ 6:46 am
Issued by Brandon Schwartz - Tahoe National Forest

Hard wind slabs continue to pose an avalanche problem leading to MODERATE avalanche danger in some near and above treeline areas. Below treeline, avalanche danger is LOW.

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
    Large
    Small

Hard wind slabs sitting on top of low density old near surface facets is the problematic snowpack structure (see videos and photos of this in the observations section). This avalanche problem is found near and above treeline most prominently on NW aspects. These wind slabs have also been found in very isolated areas on N-NE aspects where the interaction between NE wind and terrain did not result in snow surface scouring. Slab thickness has been observed to range from 6 to 18 inches. Human triggered avalanches up to size D2 remain possible. A few reports have been received of recent human triggered avalanche activity in relation to the avalanche problem. Snowpit data continues to show stubborn triggering with little day to day change.

These wind slabs are very dense, generally allowing for less than 1 inch of ski, snowboard, or snowmobile penetration into the snow surface. These slabs will allow a person to travel well out onto the slab and become fully committed to the slope prior to avalanche occurrence.

The way this avalanche problem has set up, don't expect to have much opportunity to see signs of instability in any place other than the snowpit. Avoidance is the easiest management technique. Avoid areas of dense wind packed snow. Look for evidence of past drifting, smooth wind sculpted surfaces, and avoid areas with any type of hollow sounding snow. By far the best recreation conditions exist below treeline in areas of both shade and NE wind protection.

 

recent observations

*A couple reports of recent human triggered wind slab avalanches have been received.

* Hard wind slabs resting above a weaker layer of older near surface facets have been observed on wind-loaded slopes in the Donner Summit,  Carson Pass, and Mount Rose areas. Tests targeting these stubborn, hard wind slabs continue to show unstable results with little change day to day (ECTP's and 2m ECTP's).

* Widespread hard wind packed surfaces exist on exposed N-NE-E aspects. Some areas of firm ice (rain crust) exist at the snow surface in the most heavily wind scoured areas.

* Some areas of unconsolidated snow linger below treeline in shaded areas sheltered from the E and NE winds, but the snow surface conditions are becoming increasingly 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

High pressure is firmly in place over the forecast area. Light NE to E flow aloft will remain after gusty winds die down this morning. Inversion conditions are expected to become stagnant with nightly air temperatures on the peaks significantly warmer than on the mountain valley floors. Maximum daytime air temperatures will be similar across the elevation spectrum.

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: 39 to 45 deg. F.
Average wind direction during the last 24 hours: NE
Average wind speed during the last 24 hours: 33 mph
Maximum wind gust in the last 24 hours: 71 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.
Friday Friday Night Saturday
Weather: Mostly sunny skies. Clear skies. Sunny skies.
Temperatures: 43 to 48 deg. F. 26 to 32 deg. F. 45 to 50 deg. F.
Wind Direction: Variable Variable Variable
Wind Speed: Light winds Light winds Light winds
Expected snowfall: 0 in. 0 in. 0 in.
For 8000 ft. to 9000 ft.
Friday Friday Night Saturday
Weather: Mostly sunny skies. Clear skies. Sunny skies.
Temperatures: 43 to 48 deg. F. 29 to 37 deg. F. 45 to 50 deg. F.
Wind Direction: NE to E NE to E NE to E
Wind Speed: Generally light winds. Gusts up to 25 mph in the morning. Light winds Light winds
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