THIS AVALANCHE FORECAST EXPIRED ON December 31, 2017 @ 6:51 am
Avalanche Forecast published on December 30, 2017 @ 6:51 am
Issued by Brandon Schwartz - Tahoe National Forest

Avalanche danger is MODERATE for all elevations. In isolated areas, the snowpack has weakened to the point that human triggered avalanches are now possible. Have a group discussion highlighting how managing a persistent slab avalanche problem is different from managing a wind slab avalanche problem. Adjust terrain selection and travel tactics accordingly.

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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    Very Likely
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    Possible
    Unlikely
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    Large
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Faceting, faceting, and more faceting has weakened the existing snowpack during the current dry spell. This has created isolated areas of persistent slab instability that are reactive to the weight of a person on the snowpack. It doesn't matter that it hasn't snowed since Dec 20th, metamorphic change of snow crystals on the ground is making a loose sugary weak layer get weaker by the day. NW-N-NE aspects near and above treeline hold isolated hard slabs over this weak layer. NW-N-NE aspects below treeline hold isolated soft slabs over this weak layer.

Faceted snow is widespread on NW-N-NE aspects around the forecast area, but the reactive combination of slab over faceted weak layer is isolated. This distribution makes human triggered avalanches possible in isolated areas while avalanches remain unlikely in the majority of areas on a regional scale.

Isolated persistent slabs are challenging to manage from a travel standpoint, especially when signs of instability to aid in decision making are not readily available. Previous tracks are not an indication of stability, especially when the weak layer is becoming progressively weaker. Use best practice travel techniques for minimizing exposure in and around avalanche terrain. Keep in mind that persistent slabs are often triggered much lower in the start zone than wind slabs. This tends to increase consequences, even from very small avalanches.

recent observations

* Signs of persistent slab instability on NW-N-NE aspects including snow surface cracking, whumpfing, and/or unstable snowpit test results have occurred during the past several days in the Carson Pass, Desolation Wilderness, West Shore Tahoe,  Deep Creek, and Pole Creek areas. In all cases failure occurred on faceted snow beneath either a hard or soft slab.

* Mt. Rose Ski Patrol reported unstable snowpit test results on a similar slab over facet weak layer in closed, uncontrolled terrain (natural state snowpack that is analogous to a backcountry snowpack).

* Near surface facets without an overlying slab are fairly widespread on northerly aspects in wind protected areas below treeline.

* Snow surface conditions range from wind scoured snow and firm ice near and above treeline to lingering areas of soft unconsolidated snow on sheltered northerly aspects below treeline.

* Northerly aspects above 8000 ft in the Mt. Rose area and along the Sierra Crest north of Emerald Bay hold the best snow coverage at 2 to 3+ feet. Overall less snow cover exists south of Emerald Bay, but decent coverage exists above 8,500 ft. on northerly aspects. Coverage becomes patchy on other aspects and large areas of bare ground exist on many southerly aspects at all elevations.

Weather and CURRENT CONDITIONS
weather summary

High pressure and air temperature inversion conditions will continue through this weekend. Mid to high level cloud cover of varying degrees is expected for today and tomorrow. Maximum daytime air temperatures above 7,000' will be in the 40 to 50s. Overnight lows above 7,000' will struggle to drop below freezing. The coldest overnight air temperatures will continue to occur on the mountain valley floors. Ridgetop winds will remain out of the SW. Moderate speed gusts will continue today before winds decrease tomorrow.

Weather observations from along the Sierra Crest between 8200 ft. and 8800 ft.
0600 temperature: 36 to 39 deg. F.
Max. temperature in the last 24 hours: 44 to 50 deg. F.
Average wind direction during the last 24 hours: SW
Average wind speed during the last 24 hours: 35 mph
Maximum wind gust in the last 24 hours: 56 mph
New snowfall in the last 24 hours: 0 inches
Total snow depth: 17 to 32 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Saturday Saturday Night Sunday
Weather: Partly cloudy skies. Partly cloudy skies. Partly cloudy skies, becoming mostly cloudy.
Temperatures: 51 to 57 deg. F. 28 to 33 deg. F. 47 to 52 deg. F.
Winds: SW SW SW
Expected snowfall: 0 in. 0 in. 0 in.
For 8000 ft. to 9000 ft.
Saturday Saturday Night Sunday
Weather: Partly cloudy skies. Partly cloudy skies. Partly cloudy skies, becoming mostly cloudy.
Temperatures: 46 to 54 deg. F. 29 to 34 deg. F. 42 to 48 deg. F.
Winds: SW SW SW
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