THIS AVALANCHE ADVISORY EXPIRED ON December 30, 2017 @ 6:59 am
Avalanche Advisory published on December 29, 2017 @ 6:59 am
Issued by Brandon Schwartz - Tahoe National Forest

Avalanche danger is MODERATE for all elevations. Human triggered avalanches have become possible in isolated areas. Curious as to how the avalanche danger went from LOW to MODERATE without a storm? Read up on the terms faceted snow and persistent slab.

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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    Certain
    Very Likely
    Likely
    Possible
    Unlikely
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    Historic
    Very Large
    Large
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Faceting, faceting, and more faceting has weakened the existing snowpack during this 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 as 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 near and in avalanche terrain. Keep in mind that persistent slabs are often triggered much lower in the start zone than wind slabs.

recent observations

* Signs of persistent slab instability on northerly aspects including snow surface cracking, whumpfing, and/or unstable snowpit test results have occurred during the past several days in the Carson Pass, 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

A weather system passing to the north of the forecast area will increase winds today and tomorrow. Air temperature inversion conditions persist this morning with 40s on the peaks and teens to 30s on the mountain valley floors. Maximum daytime air temperatures are expected to reach well above freezing for all elevations today. A slight amount of cooling is expected for tomorrow following a dry cold front passage tonight.

Weather observations from along the Sierra Crest between 8200 ft. and 8800 ft.
0600 temperature: 35 to 46 deg. F.
Max. temperature in the last 24 hours: 45 to 53 deg. F.
Average wind direction during the last 24 hours: SW
Average wind speed during the last 24 hours: 24 mph
Maximum wind gust in the last 24 hours: 40 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.
Friday Friday Night Saturday
Weather: Partly cloudy skies. Partly cloudy skies. Partly cloudy skies.
Temperatures: 53 to 58 deg. F. 29 to 34 deg. F. 50 to 55 deg. F.
Wind Direction: SW SW SW
Wind Speed: 10 to 15 mph with gusts to 25 mph, increasing to 15 to 25 mph with gusts to 40 mph in the afternoon. 15 to 20 mph with gusts to 40 mph. 10 to 20 mph with gusts to 35 mph.
Expected snowfall: 0 in. 0 in. 0 in.
For 8000 ft. to 9000 ft.
Friday Friday Night Saturday
Weather: Partly cloudy skies. Partly cloudy skies. Partly cloudy skies.
Temperatures: 49 to 55 deg. F. 30 to 35 deg. F. 47 to 53 deg. F.
Wind Direction: SW SW SW
Wind Speed: 20 to 35 mph. Gusts to 45 mph increasing to 70 mph in the afternoon. 20 to 30 mph. Gusts to 70 mph decreasing to 60 mph after midnight. 20 to 30 mph with gusts to 50 mph.
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