THIS AVALANCHE ADVISORY EXPIRED ON December 16, 2013 @ 6:40 am
Avalanche Advisory published on December 15, 2013 @ 6:40 am
Issued by Brandon Schwartz - Tahoe National Forest

MODERATE avalanche danger continues near treeline and below treeline on NW-N-NE aspects on slopes 32 degrees and steeper due to ongoing persistent slab instability.

Current conditions are very conducive to human triggered avalanches. Use low angle (sub 30 degree) slopes for protection. Stay clear of areas connected to steeper slopes above. Conservative terrain choices and effective communication are critical given the current instability.

1. Low

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Above Treeline
Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.

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
  • Type ?
  • Aspect/Elevation ?
  • Likelihood ?
    Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size ?
    Historic
    Very Large
    Large
    Small

Over a week has passed since the last snowfall and persistent slab instability continues. Near treeline and below treeline slopes on NW-N-NE aspects that resisted scouring by the Nov 21-23 ENE gale force wind event hold significant amounts of faceted snow below the more recent storm snow. This is the Dec 7 persistent weak layer with an overlying slab. This weak layer is easily collapsed under the weight of a person and has shown the ability to propagate from tens of feet to hundreds of feet. Sizable human triggered avalanches are possible with triggering occurring either high on the slope, low on the slope, or remotely.

recent observations

Persistent is certainly the word to describe the current snowpack instability. Observations show little to no change in stability from one day to the next. Observations made yesterday in the Deep Creek Drainage provided ongoing evidence of the instability associated with the Dec 7 faceted persistent weak layer. Widespread collapsing with audible whumphing was observed on near treeline and below treeline northerly aspects from 7,200' up to the high point of travel at 7,850'. Some of the collapses were so pronounced that a wave was visible traveling along the snow surface as the collapsing weak layer propagated away from the trigger point. Some cracking with visible vertical displacement of nearly 1 inch was also observed (see below for more info, photos, pit profile).

Barely covered rocks, stumps, and/or logs that exist on most slopes present an additional hazard to backcountry travelers. There are a few areas where the shallow snowpack has enough structure or strength to keep a person from hitting these objects. In many other areas, the snow cover serves to camouflage their presence.

Weather and CURRENT CONDITIONS
weather summary

High pressure is firmly in place over the forecast area and is expected to last through Tuesday. A very profound air temperature inversion has set up with most locations between 8,000' and 9,000' reporting air temperatures in the 40s this morning while the mountain valley floors are in the single digits and teens. Maximum daytime air temperatures are forecast to reach the mid 40s to low 50s today for areas above 7,000'. Ridgetop winds are light and variable this morning. Light to moderate speed east winds are forecast for this afternoon.

Weather observations from along the Sierra Crest between 8200 ft. and 8800 ft.
0600 temperature: 43 to 51 deg. F.
Max. temperature in the last 24 hours: 48 to 51 deg. F.
Average wind direction during the last 24 hours: East
Average wind speed during the last 24 hours: 12 mph
Maximum wind gust in the last 24 hours: 37 mph
New snowfall in the last 24 hours: 0 inches
Total snow depth: 8 to 17 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Sunday Sunday Night Monday
Weather: Sunny skies. Clear skies. Sunny skies.
Temperatures: 47 to 53 deg. F. 25 to 35 deg. F. 48 to 54 deg. F.
Wind Direction: East East East
Wind Speed: Light winds. Around 10 mph. Around 10 mph, becoming light.
Expected snowfall: 0 in. 0 in. 0 in.
For 8000 ft. to 9000 ft.
Sunday Sunday Night Monday
Weather: Sunny skies. Clear skies. Sunny skies.
Temperatures: 46 to 53 deg. F. 38 to 46 deg. F. 48 to 55 deg. F.
Wind Direction: East East East
Wind Speed: Light winds increasing to 10 to 15 mph in the afternoon. 10 to 15 mph with gusts to 25 mph. 10 to 15 mph with gusts to 25 mph in the morning, becoming 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.