THIS AVALANCHE ADVISORY EXPIRED ON January 4, 2016 @ 6:56 am
Avalanche Advisory published on January 3, 2016 @ 6:56 am
Issued by Brandon Schwartz - Tahoe National Forest

LOW avalanche danger continues for all elevations and aspects. Small wind slabs are a lingering issue today near and above treeline on NW-N-NE aspects. These wind slabs might remain sensitive to human triggering in isolated areas. Avalanche size is expected to remain too small to bury a person unless highly defined terrain traps are involved. Normal caution is advised.

 

1. Low

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

1. Low

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

1. Low

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Below Treeline
Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.
    Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.
Avalanche Problem 1: Wind Slab
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  • Aspect/Elevation ?
  • Likelihood ?
    Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size ?
    Historic
    Very Large
    Large
    Small

Strong SSW ridgetop winds yesterday created areas of blowing snow and enough wind loading to build small wind slabs near and above treeline on NW-N-NE aspects. Four small skier triggered avalanches were reported yesterday, at least two of which were intentionally triggered. In all cases avalanche size was too small to bury a person (more info below). Winds have shifted more to the S and snow on the ground available for wind transport is diminishing. Wind slabs are expected to be more difficult to trigger today than yesterday in the majority of areas. In isolated areas, triggering may remain easier today due to the presence of surface hoar, large stellar, and/or plate crystals at the weak layer interface. These are uncommon weak layers for this region and will make for more atypical remote triggering and delayed stabilization in any areas where they exist.

Terrain management to specifically avoid areas of recent wind loading is the wise choice today. Wind pillows, the leeward side of ridges where abrupt changes in snowpack depth occur, and below cornice features are simple ways to identify where unstable wind slabs might exist. More experienced travelers will be able to read micro terrain features, observe snow surface texture and feel slab over weak layer density changes to more precisely determine the boundaries of wind slabs.

recent observations

Observations made and received yesterday from Carpenter Ridge (Independence Lake area), Silver Peak (Pole Creek area), and Trimmer Peak (Luther Pass area) all indicated areas of wind loading on NW-N-NE aspects near and above treeline. On Carpenter Ridge, two small wind slab avalanches were intentionally remotely triggered near treeline on N aspect, 39 degree slopes at 8,600'. Crown height ranged from 4 to 12 inches, were 40 to 50 feet wide, with the slab extending 10 to 20 feet down slope. The avalanches ran for about 100 feet and were too small to bury a person. In this area, extended propagation beyond typical wind slab pockets combined with remote triggering pointed towards some potential persistent slab characteristics. Crown and bed surface examination revealed 1 finger hard wind slab on top fist hard near surface facets and the uncommon presence of large flat snow crystals in the form of surface hoar, large stellar, and/or plate crystals at the weak layer interface. On Trimmer Peak a party reported triggering two smaller wind slab avalanches near treeline on a N aspect at 9,300' with crown depths of 2 to 4 inches that were 10 feet wide.

The upper snowpack has remained cold this past week and there is increasing evidence of near surface faceting. In isolated areas, near crust faceting is starting to occur around the Dec 24 rain crust located 6 inches to 2 feet deep in most areas. This crust is absent in isolated areas. Both of these layers warrant continued monitoring for development into potential weak layers in the future.

Weather and CURRENT CONDITIONS
weather summary

A low pressure system passing is to the north of the forecast area. This system increased ridgetop winds yesterday and will allow for some isolated areas of light snowfall today with little to no accumulation expected. Wind direction was from the SSW yesterday and shifted to the S last night. Wind speeds are expected to remain moderate to strong into tomorrow. Wind has mixed out the air temperature inversion in some areas but it persists in other areas this morning. Maximum daytime air temperatures above 7,000' are forecast to reach the mid 20s to mid 30s today. Similar air temperatures are expected for tomorrow with a decrease in winds and better chances for an inch or two of snowfall.

Weather observations from along the Sierra Crest between 8200 ft. and 8800 ft.
0600 temperature: 22 to 28 deg. F.
Max. temperature in the last 24 hours: 24 to 35 deg. F.
Average wind direction during the last 24 hours: SSW
Average wind speed during the last 24 hours: 26 mph
Maximum wind gust in the last 24 hours: 56 mph
New snowfall in the last 24 hours: 0 inches
Total snow depth: 47 to 54 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Sunday Sunday Night Monday
Weather: Cloudy skies with isolated light snow flurries. Cloudy skies with a slight chance of snow showers. Cloudy skies with a chance of snow showers.
Temperatures: 28 to 35 deg. F. 18 to 24 deg. F. 26 to 33 deg. F.
Wind Direction: SE SE S
Wind Speed: 15 to 25 mph with gusts to 40 mph. 15 to 25 mph with gusts to 40 mph. 10 to 15 mph with gusts to 25 mph in the morning.
Expected snowfall: 0 to 1 in. 0 to 2 in. Up to 2 in.
For 8000 ft. to 9000 ft.
Sunday Sunday Night Monday
Weather: Cloudy skies with isolated light snow flurries. Cloudy skies with a slight chance of snow showers. Cloudy skies with a chance of snow showers.
Temperatures: 26 to 33 deg. F. 14 to 21 deg. F. 23 to 30 deg. F.
Wind Direction: S S S
Wind Speed: 25 to 35 mph with gusts to 60 mph. 25 to 35 mph with gusts to 60 mph. 15 to 25 mph with gusts up to 35 mph.
Expected snowfall: 0 to 1 in. 0 to 2 in. Up to 2 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.