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.


This Avalanche Advisory was published on November 23, 2009:

November 23, 2009 at 7:41 am

Areas of MODERATE danger remain above 8,700' on N and NE aspects, steeper than 32 degrees where a layer of faceted snow crystals sits above the October basal ice layer and below the recent storm snow. Human triggered avalanches remain possible in areas where this specific snowpack structure exists.

Forecast Discussion:

High pressure is building over the forecast area and is expected to remain until the end of the week. Air temperatures are in the 20s this morning in nearly all locations with a weak air temperature inversion boundary established at around 7,000'. Ridgetop winds shifted to the east yesterday evening and remain moderate in speed. Northeasterly ridgetop winds are expected to continue for the next few days.

Areas of observed instability have been limited to N and NE aspects above 8,700' that are fairly well protected at the snow surface from W and NW winds. Recent human triggered avalanche activity has occurred on limited N and NE aspects above 8,700' in both the Mount Rose area (Far east ridge of Tamarack Peak) and Carson Pass area (Sisters Peak). Crown height has ranged from 8 inches to 5 feet. This avalanche activity has occurred in areas where the specific snowpack structure is composed of an ice mass at ground level, a layer of faceted snow crystals on top of the ice mass, and recent storm snow on top of the faceted layer. Snowpack observations made yesterday on Elephant's Back (Carson Pass area) revealed continued whumpfing and collapse on a NE aspect at 9,100' on a portion of the slope where no ski tracks existed. No avalanche activity had occurred on this slope despite sufficient slope angles and many ski tracks near trigger points. Discontinuity of the faceted snow layer across the slope is expected to have kept avalanche activity from occurring on this particular slope.

With the current shallow snowpack, instability caused by the ice mass and overlying faceted weak layer is expected to persist. Moderate NE winds are expected to scour the upper portion of some avalanche start zones today on N and NE aspects, but instability will persist on isolated slopes. This ice mass/facet layer is a different and more persistent type of snowpack instability than what is commonly observed within storm snow. Instability is expected to linger on isolated slopes that are above 8,700', are N to NE aspects, hold the ice mass and faceted layer, and are fairly well protected from NW and E winds. Human triggered avalanches remain possible in these specific areas.


The bottom line:

Areas of MODERATE danger remain above 8,700' on N and NE aspects, steeper than 32 degrees where a layer of faceted snow crystals sits above the October basal ice layer and below the recent storm snow. Human triggered avalanches remain possible in areas where this specific snowpack structure exists.


Brandon Schwartz - Avalanche Forecaster, Tahoe National Forest

Weather Observations from along the Sierra Crest between 8200 ft and 8800 ft:

0600 temperature: 25 deg. F.
Max. temperature in the last 24 hours: 31 deg. F.
Average wind direction during the last 24 hours: West shifting to east.
Average wind speed during the last 24 hours: 42 mph
Maximum wind gust in the last 24 hours: 79 mph
New snowfall in the last 24 hours: 0 to 1 inches
Total snow depth: 4 to 24 inches

Two-Day Mountain Weather Forecast - Produced in partnership with the Reno NWS

For 7000-8000 ft:
  Monday: Monday Night: Tuesday:
Weather: Partly cloudy skies. Partly cloudy skies. Sunny skies.
Temperatures: 35 to 43 deg. F. 25 to 35 deg. F. 39 to 47 deg. F.
Wind direction: Northeast East East
Wind speed: Around 10 mph. Around 10 mph. Around 10 mph.
Expected snowfall: O in. O in. O in.

For 8000-9000 ft:
  Monday: Monday Night: Tuesday:
Weather: Partly cloudy skies. Partly cloudy skies. Sunny skies.
Temperatures: 32 to 39 deg. F. 25 to 33 deg. F. 35 to 42 deg. F.
Wind direction: Northeast Northeast Northeast
Wind speed: 15 to 25 mph with gusts to 40 mph. Winds decreasing to 10 to 15 mph by the afternoon. 10 to 15 mph with gusts to 40 mph after midnight. 10 to 15 mph with gusts to 40 mph in the morning.
Expected snowfall: O in. O in. O in.