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 30, 2010:


November 30, 2010 at 7:30 am

Isolated pockets of MODERATE avalanche danger exist on NW-N-NE-E aspects 35 degrees and steeper near and above treeline. Below treeline very isolated pockets of MODERATE avalanche danger exist on NW-N-NE aspects on slopes 35 degrees and steeper where a persistent weak layer exists near the base of the snowpack.


Forecast Discussion:


The winds shifted back to the southwest, cloud cover increased, and temperatures started to climb last night as a low pressure system moved into the Pacific Northwest. This system should continue to pull warmer air and clouds into the region today. However, most of the moisture associated with this system should remain north of the forecast area. Another low pressure approaching the CA coast should follow this one. By tomorrow afternoon enough moisture may push into the area for a few snow showers.

Observations:

Observations from most areas yesterday pointed to a mostly stable snowpack. Layer bonding tests, hand pits, slope cuts, and test slope data from Incline Lake Peak (Mt. Rose area) did not show any signs of instability. Observations from Silver Peak (Pole Creek drainage) also showed mostly stable snow. In the Silver Peak area and on Sunrise Bowl (north of Donner Summit), large, feathery crystals known as surface hoar had formed on the surface of the snowpack all the way up to the ridgelines. The NE winds did not prevent this surface hoar from forming nor did they transport enough snow to form new wind slabs in these areas. Farther south on Red Lake Peak near Carson Pass, the NE winds did transport snow through mid-morning; however, the new wind slabs that formed remained small and isolated. On the the north side of Red Lake Peak, snowboarders triggered two small (6-8 inch crowns, 30 ft wide), soft slab avalanches in a 37-39 degree, north facing chute (photo). The avalanches did not catch or carry either rider. These slides occurred in complex terrain near the skier-triggered avalanche from Nov. 28 (more info and photos). Layer bonding tests and snowpit data in this area indicated that these slides all failed near the base of the most recent snow and that people can still trigger slides on this interface in similar terrain. Snowpit data did not show signs of the Nov 9th facets in this area. Other recent observations across the forecast area suggest that this layer only remains weak in very isolated areas where a shallower snowpack exists on near and below treeline N-NE aspects that are sheltered from the east winds and had snow on them in early November. Observers have only found this persistent weak layer in the the Mt. Rose area so far.

Avalanche Concern #1: Lingering Storm Snow Weaknesses

The bonds between the most recent storm snow and the old snow have strengthened enough to support both the snow above them and people recreating on that snow in most areas. However, isolated pockets of terrain where these bonds remain weak do still exist. The most likely places for these isolated pockets of instability will be in more complex terrain (steep couloirs, hanging snowfields, unsupported slopes, heavily wind-loaded areas etc) on NW-N-NE-E facing slopes steeper than 35 degrees. Human triggered avalanches remain possible where these isolated pockets of weakness exist.

Avalanche Concern #2: Deep Slabs

Avalanche resulting from the failure of the weak, sugary layer of snow near the bottom of the snowpack (Nov 9th facet layer) do remain possible. These types of avalanches should be difficult to trigger unless a person finds the exact right spot on a slope where this layer exists. Any avalanches resulting from failures of this layer would be large, deep, and dangerous. Areas where this layer remains weak are very isolated at best. NW-N-NE aspects near and below treeline that are sheltered from east winds where a shallower snowpack exists and where snow existed on the slope in early November hold the best potential for this instability. Higher elevation areas in the mountains on the east side of Lake Tahoe hold most of the terrain that meets these conditions.


The bottom line:

Isolated pockets of MODERATE avalanche danger exist on NW-N-NE-E aspects 35 degrees and steeper near and above treeline. Below treeline very isolated pockets of MODERATE avalanche danger exist on NW-N-NE aspects on slopes 35 degrees and steeper where a persistent weak layer exists near the base of the snowpack.


Andy Anderson - Avalanche Forecaster, Tahoe National Forest


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

0600 temperature: 27 deg. F.
Max. temperature in the last 24 hours: 27 deg. F.
Average wind direction during the last 24 hours: Northeast shifting to the southwest after 11pm last night
Average wind speed during the last 24 hours: 15-20 mph
Maximum wind gust in the last 24 hours: 40 mph
New snowfall in the last 24 hours: O inches
Total snow depth: 43-54 inches

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

For 7000-8000 ft:

  Tuesday: Tuesday Night: Wednesday:
Weather: Mostly cloudy Mostly cloudy Mostly cloudy with a slight chance of snow showers
Temperatures: 36-43 deg. F. 20-26 deg. F. 31-38 deg. F.
Wind direction: Southwest Southwest Southwest
Wind speed: up to 10 mph 10-15 mph with gusts to 25 mph 10 mph with gusts to 25 mph in the morning
Expected snowfall: O in. O in. 0-1/4 in.

For 8000-9000 ft:

  Tuesday: Tuesday Night: Wednesday:
Weather: Mostly cloudy Mostly cloudy Mostly cloudy with a slight chance of snow showers
Temperatures: 35-42 deg. F. 17-24 deg. F. 28-35 deg. F.
Wind direction: Southwest Southwest Southwest
Wind speed: 15-25 mph with gusts to 40 mph 20-35 mph with gusts to 50 mph increasing to gusts to 60 mph after midnight 20-30 mph with gusts to 70 mph decreasing to gusts to 50 mph in the afternoon
Expected snowfall: O in. O in. 0-1/4 in.

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