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 29, 2010:


November 29, 2010 at 8:02 am

Isolated pockets of MODERATE avalanche danger exist on all aspects 35 degrees and steeper near and above treeline. Below treeline isolated pockets of MODERATE avalanche danger exist on NW-N-NE aspects on slopes 35 degrees and steeper.


Forecast Discussion:


Cold temperatures and moderate northeast winds should continue today due to a small, high-pressure ridge over the forecast area. This ridge should remain in place through today before a weak low-pressure system starts to move into the area tomorrow evening. The wind should start to shift to the southwest and clouds should start to increase tonight as this low approaches. The forecast also calls for slightly warmer temperatures during the day tomorrow.

Observations:

Another skier triggered avalanche occurred yesterday on Red Lake Peak (near Carson Pass-more info). This slide occurred on a near-treeline, NE-facing slope in complex terrain. The avalanche did injure one person. On Mt. Tallac snowpit tests showed that even though the bonds between storm snow and the old snow surfaces had gained some strength, areas of weakness remain (video and more info). Other tests such as ski cutting test slopes did not show signs of instability. On Sliver Peak (Pole Creek Drainage), in Negro Canyon (Donner Summit area), and on Pint Glass (Mt. Rose area), observations indicated that the bonds between the storm snow and the snow below it continue to gain strength. Snowpit tests on the previously weak layer of snow near the ground indicated that this layer has started to stabilize in the Negro Canyon and Silver Peak areas. However, it remains weak in the Pint Glass area. Tests on the weak, sugary layer near the ground (Nov. 9th facets) in that area indicated that if it starts to break the fracture could easily travel through that layer. Recent observations across the forecast area suggest that this layer only remains weak where the snowpack is more shallow in near and below treeline areas that are sheltered from the east winds and had snow on them in early November. In the Negro Canyon area, the NE winds had started to transport enough snow to form small, new wind slabs on the S-SE aspects near ridgelines by yesterday afternoon.

Overall the snowpack seems to be gaining strength and triggering avalanches is becoming more difficult. However, a few different isolated instabilities do still exist and warrant extra caution.

Avalanche Concern #1: Wind Slabs

The NE winds increased enough yesterday afternoon to start transporting snow and forming new wind slabs on the W-SW-S-SE aspects. These wind slabs grew more last night as the NE winds grew stronger. The added weight of a person on these newly formed wind slabs could cause them to fail today. Even though avalanches involving these wind slabs should remain relatively small, they could have serious consequences in the right locations (near cliffs, other terrain traps, or simply pushing people into shallowly buried rocks).

Avalanche Concern #2: Recent Storm Snow

In most areas the weaknesses associated with the most recent snow have strengthened. However, observations do indicate a few isolated areas where the bonds between the recent storm snow and the older snow beneath it remain weak enough for a person to break them and trigger an avalanche. The most likely places for these types of avalanches will be in more complex terrain (steep couloirs, hanging snowfields, unsupported slopes, heavily wind-loaded areas etc.) on steep NW-N-NE-E facing slopes. 

Avalanche Concern #3: 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 the right slope. Any avalanches resulting from failures of this layer would be large, deep, and dangerous. Areas where this layer remains weak are 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 are the most likely areas for this weak layer. 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 all aspects 35 degrees and steeper near and above treeline. Below treeline isolated pockets of MODERATE avalanche danger exist on NW-N-NE aspects on slopes 35 degrees and steeper.


Andy Anderson - Avalanche Forecaster, Tahoe National Forest


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

0600 temperature: 9 deg. F.
Max. temperature in the last 24 hours: 15 deg. F.
Average wind direction during the last 24 hours: Northeast
Average wind speed during the last 24 hours: 14 until 6 pm last night then increasing to 30-35 mph
Maximum wind gust in the last 24 hours: 47 mph
New snowfall in the last 24 hours: O inches
Total snow depth: 40-54 inches

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

For 7000-8000 ft:

  Monday: Monday Night: Tuesday:
Weather: Sunny Partly cloudy Mostly cloudy
Temperatures: 17-24 deg. F. 13-23 deg. F. 33-40 deg. F.
Wind direction: Northeast Southeast Southwest
Wind speed: 10-15 mph with gusts to 30 mph up to 10 mph up to 10 mph with gusts to 25 mph in the afternoon
Expected snowfall: O in. O in. O in.

For 8000-9000 ft:

  Monday: Monday Night: Tuesday:
Weather: Sunny Partly cloudy Mostly cloudy
Temperatures: 13-20 deg. F. 16-22 deg. F. 32-39 deg. F.
Wind direction: Northeast East shifting to the south Southwest
Wind speed: 15-25 mph with gusts to 40 mph decreasing to gusts to 30 mph in the afternoon 10-15 mph with gusts to 30 mph 15-25 mph with gusts to 40 mph
Expected snowfall: O in. O in. O in.

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