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 December 15, 2009:


December 15, 2009 at 7:57 am

Near and below treeline MODERATE avalanche danger exists on NW-N-NE aspects steeper than 32 degrees, today. Human triggering of large, destructive avalanches with serious consequences will be possible in these areas. Above treeline pockets of MODERATE avalanche danger exist on wind-loaded NW-N-NE-E aspects steeper than 35 degrees.


Forecast Discussion:


A low pressure system moving into the Northwest should bring increased southwest winds and some cloud cover to the forecast area today. By tomorrow this system could push some precipitation into the Central Sierra. Most of the snow should stay north of Lake Tahoe with up to 4 inches of accumulation above 8000'. Snow levels should start between 6000' and 6500'.

Observations:

Observers reported the remains of three natural avalanches that mostly likely occurred on Sunday Dec. 13th on N-NE facing terrain. One occurred in some NE facing trees between 9200' and 9600' just north of the Hourglass in the Mt. Rose area and released due to failure of the Oct. 19th facet layer. One occurred between 8200' and 8400' on a protected NE facing slope near Grouse Rocks (south of Alpine Meadows on the crest - photo). The third and largest of these slides occurred on the N-NE aspect of Round Top off Carson Pass (photo). We did not receive any new reports of backcountry avalanche activity yesterday. Snowpits and layer-bonding tests (video) continued to show the presence of persistent weak layers near the base of the snowpack in the Grouse Rocks area and near Round Top. The strength and distribution of these old weak layers remains extremely variable. These tests also showed that the new snow has started to consolidate and that the newly-formed wind slabs have started to bond to the snow below them. These wind slabs still released with moderate force in the tests.

Avalanche concern #1:

Deep slab instability due to persistent weak layers failing still represents the primary avalanche concern today. These weak layers will still react to human activity allowing large, destructive, human-triggered avalanches to remain possible. Large triggers or small triggers in the right place could easily cause slopes with these weak layers to fail. The "right place" for a small trigger could include areas near rocks, trees, shallow spots in the snowpack where the weak layer is closer to the surface, or simply a spot on the slope where the weak layer is weakest. Stability will vary slope by slope and even within one specific slope some areas may be stable while an area 5 feet away is not. This variability adds uncertainty to any slope stability assessments. The avalanches that could result from failure of these layers would be large and have dire consequences to those who trigger them. The added uncertainty and serious consequences make extra cautious decisions a more prudent choice for slopes where these weak layers exist. The Oct 19th facet layer remains the most worrisome and active of the persistent weak layers. These weak layers will most likely exist on sheltered NW-N-NE-E aspects near and below treeline above 7800'. Any slopes steeper than 32 degrees where these persistent weak layers exist hold the potential for disastrous, human-triggered avalanche activity.

Avalanche concern #2:

Even though the newly-formed wind slabs have started to bond to the snow below them, pockets of terrain where human-triggered avalanche activity remains possible still exist on the most heavily wind-loaded slopes above treeline. In many areas large, tender cornices lurk above these slopes. The additional weight of a person can easily break these cornices and send them crashing down on the the fragile slopes below. Use clues like drifted snow, cornices, ripples, and other wind created textures to determine where wind-loading has occurred.


The bottom line:

Near and below treeline MODERATE avalanche danger exists on NW-N-NE aspects steeper than 32 degrees, today. Human triggering of large, destructive avalanches with serious consequences will be possible in these areas. Above treeline pockets of MODERATE avalanche danger exist on wind-loaded NW-N-NE-E aspects steeper than 35 degrees.


Andy Anderson - Avalanche Forecaster, Tahoe National Forest


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

0600 temperature: 24 deg. F.
Max. temperature in the last 24 hours: 31 deg. F.
Average wind direction during the last 24 hours: Southwest
Average wind speed during the last 24 hours: 30 mph
Maximum wind gust in the last 24 hours: 48 mph
New snowfall in the last 24 hours: O inches
Total snow depth: 38-62 inches

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

For 7000-8000 ft:

  Tuesday: Tuesday Night: Wednesday:
Weather: Cloudy Cloudy with a slight chance of isolated snow showers north of I80 Cloudy with a chance of snow showers in the north of Lake Tahoe. Snow levels 6000-6500 ft.
Temperatures: 27-34 deg. F. 22-29 deg. F. 30-37 deg. F.
Wind direction: Southwest Southwest Southwest
Wind speed: 10-20 mph with gusts to 35 mph 10-15 mph with gusts to 35 mph 10-15 mph with gusts to 35 mph
Expected snowfall: O in. less than .5 in. North of Lake Tahoe up to 2 in.

For 8000-9000 ft:

  Tuesday: Tuesday Night: Wednesday:
Weather: Cloudy Cloudy with a slight chance of isolated snow showers north of I80 Cloudy with a chance of snow showers in the north of Lake Tahoe
Temperatures: 25-32 deg. F. 22-29 deg. F. 28-35 deg. F.
Wind direction: Southwest Southwest Southwest
Wind speed: 25-35 mph increasing to 30-45 mph with gusts to 70 mph 25-40 mph decreasing to 20 -30 mph with gusts to 65 mph after midnight 20-30 mph with gusts to 55 mph decreasing to 45 mph in the afternoon
Expected snowfall: O in. less than .5 in. North of Lake Tahoe up to 3 in.