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.

Even though only 1-3 inches of snow fell yesterday, the southwest winds transported snow all day and continued to wind-load leeward slopes. A weak, high-pressure ridge over the forecast area should keep the winds calmer and skies clearer today. Overnight the winds and cloud cover should increase again due to a low pressure system moving into the Pacific Northwest. This system may bring some light snow showers to areas north of I-80 tonight and tomorrow.

Observations:

Yesterday, on N-NE facing aspects below treeline (8800 ft. to 9000 ft.) in the Mt. Rose area, two slopes fractured and collapsed due to failure of the Oct. 19th facet layer. On each of these slopes the fractures started at a single point and propagated across an area over 500 ft. across the slope and 300 ft. upslope (see photos). Shoveling while digging a snowpit caused the first slope to collapse. A skier stepping onto a shallow spot near a tree caused the second slope to fail. The only thing that kept these collapses from becoming large avalanches with crowns 3-4 ft deep and 500 ft wide was that these 26-29 degree slopes were not steep enough. As if these very obvious clues were not enough, snowpit tests from the pit that caused the first collapse also showed that fracture propagation along the Oct. 19th facet layer remains likely (see video). Near and above treeline (around 9200 ft.) several small avalanches occurred on 37-39-degree, N-NE aspects when a skier dropped oven to refrigerator sized cornices onto the slopes below (photos). The recently formed cornices failed several feet away from their edges due to very small kicks or just stepping lightly onto the snow. In the Donner Summit area between 7000 ft. and 7800 ft. tender cornices also formed over wind-loaded slopes. When kicked onto the slopes below they did not cause avalanche activity. Snowpit tests in this area showed more stable results than in the Mt. Rose area (see observation).

Avalanche concern #1:

Deep slab instability due to persistent weak layers failing continues to pose a serious threat to backcountry travelers. The Oct 19th facet layer or the facets near the Nov. 22 rain crust remain the two most worrisome of these layers. These weak layers will still react to human activity making large, destructive, human-triggered avalanches possible today. Instability associated with these weak layers is not uniform. 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. These weak layers will most likely exist on sheltered NW-N-NE-E aspects near and below treeline. Any slopes steeper than 32 degrees where these persistent weak layers exist hold the potential for disastrous, human-triggered avalanche activity.

Avalanche concern #2:

Human triggered avalanche activity will also remain possible on wind-loaded slopes near and above treeline. Recent observations and stability tests indicate that the newly formed wind slabs have not bonded well to the snow below them. 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. Wind-loaded slopes steeper than 35 degrees will hold avalanche potential today.

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. Near and above treeline MODERATE avalanche danger exists on wind-loaded 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:	26 deg. F.
Average wind direction during the last 24 hours:	Southwest
Average wind speed during the last 24 hours:	25-30 mph
Maximum wind gust in the last 24 hours:	48 mph
New snowfall in the last 24 hours:	1-3 inches
Total snow depth:	43-63 inches

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

For 7000-8000 ft:
	Monday:	Monday Night:	Tuesday:
Weather:	Mostly cloudy	Cloudy with a slight chance of snow showers after midnight	Cloudy with a chance of snow showers
Temperatures:	28-35 deg. F.	17-24 deg. F.	29-36 deg. F.
Wind direction:	Southwest	Southwest	Southwest
Wind speed:	10 mph	10-15 mph with gusts to 30 mph	15-25 mph with gusts to 35 mph
Expected snowfall:	O in.	O in.	O in.
For 8000-9000 ft:
	Monday:	Monday Night:	Tuesday:
Weather:	Mostly cloudy	Cloudy with a slight chance of snow showers after midnight	Cloudy with a chance of snow showers
Temperatures:	27-34 deg. F.	17-24 deg. F.	27-34 deg. F.
Wind direction:	Southwest	Southwest	Southwest
Wind speed:	10-20 mph with gusts to 40 mph	20-35 mph with gusts to 60 mph	25-35 mph with gusts to 70 mph increasing to 30-45 mph with gusts to 80 mph in the afternoon
Expected snowfall:	O in.	O in.	O in.