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.

North of Lake Tahoe snow showers have deposited about 1 inch of snow along the Sierra Crest. A high pressure ridge building over the forecast area should put an end to these showers. The forecast calls for very little additional accumulation, today. The strong southwest winds should also start to decrease this afternoon. By tomorrow the forecast area should see warmer, drier, calmer weather.

Yesterday observers reported another natural avalanche that probably occurred on Dec 13th near Round Top off of Carson Pass (photo). We did not receive any reports of new backcountry avalanche activity. Snowpits and observations in the Carson Pass area showed that the Oct. 19th facet layer still exists on NW-N-NE aspects that are protected from the east winds. Tests on this layer showed that if it breaks, the fracture can propagate across the snowpack along this layer. Layer bonding tests also indicated that this layer needs a larger trigger to cause it to break in most areas. Observations also showed that the strong southwest winds had started to form pockets of dense, hard wind slabs on exposed N-NE-E aspects near and above treeline. Most of these slabs sit on top of less consolidated snow. Layer bonding tests (video) showed that the bonds between these hard slabs and the layers below them would break due to moderate force.

Avalanche concern #1:

The newly-formed, hard wind slabs on wind-loaded and cross-loaded NW-N-NE-E aspects near and above treeline will represent the primary avalanche concern today. Most of these hard slabs would require a large trigger to cause them to fail. In many areas, just such a large trigger lurks above these slabs in the form of tender cornices. The additional weight of a person can easily break these cornices and send them crashing down on the the slopes below. A small trigger (like a person) in the  right place could also result in an avalanche involving one of these hard slabs. If one of these hard slabs does release today, it will likely break above the person who triggers it making it much harder to escape from the slab before it gains enough momentum to entrain that person. Use clues like drifted snow, cornices, ripples, hollow sounding areas, and other wind created textures to determine where wind-loading has occurred.

Avalanche concern #2:

Deep slab instability due to persistent weak layers failing remains a threat to backcountry travelers. These weak layers will still react to human activity allowing large, destructive, human-triggered avalanches to remain possible. Like the hard wind-slabs mentioned above 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. The strength, distribution and depth of these weak layers varies greatly across the forecast area and across specific slopes. This variability adds uncertainty to any slope stability assessments where these 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 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.

 

Even though it may be slightly harder to trigger either of these avalanche concerns today, human-triggered avalanche activity resulting from them would have serious consequences. These consequences and the added uncertainty resulting from the variability of the persistent weak layers make conservative backcountry travel decisions a more prudent choice for today.

The bottom line:

Near and below treeline MODERATE avalanche danger exists on sheltered 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 on slopes steeper than 35 degrees, MODERATE avalanche danger exists on wind-loaded N-NE-E aspects with pockets of MODERATE danger on cross-loaded NW  aspects.

Andy Anderson - Avalanche Forecaster, Tahoe National Forest

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

0600 temperature:	26 deg. F.
Max. temperature in the last 24 hours:	30 deg. F.
Average wind direction during the last 24 hours:	Southwest
Average wind speed during the last 24 hours:	35-40 mph
Maximum wind gust in the last 24 hours:	85 mph
New snowfall in the last 24 hours:	North of Lake Tahoe 1 inches
Total snow depth:	39-56 inches

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

For 7000-8000 ft:
	Wednesday:	Wednesday Night:	Thursday:
Weather:	Cloudy with a scattered snow showers in the morning. Showers should taper off by the afternoon.	Mostly cloudy	Mostly cloudy
Temperatures:	35-41 deg. F.	23-29 deg. F.	32-39 deg. F.
Wind direction:	Southwest	Southwest	Southwest
Wind speed:	10-20 mph with gusts to 25 mph increasing to gusts to 35 mph this afternoon	10-15 mph	10-15 mph becoming light in the afternoon
Expected snowfall:	trace to 1 in.	O in.	O in.
For 8000-9000 ft:
	Wednesday:	Wednesday Night:	Thursday:
Weather:	Cloudy with a scattered snow showers in the morning. Showers should taper off by the afternoon.	Mostly cloudy	Mostly cloudy
Temperatures:	27-34 deg. F.	20-29 deg. F.	32-39 deg. F.
Wind direction:	Southwest	West	Southwest
Wind speed:	25-35 mph with gusts to 75 mph	15-25 mph with gusts to 35 mph decreasing to 10-15 mph after midnight	up to 10 mph
Expected snowfall:	trace to 1 in.	O in.	O in.