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.

Yesterday's weather disturbance only brought up to 3 inches of new snow to some parts of the forecast area. Most of the region only saw a trace to 2 inches of snow over the last 24 hours. The winds did increase yesterday and should continue to gain strength as another small storm begins to affect the region today. Above 8000 ft. the forecast calls for southwest winds in the 45-75 mph range with gusts to 110 mph tonight and tomorrow. Even though this system only brings a small amount of moisture with it, snowfall totals could range between 7 and 13 inches by the end of the day tomorrow. This storm should bring slightly warmer temperatures with daytime highs in the mid to upper 20's for both today and tomorrow. After this wave of weather, another stronger cold front should move into the area on Monday night.

Recent Observations:

Observations on Mt. Judah, Castle Peak, and from Silver Peak all showed new wind slabs forming on the N-NE aspects in near and above treeline terrain. In all three of these areas the nascent wind slabs did not extend far down slope. On Mt Judah these new wind slabs ranged from 10-18 inches deep. Ski cuts and ski kicks from above could easily trigger small avalanches due to their failure (photos, more info). As the winds increased yesterday afternoon, wind loading also increased allowing the wind slabs to become more extensive and deeper. 

Data concerning the buried crust/facet combination collected from Tamarack Peak, Silver Peak (video, snowpit), and on Mt. Judah (video) showed that the crust above the facets remains strong in some areas and weak in others. It appears that this crust becomes weaker and thinner at the higher elevations. The data also showed that the facets continue to weaken. Tests on Silver Peak at 8000 ft. showed that if enough force impacts the snowpack to break the crust, the facets below it will also fail and resulting fracture can travel along the facet layer. On Mt Judah at a slight lower elevation, similar tests showed that propagation remained less likely.

Avalanche Problem #1: Wind Slabs

More new snow combined with more strong southwest winds will increase the overall size and depth of the wind slabs that started forming yesterday. These wind slabs already started to fail in response to human triggers yesterday and should remain fragile today. Triggering a wind slab avalanche large enough to bury or injure a person will be possible today. The largest and most sensitive wind slabs will exist on near and above treeline NW-N-NE-E aspects. Some small wind slabs may form on wind loaded slopes in below treeline terrain as well due to the strength of the winds.

Avalanche Problem #2: Persistent Slabs

A buried layer of weak faceted snow exists in the snowpack. In most areas the rain crust above this layer remains strong enough to prevent forces from reaching the weak facet layer and thus prevent widespread avalanche activity from occurring due to the failure of the facets. The most well developed and weakest facets exist on the NW-N-NE aspects between 7,500 ft. and 10,000 ft. Triggering an avalanche on this layer remains unlikely in most areas today, but areas where deep wind slabs form may add enough load to allow failure of this layer to become possible. As the crust continues to weaken and as new load continues to pile up on the snowpack the failure of the Dec. 2nd facet layer becomes more possible. This layer is the sugary snow located just below the first crust encountered when digging down into the snowpack.

The bottom line:

Near and above treeline, the avalanche danger remains MODERATE on NW-N-NE-E aspects on slopes 35 degrees and steeper due to continued wind and snow. Below treeline, the avalanche danger is LOW with pockets of MODERATE danger on wind loaded NW-N-NE-E 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:	19-23 deg. F.
Max. temperature in the last 24 hours:	19-23 deg. F.
Average wind direction during the last 24 hours:	West Southwest
Average wind speed during the last 24 hours:	25-30 mph
Maximum wind gust in the last 24 hours:	50-70 mph
New snowfall in the last 24 hours:	0-2 inches
Total snow depth:	42-50 inches

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

For 7000-8000 ft:
	Sunday:	Sunday Night:	Monday:
Weather:	A chance of snow in the morning with snow becoming more likely in the afternoon	Snow likely	Snow likely
Temperatures:	26-31 deg. F.	25-30 deg. F.	26-31 deg. F.
Wind direction:	Southwest	Southwest	Southwest
Wind speed:	15-25 mph with gusts to 40 mph	25-40 mph with gusts between 50 and 60 mph	25-40 mph with gusts to 55 mph
Expected snowfall:	2-4 in.	2-4 in.	3-5 in.
For 8000-9000 ft:
	Sunday:	Sunday Night:	Monday:
Weather:	A chance of snow in the morning with snow becoming more likely in the afternoon	Snow likely	Snow likely
Temperatures:	23-28 deg. F.	23-28 deg. F.	23-28 deg. F.
Wind direction:	Southwest	Southwest	Southwest
Wind speed:	35-40 mph with gusts to 60 mph increasing to 45-50 mph with gusts to 75 mph in the afternoon	45-60 mph with gusts to 90 mph increasing to 60-75 mph with gusts to 110 mph after midnight	60-75 mph with gusts to to 110 mph
Expected snowfall:	2-4 in.	2-4 in.	3-5 in.