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.

In the last 24 hours, 4-6 inches of new snow has accumulated north of Highway 50 above 7000'. Areas south of Highway 50 received 8-10 inches. Temperatures have been steadily dropping since yesterday morning. The southwest winds have averaged between 30 and 40 mph with gusts to 76 mph along the ridgetops. As the current low-pressure system moves east today, a high-pressure ridge will start to replace it. The winds should shift to the north and east, and temperatures should stay cold. Some lingering snow showers could drop another inch of snow on the Sierra above 8000'. However, most of the snow showers should begin to taper off today.

Observations:

The layer of weak, sugary snow from Oct 19th (photo) still lurks near the base of the snowpack on sheltered, NW-N-NE aspects above 8000'. On Mt. Tallac yesterday snowpit tests showed that initiating a fracture in this layer from the surface is becoming more difficult. However, tests in the Mt. Rose area, on Mt. Lola, on Picket Peak, and on Castle Peak over the last several days still showed that fracture propagation through the snowpack on this layer remains possible where more well developed facets (larger, more sugary snow grains) exist.  Observations also show mix of old snow surfaces beneath the new snow that has fallen over the last 24 hours. Above treeline on N-NE-E aspects, a hard, wind-packed surface exists in many areas. In most areas up to 8500', a thin breakable crust exists on northerly slopes. In the Carson Pass area a thin layer of weak snow existed near this thin crust. A thicker melt-freeze crust exists on most of the southerly aspects. On Mt. Tallac and in the Carson Pass area yesterday afternoon, the new snow seemed to be bonding well to these old snow surfaces.

Avalanche concern #1:

The strong southwest winds and new snow have formed new wind slabs and cornices on the N-NE-E aspects. The largest and most widespread of these slabs should exist south of Hwy 50 on exposed, wind-loaded slopes above treeline. North of Hwy 50 these slabs should be smaller due to less snow accumulation. Failure within the new snow or due to the bonds between the new snow and the snow surfaces mentioned above could result in avalanche activity involving these newly-formed wind slabs. The possibility for human-triggered avalanches will exist on wind-loaded slopes today.

Avalanche Concern #2:

Avalanches remain possible on NW-N-NE slopes above 8000' where the Oct. 19th facet layer exists. The new snow could reactivate this layer today. Even though the thick, well-consolidated snow above this layer makes it harder to break these facets, the right trigger in the right place could still initiate a fracture in this layer. Once that happens the fracture can propagate through this layer making the 3-4 feet of well consolidated snow above it a dense, destructive, 3-4-foot-deep slab. Areas near exposed rocks, trees, or on the edges of the slab where a shallower snowpack exists will represent the most likely places where people could trigger an avalanche due to failure of this layer. Avalanches that due occur resulting from failure of this layer will do so without typical warning signs of whumpfing, shooting cracks, or other avalanche activity. Uncertainty in this layer's strength and distribution make stability evaluations on slopes with this layer unreliable and difficult.

The bottom line:

Near treeline and below treeline, pockets of MODERATE avalanche danger exist above 8,000' on NW-N-NE aspects, 35 degrees and steeper due to persistent deep slab instability. MODERATE avalanche danger exists above treeline 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:	20 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:	30-35 mph
Maximum wind gust in the last 24 hours:	76 mph
New snowfall in the last 24 hours:	North of Hwy 50 5 inches - South of Hwy 50 8-10 inches
Total snow depth:	36-57 inches

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

For 7000-8000 ft:
	Tuesday:	Tuesday Night:	Wednesday:
Weather:	Mostly cloudy with scattered snow showers in the morning. Snow should become isolated and taper off in the afternoon.	Partly cloudy	Sunny
Temperatures:	18-25 deg. F.	10-16 deg. F.	27-34 deg. F.
Wind direction:	West shifting to the north	East	East
Wind speed:	15-20 mph with gusts to 40 mph decreasing in the afternoon	10-15 mph with gusts to 30 mph	10-15 mph with gusts to 35 mph
Expected snowfall:	trace in.	O in.	O in.
For 8000-9000 ft:
	Tuesday:	Tuesday Night:	Wednesday:
Weather:	Mostly cloudy with scattered snow showers in the morning. Snow should become isolated and taper off in the afternoon.	Partly cloudy	Sunny
Temperatures:	17-23 deg. F.	11-17 deg. F.	27-34 deg. F.
Wind direction:	West shifting to the north	East	East
Wind speed:	25-35 mph with gusts to 60 mph decreasing to 50 mph this afternoon	20-35 mph with gusts to 65 mph	20-30 mph with gusts to 60 mph decreasing to 50 mph in the afternoon
Expected snowfall:	up to 1 in.	O in.	O in.