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.

A cold, fast, low-pressure system should arrive over the forecast area this afternoon. Widespread moderate to heavy snow should begin this afternoon and continue across the forecast area through tomorrow morning. The forecast calls for 8-14 inches of new snow above 7000'. Snow levels should start around 6000' today and rapidly fall to the valley floors as the cold Arctic air associated with this system moves into the region. The southwest winds have already started to increase. They should remain strong throughout the storm. The forecast calls for the snow to start tapering off tomorrow during the day and for the winds to shift to the northwest and decrease.

Observations:

On sheltered, NW-N-NE aspects above 8000', snowpits and observations continue to show that a layer of weak sugary snow exists near the base of the snowpack (the Oct. 19th facet layer). Yesterday, observations on the far east ridge of Tamarack Peak showed that this layer has started to gain some strength. Other recent observations and snowpit tests on this layer from across the forecast area also show that human-triggering of this layer is becoming more difficult. However, tests in the Mt. Rose area, on Mt. Lola, on Picket Peak, and on Castle Peak still showed that fracture propagation through the snowpack on this layer remains possible where more well developed facets (larger, more sugary snow grains) exist.  A mix of snow surfaces exists across the forecast area for the forecasted new snow to land on top of. 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. A thicker melt-freeze crust exists on most of the southerly aspects.

Avalanche concern #1:

The strong southwest winds and new snow will start to form new wind slabs and cornices on the N-NE-E aspects this afternoon. Any of the snow surfaces mentioned above could serve as a weak layer or a bed surface for avalanche activity involving these newly-formed wind slabs. As these slabs and cornices grow this afternoon, the possibility for human-triggered avalanches will increase on wind-loaded slopes.

Avalanche Concern #2:

Avalanches remain possible on NW-N-NE slopes above 8000' where the Oct. 19th facet layer exists. This layer has barely started to adjust to the snow load above it. Adding more snow to the snowpack will increase the load on this layer and could reactivate it. 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.

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. The avalanche danger should increase this afternoon due to new snow and wind. Areas of MODERATE danger will develop this afternoon and evening 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:	29 deg. F.
Max. temperature in the last 24 hours:	37 deg. F.
Average wind direction during the last 24 hours:	Southwest
Average wind speed during the last 24 hours:	45 mph
Maximum wind gust in the last 24 hours:	71 mph
New snowfall in the last 24 hours:	trace to 1 inches
Total snow depth:	33-52 inches

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

For 7000-8000 ft:
	Monday:	Monday Night:	Tuesday:
Weather:	Isolated snow showers in the morning. Then steady snow in the afternoon.	Snow	Mostly cloudy with scattered snow showers in the morning. Snow should taper off in the afternoon.
Temperatures:	32-39 deg. F.	11-18 deg. F.	18-25 deg. F.
Wind direction:	Southwest	Southwest	West shifting to Northwest
Wind speed:	15-25 mph with gusts to 45 mph	15-25 mph with gusts to 45 mph	15-20 mph with gusts to 40 mph decreasing to 10-15 mph in the afternoon
Expected snowfall:	up to 2 in.	4-10 in.	trace in.
For 8000-9000 ft:
	Monday:	Monday Night:	Tuesday:
Weather:	Isolated snow showers in the morning. Then steady snow in the afternoon.	Snow	Mostly cloudy with scattered snow showers in the morning. Snow should taper off in the afternoon.
Temperatures:	30-38 deg. F.	5-12 deg. F.	17-23 deg. F.
Wind direction:	Southwest	Southwest	Northwest
Wind speed:	30-45 mph with gusts to 75 mph	30-45 mph with gusts to 70 mph	20-35 mph with gusts to 60 mph decreasing to 45 mph in the afternoon
Expected snowfall:	up to 3 in.	5-10 in.	trace in.