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.

The last wave in the series of winter storms dropped another 10-20 inches of snow across the forecast area yesterday and last evening. The snowfall tapered off overnight as this system began exiting the region. Some isolated snow showers may continue this morning while a short-lived high pressure ridge builds over the area. This ridge should bring clearer skies and decreased winds to the region through Christmas day. By Christmas afternoon the winds and clouds should begin to increase again as another winter storm approaches the area. Snowfall associated with this storm should begin late in the day on Christmas and continue through the 26th. The forecast calls for snow levels to climb to 5500-6500 ft. at the start of this storm before falling back down to around 5000 ft. or lower on Wednesday. The forecast calls for an additional 1-2 ft. of snow by Thursday morning.

Recent Observations:

Widespread shooting cracks, collapses, and test slope failures continued to occur in below and above treeline terrain yesterday on Hidden Peak (photos), Andesite Ridge, and in the Wood Creek drainage (more info). On Andestie Ridge and Hidden Peak, these failures occurred within the storm snow. Some of the shooting cracks in wind loaded near treeline terrain on Andestie propagated as far as 50 ft away from the person who triggered them. Ski cuts on steep below treeline test slopes on Hidden Peak triggered failures within the new snow as well (photos more info). In the Wood Creek drainage, collapses and whumphing occurred within the new snow and on the Dec 12 facets on low elevation S facing slopes (more info). Snowpit tests on Hidden Peak (video, snowpit, more info), Powderhouse Peak (videos, more info), and in the Wood Creek drainage (more info) all showed that the persistent weak layers (the Dec 12th facets above the rain crust and the Dec 2nd facets below it) remain fragile. If these layers break, tests on Hidden Peak and Powderhouse Peak indicated the resulting fractures can travel along the weak layer. Recent observations around the forecast area have shown similar snowpack characteristics. Overall data indicates a snowpack teetering on the edge of failure.

Avalanche Problem #1: Wind Slabs and Storm Slabs

Fragile wind slabs and storm slabs that formed during the last few days still exist across the forecast area. These slabs exist in widespread areas below and above treeline and rest on top of a variety of weak layers ranging from storm snow weaknesses to density changes to deeply buried persistent weak layers. Widespread human triggered avalanches involving these slabs will remain likely today and tomorrow. The larger and more destructive avalanches will result from the failure of large wind slabs. The largest and most fragile wind slabs will exist on wind loaded and cross loaded NW-N-NE-E-SE aspects in both above and below treeline terrain. In more protected areas, the softer and smaller storm slabs exist. Even though widespread natural avalanches are less likely some natural avalanches will remain possible.

Avalanche Problem #2: Persistent Slabs

In some areas the persistent weak layers (the Dec 12th and the Dec 2 facets mentioned above) now buried deep in the snowpack have started to fail. In other places they are still holding on. Variability in the strength and distribution of these layers means uncertainty remains as to exactly where and when these facet layers will fail, but avalanches resulting from the failure of these layers could still occur. As the snow on top of them consolidates into more of a slab layer, these kind of avalanches could become more widespread. The most well developed and weakest facets exist on the NW-N-NE aspects between 7,500 ft. and 10,000 ft. However, they have also been noted on some southerly and easterly aspects. The avalanches that result form the failure of these layers will fracture across wider areas and could easily break above or far to the side of the person who triggers them. Other avalanches could also step down to these deeper weaknesses. Any avalanches that fail on these facet layers will be large and destructive.

The bottom line:

This forecast is vaild for 48 hours. We will update it again on Dec. 26th.

Below and above treeline CONSIDERABLE the avalanche danger exists on all aspects on slopes steeper than 33 degrees due to wind slabs and heavy snow loads sitting on an a weak snowpack. Large, destructive human triggered avalanches will remain likely. Natural avalanches will remain possible. Dangerous avalanche conditions exist. Careful snowpack evaluation, cautious route finding, and conservative decision making are essential.

Andy Anderson - Avalanche Forecaster, Tahoe National Forest

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

0600 temperature:	18-22 deg. F.
Max. temperature in the last 24 hours:	23-30 deg. F.
Average wind direction during the last 24 hours:	Southwest
Average wind speed during the last 24 hours:	35-45 mph
Maximum wind gust in the last 24 hours:	95 mph
New snowfall in the last 24 hours:	10-20 inches
Total snow depth:	62-94 inches

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

For 7000-8000 ft:
	Monday:	Monday Night:	Tuesday:
Weather:	Mostly cloudy with some isolated snow showers in the morning.	Mostly cloudy	Cloudy with a 10% chance of snow in the afternoon
Temperatures:	24-31 deg. F.	17-22 deg. F.	27-32 deg. F.
Wind direction:	Southwest shifting to the west in the afternoon	East	Southwest
Wind speed:	20-25 mph with gusts to 35 mph decreasing to 10-15 mph with gusts to 25 mph in the afternoon	10-15 mph becoming light after midnight	10-15 mph increasing to 20-25 mph with gusts to 35 mph in the afternoon
Expected snowfall:	less than 1 in.	O in.	O in.
For 8000-9000 ft:
	Monday:	Monday Night:	Tuesday:
Weather:	Mostly cloudy with some isolated snow showers in the morning.	Mostly cloudy	Cloudy with a 10% chance of snow in the afternoon
Temperatures:	21-28 deg. F.	17-22 deg. F.	24-29 deg. F.
Wind direction:	Southwest shifting to the west in the afternoon	South	Southwest
Wind speed:	40-45 mph with gusts to 65 mph decreasing to 25-35 mph with gusts to 45 mph in the afternoon	15-25 mph with gusts to 40 mph	20-25 mph with gusts to 35 mph increasing to 30-35 mph with gusts to 55 mph in the afternoon
Expected snowfall:	less than 1 in.	O in.	O in.