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 fairly stable weather pattern has set up over the forecast area. High pressure offshore will allow for a mix of sun and clouds over the next few days. Cool upper atmosphere flow from the north to northeast will continue to keep daytime highs in the 20s and overnight lows in the single digits and teens for nearly all areas. Ridgetop winds are forecast to remain mainly out of the east to northeast today and tomorrow, with a period of west to northwest winds this afternoon and evening. Winds will be strong enough to move snow today and tomorrow.

Recent Observations:

Recent observations from around the forecast area show stable snow conditions in some areas while folks are getting caught in avalanches in other areas. Near and above treeline areas continue to hold pockety wind slabs, especially in complex terrain. These winds slabs generally range from 6 inches to 2 feet deep and can be found on all aspects due to wind shifts on Sunday (photos, pit profiles, videos, more info).

Observations made Sunday on northerly aspects on Trimmer Peak (Luther Pass area) and on southerly aspects on Castle Peak (Donner Summit area) revealed stable snow conditions. In contrast, observations made Sunday on Tamarack Peak (Mount Rose area) and at the site of the Dec 29 skier triggered avalanche on Echo Peak (Echo Summit area) showed continued instability on new and old wind slabs. Shooting cracks and unstable snowpit test results were prevalent in near and above treeline terrain on NE aspects. While significant wind scouring has occurred in some locations, unstable wind slabs exist in other locations on the same aspect (photos, pit profiles, videos, more info).

Avalanche Problem #1: Wind Slabs

Here is the rub. In most locations out there the snowpack looks stable, it feels stable, the snow quality is great, and it lulls one into a sense of security. Interspersed within those areas of stable snow are pockets of unstable wind slabs needing only a human trigger. Near and above treeline areas on all aspects hold pockets of new and old wind slabs due to recent wind shifts from SW to E and NE winds. Below average air temperatures have also worked to slow stabilization of lower density snow layers beneath the wind slabs. Areas of complex terrain in and around steep chutes, gullies, couloirs, cliffs and rock bands hold the greatest instability. Terrain traps often associated with these types of terrain features can lead to deep burial depths resulting from fairly small avalanches. Continue to exercise good travel techniques and don't let the sense of stable snow security keep travel companions from moving fully into a safe zone while waiting for others. Carefully evaluate secondary exposures (cliffs below, terrain traps, etc.) of both ascent and descent lines.

Avalanche Problem #2: Persistent Deep Slabs

Many large crowns ranging from 3 to 7 feet deep exist around the forecast area from the deep slab avalanche cycle that occurred Dec 22 through 26. The Dec 2 and Dec 12 near crust facet layers failed during this period of high intensity new snow loading. At this time triggering additional deep slab avalanches has become unlikely. The weight of a single skier or snowmobiler on a slope will have a difficult time stressing the snowpack at the current depth of these weak layers. Large triggers such as multiple individuals close together on the same slope, large cornice collapses, or avalanches moving within the upper snowpack are more realistic triggers for additional deep slab avalanches. Recent snowpit tests targeting these faceted weak layers have shown that wide propagating, large and destructive deep slab avalanches remain possible if sufficient triggering force is transmitted deep into the snowpack, deeper than that typically transmitted by a single skier or snowmobiler.

The bottom line:

Near and above treeline, pockets of MODERATE avalanche danger exist on all aspects on slopes 35 degrees and steeper. For all other areas, avalanche danger is LOW. Expect to find isolated areas of unstable snow in wind affected complex terrain in and around areas of chutes, cliffs, and exposed rocks.

This advisory is valid for 48 hours from the time of issue. The next scheduled update will occur on January 2, 2013 by 7am.

Brandon Schwartz - Avalanche Forecaster, Tahoe National Forest

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

0600 temperature:	8 to 16 deg. F.
Max. temperature in the last 24 hours:	13 to 21 deg. F.
Average wind direction during the last 24 hours:	East
Average wind speed during the last 24 hours:	19 mph
Maximum wind gust in the last 24 hours:	40 mph
New snowfall in the last 24 hours:	0 to trace inches
Total snow depth:	66 to 93 inches

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

For 7000-8000 ft:
	Monday:	Monday Night:	Tuesday:
Weather:	Sunny skies, becoming partly cloudy.	Partly cloudy skies.	Partly cloudy skies, becoming sunny.
Temperatures:	19 to 26 deg. F.	1 to 11 deg. F.	23 to 30 deg. F.
Wind direction:	E	W shifting to E after midnight.	E
Wind speed:	10 to 20 mph with gusts to 35 mph, decreasing to around 10 mph in the afternoon.	10 to 15 mph with gusts to 25 mph.	10 to 20 mph with gusts to 30 mph.
Expected snowfall:	O in.	O in.	O in.
For 8000-9000 ft:
	Monday:	Monday Night:	Tuesday:
Weather:	Sunny skies, becoming partly cloudy.	Partly cloudy skies.	Partly cloudy skies, becoming sunny.
Temperatures:	23 to 29 deg. F.	7 to 13 deg. F.	20 to 27 deg. F.
Wind direction:	NE	NW shifting to NE after midnight.	E
Wind speed:	25 to 30 mph with gusts to 45 mph, decreasing to 15 to 20 mph with gusts to 35 mph in the afternoon.	20 to 25 mph with gusts to 40 mph.	25 to 35 mph with gusts to 45 mph.
Expected snowfall:	O in.	O in.	O in.