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.

High pressure is firmly in place over the forecast area. Ridgetop winds that were strong out of the northeast for much of yesterday have begun to decrease. Light to moderate speed ridgetop winds are expected today and tomorrow.  A strong air temperature inversion has set up with single digits and below zero readings on the mountain valley floors. Locations above 6,500' to 7,000' are in the upper teens to upper 20s this morning. Air temperature inversion is expected to persist for at least the next several days.

Recent Observations:

Observations made yesterday on Porcupine Ridge (Luther Pass area), along the Lincoln/Anderson Ridge (Donner Summit area), and in the Deep Creek drainage (Deep Creek area) all revealed fairly stable snowpack conditions. Wind transport of old snow on the ground was prevalent along the higher ridgelines (photos, more info). In many areas snow was pulled high into the atmosphere and not deposited onto leeward slopes. One exception was along the Lincoln/Anderson Ridge where wind deposits up to 1.5' deep were noted in some cross loaded SE to S aspect gullies. No additional evidence of instability was observed in the upper snowpack in any of these specific locations.

Observations made along Porcupine Ridge targeting the Dec 2 and Dec 12 facet layers continued to show persistent instability. Fracture propagation tests continue to show that if collapse of the weak layer can be initiated, propagating failure of the weak layer remains likely to occur. In this area these December facet layers are much closer to the snow surface (around 3' deep) than in other areas, especially when compared to locations near the Sierra Crest (video, pit profile, more info).

Avalanche Problem #1: Wind Slabs

E to NE ridgetop winds over the past few days have significantly stripped snow from N-NE-E aspect areas above treeline. Limited areas of new wind slab formation have been observed in lee areas on SE-S-SW-W-NW aspects. Complex terrain in and around steep chutes, gullies, couloirs, cliffs and rock bands remain the most suspect for isolated pockets of unstable wind slabs. Continue to exercise good travel techniques and don't let the sense of stable snow security influence travel companions into half efforts at safe travel actions.

Avalanche Problem #2: Persistent Deep Slabs

The story here is much the same, unlikely triggering but with catastrophic consequences. 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. Snowpit tests targeting these faceted weak layers continue to show that wide propagating, large and destructive deep hard 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:

Avalanche danger is LOW for all elevations and aspects. Isolated areas of unstable snow may exist in wind affected complex terrain in and around areas of chutes, gullies, cliffs, and exposed rocks. Normal caution is advised.

Brandon Schwartz - Avalanche Forecaster, Tahoe National Forest

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

0600 temperature:	19 to 25 deg. F.
Max. temperature in the last 24 hours:	22 to 29 deg. F.
Average wind direction during the last 24 hours:	Northeast
Average wind speed during the last 24 hours:	29 mph
Maximum wind gust in the last 24 hours:	68 mph
New snowfall in the last 24 hours:	O inches
Total snow depth:	64 to 91 inches

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

For 7000-8000 ft:
	Wednesday:	Wednesday Night:	Thursday:
Weather:	Sunny skies.	Partly cloudy skies.	Partly cloudy skies.
Temperatures:	24 to 30 deg. F.	3 to 13 deg. F.	24 to 31 deg. F.
Wind direction:	SE	Variable	Variable
Wind speed:	10 to 15 mph in the morning, becoming light.	Light winds	Light winds
Expected snowfall:	O in.	O in.	O in.
For 8000-9000 ft:
	Wednesday:	Wednesday Night:	Thursday:
Weather:	Sunny skies.	Partly cloudy skies.	Partly cloudy skies.
Temperatures:	23 to 29 deg. F.	12 to 18 deg. F.	26 to 32 deg. F.
Wind direction:	SE	SE	SW
Wind speed:	15 to 20 mph. Gusts up to 30 mph in the morning.	10 to 15 mph with gusts to 25 mph.	10 to 15 mph with gusts to 25 mph.
Expected snowfall:	O in.	O in.	O in.