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.

Wind speeds are increasing as mid and high level cloud cover moves over the forecast area ahead of a well advertised multi day storm event. Ridgetop winds shifted from east to southwest yesterday morning and have steadily increased in speed. Winds will continue to increase, becoming strong to gale force today with ridgetop gusts up to 100 mph. Air temperatures this morning are running in the mid teens to mid 20s for areas above 7,000'. Winds overnight have mixed out any inversion layers. Daytime highs for today are expected in the 20s for areas above 7,000'. A few light snow showers may occur today ahead of high intensity snowfall expected to begin early tomorrow morning.

Recent Observations:

Observations made yesterday on Becker Peak (Echo Summit area), Rubicon Peak (West Shore Tahoe area), Castle Peak (Donner Summit area) and on Tamarack Peak (Mount Rose area) all showed ongoing concerns for wind slabs and/or persistent facet layers. No new avalanches were observed, but snowpit data continued to reveal lingering wind slab instability failing on a layer of lower density snow about 1 foot below the snow surface. The force required to trigger failure and propagation of these lingering wind slab instabilities varied significantly from one location to another (more info, photos, videos).

Snowpit tests targeting the Dec 2 and Dec 12 facet layers gave variable results, but indicated ongoing instability concerns in a variety of areas. The Dec 2 facet layer (located below the uppermost prominent rain crust in the snowpack) is widespread and exists in nearly all areas on NW-N-NE aspects between 7,500' and 10,000'. Over the past few days, tests on this layer have shown a high degree of variability with this layer stable in some areas and continued unstable in other areas. The Dec 12 facet layer (located on top of the rain crust that caps the Dec 2 facet layer) is more spotty in distribution, having been observed on NW-N-NE aspects on Powerhouse Peak (Luther Pass area), in the Davis and Winters Creek drainages (Mount Rose area), and possibly on the north side of Castle Peak (Donner Summit area). In all areas where this layer has been noted it has show unstable snowpit test results (more info, photos, video).

Avalanche Problem #1: Wind Slabs

Strong SW winds today will move snow that fell Monday night and Tuesday under light wind conditions, creating a fresh round of wind slab formation. This will occur on top of lingering instability of older wind slabs observed over the past few days. New wind slabs will be most prominent in directly wind loaded areas on N-NE-E aspects mainly near and above treeline. Expect some small wind slabs to form below treeline due to the strength of the winds, but slab size will be smaller. New slab thickness could grow to around 1 foot in many areas. This will be on top of already existing wind slabs that are also around 1 foot thick.

Avalanche Problem #2: Persistent Slabs

Faceted snow crystals that exist below the rain crust that formed out of the Dec 2nd rain and snow event present a weak layer that is widespread around the forecast area on NW-N-NE aspects between 7,500' and 10,000'. This combined with isolated areas where the more newly formed Dec 12 facet layer exists, presents multiple persistent weak layers of concern. The snow that exists above these layers has become an increasingly cohesive slab. In most areas, slabs around 2-3 feet thick sit on top of these facet layers. The most heavily wind loaded areas hold slabs that may approach 6 feet thick. There is significant uncertainty as the where and when these facet layers will fail, but the door remains open for the possibility of large human triggered avalanches today, followed by the possibility of large natural avalanches during the upcoming storm cycle.

The bottom line:

Both above and below treeline, avalanche danger is MODERATE on NW-N-NE-E aspects on slopes 35 degrees and steeper due to wind slabs and persistent slabs. Large and destructive human triggered avalanches are possible in these areas. For other areas, avalanche danger is LOW.

Brandon Schwartz - Avalanche Forecaster, Tahoe National Forest

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

0600 temperature:	15 to 25 deg. F.
Max. temperature in the last 24 hours:	22 to 31 deg. F.
Average wind direction during the last 24 hours:	East shifting to southwest
Average wind speed during the last 24 hours:	32 mph
Maximum wind gust in the last 24 hours:	56 mph
New snowfall in the last 24 hours:	O inches
Total snow depth:	45 to 56 inches

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

For 7000-8000 ft:
	Thursday:	Thursday Night:	Friday:
Weather:	Mostly cloudy skies with a slight chance of snow.	Cloudy skies with a slight chance of snow.	Cloudy skies with snow.
Temperatures:	24 to 30 deg. F.	15 to 25 deg. F.	24 to 30 deg. F.
Wind direction:	SW	SW	SW
Wind speed:	20 to 35 mph with gusts to 50 mph.	20 to 35 mph with gusts to 50 mph.	15 to 30 mph with gusts to 45 mph.
Expected snowfall:	0 to trace in.	0 to trace in.	6 to 12 in.
For 8000-9000 ft:
	Thursday:	Thursday Night:	Friday:
Weather:	Mostly cloudy skies with a slight chance of snow.	Cloudy skies with a slight chance of snow.	Cloudy skies with snow.
Temperatures:	20 to 27 deg. F.	13 to 20 deg. F.	22 to 27 deg. F.
Wind direction:	SW	SW	SW
Wind speed:	25 to 35 mph with gusts to 50 mph, increasing to 55 to 65 mph with gusts to 100 mph.	50 to 70 mph with gusts to 95 mph.	40 to 50 mph with gusts to 80 mph.
Expected snowfall:	0 to trace in.	0 to trace in.	6 to 12 in.