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.

Fairly benign weather is in store for today. Transitional weather is expected for tomorrow ahead of a significant multi-day storm event forecast to start late Thursday night. As the forecast area wakes from the deep freeze this morning, air temperatures are in the single digits across the region. As ridge level winds shift to the S and SW today and tonight, air temperatures above 7,000' will warm into the mid 20s to low 30s and remain there through Thursday. Ridgetop winds are forecast at moderate speed today before increasing tonight and tomorrow with gusts to 100mph expected Thursday.

Recent Observations:

Observations made yesterday on Powderhouse Peak (Luther Pass area) and near Frog Lake (Carson Pass area) revealed significant variability in snowpack instability. On Powderhouse Peak, snowpit data revealed lingering wind slab instability in wind loaded areas at treeline. Deeper in the snowpack, a very weak layer of faceted crystals were noted on top of the rain crust that caps the Dec 2nd facet layer. This is a new facet layer that formed just above the rain crust and along the bottom edge of the snow that fell on Dec 12. This layer was observed at treeline on a NW aspect at 9,200'. At this time it is unknown just how widespread this new facet layer is (videos, pit profile, more info). Near Frog Lake, much better stability was observed in snowpit data with good bonding of wind slabs to the lower density snow below (pit profile, more info). The Dec 2nd facet layer continues to persist in a widespread distribution with variable degrees of strength and weakness around the forecast area.

Avalanche Problem #1: 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 at least isolated areas where the newly formed Dec 12 facet layer exists, presents multiple persistent weak layers of concern. The snow that exists above these layers is becoming an increasingly cohesive slab as storm snow and wind slab instabilities diminish. In most areas, slabs around 2 feet thick sit on top of these facet layers while the most heavily wind loaded areas hold slabs that may approach 6 feet thick. As the upper snowpack gains strength, the area of greatest relative weakness may move down to the facet layers. There is significant uncertainty as the where and when these facet layers will fail, but the door is open for the possibility of large human triggered avalanches today and tomorrow, followed by the possibility of large natural avalanches during the upcoming storm cycle.

Avalanche Problem #2: Wind Slabs

Lingering instability associated with surface wind slabs is expected today in isolated areas. Cold air temperatures yesterday afternoon and overnight are expected to have slowed stabilization of these slabs in areas where instability remained yesterday. Lingering instability is most likely in wind affected areas on N-NE aspects both above and below treeline where higher density wind slabs sit on top of markedly lower density snow. Slab thickness is around 1 foot in most areas.

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 persistent slabs and lingering wind 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:	3 to 7 deg. F.
Max. temperature in the last 24 hours:	17 to 18 deg. F.
Average wind direction during the last 24 hours:	West
Average wind speed during the last 24 hours:	16 mph
Maximum wind gust in the last 24 hours:	28 mph
New snowfall in the last 24 hours:	1 to 4 inches
Total snow depth:	48 to 57 inches

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

For 7000-8000 ft:
	Wednesday:	Wednesday Night:	Thursday:
Weather:	Partly cloudy skies.	Mostly cloudy skies.	Mostly cloudy skies with a slight chance of snow.
Temperatures:	23 to 30 deg. F.	15 to 25 deg. F.	25 to 31 deg. F.
Wind direction:	E shifting to S in the afternoon.	SW	SW
Wind speed:	10 to 15 mph with gusts to 30 mph.	10 to 20 mph with gusts to 35 mph.	20 to 35 mph with gusts to 50 mph.
Expected snowfall:	O in.	O in.	0 to trace in.
For 8000-9000 ft:
	Wednesday:	Wednesday Night:	Thursday:
Weather:	Partly cloudy skies.	Mostly cloudy skies.	Mostly cloudy skies with a slight chance of snow.
Temperatures:	20 to 27 deg. F.	20 to 27 deg. F.	23 to 30 deg. F.
Wind direction:	E shifting to S in the afternoon.	SW	SW
Wind speed:	15 to 25 mph with gusts to 40 mph.	30 to 40 mph with gusts to 60 mph.	55 to 70 mph with gusts to 100 mph.
Expected snowfall:	O in.	O in.	0 to trace in.