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.

This advisory was posted on December 21, 2007 6:57 AM
Click here for a detailed map of the SAC forecast area

2_moderate.jpg
Today's Advisory:
Near and above treeline, avalanche danger is MODERATE on slopes 35 degrees and steeper. Below treeline, avalanche danger is MODERATE in open areas, 35 degrees and steeper.

Skies have cleared overnight, bringing cold temperatures to the forecast area this morning. The recent storms of 12/18-12/20 have deposited 12 to 24 inches of new snow throughout the area. Ridgetop winds shifted to east during the overnight hours. Moderate to strong ridetop winds are expected to continue this morning before beginning to decrease in speed by mid day. The forecast area is expected to remain under the influence of northeast flow today, keeping air temperatures cool.

Yesterday, widespread collapsing and whumphing were observed in a variety of areas around Donner Summit (Sierra Crest) and on Tamarack Peak (Mount Rose area).Whumphing was observed below treeline in the Donner Summit area and above and below treeline in the Tamarack Peak area. Additionally, east aspects near and above treeline on Tamarack Peak showed greater signs of snowpack instability. The east aspect Proletariat face displayed shooting cracks that were skier triggered by traversing the ridgetop and an avalanche occurred on the east face of Hourglass Bowl. It is unclear if the avalanche was remotely triggered by a skier several hundred feet away, mid slope on the northeast aspect of Hourglass Bowl or if the avalanche released naturally under continued wind loading conditions at that time.

The soft slab avalanche occurred around 12pm on a 36 degree, east aspect slope above treeline. The slab failed at ground level. The crown averaged 1 foot deep with a maximum depth of 2 feet deep. The crown measured approximately 500 feet in length and the avalanche ran around 700 linear feet to the bottom of the bowl, 400 vertical feet below.

Several different weak layers were identified yesterday in the above locations. Multiple layers of needle crystals within the new snow yielded easy to moderate shears in the Donner and Tamarack areas. In the Tamarack area two unstable layers were observed deeper in the snowpack. Cracks were observed to penetrate down to the old/new snow interface where buried surface hoar has been noted in some areas. Still deeper in the snowpack, collapse and cracking into the upper portion of basal layer faceted crystals was observed.

A variety of continued snowpack instability is expected today. Above treeline, east winds will scour snow from N-NE-E aspect avalanche start zones, reducing the lingering instability in these areas. The snow removed from these slopes will be deposited on S-SW-W aspects and may create some small and shallow pockets of instability. At and below treeline, the possibility of human triggered avalanches remains greater. The most hazard exists on slopes 35 degrees and steeper where new snow sits on top of buried surface hoar and faceted snow layers. These crystal types tend to cause more persistent and longer lived instabilities than the new snow needle form crystals that exist within the recent storm snow. Natural avalanche activity is not expected today. Human triggered avalanches remain possible on a variety of slopes 35 degrees and steeper.

The bottom line: Near and above treeline, avalanche danger is MODERATE on slopes 35 degrees and steeper. Below treeline, avalanche danger is MODERATE in open areas, 35 degrees and steeper.

Please send us your snow, weather, and avalanche observations by clicking the submit observations link on our contact page.

Brandon Schwartz, Avalanche Forecaster

Today's Central Sierra Weather Observations:
0600 temperature at Sierra Crest (8,700 feet): 9 deg. F
Max. temperature at Sierra Crest past 24 hours: 22 deg. F
Average wind direction at Sierra Crest past 24 hours: Southwest shifting to east
Average wind speed at Sierra Crest past 24 hours: 29 mph
Maximum wind gust at Sierra Crest past 24 hours: 76 mph
New snow fall at 8,200 feet past 24 hours: 4-6 inches
Total snow depth at 8,200 feet: 36 inches

Mountain Weather Forecast For Today:
Partly cloudy skies.
Temperature forecast for 8,000 to 9,000 feet: 21 to 26 deg. F.
Ridgetop winds forecast for the Sierra Crest: East at 25 to 35 mph with gusts to 60 mph.
Snowfall expected in the next 24 hours: 0 inches

2 Day Mountain Weather Forecast:

7000 to 8000 Feet:
Today, partly cloudy skies with daytime highs 24 to 29 deg. F. East winds at 10 to 15 mph with gusts to 25 mph.

Tonight, partly cloudy skies with overnight lows 9 to 17 deg. F. Southeast winds at 10 mph.

For Saturday, mostly cloudy with daytime highs 33 to 39 deg. F. South winds at 10 mph.

Above 8000 Feet:
Today, partly cloudy skies with daytime highs 21 to 26 deg. F. East winds at 25 to 35 mph with gusts to 60 mph.

Tonight, partly cloudy skies with overnight lows 12 to 17 deg. F. East winds at 10 to 15 mph with gust to 25 mph.

For Saturday, mostly cloudy with daytime highs 28 to 34 deg. F. East winds at 10 mph.

The bottom line:

Andy Anderson - Avalanche Forecaster, Tahoe National Forest

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

0600 temperature:	deg. F.
Max. temperature in the last 24 hours:	deg. F.
Average wind direction during the last 24 hours:
Average wind speed during the last 24 hours:	mph
Maximum wind gust in the last 24 hours:	mph
New snowfall in the last 24 hours:	O inches
Total snow depth:	inches

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

For 7000-8000 ft:
	Friday:	Friday Night:	Saturday:
Weather:
Temperatures:	deg. F.	deg. F.	deg. F.
Wind direction:
Wind speed:
Expected snowfall:	O in.	O in.	O in.
For 8000-9000 ft:
	Friday:	Friday Night:	Saturday:
Weather:
Temperatures:	deg. F.	deg. F.	deg. F.
Wind direction:
Wind speed:
Expected snowfall:	O in.	O in.	O in.