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 20, 2007 7:00 AM
Click here for a detailed map of the SAC forecast area

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Today's Advisory:
Near and above treeline, avalanche danger is CONSIDERABLE on wind loaded NW-N-NE-E-SE aspects, 35 degrees and steeper. Below treeline, avalanche danger is MODERATE in protected open areas, 35 degrees and steeper.

The current storm system began affecting the forecast area yesterday afternoon. New snowfall amounts of 8 to 16 inches have occurred overnight throughout the forecast area. Heavy snowfall is continuing this morning with another 10 to 16 inches expected. Southwest ridgetop winds have remained moderate to strong for the past several days. Air temperatures at the upper elevations have remained in the upper teens and low 20s during that time. Ridgtop winds are expected to remain strong through this morning before decreasing and shifting to west and then north this afternoon and evening. Snowfall is expected to taper to showers this afternoon.

Avalanche start zones on N-NE-E aspects have filled in with new snow over the past two days. The snowpack in areas above 7,000' now ranges from 2 to 5 feet deep. Snow that fell last night is less subject to the influence of anchors as the snowpack becomes deeper and objects such as rocks, down trees, and stumps become increasingly buried.

The combination of southwest ridgetop winds and significant new snowfall has created areas of instability on wind loaded N-NE-E aspects as well as on cross loaded NW and SE aspects. Recently formed wind slabs in these areas began to show some sensitivity to human triggering yesterday in both the Castle Peak (Donner Summit) and Mount Rose areas. This was observed as infrequent test slope failure, with one very small human triggered avalanche reported yesterday from the Mount Rose area. A skier triggered a small slab on a north aspect, 40 degree gully feature in the area between Incline Lake and Relay Peak. The avalanche had a 1 foot crown, measured 25 feet across and ran 15 feet down slope. With the addition of more snow overnight, wind loaded areas near and above treeline are expected to show significantly more sensitivity to human triggering today with natural avalanche activity possible.

Below treeline, two significant weak layers have been identified within the snowpack. In the Castle Peak area and at ski area snow study plots along the Sierra Crest to the south, easy to moderate force shears and compression failure was noted in the top 10 to 15cm of the snowpack yesterday. This layer of new snow crystals in now buried under additional new snow that fell last night and is below the depth of ski, snowboard, of snowmobile penetration into the snowpack. A layer of buried surface hoar exists in many areas around Carson Pass, Mt. Judah (Donner Summit) and Mount Rose. Clean and easy shears as well as some small cracking without fracture propagation was reported yesterday on a NNW aspect just below treeline on Tamarack Peak (Mount Rose area). New snow load on these layers has increased overnight and so has the likelihood of human triggered avalanches in steep open areas below treeline. The only way to safely verify the presence or absence of these layers in your location is by digging.

The bottom line: Near and above treeline, avalanche danger is CONSIDERABLE on wind loaded NW-N-NE-E-SE aspects, 35 degrees and steeper. Below treeline, avalanche danger is MODERATE in protected 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): 22 deg. F
Max. temperature at Sierra Crest past 24 hours: 25 deg. F
Average wind direction at Sierra Crest past 24 hours: Southwest
Average wind speed at Sierra Crest past 24 hours: 60 mph
Maximum wind gust at Sierra Crest past 24 hours: 95 mph
New snow fall at 8,200 feet past 24 hours: 12-16 inches
Total snow depth at 8,200 feet: 37 inches

Mountain Weather Forecast For Today:
Cloudy with heavy snowfall in the morning, decreasing to snow showers in the afternoon.
Temperature forecast for 8,000 to 9,000 feet: 16 to 23 deg. F.
Ridgetop winds forecast for the Sierra Crest: Southwest at 45 to 55 mph with gusts to 100 mph. Shifting to west and decreasing in the afternoon.
Snowfall expected in the next 24 hours: 10-16 inches

2 Day Mountain Weather Forecast:

7000 to 8000 Feet:
Today, cloudy with heavy snowfall before 10 am, tapering to showers in the afternoon. Snow accumulation 4 to 8 inches in the morning with up to 2 additional iinches in the afternoon. Daytime highs 23 to 28 deg. F. Southwest winds at 20 to 30 mph with gusts to 45 mph. Winds expected to shift to west at 15 to 20 mph in the afternoon.

Tonight, mostly cloudy in the evening then clearing. Scattered snow showers with snow accumulation up to 1 inch. Overnight lows 7 to 13 deg. F. Northwest winds at 10 to 15 mph, shifting to north at 10 mph after midnight.

For Friday, partly cloudy in the morning with increasing clouds during the day. Daytime highs 23 to 28 deg. F. East winds at 10 to 15 mph with gusts to 25 mph are expected.

Above 8000 Feet:
Today, cloudy with heavy snowfall before 10 am, tapering to showers in the afternoon. Snow accumulation 6 to 10 inches in the morning with 2 to 4 additional inches in the afternoon. Daytime highs 16 to 23 deg. F. Southwest winds at 45 to 55 mph with gusts 80 to 100 mph. Winds expected to shift to west at 25 to 35 mph with gusts to 55 mph in the afternoon.

Tonight, mostly cloudy in the evening then clearing. Scattered snow showers with snow accumulation up to 2 inches. Overnight lows around 11 deg. F. North winds at 20 to 30 mph with gust to 50 mph.

For Friday, partly cloudy in the morning with increasing clouds during the day. Daytime highs 21 to 26 deg. F. Northeast winds at 20 to 30 mph with gusts to 55 mph, decreasing to 40 mph in the afternoon are expected.

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