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 January 6, 2008 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 the avalanche danger is CONSIDERABLE on NW-N-NE-E aspects, 32 degrees and steeper. Below treeline, avalanche danger is MODERATE with pockets of CONSIDERABLE in open areas steeper than 35 degrees.

Snow showers should continue today as the last of this series of storm systems leaves the area. Snow accumulation should be much less intense today. The forecast calls for another 6 to 12 inches during the day. The southwest winds should continue, and they should start to diminish this afternoon and overnight tonight. Temperatures should stay cold as a cold air mass remains over the forecast area. Tomorrow there should be a break in the storm before a weak system moves into the area on Tuesday.

This storm deposited 46 to 54 inches of snow on the Sierra Crest and 50 to 60 inches in the Mount Rose area over the last 50 hours. The storm started out with a short burst of cold, light snow on Thursday night followed by several inches of much heavier, warmer snow and rain at lower elevations on Friday. By Friday afternoon the temperatures began a cooling trend that continues through today. The snow that has fallen since then has been progressively colder and less dense. These changing snow levels and the rain event created two weaknesses near the bottom of the storm snow: a density inversion and a rain crust. Observations yesterday from Alpine Meadows, near Kirkwood, and from Tamarack Ridge in the Mount Rose area all indicate weak bonding between these layers and the new dense snow on top of them. With less snow and wind forecast for tady these bonds should begin to strengthen. Weaknesses (small density changes, changes in snow crystal type, and rimed snow crystals) formed in the upper layers of the new snow as a result of subtle changes in the storm's winds, snowfall intensity, and temperature. These weaknesses should still be active today, but they should strengthen as the snow settles over the next few days. Significant cracking, collapsing, and test slope failures occurred at these layers in the Mount Rose area yesterday. Weak layers are also lurking deeper in the snowpack in the form of old crusts and basal facets (weak sugary grains near the bottom of the snowpack).

On top of these weak layers the winds have deposited large wind slabs. The winds have been shifting from the SSW to SW to WSW through out the storm. They have been consistently strong with an average speed over the past 50 hours at approximately 50 mph. These winds have transported snow onto the leeward slopes and formed wind slabs on the NW-N-NE-E aspects and large cornices on the lee side of ridgelines. The largest wind slabs and cornices formed near and above treeline; however, these strong winds also formed wind slabs in open areas below treeline. The winds moved enough snow to form drifts and wind slabs in some more protected areas as well. Even in the most protected areas without the help of wind loading the layer of dense, heavy snow near the base of the new snow could be able to behave as a slab. The combination of these slabs sitting on the weak layers mentioned above should be a good recipe for avalanche activity today.
Yesterday two natural avalanches were observed in the Mount Rose area and one near Caples Lake (southern Sierra Crest). These slides failed in the new snow and were relatively small. Many more slides probably occurred without being witnessed due to the poor visibility. Today avalanche activity should be less widespread due to less dense, less intense snow and more time for the snowpack to adjust to its new load. The majority of avalanche activity should occur within the new snow. Any avalanches that do occur today could also step down to near the old/new snow interface where a density inversion, a melt-freeze crust, a wind crust, or near surface faceted snow crystals exist, depending on location. Snowpack failure into the basal faceted layer at ground level is still a possibility, especially once a large avalanche is already in motion.

Near and above treeline human triggered avalanches are probable today on all wind loaded slopes steeper than 32 degrees. NW-N-NE-E aspects will be the most likely to wind load from the southwest wind. Natural avalanches are still possible on these slopes as well. Below treeline, natural avalanches are possible on any open slopes steeper than 35 degrees. Human triggered avalanche are probable in these areas as well.

Near and above treeline the avalanche danger is CONSIDERABLE on NW-N-NE-E aspects, 32 degrees and steeper. Below treeline, avalanche danger is MODERATE with pockets of CONSIDERABLE 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.

Andy Anderson, Avalanche Forecaster

Today's Central Sierra Weather Observations:
0600 temperature at Sierra Crest (8,700 feet): 11 deg. F
Max. temperature at Sierra Crest past 24 hours: 18 deg. F
Average wind direction at Sierra Crest past 24 hours: Southwest
Average wind speed at Sierra Crest past 24 hours: 48 mph
Maximum wind gust at Sierra Crest past 24 hours: 99 mph
New snow fall at 8,200 feet past 24 hours: 18 to 20 inches
Total snow depth at 8,200 feet: 65-75 inches
Mountain Weather Forecast For Today:
Cloudy with snow showers.
Temperature forecast for 8,000 to 9,000 feet: 16 to 22 deg. F
Ridgetop winds forecast for the Sierra Crest: Southwest at 20 to 35 mph with gusts to 80 mph.
Snowfall expected in the next 24 hours: 11 to 18 inches
2 Day Mountain Weather Forecast:

7000 to 8000 Feet:
Today, cloudy skies with snow showers. Snow accumulation 6 to 10 inches. Daytime highs 21 to 27 deg. F. Southwest winds at 10 to 15 mph, with gusts to 30 mph.

Tonight, cloudy skies with light snow. Snow accumulation 3 to 5 inches. Overnight lows 10 to 14 deg. F. Southwest winds at 10 to 15 mph with gusts to 25 mph.

For Monday, cloudy skies with a slight chance of snow showers. Snow accumulation should be less than 1 inch. Daytime highs 23 to 27 deg. F. Southwest winds shifting to the west at 10 to 15 mph with gusts to 25 mph.

Above 8000 Feet:
Today, cloudy skies with snow showers. Snow accumulation 8 to 12 inches. Daytime highs 16 to 22 deg. F. Southwest winds at 20 to 35 mph, with gusts to 80 mph decreasing to 50 mph gusts in the afternoon.

Tonight, cloudy skies with light snow. Snow accumulation 3 to 6 inches. Overnight lows 6 to 10 deg. F. Southwest winds at 15 to 25 mph with gusts to 35 mph decreasing to 10 to 15 mph with gusts to 25 mph after midnight.

For Monday, cloudy skies with a slight chance of snow showers. Snow accumulation should be less than 1 inch. Daytime highs 18 to 24 deg. F. Southwest winds shifting to the northwest at 10 to 15 mph with gusts to 25 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:
	Sunday:	Sunday Night:	Monday:
Weather:
Temperatures:	deg. F.	deg. F.	deg. F.
Wind direction:
Wind speed:
Expected snowfall:	O in.	O in.	O in.
For 8000-9000 ft:
	Sunday:	Sunday Night:	Monday:
Weather:
Temperatures:	deg. F.	deg. F.	deg. F.
Wind direction:
Wind speed:
Expected snowfall:	O in.	O in.	O in.