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 Avalanche Advisory was published on December 17, 2006:


December 17, 2006 at 1:00 am

Forecast Discussion:


This advisory was posted on Sunday, December 17, 2006 at 7:00 am

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A cold high pressure system is sitting over the forecast area. Temperatures will remain in the teens and twenties for the next few days. The winds shifted to the west southwest last night, but as the day progresses we should see a light NW - N wind develop. By Monday the winds will strengthen and be blowing out of the east.

We picked up about 6" of cold, dry, light snow during the night. With the moderate west southwest winds, some wind loading occurred on the north through east aspects. For the most part the new snow has very little cohesion and exhibits few slab characteristics. It is also very light and added little additional load to the snowpack. Combined with the wind, it may be able to form some some shallow soft slabs on the north through east aspects on exposed slopes. On the most windloaded north through east facing slopes that are steeper than 35 degrees, there may be a few small, isolated pockets of instability that could possibly be triggered by a person. If triggered these slides are likely to be small and should not run very far. As the winds shift to the north and east today these slopes will be scoured and the chances of finding one of these small pockets of instability will diminish.

The cold temperatures have caused all the water in the snowpack to refreeze and have created a very strong base in most areas below 8500' where the snow was wet from top to bottom. Above 8,500', only the upper half of the snowpack was affected by the rain; therefore, we now have a well-bonded, refrozen crust sitting on top of basal layer facets and near crust facets. This crust should serve as a "bridge" that will allow less force exerted by skiers and snowmobiles to reach the weak facetted grains. One could transmit force through the crust at places where the snowpack is shallow or with a large trigger. Right now many of the shallow trigger points in the snowpack are very poorly connected to larger starting zones so the likely hood of triggering anything on these layers is small. As the season progresses and we get more snow, the starting zones will be more connected to these trigger points. This interface between the crust and the facetted layers underneath will be one to keep an eye on.

Near and above treeline, avalanche danger is LOW with small, isolated pockets of MODERATE danger on the most wind loaded N-E aspects, 35 degrees and steeper. Below treeline, avalanche danger is LOW.

Andy Anderson , 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:
18 deg. F
Average wind direction at Sierra Crest past 24 hours:
east shifting to west southwest after midnight
Average wind speed at Sierra Crest past 24 hours:
30 mph
Maximum wind gust at Sierra Crest past 24 hours:
58 mph
New snow fall at 8,200 feet past 24 hours:
7 inches
Total snow depth at 8,200 feet:
26 inches
Mountain Weather Forecast For Today:
Mostly cloudy skies with snow showers ending this afternoon. Accumulations up to 2"
Temperature forecast for 8,000 to 9,000 feet:
16-24 deg. F
Ridgetop winds forecast for the Sierra Crest:
north 15 - 20 mph, G 25 mph
Snowfall expected in the next 24 hours:
2 inches
2 Day Mountain Weather Forecast:

7000 to 8000 Feet
For today, mostly cloudy skies with snow showers ending this afternoon. Daytime highs of 20 to 28 degrees F. Snow accumulation up to 1 inch. Winds shifting to the NW at 10-15 mph. Overnight lows around 5 degrees F with partly cloudy skies. Winds shifting to the NE at 10-20mph. Monday will be sunny with daytime highs 23 to 31 degrees F. The winds will pick up and shift to the east at 15-25 mph with gusts to 40mph.

Above 8000 Feet
For today, mostly cloudy skies with snow showers ending this afternoon. Daytime highs of 16 to 24 degrees F. Snow accumulation up to 2 inches. Winds shifting to the N at 15-20 mph. Overnight lows 3 to 9 degrees F with partly cloudy skies. Winds shifting to the NW at 15-25mph with gusts to 35mph. Monday will be sunny with daytime highs 19 to 27 degrees F. The winds will pick up and shift to the east at 25-30 mph with gusts to 60mph.


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.