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 21, 2006:


December 21, 2006 at 1:00 am

Forecast Discussion:


This advisory was posted on Thursday, December 21, 2006 at 6:59 am

The storm system that is currently approaching the forecast area has slowed down and will not make its biggest impact until late today and tonight. A couple inches of snow will likely fall this morning, but the heavy precipitation is not expected to begin until between 2pm and 5pm today. Ridgetop winds have shifted to westerly this morning and wind speed will increase as the day progresses. Snow level around 6,000' this morning is expected to rise to 7,000' to 7,500' during the day. Snow level will drop back to 6,000' this evening. Avalanche danger will not begin to rapidly increase until the late afternoon or evening hours of today when heavy snowfall rates begin.

In all areas, snow surface conditions consist of either a hard surface crust or pockets of unconsolidated snow sitting on top of a very strong and supportable crust layer. Good bonding has been observed at this layer interface throughout the forecast area at this time. Despite some faceting of snow crystals near crust layers and depth hoar basal layers above 8,500' on northerly aspects, the snowpack is in good shape to handle additional new snow loading from the approaching storm system.

A fairly stable snowpack will remain for almost all of the day today until the onset of heavy snowfall this afternoon or evening. Only 1 to 2 inches of snow is expected below 7,000' this morning before rising snow levels bring rain to the lower elevations of the forecast area. This is not enough new snow to create significant instability from a rain on new snow event. Some roller ball activity is the greatest extent of instability expected. Above 7,500', 3 to 6 inches of snow are possible during the day today. Increasing westsouthwesterly ridgetop winds will cause wind loading on N-NE-E aspects. The combination of new snow and wind loading will create some pockets of instability in the most heavily wind loaded areas by late today. Small human triggered avalanches will become possible in these areas.

Above 7,500', avalanche danger is LOW this morning for all aspects. Areas of MODERATE danger will develop by late today in near and above treeline areas on N-NE-E aspects 35 degrees and steeper. Below 7,500', avalanche danger will remain LOW today on all aspects.

Avalanche danger will increase overnight with heavy snowfall. Natural and human triggered avalanches will become likely.

Brandon Schwartz, Avalanche Forecaster

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Today's Central Sierra Weather Observations:
0600 temperature at Sierra Crest (8,700 feet): 26 deg. F
Max. temperature at Sierra Crest past 24 hours: 31 deg. F
Average wind direction at Sierra Crest past 24 hours: westerly
Average wind speed at Sierra Crest past 24 hours: 15 mph
Maximum wind gust at Sierra Crest past 24 hours: 28 mph
New snow fall at 8,200 feet past 24 hours: 0 inches
Total snow depth at 8,200 feet: 22 inches

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Mountain Weather Forecast For Today:
Cloudy skies with rain and snow
Temperature forecast for 8,000 to 9,000 feet: 28 - 33 deg. F
Ridgetop winds forecast for the Sierra Crest: westsouthwesterly 50 - 60 mph, G 80 mph, by afternoon
Snowfall expected in the next 24 hours: 12 - 20 inches mainly overnight tonight

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2 Day Mountain Weather Forecast:
7000 to 8000 Feet
For today, cloudy skies with rain and snow. Snow becoming heavy late day. Snow level 7,000' to 7,500'. Snow accumulation 2 to 4 inches. Daytime highs 32 to 35 degrees F. Winds out of the westsouthwest at 25 to 40 mph increasing to 50 to 65 mph in the late afternoon. Overnight lows 15 to 20 degrees F with heavy snowfall. Snow level lowering to 6,000'. Snow accumulation 8 to 12 inches. Westsouthwesterly winds 40 to 55 mph with gusts to 70 mph. Friday will be partly cloudy with snow showers. Daytime highs around 30 degrees F. Northwesterly winds 25 to 40 mph.

Above 8000 Feet
For today, cloudy skies with snow becoming heavy late day. Snow accumulation 3 to 6 inches. Daytime highs 28 to 32 degrees F. Winds out of the westsouthwest at 25 to 35 mph with gusts to 50 mph. Winds increasing to 50 to 60 mph with gusts to 80 mph in the late afternoon. Overnight lows 11 to 17 degrees F with heavy snowfall. Snow accumulation 9 to 14 inches. Westsouthwesterly winds 60 to 70 mph with gusts to 90 mph. Friday will be partly cloudy with snow showers. Daytime highs around 28 degrees F. Northwesterly ridgetop winds 35 to 55 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:

  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.