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 November 29, 2012:


November 29, 2012 at 8:00 am

MODERATE avalanche danger exists on near and above treeline NW-N-NE-E aspects steeper than 35 degrees due to new snow, strong winds, and some areas of persistent weak layers. In other areas the avalanche danger should remain LOW. Expect the avalanche danger to increase tonight and continue to rise as a large storm impacts the region over the next few days.


Forecast Discussion:


.5 to 1.2 inches of liquid precipitation fell across the forecast area yesterday. Those areas closest to the Sierra crest received the highest precipitation amounts and areas farther east received the lower end of those amounts. Snow levels mostly stayed near 7000 ft with a brief drop down to 6500 ft during the afternoon. Above 7000 ft. 5-7 inches of new snow accumulated across the mountains. The forecast calls for break in precipitation today. Some showers could continue to produce up to an inch of new snow above 7000 ft. today. The gale force southwest winds that accompanied yesterday's storm will continue today. These winds and precipitation should begin to increase tonight as the second wave of this series of storms arrives over the region. The forecast calls for wind speeds along the Sierra Crest in the 75-90 mph range with gusts over 130 mph for tonight and tomorrow. This wave should also bring another 1-3 inches of liquid to the region tonight and tomorrow. with snow levels between 7000 ft. and 7500 ft.  2-3 ft of wet heavy snow could easily accumulate at the higher elevations. The lower elevations will see a mix of rain and very wet snow. Another even wetter and warmer storm should follow this weekend. For more information check out this weather briefing from the Reno NWS.

Recent Observations:

Yesterday observations in the Mt. Rose backcountry showed that new wind slabs and small cornices began forming as soon as the snowfall started. By 3 pm some of these wind slabs had already reached 6-8 inches in depth. They did not extend very far away from ridgelines nor had they become very widespread at that time. Skier triggered shooting cracks up to 4 ft in length did occur in the most heavily wind loaded areas (photos more information). In less wind affected areas, observations also showed poor bonding between the new snow and the old snow surfaces of crusts and less consolidated snow. Ski cuts on steep test slopes caused the new snow to easily sluff downhill.

Recent observations across the forecast area prior to yesterday's snowfall revealed a shallow and consolidated snowpack capped by thick firm crusts at the mid elevations, and patchy snow coverage at the lower elevations. Some persistent weak layers have formed in the snowpack at the upper elevations. These weak layers consist of weak sugary snow (facets) that exists below a crust on northerly aspects above 8200 ft along the Sierra Crest and near the base of the snowpack on northerly aspects above 9300 ft in the Mt. Rose area and in the Freel Peak area.

Today's Primary Avalanche Concern: Wind Slabs

Human triggered avalanches involving the wind slabs that formed yesterday will remain possible today. Some of these wind slabs could have grown to a few feet in depth due to the continued strong winds and snowfall. Today's strong winds will continue to move snow and may create new wind slabs in some unexpected places. These wind slabs sit on top of ice crusts and unconsolidated snow in some areas. Yesterday's data indicated that they have not bonded well to these old snow surfaces. The largest and most fragile wind slabs will exist on near and above treeline NW-N-NE-E aspects where the most wind loading has and continues to occur. Some smaller but still fragile wind slabs may exist in more protected areas due to the strength of the winds.

Today's Secondary Avalanche Concern: Persistent Slabs

This problem results from the existence the persistent weak layers in isolated areas above 9,000' on NW-N-NE aspects. These kind of avalanches should remain small and difficult to trigger today and any areas of instability that do exist will be hidden within surrounding areas of stable snow. As more storms arrive and add significant load to the snowpack over the next several days, triggering these kind of avalanches could become easier and this problem could become more widespread.


The bottom line:

MODERATE avalanche danger exists on near and above treeline NW-N-NE-E aspects steeper than 35 degrees due to new snow, strong winds, and some areas of persistent weak layers. In other areas the avalanche danger should remain LOW. Expect the avalanche danger to increase tonight and continue to rise as a large storm impacts the region over the next few days.


Andy Anderson - Avalanche Forecaster, Tahoe National Forest


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

0600 temperature: 28-32 deg. F.
Max. temperature in the last 24 hours: 32-38 deg. F.
Average wind direction during the last 24 hours: Southwest
Average wind speed during the last 24 hours: 55 mph
Maximum wind gust in the last 24 hours: 105 mph
New snowfall in the last 24 hours: 5-8 inches
Total snow depth: 21-35 inches

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

For 7000-8000 ft:

  Thursday: Thursday Night: Friday:
Weather: Scattered showers Cloudy with a mix of snow and rain. Snow levels between 7000 ft and 7500 ft. Cloudy with a mix of snow and rain. Snow levels between 7000 ft and 7500 ft.
Temperatures: 34-41 deg. F. 27-34 deg. F. 33-40 deg. F.
Wind direction: Southwest South Southwest
Wind speed: 30-45 mph with gusts to 75 mph 35-50 mph with gusts to 80 mph 50-65 mph with gusts to 85 mph
Expected snowfall: up to 1 in. 2-6 in. 6-10 in.

For 8000-9000 ft:

  Thursday: Thursday Night: Friday:
Weather: Scattered snow showers Snow Snow
Temperatures: 34-40 deg. F. 24-31 deg. F. 32-38 deg. F.
Wind direction: Southwest Southwest Southwest
Wind speed: 60-65 mph with gusts to 100 mph increasing to 70-75 mph with gusts to 115 mph in the afternoon 75-90 mph with gusts to 130 mph 75-90 mph with gusts to 135 mph
Expected snowfall: up to 1 in. 4-10 in. 12-24 in.

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