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 28, 2012:


November 28, 2012 at 7:54 am

Today the avalanche danger will rise to MODERATE on near and above treeline NW-N-NE-E aspects steeper than 35 degrees due to new snow and wind impacting the forecast area. In other areas the avalanche danger should remain LOW. A few very isolated areas of unstable snow may exist above 9,000' on NW-N-NE aspects in complex or extreme terrain in areas of LOW danger.


Forecast Discussion:


The forecast calls for a fast-moving and small winter storm to arrive over the forecast area today and tonight. The southwest winds have already increased ahead of this storm and should continue to increase through tonight before this system departs the area tomorrow morning. The winds should decrease some tomorrow morning before increasing again tomorrow afternoon. The forecast calls for this storm to produce between 2 and 7 inches of new snow above 7000 ft by tomorrow morning. Most of this snow should occur this afternoon and evening. Snow levels with this system should remain between 6500 ft. and 7500 ft. so some of the precipitation could fall as rain. This storm represents the first of a series of progressively stronger, wetter, and warmer storms predicted to affect the forecast area through this weekend (for more information check out this weather briefing from the Reno NWS).

Recent Observations:

Observations across the forecast area have shown 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 layers of weak sugary snow (facets) that exist below a crust in the upper portion of the snowpack along the Sierra Crest, near the base of the snowpack in the Mt. Rose area and in the Freel Peak area (more info, video and snowpit). Along the Sierra Crest observations near Carson Pass, Silver Peak, and yesterday on Castle Peak all showed this weakness existing above 8200ft on NW-N-NE aspects. In the Mt. Rose area data indicates that the weakest and most well developed facets exist above 9500 ft on isolated northerly aspects that received little to no traffic prior to burial of this layer on November 16th (photos, videos, pit profiles, more info). This layer does exist in other places down to around 9200 ft. in the Mt. Rose area; however, it seems to be less well developed at these lower elevations.

Today's Primary Avalanche Concern: Wind Slabs

As new snow and strong winds impact the forecast area today, wind slabs will start to form on the leeward aspects. The size and extent of these wind slabs should remain limited due to the small amount of new snow predicted to fall today.  However, some of these wind slabs could grow to a few feet in depth by the end of the day and would become large enough to pose a problem especially if the higher end of the predicted snowfall amounts occurs. These wind slabs will form on top of ice crusts and may not bond very well to the existing snowpack making human triggered avalanches involving these wind slabs possible later today. The largest and most fragile wind slabs will exist on near and above treeline NW-N-NE-E aspects where the most wind loading occurs.  

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 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:

Today the avalanche danger will rise to MODERATE on near and above treeline NW-N-NE-E aspects steeper than 35 degrees due to new snow and wind impacting the forecast area. In other areas the avalanche danger should remain LOW. A few very isolated areas of unstable snow may exist above 9,000' on NW-N-NE aspects in complex or extreme terrain in areas of LOW danger.


Andy Anderson - Avalanche Forecaster, Tahoe National Forest


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

0600 temperature: 32-38 deg. F.
Max. temperature in the last 24 hours: 39-45 deg. F.
Average wind direction during the last 24 hours: Southwest
Average wind speed during the last 24 hours: 35 mph
Maximum wind gust in the last 24 hours: 65 mph
New snowfall in the last 24 hours: O inches
Total snow depth: 15-27 inches

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

For 7000-8000 ft:

  Wednesday: Wednesday Night: Thursday:
Weather: Cloudy with snow showers through out the day. Cloudy with scattered snow showers in the evening. Showers becoming isolated after midnight. Cloudy with a chance of snow showers in the morning. Rain and snow likely in the afternoon.
Temperatures: 38-45 deg. F. 23-30 deg. F. 39-45 deg. F.
Wind direction: South Southwest Southwest
Wind speed: 25-40 mph with gusts to 55 mph 30-40 mph with gusts to 55 mph 30-40 mph with gusts to 60 mph
Expected snowfall: 1-4 in. up to 1 in. up to 2 in.

For 8000-9000 ft:

  Wednesday: Wednesday Night: Thursday:
Weather: Cloudy with snow showers through out the day. Cloudy with scattered snow showers in the evening. Showers becoming isolated after midnight. Cloudy with a chance of snow showers in the morning. Snow becoming more widespread in the afternoon.
Temperatures: 37-43 deg. F. 22-29 deg. F. 35-41 deg. F.
Wind direction: South Southwest Southwest
Wind speed: 40-50 mph with gusts to 75 mph increasing to gusts to 95 mph in the afternoon 40-50 mph with gusts to 80 mph decreasing to gusts to 70 mph after midnight 50-55 mph with gusts to 85 mph increasing to 60-65 mph with gusts to 100 mph in the afternoon
Expected snowfall: 1-5 in. up to 1 in. up to 2 in.

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