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, 2011:


November 28, 2011 at 7:57 am

Above 8,000', areas of MODERATE avalanche danger exist both above and below treeline on N-NE aspects on slopes 32 degrees and steeper. Avalanche danger is LOW with pockets of MODERATE danger above 8,000' in both above and below treeline terrain on NW and E aspects on slopes 32 degrees and steeper. For all other areas, avalanche danger is LOW.


Forecast Discussion:


A weak weather system will pass well to the north of the forecast area today. High cloud cover through this morning is expected to give way to partly cloudy skies this afternoon. An air temperature inversion is in place this morning with the coldest air generally below 7,000'. Remote sensors above 8,500' are reporting air temperatures this morning in the upper 30s to low 40s. Another warm day is forecast for today with daytime highs in the 40s to 50s for most elevations. Ridgetop winds have shifted from south to west and have decreased from strong to moderate in speed. A further decrease in wind speed is forecast for this afternoon, especially at the mid and lower elevations.

Observations:

The overall snowpack across the forecast area is shallow and usable for over snow recreation only in select areas. Observations made yesterday on Castle Peak continue to show ongoing instability within the snowpack (pit profile, video, more info). Skier triggered whumpfing and shooting cracks are still occurring in areas where the November 18 facet layer remains highly developed (pit profile, photo, more info). In areas where the facets are less developed, snowpit tests continue to show that once fracture is initiated along this weak layer, propagation remains possible. Areas where significant anchoring exist have shown better stability, but the snowpack in these areas is generally too shallow for over snow recreation. Some lingering instability of shallow wind slabs that formed last week have been observed in isolated ares of previous wind loading near and above treeline. A thin and breakable rain crust that formed on Thanksgiving Day exists on the snow surface along the Sierra Crest up to around 9,000' in most areas.

Avalanche Concern #1: Persistent slabs

The November 18 facet layer has shown some gains in strength, but they are slow. This is a persistent weak layer and it will take at least several more weeks for this layer to stabilize in all areas. In locations where the snowpack is deepest, a one to two foot thick slab exists on top of the facet layer. The greatest areas of instability have been observed on N-NE aspects above 8,000', with pockets of instability on NW and E aspects. Heavily shaded, colder slopes with a relatively deeper snowpack and few visible anchors protruding through the snow surface are the most suspect.

Isolated pockets of lingering wind slab are also out there in previously wind loaded areas near and above treeline on NW-N-NE-E aspects. These slabs are generally less than one foot thick.


The bottom line:

Above 8,000', areas of MODERATE avalanche danger exist both above and below treeline on N-NE aspects on slopes 32 degrees and steeper. Avalanche danger is LOW with pockets of MODERATE danger above 8,000' in both above and below treeline terrain on NW and E aspects on slopes 32 degrees and steeper. For all other areas, avalanche danger is LOW.


Brandon Schwartz - Avalanche Forecaster, Tahoe National Forest


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

0600 temperature: 37 to 43 deg. F.
Max. temperature in the last 24 hours: 44 to 51 deg. F.
Average wind direction during the last 24 hours: South shifting to west
Average wind speed during the last 24 hours: 32 mph
Maximum wind gust in the last 24 hours: 51 mph
New snowfall in the last 24 hours: O inches
Total snow depth: 7 to 23 inches

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

For 7000-8000 ft:

  Monday: Monday Night: Tuesday:
Weather: Mostly cloudy skies in the morning, becoming partly cloudy. Clear skies. Sunny skies.
Temperatures: 46 to 54 deg. F. 28 to 36 deg. F. 47 to 55 deg. F.
Wind direction: SW SW SW
Wind speed: 10 to 15 mph with gusts to 30 mph. Decreasing to around 10 mph in the afternoon. Up to 10 mph. Around 10 mph.
Expected snowfall: O in. O in. O in.

For 8000-9000 ft:

  Monday: Monday Night: Tuesday:
Weather: Mostly cloudy skies in the morning, becoming partly cloudy. Clear skies. Sunny skies.
Temperatures: 41 to 51 deg. F. 30 to 37 deg. F. 42 to 52 deg. F.
Wind direction: SW SW SW
Wind speed: 10 to 15 mph with gusts 25 to 35 mph. 10 to 15 mph with gusts to 25 mph in the evening. 10 to 15 mph with gusts to 30 mph.
Expected snowfall: O in. O in. O in.