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 March 22, 2012:


March 22, 2012 at 6:35 am

Very isolated pockets of MODERATE avalanche danger exist both above and below treeline on NW-N-NE aspects on slopes 35 degrees and steeper due to ongoing persistent deep slab instability. Large destructive human triggered deep slab avalanches remain possible in these areas. For all other areas, avalanche danger is generally LOW with isolated pockets of MODERATE danger below 8,500' on slopes 37 degrees and steeper due to wet snow instability.


Forecast Discussion:


Cloud cover moved into much of the forecast area overnight, keeping air temperatures above freezing in many areas for the second night in a row. Air temperatures below 8,000' remain above freezing this morning while air temperatures above 8,000' are at or slightly below freezing. Maximum daytime air temperatures above 7,000' are forecast to reach the low 30s to mid 40s today. Ridgetop winds remain out of the southwest this morning and are moderate in speed. Winds are forecast to become strong as the day progresses and continue through tonight. Isolated snow showers today are expected to bring trace amounts of accumulation at best. Reduced cloud cover and cooler air temperatures are forecast for tomorrow.

Recent Observations:

Wet surface snow remained widespread yesterday with areas of sun, cloud cover, and well above freezing air temperatures affecting snow on all aspects. The recent new snow is showing signs of adjusting to the melt conditions, but is far from becoming any form of well established and supportable melt-freeze layer due to a lack of strong overnight refreeze. Cloud cover for much of last night kept air temperatures above freezing in many mid and low elevation areas. Any significant amount of supportable snow surface refreeze this morning will be limited to areas above 8,500'.

Snowpit data was collected yesterday on Silver Peak (Pole Creek area) just above the shallowest portion of the crown of the natural deep slab avalanche that occurred during the pre dawn hours on March 17 (info on avalanche). At this location just above 7,700' on a N aspect at treeline, increasing moisture within and strengthening of the persistent weak layer of crusts and facets that exist about 4 feet below the snow surface was observed. Propagation tests performed on the persistent weak layer at this location indicated that propagation was now unlikely (pit profile). This is outlier data from what has been seen around the rest of the forecast area over the past week indicating continued likely propagation. This new data should be treated with caution regarding current strength of the persistent weak layer on a regional scale.

Today's Primary Avalanche Concern: Wet Snow Instability

The cumulative effects of a poor snow surface refreeze the last two nights in most areas, strong late March solar radiation, and well above freezing air temperatures will fuel wet surface snow instability again today. Most wet snow instability is expected to occur on E-SE-S-SW-W aspects. Wet snow instability could also occur on NW-N-NE aspects, especially below 8,000' due to poor overnight refreeze. Wet loose snow avalanche activity rather than wet slab avalanche activity is expected. Secondary terrain hazards such as cliffs and terrain traps will greatly magnify the consequence of becoming caught and carried in an otherwise small avalanche.

Today's Secondary Avalanche Concern: Persistent Deep Slabs

The persistent weak layer of crusts and facets generally lurks between five and eight feet deep in the snowpack. In the vast majority of areas this weak layer is buried too deep for a single skier or snowmobiler to transmit force through the snowpack sufficient to collapse and cause propagating failure of the snowpack along this layer. In isolated areas, this weak layer exists within 3 to 4 feet of the snow surface, well within the possibility of human triggering by a skier of snowmobiler. As a result, concerns remain ongoing for isolated deep slab avalanches both above and below treeline on NW-N-NE aspects. Any deep slab avalanches that occur at this time will be large and destructive. Spot probing for this low density snow layer is an effective way to determine the depth of the persistent weak layer in a given location.


The bottom line:

Very isolated pockets of MODERATE avalanche danger exist both above and below treeline on NW-N-NE aspects on slopes 35 degrees and steeper due to ongoing persistent deep slab instability. Large destructive human triggered deep slab avalanches remain possible in these areas. For all other areas, avalanche danger is generally LOW with isolated pockets of MODERATE danger below 8,500' on slopes 37 degrees and steeper due to wet snow instability.


Brandon Schwartz - Avalanche Forecaster, Tahoe National Forest


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

0600 temperature: 29 to 34 deg. F.
Max. temperature in the last 24 hours: 36 to 45 deg. F.
Average wind direction during the last 24 hours: Southwest
Average wind speed during the last 24 hours: 34 mph
Maximum wind gust in the last 24 hours: 63 mph
New snowfall in the last 24 hours: O inches
Total snow depth: 44 to 80 inches

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

For 7000-8000 ft:

  Thursday: Thursday Night: Friday:
Weather: Mostly cloudy skies with isolated snow showers. Mostly cloudy skies, becoming partly cloudy. Partly cloudy skies.
Temperatures: 36 to 43 deg. F. 25 to 30 deg. F. 32 to 37 deg. F.
Wind direction: SW SW SW
Wind speed: 20 to 30 mph with gusts 45 to 60 mph. 15 to 30 mph with gusts to 45 mph. 15 to 30 mph with gusts to 45 mph.
Expected snowfall: 0 to trace in. O in. O in.

For 8000-9000 ft:

  Thursday: Thursday Night: Friday:
Weather: Mostly cloudy skies with isolated snow showers. Mostly cloudy skies, becoming partly cloudy. Partly cloudy skies.
Temperatures: 30 to 35 deg. F. 20 to 25 deg. F. 25 to 30 deg. F.
Wind direction: SW SW SW
Wind speed: 40 to 55 mph with gusts to 85 mph, decreasing to 30 to 45 mph with gusts to 70 mph in the afternoon. 30 to 40 mph with gusts to 70 mph. 20 to 40 mph with gusts to 65 mph.
Expected snowfall: 0 to trace in. O in. O in.