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 April 14, 2012:


April 14, 2012 at 6:40 am

Both above and below treeline, avalanche danger is LOW with pockets of MODERATE danger in wind loaded areas on all aspects on slopes 35 degrees and steeper. Areas of unstable snow will exist within surrounding areas of seemingly stable snow.


Forecast Discussion:


The last in the series of storm systems that impacted the forecast area over the past 4 days is slowly existing the region. New snow amounts above 7,000' from this past week are generally 1 to 2 feet. Scattered snow showers today are forecast to end by this evening. Remote sensors are reporting air temperatures above 8,000' in the mid teens to mid 20s this morning. Maximum daytime air temperatures for areas above 7,000' are expected in the mid 20s to mid 30s this afternoon. Ridgetop winds were out of the southwest yesterday and have shifted to the north early this morning. Light winds during the shift early this morning will increase to moderate in speed. Decreased cloud cover, a significant increase in air temperature, and light to moderate winds are forecast for tomorrow.

Recent Observations:

Observations made yesterday on Silver Peak (Pole Creek area) revealed surface wind slabs and density inversions within the recent new snow. This allowed for widespread skier triggered shooting cracks producing small slab and loose snow avalanches on test slopes on N-NE aspects in near and below treeline terrain. By afternoon, snowpack instability was on a decreasing trend in this area, but skier triggered shooting cracks and unstable snowpit test results continued to occur (photos, more info). Along the east ridge of Tamarack Peak (Mount Rose area), evidence of instability was limited to wind loaded areas. Wind slabs reacted to intentional cornice collapse of refrigerator size blocks by producing avalanches 2 feet deep failing 2 to 3 inches above the old/new snow interface. All of these avalanches occurred in near treeline terrain on N aspect 35 to 40 degree slopes. During the afternoon hours avalanche activity in response to cornice collapse became smaller with propagation less widespread. New snow and wind loading were noted to come to an end in this area by noon (photos, video, more info). Yesterday evening on Donner Ridge (Donner Summit area) significant ongoing wind loading, wind slab formation, cornice formation, and skier triggered shooting cracks were observed near and above treeline on NE-E aspects at 7pm.

Today's Primary Avalanche Concern: Wind Slabs

Unstable wind slabs are expected to linger today in recently wind loaded areas. Areas of instability are expected to exist in pockets both above and below treeline on all aspects. The largest wind slabs capable of contributing to larger human triggered avalanches are expected on NW-N-NE-E-SE aspects. Small new wind slabs will form today on SE-S-SW-W aspects as N to NW winds scour snow that was deposited yesterday on northerly and easterly aspects, moving it back over the ridgetop. Expect to find areas of instability existing within surrounding areas of seemingly stable snow. Today is the type of day to approach snowpack stability on a slope by slope basis.

Today's Secondary Avalanche Concern: Wet snow

Very limited areas of wet snow instability may form today below 7,500' on E-SE-S-SW-W aspects. Roller balls and wet loose snow avalanches are possible in isolated areas, especially if the sun breaks through the clouds this afternoon. More widespread wet snow instability is expected to occur tomorrow.

Other things to think about: The Persistent Weak Layer

Due to the depth of the persistent weak layer (5 to 8 ft down) and the strength of the snow above it, triggering a deep slab avalanche on NW-N-NE aspects has become unlikely. At this time data indicates that collapsing this layer would require an extremely large trigger or significant weakening of the snowpack. Spring melting could eventually allow free water to percolate down through the snowpack and cause sufficient weakening of this layer for a deep wet slab avalanche cycle to occur. Conditions that would create enough melting for this scenario remain several weeks to a month or more away.


The bottom line:

Both above and below treeline, avalanche danger is LOW with pockets of MODERATE danger in wind loaded areas on all aspects on slopes 35 degrees and steeper. Areas of unstable snow will exist within surrounding areas of seemingly stable snow.


Brandon Schwartz - Avalanche Forecaster, Tahoe National Forest


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

0600 temperature: 15 to 23 deg. F.
Max. temperature in the last 24 hours: 24 to 29 deg. F.
Average wind direction during the last 24 hours: Southwest
Average wind speed during the last 24 hours: 25 mph
Maximum wind gust in the last 24 hours: 52 mph
New snowfall in the last 24 hours: 1 to 3 inches
Total snow depth: 43 to 102 inches

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

For 7000-8000 ft:

  Saturday: Saturday Night: Sunday:
Weather: Cloudy skies with scattered snow showers. Partly cloudy skies. Partly cloudy skies.
Temperatures: 30 to 37 deg. F. 25 to 35 deg. F. 44 to 50 deg. F.
Wind direction: NW N N shifting to W
Wind speed: 10 to 15 mph. 10 to 15 mph with gusts to 25 mph in the evening, becoming light. 10 to 15 mph with gusts to 25 mph.
Expected snowfall: 0 to trace in. O in. O in.

For 8000-9000 ft:

  Saturday: Saturday Night: Sunday:
Weather: Cloudy skies with scattered snow showers. Partly cloudy skies. Partly cloudy skies.
Temperatures: 25 to 30 deg. F. 24 to 29 deg. F. 38 to 45 deg. F.
Wind direction: N N NW
Wind speed: 20 to 30 mph with gusts to 45 mph. 15 to 20 mph with gusts to 30 mph. 15 to 20 mph with gusts to 35 mph.
Expected snowfall: 0 to trace in. O in. O in.