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 February 20, 2012:

February 20, 2012 at 7:48 am

Pockets of MODERATE avalanche danger persist near treeline and below treeline on NW-N-NE aspects on slope 35 degrees and steeper due to a persistent weak layer at the base of the recent new snow. For all other areas, avalanche danger is LOW.

Forecast Discussion:

A weak weather disturbance will pass through the region today, bringing mostly cloudy to cloudy skies and a chance of snow showers. Little to no new snow accumulation is expected. Remote sensors between 6,200' and 9,600' are reporting air temperatures in the 20s this morning across the forecast area. Ridgetop winds remain out of the southwest. Winds speed is light this morning, but is forecast to increase to moderate in speed as the morning progresses. Maximum daytime air temperatures are expected in the mid 20s to mid 30s today for areas above 7,000'. Decreased cloud cover, moderate speed north winds, and air temperatures in the 30s and 40s are forecast for tomorrow.

Recent Observations:

Recent observations from around the forecast area continue to show the presence of a persistent weak layer of faceted snow and crust at the base of the recent new snow. This combination of weak layer and overlying slab exists in near treeline and below treeline areas on NW-N-NE aspects that were protected from scouring by strong NE winds on Feb 15. Most above treeline areas were heavily wind scoured during this event, removing the surface slab and exposing the crust at the snow surface. For each of the last 5 days, there have been reports of human triggered snowpack collapse and subsequent whumphing. Snowpit tests including those performed yesterday on Lincoln Ridge (Yuba Pass area) on N aspects near and below treeline between 7,200 and 7,400' have produced varying results with propagation of initiated fractures indicated as likely in some areas and unlikely in other areas of similar terrain and snowpack. Several ski and snowmobile cuts in this area on suspect 32 to 42 degree slopes did not produce any visible signs of snowpack failure.

Today's Primary Avalanche Concern: Persistent Slabs

The snowpack instability that has been observed throughout the forecast area over the last 5 days will continue with little change expected for today. Areas of instability exist in pockets surrounded by areas of seemingly stable snow. The persistent weak layer of facets and crust exists at the base of surface slabs that are anywhere from 6 to 18 inches thick. Any avalanche activity that occurs today will involve all of the recent new snow from this past week, creating avalanches that are large enough to bury or injure a person. Propagation tests such as the Extended Column Test are a quick and easy way to aid in the decision making process for travel in avalanche terrain (click here for a video explaining this test and how to perform it). Continue to use caution and exercise good travel habits in order to minimize risk during this period of ongoing snowpack instability.


The bottom line:

Pockets of MODERATE avalanche danger persist near treeline and below treeline on NW-N-NE aspects on slope 35 degrees and steeper due to a persistent weak layer at the base of the recent new snow. 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: 20 to 24 deg. F.
Max. temperature in the last 24 hours: 25 to 34 deg. F.
Average wind direction during the last 24 hours: Southwest
Average wind speed during the last 24 hours: 19 mph
Maximum wind gust in the last 24 hours: 53 mph
New snowfall in the last 24 hours: 0 to trace inches
Total snow depth: 26 to 36 inches

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

For 7000-8000 ft:
  Monday: Monday Night: Tuesday:
Weather: Cloudy skies becoming mostly cloudy as the day progresses. Isolated snow showers in the morning. Mostly cloudy skies. Partly sunny skies.
Temperatures: 31 to 37 deg. F. 19 to 26 deg. F. 37 to 44 deg. F.
Wind direction: W NW N
Wind speed: 10 to 20 mph with gusts to 30 mph. 10 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.

For 8000-9000 ft:
  Monday: Monday Night: Tuesday:
Weather: Cloudy skies becoming mostly cloudy as the day progresses. Isolated snow showers in the morning. Mostly cloudy skies. Partly sunny skies.
Temperatures: 26 to 32 deg. F. 18 to 25 deg. F. 37 to 43 deg. F.
Wind direction: W N N
Wind speed: 20 to 30 mph with gusts to 50 mph. 25 to 35 mph with gusts to 50 mph. 20 to 30 mph with gusts to 50 mph.
Expected snowfall: 0 to trace in. O in. O in.