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 21, 2009:

November 21, 2009 at 7:59 am

On slopes steeper than 30 degrees, the avalanche danger is MODERATE on N-NE aspects above 8700 ft. Human-triggered avalanches will be possible on these slopes today.

Forecast Discussion:

6 to 12 inches of snow fell across the forecast area since yesterday afternoon. Snow showers started to taper off last night as the low-pressure system continued moving east. Along with the snow, this system brought sustained winds between 40 and 70 mph with some gusts reaching 116 mph. As this system exits the region today, the winds should decrease, and the skies should begin to clear. Another smaller low pressure system passing to the north of the forecast area should cause the wind to increase again overnight and through Sunday. This second system does not have much water associated with it and should not produce any additional accumulation.

Observations from Elephant's Back, Mt. Lola, Mt. Rose, and Barker Peak prior to the storm revealed only a few areas on the N-NE aspects above 8700' where enough snow existed to cover the ground obstacles. On these slopes the snowpack consisted of 3-12 inches of ice frozen to the ground with a mix of weak, poorly-bonded, sugar-like snow (facets) and thin wind slabs sitting on top of the ice. In some areas both of these upper layers existed and in others only one. Observers triggered shooting cracks and whumpfing as they walked across these areas over the last three days. Layer bonding tests also showed that the facets on top of the ice layer can barely support themselves. Adding any additional weight to these facets could easily cause them to fail. Prior to this storm the N-E aspects below 8700' only had a few inches of snow on them. This snow did not cover the terrain roughness and many rocks, shrubs, and patches of dirt remained on these slopes. All other aspects had more dirt than snow prior to this storm.

Even though the snowpack remains shallow, avalanche concerns do exist today. The snowpack prior to the storm already had a weak layer that humans could break (the facets) and a bed surface for an avalanche to slide on (the smooth slippery ice) on some of the N-NE aspects above 8700'. The new storm snow combined with strong winds will have added new wind slabs on top of the already fragile snowpack. The thickest and heaviest wind slabs will exist on high-elevation N-NE-E aspects where the weakest snowpack existed prior to the storm. The combination of these factors makes human-triggered avalanche activity possible today on the N-NE aspects above 8700' that had snow cover before this storm. The shallow snowpack makes any avalanches that do occur today even more dangerous because anyone who gets caught will hit rocks and other barely covered obstacles during his/her ride.

Use extra caution today. The slopes that will attract people for recreation will also be the slopes that are most prone to avalanches. Early season conditions exist with many exposed rocks, trees, and other obstacles lurking just below the surface waiting to take people out for the season.


The bottom line:

On slopes steeper than 30 degrees, the avalanche danger is MODERATE on N-NE aspects above 8700 ft. Human-triggered avalanches will be possible on these slopes today.


Andy Anderson - Avalanche Forecaster, Tahoe National Forest

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

0600 temperature: 18 deg. F.
Max. temperature in the last 24 hours: 35 deg. F.
Average wind direction during the last 24 hours: West Southwest
Average wind speed during the last 24 hours: 45-50 mph
Maximum wind gust in the last 24 hours: 116 mph
New snowfall in the last 24 hours: 6-12 inches
Total snow depth: 6-26 inches

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

For 7000-8000 ft:
  Saturday: Saturday Night: Sunday:
Weather: Partly cloudy this morning becoming mostly cloudy this afternoon Mostly cloudy Mostly cloudy with a chance for isolated snow showers in the morning
Temperatures: 26-34 deg. F. 20-27 deg. F. 27-37 deg. F.
Wind direction: Northwest South shifting to West after midnight West
Wind speed: 10 mph 10 mph with gusts to 30 mph 15-30 mph with gusts to 45 mph
Expected snowfall: O in. O in. Trace in.

For 8000-9000 ft:
  Saturday: Saturday Night: Sunday:
Weather: Partly cloudy this morning becoming mostly cloudy this afternoon Mostly cloudy Mostly cloudy with a chance for isolated snow showers in the morning
Temperatures: 20-27 deg. F. 16-23 deg. F. 24-34 deg. F.
Wind direction: Northwest West West
Wind speed: 10-20 mph with gusts to 35 mph 20-30 mph with gusts to 45 mph increasing to 35-50 mph with gusts to 70 mph after midnight 35-50 mph with gusts to 75 mph
Expected snowfall: O in. O in. Trace in.