THIS AVALANCHE ADVISORY EXPIRED ON December 14, 2013 @ 6:54 am
Avalanche Advisory published on December 13, 2013 @ 6:54 am
Issued by Andy Anderson - Tahoe National Forest

On slopes steeper than 32 degrees, MODERATE avalanche danger exists on near and below treeline NW-N-NE aspects due to lingering persistent slabs. Human triggered persistent slabs remain possible. Remotely triggered avalanches can occur under these conditions. Some isolated areas of unstable snow may still remain on above treeline slopes as well.

In addition to the avalanche danger a myriad of season-ending obstacles still lurks just below the surface. Conservative terrain selection, careful snowpack evaluation, and conservative decision-making are essential.

1. Low

?

Above Treeline
Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.

2. Moderate

?

Near Treeline
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.

2. Moderate

?

Below Treeline
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
    Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
Avalanche Problem 1: Persistent Slab
  • Type ?
  • Aspect/Elevation ?
  • Likelihood ?
    Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size ?
    Historic
    Very Large
    Large
    Small

Human triggered avalanches remain possible in near and below treeline terrain on NW-N-NE aspects where weak sugary snow exists under the snow that fell on Dec. 7th. While the vast majority of these persistent slabs exist in near and below treeline terrain, the possibility remains that some isolated areas above treeline where snow did not get scoured away earlier this season could pose a problem. The recent snow has enough cohesion to act as a soft slab sitting above the Dec 7th facets. Even though some data indicates that the snowpack has become less fragile, the additional weight of a person on a slope where this persistent weak layer/soft slab combination exists can still cause the slope to fracture. Unstable conditions like these take a long time to change and often catch people off-guard.

Since fractures can travel through the persistent weak layer, people can trigger persistent slabs remotely while standing in low angle terrain or on ridgelines or anywhere that the snowpack under their feet connects to steeper and/or deeper terrain. Wide propagation and avalanche activity on lower angle slopes to around 32 degrees remains possible below treeline.

recent observations

Another day of widespread collapsing and whumpfing yesterday near Red Lake on Carson Pass (more info, snowpit, video). These collapses happened when the weak sugary snow (Dec. 7th facets) below the recent new snow broke and a fracture propagated along the weak layer. Several of these whumpfs and and collapses happened near areas that had previously collapsed. Snowpit tests also indicated that when the facet layer breaks, the resulting fracture can travel along this persistent weak layer. Data also indicates that the snow layer above the Dec. 7th facets continues to represents a soft slab.

Barely covered rocks, stumps, logs, and other hard, immobile, season-ending, equipment-breaking objects still exist on all slopes. The shallow snowpack does not have enough structure or strength to keep a person above all these things. It only camouflages their existence.

Weather and CURRENT CONDITIONS
weather summary

A weak cold front passing through the region caused the WSW winds to increase over the last 24 hours and brought a few clouds to the area. As it departs today, the winds should shift to the N and E behind the front and temperatures should remain cooler today and tonight. The forecast calls for highs in the low 30's above 7000 ft today. By tomorrow afternoon the winds should begin to decrease and temperatures above 7000 ft. should start to warm up again as a high pressure ridge returns to the region.

Weather observations from along the Sierra Crest between 8200 ft. and 8800 ft.
0600 temperature: 20 to 29 deg. F.
Max. temperature in the last 24 hours: 35-44 deg. F.
Average wind direction during the last 24 hours: West Southwest shifting towards the North early this morning
Average wind speed during the last 24 hours: 10 to 20 mph
Maximum wind gust in the last 24 hours: 50 mph
New snowfall in the last 24 hours: 0 inches
Total snow depth: 10 to 20 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Friday Friday Night Saturday
Weather: Partly cloudy this morning then becoming sunny as the day progresses Clear Sunny
Temperatures: 29-36 deg. F. 16-24 deg. F. 37-44 deg. F.
Wind Direction: North Northeast Southeast
Wind Speed: 10-15 mph with gusts to 25 mph 15-20 mph with gusts to 30 mph 10-15 mph with gusts to 25 mph in the morning becoming light in the afternoon
Expected snowfall: 0 in. 0 in. 0 in.
For 8000 ft. to 9000 ft.
Friday Friday Night Saturday
Weather: Partly cloudy this morning then becoming sunny as the day progresses Clear Sunny
Temperatures: 28-35 deg. F. 17-24 deg. F. 39-46 deg. F.
Wind Direction: North Northeast Southeast shifting to the East in the afternoon
Wind Speed: 25-35 mph with gusts to 50 mph 25-35 mph with gusts to 55 mph decreasing to 45 mph after midnight 25-30 mph with gusts to 50 mph decreasing to 15-20 mph with gusts to 30 mph in the afternoon
Expected snowfall: 0 in. 0 in. 0 in.
Disclaimer

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.