THIS AVALANCHE ADVISORY EXPIRED ON December 13, 2015 @ 6:54 am
Avalanche Advisory published on December 12, 2015 @ 6:54 am
Issued by Andy Anderson - Tahoe National Forest

MODERATE avalanche danger exists on slopes steeper than 32 degrees on NW-N-NE-E-SE aspects in near and above treeline terrain and on NW-N-NE aspects in below treeline terrain. Wind slabs will linger on wind loaded slopes, and the potential for persistent slabs still remains on some sheltered northerly aspects today. Evaluate snow and terrain carefully, and identify features of concern before committing to a slope. While avalanches will be more difficult to trigger today, human triggered avalanches will remain possible.

2. Moderate

?

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

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: Wind Slab
  • Type ?
  • Aspect/Elevation ?
  • Likelihood ?
    Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size ?
    Historic
    Very Large
    Large
    Small

The wind slabs that formed during the storm still remain on wind loaded N-NE-E aspects and cross loaded NW and SE aspects. Data and observations have indicated that these wind slabs have formed good bonds within themselves and with the old snow surfaces in many places, but not in all places. Triggering an avalanche involving one of these wind slabs has become more difficult, but human triggered wind slab avalanches remain possible today. Open, exposed, near and above treeline terrain will represent the most common locations for these wind slabs. The most fragile wind slabs will exist in steep, wind-loaded couliors, on unsupported slopes above rocks or cliffs, in very heavily wind loaded areas, or in other complex or extreme terrain. Avalanches involving these wind slabs could still involve enough snow to bury a person. If these wind slabs release on slopes where the persistent weak layers mentioned below exist, they could overload that deeper layer and involve more snow. Today's shifting winds may also deposit some very small and isolated wind slabs on other aspects (like the S-SW-W).

Avalanche Problem 2: Persistent Slab
  • Type ?
  • Aspect/Elevation ?
  • Likelihood ?
    Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size ?
    Historic
    Very Large
    Large
    Small

Observations continue to show a persistent weak layer lurking near the base of the snowpack on NW-N-NE aspects above 8000 ft. Data still indicates that if this layer does break the resulting fracture can travel through the persistent weak layer and that a person in the right spot on the right slope could break that layer. In many areas these persistent slabs remain small and isolated since anchors poking through the snowpack interrupt the weak layer. However, these persistent slabs could fracture across wider areas and involve more snow on slopes with smooth ground cover and less anchors. Cracking, collapsing, whumphing, and recent avalanche activity can all provide clues as to where these persistent slabs may be an issue, but persistent slabs often release on slopes and days with none of these obvious clues. Digging into the snowpack to find this persistent weak layer and avoiding slopes where it exists, traveling on lower angle terrain, or traveling on well anchored terrain represent the best ways to avoid this avalanche problem.

recent observations

Yesterday observations on NW-N-NE aspects above 8000 ft. from Indian Valley (between Carson Pass and Bear Valley) and Castle Peak (Donner Summit area) showed new snow resting above an old snowpack that consisted of crusts surrounding loose weak snow (facets). In both areas tests targeting the persistent weak layers failed and indicated that if the weak layer breaks the resulting fracture can travel through the persistent weak layer. In the Indian Valley area, one low angle slope near a creek collapsed twice: first under the weight of snowmobiler when he stepped off his sled and second when another snowmobiler approached the slope on foot. These whumps, snowpit tests, and the upside down structure of the snowpack represented the only signs of instability concerning the persistent weak layer on Castle Peak and in the Indian Valley area. On Silver Peak (Pole Creek area) in terrain below 8000 ft. the persistent weak layer had become very moist and would easily clump together. Observations near ridglines on Silver Peak and Castle Peak showed some wind slabs that had bonded well to themselves and the old snow surfaces. Some minor cracking occurred in both places on wind loaded test slopes and one small 4 to 8 inch wind slab did release on one wind loaded test slope on Silver Peak. Overall signs of instability remained limited and hard to find yesterday. 

Weather and CURRENT CONDITIONS
weather summary

The forecast calls for mostly cloudy skies and warmer temperatures for today due to a small high pressure ridge bringing some wamer air to the region. The winds started shifting to the north early this morning, and they should remain in the light to moderate range. This period of calmer weather will not last long. By this afternoon the winds should shift back to the west and start increasing ahead of another winter storm. The forecast calls for this storm to arrive over the area tomorrow. This storm should bring another round of strong southwest winds with average speeds in the the 70's and gusts up to 120 at the upper elevations. Heavy snowfall and falling temperatures will also accompany this storm tomorrow. Another 6 to 12 inches of snow may accumulate during the day tomorrow, and snowfall rates could approach 2-3 inches per hour between 10am and 4pm. Check in with the Reno NWS for more details. 

Weather observations from along the Sierra Crest between 8200 ft. and 8800 ft.
0600 temperature: 13 to 20 deg. F.
Max. temperature in the last 24 hours: 18 to 26 deg. F.
Average wind direction during the last 24 hours: Southwest shifting to NE
Average wind speed during the last 24 hours: 10 to 20 mph
Maximum wind gust in the last 24 hours: 34 mph
New snowfall in the last 24 hours: 2 to 4 inches
Total snow depth: 28 to 36 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Saturday Saturday Night Sunday
Weather: Mostly cloudy Cloudy with a 20% chance of snow Snow
Temperatures: 30 to 35 deg. F. 26 to 31 deg. F. 30 to 36 deg. F. in the morning then falling to 20 to 25 deg. F.
Wind Direction: Northwest shifting to west in the afternoon West shifting to southwest Southwest
Wind Speed: 10 to 15 mph increasing to 10 to 20 mph with gusts to 30 mph in the afternoon 10 to 20 mph increasing to 20 to 30 mph with gusts to 50 mph 40 to 50 mph with gusts to 80 mph
Expected snowfall: 0 in. 0 in. 6 to 12 in.
For 8000 ft. to 9000 ft.
Saturday Saturday Night Sunday
Weather: Mostly cloudy Cloudy with a 20% chance of snow Snow
Temperatures: 26 to 33 deg. F. 25 to 30 deg. F. 27 to 34 deg. F. in the morning then falling to 18 to 23 deg. F.
Wind Direction: North shifting to west in the afternoon West Southwest
Wind Speed: 20 to 30 mph with gusts to 40 mph increasing to 30 to 40 mph with gusts to 60 mph in the afternoon 50 to 60 mph with gusts to 85 mph 70 to 80 mph with gusts to 120 mph
Expected snowfall: 0 in. 0 in. 6 to 12 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.