THIS AVALANCHE ADVISORY EXPIRED ON December 18, 2016 @ 7:00 am
Avalanche Advisory published on December 17, 2016 @ 7:00 am
Issued by Andy Anderson - Tahoe National Forest

MODERATE avalanche danger exists due to possible wind slabs on all aspects above treeline and on NW-N-NE-E-SE aspects in near and below treeline terrain. Some unlikely but not impossible deep slabs may still linger on high elevation NW-N-NE aspects on slopes steeper than 32 degrees as well. At elevations below 9000 ft. less avalanche danger exists, but firm icy slopes with a dusting of snow may exist on all aspects.

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

East and northeast winds may transport some snow today and wind load some of the W-SW-S aspects in above treeline terrain. These newly formed wind slabs should remain relatively shallow, not extend very far down-slope, and be limited to upper elevation areas with available snow where the wind may be strong enough to transport snow.  They will form on firm crusts and could be fragile today. Even though they should not grow very large these small slabs could knock a person off his/her feet. In addition to these small newly formed wind slabs, some larger, more consequential wind slabs that formed on the wind-loaded N-NE-E aspects and cross-loaded NW and SE aspects during the storm may also remain. The largest of these wind slabs will exist above 9000 ft. along the Sierra Crest, in the Carson Range, and in the Mt. Rose area in places that received more snow than rain during the storm. While these wind slabs should have become more difficult to trigger in many places, human triggering of these wind slabs may remain possible in specific areas like the most heavily wind loaded slopes, couloirs, unsupported slopes above cliff bands, and other complex or extreme terrain. The cornices that exist above many of these wind loaded slopes will also remain fragile today, and if they break they could provide large triggers for the slopes below them. If these wind slabs release on northerly aspects above 9000 ft, the possibility remains for them to provide a large enough trigger to initiate a deep slab avalanche

Clues like blowing snow, cornices, snow surface scouring, hollow sounding snow, and wind pillows can help identify which slopes may be wind loaded and where wind slabs may exist. Give cornices a wide berth since they can break well away from their edges. 

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

Persistent weak layers still exist near the base of the snowpack on NW-N-NE aspects above 9000 ft. Trigging a deep slab avalanche that fails on a persistent weak layer near the base of the snowpack may have become unlikely due to the strength of the snow above the weak layer in many areas. However, if this layer does break, fractures can still travel along the layer and the resulting avalanche would be large, destructive, and likely unsurvivable. Even though these deep slab avalanches may have become difficult to trigger a person's weight above a trigger point where a shallower snowpack exists or a larger/deeper trigger may be able to break this layer in some isolated areas. Trigger points could include areas near rocks or cliffs, areas where a shallow snowpack exists, buried stumps, trees, convex rollovers, and areas where the weak layer is weakest. Large/deep triggers could include other avalanches, cornice failures, multiple people on a slope, stuck snowmobiles, or even boot-packing. 

Due to the consequences associated with the large destructive avalanches that could occur if the persistent weak layer fails and the high degree of uncertainty associated with it, this unlikely but not impossible avalanche problem still warrants extra caution. These avalanches could connect multiple start zones, could be triggered remotely, and could run farther than expected. When a persistent weak layer like this exists, most informal observations will not provide reliable information about slope stability. Since these avalanches may not fail until someone hits the right spot on the slope, multiple tracks could exist on the slope before it slides. Snowpits can determine if the layer exists and tests can help to see how weak it is, but remember, snowpit work never proves stability, it shows potential signs of instability. 

recent observations

Yesterday observations from Relay Peak and Tamarack Peak in the Mt. Rose backcountry showed 18 to 24 inches of new snow sitting on top of a refreezing rain crust. A slightly more dense layer of snow existed near the top of this recent snow. In wind loaded areas, large tender cornices existed and ski cuts on wind loaded test slopes did produce cracking on newly forming 4 to 6 inch wind slabs. Below the upper rain crust, a variety of old snow existed including the weak layer of facets near the base of the snowpack. Snowpit tests on a N aspect on Tamarack Peak indicated weakness in the snowpack just below the upper rain crust that fractures could propagate along and that humans on the surface could still break. Tests targeting the deeply buried facet layer that exists near the base of the snowpack indicated that it would be difficult to break this layer but that if it does break fractures could still travel along the facets.

Across the Lake on the Sierra Crest, much less new snow existed below 9000 ft. with observations from Andesite Peak showing only 4 to 5 inches of new snow at 8200 ft. and observations on Carson Pass near Frog Lake showed about 12 inches of new snow around 8700 ft. In both these areas, the new snow existed on top of dense wet snow that had not refrozen yet. Ski kicks on wind loaded test slopes in both these areas did produce some shooting cracks. On Andesite Peak the wind slabs did not extend very far down-slope and measured 6 to 8 inches in depth, while on Carson Pass larger deeper wind slabs existed. Snowpit data from Andesite found a shallow snowpack with moist snow to the ground. Near Frog Lake a deeper snowpack existed with evidence of facets in the bottom half of the snowpack. Observations from Red Lake Peak reported, "shallow sastrugi or bare ground on windward aspects, and heavy wind loading and active wind transport onto leeward aspects." This party also reported seeing "avalanche activity on E aspects both off of Red Lake Peak and the ridge toward Little Round Top."

Weather and CURRENT CONDITIONS
weather summary

Winds shifted to the east and northeast last night as a high-pressure ridge started to move into the area. These light to moderate N-NE-E winds should continue through tomorrow. The high-pressure combined with a mass of cold air should allow cold dry weather to prevail over the region. The forecast calls for sunny skies with daytime highs in the 20's today and tomorrow with overnight lows in the single digits above 7000 ft. Expect inversion conditions to exist as cold air gets trapped in the valleys. 

Weather observations from along the Sierra Crest between 8200 ft. and 8800 ft.
0600 temperature: 5 to 11 deg. F.
Max. temperature in the last 24 hours: 18 to 29 deg. F.
Average wind direction during the last 24 hours: Southwest shifting to northeast
Average wind speed during the last 24 hours: 20 to 25 mph
Maximum wind gust in the last 24 hours: 59 mph
New snowfall in the last 24 hours: 1 to 3 inches
Total snow depth: Along the Sierra Crest: 20 to 35 inches | In the Mt. Rose area: 58 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Saturday Saturday Night Sunday
Weather: Sunny Clear Sunny
Temperatures: 24 to 29 deg. F. 7 to 13 deg. F. 25 to 30 deg. F.
Wind Direction: North West shifting to the northeast after midnight East
Wind Speed: Light winds with gusts to 30 mph in the morning Light Light winds with gusts to 25 mph in the afternoon
Expected snowfall: 0 in. 0 in. 0 in.
For 8000 ft. to 9000 ft.
Saturday Saturday Night Sunday
Weather: Sunny Clear Sunny
Temperatures: 22 to 27 deg. F. 11 to 16 deg. F. 25 to 30 deg. F.
Wind Direction: North West shifting to the northeast after midnight East
Wind Speed: 15 to 25 mph with gusts to 50 mph decreasing to gusts to 35 mph in the afternoon 10 to 15 mph with gusts to 40 mph decreasing to gusts to 30 mph after midnight 15 to 25 mph with gusts to 35 mph increasing to gusts to 45 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.

For a recorded version of the Avalanche Advisory call (530) 587-3558 x258