THIS AVALANCHE FORECAST EXPIRED ON December 12, 2018 @ 6:59 am
Avalanche Forecast published on December 11, 2018 @ 6:59 am
Issued by Andy Anderson - Tahoe National Forest - Sierra Avalanche Center

New wind slabs could have formed during the night and old wind slabs with a fragile weak layer (surface hoar) below them may still linger on isolated slopes in very wind-exposed near and above treeline terrain. The avalanche danger is MODERATE. Human triggered avalanches may be possible today.

2. Moderate

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

2. Moderate

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

1. Low

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Below Treeline
Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.
    Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
Avalanche Problem 1: Wind Slab
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Fragile wind slabs that formed a few days ago may still linger on isolated NW-N-NE-E aspects in near and above treeline terrain. Northeast winds during the night may have been strong enough to create new small wind slabs on the NW-W-SW-S-SE aspects in the most NE wind exposed areas. Very exposed upper elevation terrain and complex terrain like couloirs, cliffy areas, and areas with multiple intersecting steep slopes represent the best places to find one of these unstable wind slabs. In some cases, these wind slabs formed on top of weak feathery snow known as surface hoar. Most avalanches resulting from these wind slabs should remain small. Small avalanches can have serious consequences especially in complex terrain or during early season with all the obstacles lurking near the snow surface. It is not impossible for a wind slab avalanche to involve enough snow to bury or injure a person (size D2) in isolated areas. 

Signs that wind slabs may exist on a slope include cornices above a slope, wind drifts/pillows, ripples/wind-created textures on the snow surface, hollow sounding firm snow, and blowing snow. Buried surface hoar as a weak layer will make any wind slabs that do exist more difficult to manage. Surface hoar can allow remote triggering of avalanches and can allow avalanches to propagate across wider areas and run farther downslope. If there are any doubts or uncertainty about the existence of an unstable wind slab on a slope, assume the slope is unstable and avoid it. Avoiding wind-loaded terrain where problematic wind slabs may reside represents a great way to manage today's potential avalanche problem and to enjoy the cold soft snow that still remains in many more-sheltered, non-wind-exposed areas.

recent observations
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* Tests on wind-exposed slopes on Janine's Peak (Desolation Wilderness) indicated that small wind-slabs resting above weak surface hoar remain fragile. 

* Observers on Becker Peak (Echo Summit) found the 12/4 surface hoar layer buried 4 to 6 inches below the surface but did not find any signs of a slab layer above it. 

* On Maggies Peak (Emerald Bay), observations did not find any slab layers or buried surface hoar, but the upper few inches of the snowpack has become much less cohesive and some loose dry sluffing occurred in response to snowboard cuts.

* Observations from Relay Peak (Mt. Rose Backcountry) indicated that the buried surface hoar layer was decomposing in that area. Minor evidence of wind transport existed near the summit, but no wind slab formation had occurred. 

** A skier was knocked off their feet above a cliff by a small slab avalanche on Mt. Tallac on Sunday. They went over the cliff and suffered serious injuries as a result of the 200 to 300 ft. fall (some information about the rescue can be found here).

* Reasonable snow coverage exists above 7,000' to 7,500' along the Sierra Crest in most places. On the east side of the lake coverage is not as good but still adequate for careful recreation. Early season conditions still exist and even in areas with ok coverage numerous barely covered or still exposed obstacles remain.

Weather and CURRENT CONDITIONS
weather summary

The winds shifted to the NE during night as yesterday's weak system left the area. They should start to decrease some today before they shift back to the west and southwest tonight as another weak cold front moves through the area. The forecast calls for little to no precipitation with this system.  Partly cloudy skies and light to moderate winds should continue over the region through tomorrow.

CONDITIONS ALONG THE SIERRA CREST BETWEEN 8200 FT. AND 9200 FT. IN THE LAST 24 HOURS
Weather Station Map
Weather Station Table
6am temperature: 19 to 26 deg. F.
Max. temperature: 31 to 41 deg. F.
Average ridgetop wind direction: SW to NE
Average ridgetop wind speed: 15 to 20 mph
Maximum ridgetop wind gust: 39 mph
New snowfall: up to 1 inches
Total snow depth: 22 to 34 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Tuesday Tuesday Night Wednesday
Weather: Partly cloudy. Snow levels below 7000 feet. Chance of precipitation is 5%. Partly cloudy then becoming mostly cloudy. Snow levels below 7000 feet. Chance of precipitation is 10%. Mostly cloudy then becoming partly cloudy. Slight chance of snow in the morning. Snow levels below 7000 feet. Chance of precipitation is 10%.
Temperatures: 38 to 43. deg. F. 20 to 26. deg. F. 34 to 39. deg. F.
Mid Slope Winds: Light winds. Light winds becoming southwest 10 to 15 mph with gusts to 40 mph after midnight. West 10 to 15 mph with gusts to 35 mph in the morning becoming light.
Expected snowfall: No accumulation. | SWE = none. No accumulation. | SWE = none. 90% probability no accumulation. 10% probability up to 1 inch. | SWE = none.
For 8000 ft. to 9000 ft.
Tuesday Tuesday Night Wednesday
Weather: Partly cloudy then becoming sunny. Snow levels below 7000 feet. Chance of precipitation is 5%. Partly cloudy then becoming mostly cloudy. Snow levels below 7000 feet. Chance of precipitation is 10%. Mostly cloudy then becoming partly cloudy. Snow levels below 7000 feet. Chance of precipitation is 10%.
Temperatures: 35 to 41. deg. F. 19 to 24. deg. F. 31 to 36. deg. F.
Ridge Top Winds: Northeast 10 to 15 mph with gusts to 30 mph. West 15 to 25 mph. Gusts up to 30 mph increasing to 55 mph after midnight. North 15 to 30 mph with gusts to 65 mph.
Expected snowfall: No accumulation. | SWE = none. No accumulation. | SWE = none. 90% probability no accumulation. 10% probability up to 1 inch. | SWE = none.
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