THIS AVALANCHE ADVISORY EXPIRED ON December 28, 2016 @ 7:00 am
Avalanche Advisory published on December 27, 2016 @ 7:00 am
Issued by Brandon Schwartz - Tahoe National Forest

Areas of MODERATE avalanche danger exist at all elevations on slopes 32 degrees and steeper due to a combination of wind slabs and persistent slabs. Several human triggered avalanches occurred yesterday with signs of persistent instability present in other areas. Human triggered avalanches are possible again today in a variety of areas. Isolated natural avalanche activity is unlikely but not impossible today. Exercise caution and conservative decision making during backcountry travel.

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.

2. Moderate

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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
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A new round of wind slab formation will add to existing avalanche problems today. Prior to last night's increase in winds, 4 to 12 inches of unconsolidated snow was on the ground on all aspects and available for wind transport. Redistribution and subsequent wind loading is expected to have occurred last night and continue today. New wind slabs will be most prevalent in near and above treeline terrain on NW-N-NE-E-SE aspects. New wind slabs may form in more isolated areas on S aspects as well. In some locations these new wind slabs will form of top of older wind slabs that remained unstable yesterday.

Identify areas of recent wind loading and avoid steep or consequential slopes that likely hold wind slabs. Many clues exist to identify the likely location of wind slabs. Looking for areas of blowing snow. Look for rough or scoured snow surfaces on windward aspects. Look for cornices, wind pillows, and areas of smooth snow in leeward areas to identify where wind slab development has occurred.

Avalanche Problem 2: Persistent Slab
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Much of the instability observed yesterday was associated with old storm slabs and in some cases wind slabs that have now become persistent slabs. In many but not all areas, faceted snow sitting above the Dec 15 rain crust and below recent storm snow is a problematic weak layer in near treeline and below treeline areas on W-NW-N-NE-E aspects. Locations were avalanches have occurred on this weak layer are frequently low on the slope and away from the traditional ridgetop wind slab start zones. The icy Dec 15 rain crust is generally easily identifiable at the bed surface.

Managing persistent slab problems is more challenging than managing wind slab or within new snow avalanche problems. Time will tell, but as recent storm snow above the faceted weak layer becomes increasingly cohesive and slab like, the potential exists for instability to become more widespread and avalanches to possibly become more easily triggered. Take home point: Anticipate avalanche activity lower on the slope than usual. Adjust travel tactics to account for avalanches in less than typical places. Think slope angle, not historical start zone. Don't accept previous tracks or the passage of time as the sole argument for the absence of instability.

recent observations

Four human triggered avalanches were reported yesterday on the N aspect of Ralston Peak (Echo Summit area). One person was caught and carried. Three of the avalanches occurred low on the slope. Details are limited but at least some of the avalanches are reported to have failed on top of the Dec 15 rain crust. On Andesite Peak (Donner Summit area), snowpit work at the site of the Dec 24 avalanches revealed ongoing signs of instability on faceted snow on top of the Dec 15 rain crust. Similar instability was noted on W aspects in the area. Snowpit work at Upper Blue Lake, Tamarack Junction, and Indian Valley (Blue Lakes/Carson Pass area) revealed less instability than was present at the same sites on Dec 24. Observations received from National Geographic Bowl (Backcountry terrain on Granite Chief, outside the boundary of Squaw Valley) did not reveal signs of unstable slabs.

Weather and CURRENT CONDITIONS
weather summary

A weather system is passing to the north of the forecast area this morning.  Ridgetop winds out of the SW to W increased overnight to moderate in speed. Mid and high level cloud cover has also increased. Cloud cover is expected to decrease as the day progresses, but winds will continue. Ridgetop gusts are forecast to reach 60 to 75 mph today through this evening. Maximum daytime air temperatures are expected to climb above freezing for nearly all areas today. For tomorrow, expect decreasing winds and continued warming air temperatures at all elevations.

Weather observations from along the Sierra Crest between 8200 ft. and 8800 ft.
0600 temperature: 27 to 33 deg. F.
Max. temperature in the last 24 hours: 29 to 41 deg. F.
Average wind direction during the last 24 hours: SW
Average wind speed during the last 24 hours: Prior to 6 pm yesterday: 4 mph | Since 6 pm yesterday: 20 mph
Maximum wind gust in the last 24 hours: Prior to 6 pm yesterday: 9 mph | Since 6 pm yesterday: 51 mph
New snowfall in the last 24 hours: 0 inches
Total snow depth: Along the Sierra Crest: 28 to 43 inches | In the Mt. Rose area: 59 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Tuesday Tuesday Night Wednesday
Weather: Mostly cloudy skies, becoming partly cloudy. Partly cloudy skies. Sunny skies.
Temperatures: 36 to 41 deg. F. 16 to 22 deg. F. 40 to 45 deg. F.
Wind Direction: SW W Variable
Wind Speed: 10 to 15 mph in the morning, becoming light. 10 to 15 mph with gusts to 45 mph in the evening, becoming light. Light winds
Expected snowfall: 0 in. 0 in. 0 in.
For 8000 ft. to 9000 ft.
Tuesday Tuesday Night Wednesday
Weather: Mostly cloudy skies, becoming partly cloudy. Partly cloudy skies. Sunny skies.
Temperatures: 34 to 39 deg. F. 17 to 22 deg. F. 38 to 44 deg. F.
Wind Direction: W W W
Wind Speed: 20 to 30 mph with gusts to 60 mph. 20 to 35 mph. Gusts to 75 mph decreasing to 60 mph after midnight. 15 to 25 mph with gusts to 45 mph in the morning, becoming light.
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