THIS AVALANCHE ADVISORY EXPIRED ON January 19, 2016 @ 7:02 am
Avalanche Advisory published on January 18, 2016 @ 7:02 am
Issued by Brandon Schwartz - Tahoe National Forest

CONSIDERABLE avalanche danger exists today both above treeline on NW-N-NE-E-SE aspects and near and below treeline on N-NE-E aspects on slopes 32 degrees and steeper due to a combination of wind slabs, storm slabs, and persistent slabs. For all other areas avalanche danger is MODERATE. Natural avalanche activity is possible during the morning hours. Human triggered avalanches will remain possible to likely all day. In some cases, avalanches could be large and destructive.

3. Considerable

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Above Treeline
Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.

3. Considerable

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Near Treeline
Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.

3. Considerable

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Below Treeline
Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.
    Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.
Avalanche Problem 1: Wind Slab
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New wind slabs began forming late yesterday afternoon and evening in near and above treeline areas. Gale force winds were reported as erratic in direction in some areas, with irregular wind loading patterns occurring. The vast majority of new wind slabs are expected to exist near and above treeline on NW-N-NE-E-SE aspects today with some more isolated wind slabs possible above treeline on S-SW-W aspects in areas where wind direction was erratic. Evidence of recently drifted snow in the form of cornice formations, wind pillows, and snow surface patterning will allow for identification of the areas where unstable wind slabs most likely exist.

Avalanche Problem 2: Storm Slab
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As new snow accumulates through the morning hours, new storm slabs may build in wind protected areas on all aspects near treeline and below treeline. New snow is falling on wet snow and weak rain crust below 8,000' and onto a drier old snow surface at the upper elevations. Hand pits will easily identify the strength of new snow bonding to the old snow surface as well as identify any easily shearing weak layers within the new snow.

Avalanche Problem 3: Persistent Slab
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Persistent weak layers of reactive buried surface hoar exist 2.5 to 3.5 feet below the snow surface on some but not all near and below treeline slopes on N-NE-E aspects and potentially in isolated areas on NW and SE aspects. The number of slopes with unstable persistent slabs in diminishing as avalanche activity continues to occur and to some degree eliminate the hazard on specific slopes. In other areas the surface hoar layers have already collapsed or have been compressed and are now less reactive.

In the areas where snowpit tests show lingering unstable buried surface hoar layers, recent rain and new snow has continued to load these weak layers. Avalanche activity failing on these buried surface hoar layers has the potential to be large and destructive with very wide propagation potential. Remote triggering remains possible. Snowpack collapse and signs of past avalanche avalanche activity in the form of crown lines and debris on nearby slopes are some of the obvious clues that may be present in areas of buried surface hoar layers. Otherwise multiple, highly targeted snowpits will be required to gain an insight on this avalanche problem.

 

recent observations

Signs of recent avalanche activity remain widespread in the backcountry. Evidence of a small natural avalanche believed to have been triggered by cornice collapse was observed yesterday near Carson Pass in the Frog Lake Cliffs area on a NE aspect (more info below). The last reported skier triggered avalanche activity occurred on Saturday on a NE aspect on Andesite Ridge in the Donner Summit area (more info below). Explosive control work performed yesterday on a backcountry snowpack on a cross loaded N aspect just above Donner Pass as part of an ongoing SAR mission produced small slab avalanche results on a slope with a start zone thought to be absent of buried surface hoar.

Observations made and received yesterday from the Donner Pass, Mount Rose Summit, Barker Pass, Luther Pass, and Carson Pass areas indicated that where buried surface hoar layers are found, snowpit tests produce either unstable results or mixed, less clear results. In areas where buried surface hoar layers are absent or have already been collapsed or compressed, little to no evidence of instability was observed in the snowpits.

In areas below about 8,000' where rain has fallen on the snowpack in recent days, the snow surface remained wet yesterday, with some new crust formation occurring as the day progressed.

Snowpit data from the Mount Rose Summit and Carson Pass areas yesterday also indicated some potentially problematic faceted snow layers 2 to 3 feet below the snow surface on NE aspects in the 8,700' to 9,200' elevation range.

Weather and CURRENT CONDITIONS
weather summary

A weather system is currently moving through the region. Peak snowfall intensity is occurring during the pre dawn hours this morning with snow level around 5,500'. Snowfall is expected to taper off from north to south, becoming light showers in most locations by 9 or 10 am. Locations along the Sierra Crest have seen the greatest snowfall accumulation at 4 to 6 inches thus far. Snowfall totals along the Sierra Crest will likely match north and south by late morning with 6 to 12 inches for many areas. Snowfall totals will decrease to the east of the Sierra Crest. Ridgetop winds remain out of the SW and are decreasing in speed this morning. Gale force winds over the upper elevation ridgetops yesterday afternoon and last night have decreased to moderate in speed. Maximum daytime air temperatures will reach the upper 20s to mid 30s today for areas above 7,000'. The next storm system will arrive tomorrow with additional snowfall, gale force SW winds. Rising snow levels are expected tomorrow, currently forecast to reach 6,000' to 6,500'.

Weather observations from along the Sierra Crest between 8200 ft. and 8800 ft.
0600 temperature: 27 to 30 deg. F.
Max. temperature in the last 24 hours: 32 to 35 deg. F.
Average wind direction during the last 24 hours: SW
Average wind speed during the last 24 hours: 45 mph
Maximum wind gust in the last 24 hours: 91 mph
New snowfall in the last 24 hours: 2 to 6 inches
Total snow depth: 59 to 63 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Monday Monday Night Tuesday
Weather: Cloudy skies, becoming mostly cloudy. Snow tapering by mid morning. Cloudy skies with a slight chance of snow showers after midnight. Cloudy skies with snow likely.
Temperatures: 32 to 37 deg. F. 22 to 27 deg. F. 32 to 37 deg. F.
Wind Direction: SW SW SW
Wind Speed: 10 to 20 with gusts to 35 mph, decreasing to 10 to 15 mph with gusts to 25 mph in the afternoon. 15 to 25 mph with gusts to 35 mph. 25 to 35 mph with gusts to 50 mph, increasing to 35 to 45 mph with gusts to 70 mph in the afternoon.
Expected snowfall: 3 to 5 in. 0 to trace in. 3 to 8 in.
For 8000 ft. to 9000 ft.
Monday Monday Night Tuesday
Weather: Cloudy skies, becoming mostly cloudy. Snow tapering by mid morning. Cloudy skies with a slight chance of snow showers after midnight. Cloudy skies with snow likely.
Temperatures: 27 to 32 deg. F. 20 to 26 deg. F. 27 to 32 deg. F.
Wind Direction: W SW SW
Wind Speed: 25 to 35 mph with gusts to 50 mph, decreasing to 20 to 25 mph with gusts to 40 mph in the afternoon. 20 to 25 mph with gusts to 40 mph, increasing to 30 to 35 mph with gusts to 50 mph. 50 to 55 mph with gusts to 85 mph, increasing to 65 to 70 mph with gusts to 105 mph in the afternoon.
Expected snowfall: 3 to 5 in. 0 to trace in. 3 to 8 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.