THIS AVALANCHE ADVISORY EXPIRED ON January 25, 2016 @ 6:59 am
Avalanche Advisory published on January 24, 2016 @ 6:59 am
Issued by Brandon Schwartz - Tahoe National Forest

Areas of MODERATE avalanche danger remain near and above treeline on NW-N-NE-E-SE aspects on slopes 35 degrees and steeper due to recently formed wind slabs. Below treeline, there is LOW avalanche danger with pockets of MODERATE avalanche danger in open areas below treeline and near treeline on N-NE-E aspects on slopes 32 degrees and steeper due to deep persistent slabs.

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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Lingering wind slabs that formed from the most recent storm remain in near and above treeline areas on NW-N-NE-E-SE aspects. Human triggered avalanches 12 to 18 inches deep occurred yesterday in some wind loaded areas (more info below). Wind slabs will be more difficult to trigger today than yesterday, but could still bury or injure a person. Use clues such as recent avalanche activity, cornice formations, wind pillows, and snow surface texture to identify suspect slopes.

Avalanche Problem 2: Deep Slab
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Areas where deep persistent slabs remain a problem are becoming more isolated. Buried surface hoar layers are now 2.5 to 4.5 feet deep in the snowpack and remain unstable in some areas. A widespread persistent slab avalanche cycle occurred Jan 5 through 19 as these weak layers collapsed. Distribution of these weak layers are changing. In some areas, avalanche activity has cleaned them out, in some areas they have already collapsed or been compressed and are no longer reactive, in other areas the buried surface hoar crystals are rounding and gaining strength. In some areas snowpit data indicates that instability is ongoing. Open areas near treeline and below treeline on N-NE-E aspects is where this avalanche problem is found, not up near the ridgetops in the traditional avalanche start zones.

Even though avalanche occurrence has become unlikely, consequences remain high. Due to the depth of the weak layer and density of the snow above, any deep persistent slab avalanches that do occur could be large, destructive, and difficult to survive. As with all weak layers, slab avalanches are most likely to be triggered in areas where the weak layer is closer to the snow surface. This often occurs near isolated rocks and trees. This is certainly the type of avalanche problem where a slope could avalanche after it has already been traveled by numerous individuals who left tracks without finding the shallow spot trigger point. Perform your own evaluation, keeping in mind that previous tracks are not a sign of intelligent life. For more info on managing deep persistent slabs, see the Avalanche Problems Toolbox.

recent observations

Human triggered wind slab avalanches 12 to 18 inches deep occurred yesterday on Andesite Ridge (Donner Summit area) in above treeline areas on NE aspects. A small natural avalanche was observed in a creek bed in terrain south of Carson Pass between Blue Lakes and Indian Valley (more info on these avalanches in the specific obs below).

Encountered snowpack stability varied widely yesterday in the backcountry with some parties seeing little to no signs of instability and others seeing avalanches. Significant amounts of blowing snow and wind transport were observed across the forecast area. Most evidence of instability that was observed occurred within the top 2 feet of the snowpack. In isolated areas, snowpit data continues to point to ongoing deep instability down 2.5 to 4.5 feet where the Jan 5, Jan 9, and Jan 13 buried surface hoar layers linger. Numerous rain crusts now exist within the top 3 feet of the snowpack, especially below 8,000'. Problematic near crust faceting has yet to be reported, but these crusts warrant monitoring for potential weak layer development.

Weather and CURRENT CONDITIONS
weather summary

Partly sunny to mostly cloudy skies are forecast for today ahead of an inside slider type weather system that will focus its impacts on western Nevada. Ridgetop winds remain out of the SW this morning and are decreasing in speed. Light to moderate speed W winds are forecast for today. Light snow showers are likely tonight with little to no accumulation expected. Ridgetop winds are forecast to shift to the N to NE tomorrow and reach moderate speeds during the morning hours tomorrow as high pressure builds over the region.

Weather observations from along the Sierra Crest between 8200 ft. and 8800 ft.
0600 temperature: 20 to 25 deg. F.
Max. temperature in the last 24 hours: 26 to 31 deg. F.
Average wind direction during the last 24 hours: SW
Average wind speed during the last 24 hours: 28 mph
Maximum wind gust in the last 24 hours: 62 mph
New snowfall in the last 24 hours: trace to 1 inches
Total snow depth: 74 to 83 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Sunday Sunday Night Monday
Weather: Partly sunny to mostly cloudy skies with a slight chance of snow showers in the afternoon. Mostly cloudy skies with a chance of snow showers in the evening. A slight chance of snow showers after midnight. Partly cloudy skies, becoming sunny.
Temperatures: 27 to 34 deg. F. 14 to 21 deg. F. 29 to 36 deg. F.
Wind Direction: W W NE
Wind Speed: 10 to 15 mph. Gusts to 25 mph in the morning. 15 to 25 mph with gusts to 35 mph. 15 to 20 mph with gusts to 30 mph.
Expected snowfall: 0 to trace in. 0 to trace in. 0 in.
For 8000 ft. to 9000 ft.
Sunday Sunday Night Monday
Weather: Partly sunny to mostly cloudy skies with a slight chance of snow showers in the afternoon. Mostly cloudy skies with a chance of snow showers in the evening. A slight chance of snow showers after midnight. Partly cloudy skies, becoming sunny.
Temperatures: 23 to 30 deg. F. 14 to 21 deg. F. 27 to 34 deg. F.
Wind Direction: W NW N to NE
Wind Speed: 15 to 20 mph with gusts to 30 mph. 15 to 25 mph with gusts to 40 mph. 20 to 25 mph with gusts to 45 mph, decreasing to 15 to 20 mph with gusts to 35 mph in the afternoon.
Expected snowfall: 0 to trace in. 0 to trace 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.