THIS AVALANCHE ADVISORY EXPIRED ON February 1, 2014 @ 7:00 am
Avalanche Advisory published on January 31, 2014 @ 7:00 am
Issued by Andy Anderson - Tahoe National Forest

Pockets of CONSIDERABLE avalanche danger may still exist on slopes steeper than 30 degrees in near and below treeline terrain on NW-N-NE aspects above 9000 ft. due to recent snow slabs sitting on top of persistent weak layers. Human triggered avalanches could remain likely on these slopes today. MODERATE avalanche danger exists on similar slopes on those aspects below 9000 ft. due to buried wet persistent weak layers. MODERATE avalanche danger also exists and on the wind loaded NW-N-NE-E-SE aspects steeper than 35 degrees in near and above treeline terrain due to the presence of wind slabs.

The unusual conditions that exist in the mountains right now require extra caution and superb travel skills.

2. Moderate

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

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: Wet Slab
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Human triggered wet slab avalanches remain possible on near and below treeline NW-N-NE aspects due to the wet and weak facet layers buried under the new snow. During the storm rain soaked these persistent weak layers up to at least 8800 ft. These rain soaked layers will remain very weak until the water in them refreezes. Even though air temperatures have dropped below freezing, an insulating blanket of new snow now exists above these wet facet layers, so they may take a few days to refreeze. Once they do refreeze they will gain strength. Until that time wet persistent slabs will remain a problem. These avalanches could involve the entire snowpack. Fortunately as the wet layers refreeze not only will the snowpack stability improve, but the on the snow recreation quality will also improve.

Avalanche Problem 2: Persistent Slab
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In higher elevation areas, where rain did not penetrate to the ground and saturate the old persistent weak layers, human triggered avalanche activity may remain likely today. New snow slabs resting above old persistent weak layers that could barely support themselves prior to the storm create serious avalanche conditions. These kind of avalanches can be triggered from a distance or from "safe" slopes connected to steeper terrain. These avalanches could also involve the entire snowpack.

Based on snow levels during the storm this avalanche problem should only exist on NW-N-NE aspects in near and below treeline terrain above 9000 ft. that had previous snow cover. Since these limitations greatly restrict the areas where this problem could exist, it remains an isolated and "pockety" problem. Digging snowpits to determine if the persistent weak layers exist below the surface and whether or not they have been soaked by rain is a must to determine where this problem could remain. We have very limited field data concerning locations where the persistent weak layers may remain dry; therefore, confidence is lower and uncertainty is higher concerning where and how widespread this problem may still be.

Avalanche Problem 3: Wind Slab
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Some fragile wind slabs may still linger on the near and above treeline NW-N-NE-E-SE aspects. Human triggered avalanches involving these wind slabs remain possible. On the NW-N-NE aspects where these slabs exist above the old persistent weak layers, they will behave more like the persistent slabs and wet persistent slabs discussed above.

recent observations

Yesterday observations from Silver Peak and Chickadee Ridge (Mt. Rose backcountry) showed a similar snowpack structure: 7 to 12 inches of new snow resting on top of wet facets. In both of these areas human-triggered collapsing and cracking occurred as the weight of a person on top of the snow caused the wet facets to fail. Snowpit tests also indicated instability and some tests showed that fractures can still travel through the wet facet layers. Observers on skis sank through the new snow into the rain soaked facets and would sink to the ground if they stepped off their skis. A large loose wet snow avalanche had naturally released during the early morning hours yesterday on Silver Peak, but we did not see any other avalanche activity nor did observers did not report any.

Weather and CURRENT CONDITIONS
weather summary

In the last 24 hours, another 3 to 5 inches of new snow accumulated in areas north of Emerald Bay. South of Emerald Bay 4 to 9 inches accumulated. Most of this snow occurred yesterday morning with another smaller burst of snowfall happening last night. The winds decreased yesterday and shifted to the North and Northeast last night. The forecast calls for the light to moderate Northeast and North winds to continue through tomorrow. Some scattered snow showers may continue today under cloudy to partly cloudy skies especially in areas south of Hwy 50, and temperatures should remain cool in the 20's. As the low pressure moves farther away from the region, skies should begin to clear. By tomorrow a high pressure ridge should reform over the area bringing sunny skies and slightly warmer temperatures. 

Weather observations from along the Sierra Crest between 8200 ft. and 8800 ft.
0600 temperature: 16 to 21 deg. F.
Max. temperature in the last 24 hours: 29 to 31 deg. F.
Average wind direction during the last 24 hours: Before midnight: West | After midnight: Northeast
Average wind speed during the last 24 hours: 10 to 15 mph
Maximum wind gust in the last 24 hours: 33 mph
New snowfall in the last 24 hours: North of Emerald Bay: 3 to 5 inches | South of Emerald Bay: 4 to 9 inches
Total snow depth: 15 to 34 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Friday Friday Night Saturday
Weather: Cloudy with a slight chance of scattered snow showers in the morning. Becoming partly cloudy in the afternoon. Areas south of Hwy 50 have the best chances for snow showers. Partly cloudy with a slight chance of snow showers in the evening. Becoming clear overnight. Sunny
Temperatures: 23 to 30 deg. F. 11 to 17 deg. F. 25 to 32 deg. F.
Wind Direction: Northeast North Variable
Wind Speed: 10 to 15 mph with gusts to 25 mph in the morning 10 to 15 mph with gusts to 25 mph Light
Expected snowfall: North of Hwy 50: 0 in. | South of Hwy 50: up to 1 in. 0 in. 0 in.
For 8000 ft. to 9000 ft.
Friday Friday Night Saturday
Weather: Cloudy with a slight chance of scattered snow showers in the morning. Becoming partly cloudy in the afternoon. Areas south of Hwy 50 have the best chances for snow showers. Partly cloudy with a slight chance of snow showers in the evening. Becoming clear overnight. Sunny
Temperatures: 18 to 25 deg. F. 7 to 14 deg. F. 20 to 28 deg. F.
Wind Direction: Northeast North Northeast
Wind Speed: 15 to 20 mph with gusts to 30 mph 15 to 20 mph with gusts to 35 mph 10 to 15 mph in the morning becoming light in the afternoon
Expected snowfall: North of Hwy 50: 0 in. | South of Hwy 50: up to 1 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.