THIS AVALANCHE ADVISORY EXPIRED ON December 11, 2015 @ 7:00 am
Avalanche Advisory published on December 10, 2015 @ 7:00 am
Issued by Andy Anderson - Tahoe National Forest

CONSIDERABLE avalanche danger exists on slopes steeper than 32 degrees on NW-N-NE-E-SE aspects due to a combination of new wind slabs and persistent slabs. Dangerous avalanche conditions exist. Careful snowpack evaluation, cautious route-finding and conservative decision-making are essential. Large human triggered avalanches will be likely today and some natural avalanches will be possible.

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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Intense snowfall and strong winds will build wind slabs on the NW-N-NE-E-SE aspects faster than those new wind slabs can form bonds within themselves or with the old snow surfaces. Human triggering of these wind slabs will be likely today and natural avalanches involving these wind slabs will be possible especially during the periods of heaviest snowfall this morning and tonight. Wind slabs will accumulate on wind loaded and cross loaded slopes and clues like blowing snow, cornices above a slope, pillows of drifted snow, and other wind created textures can help identify where these fragile wind slabs exist. While open near and above treeline terrain will represent the most common locations for new wind slabs, they could also form in some more traditionally sheltered areas such as open slopes in below treeline terrain due to the strength of the winds at all elevations during this storm. Wind slabs could easily grow to more than 3 ft in depth today as new snow and wind combine to multiply the accumulation amounts. If these wind slabs release on slopes where a persistent weak layer exists in the the old snowpack, they could overload that deeper layer and involve even more snow. 

Avalanche Problem 2: Persistent Slab
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Persistent slabs may form on NW-N-NE aspects today as the new snow accumulates above a snowpack with weak layers of sugary snow in the bottom half of the snowpack. Data indicates that these facet layers remain weak. The additional load of new snow may push them so close to the breaking point that human triggered avalanches resulting from the failure of these persistent weak layers will become possible today. In many areas the persistent slabs remain confined by anchors poking through the buried weak layers, so these avalanches may not fracture over large areas. However, these persistent slab avalanches could fracture across wider areas and involve much more snow on slopes with smooth ground cover. Cracking, collapsing, whumphing, and recent avalanche activity can all provide clues as to where these persistent slabs may be an issue, but they could still occur on slopes without those clues. Digging into the snowpack to find this persistent weak layer and avoiding slopes where it exists, traveling on lower angle terrain, or traveling on well anchored terrain represents the best way to avoid this avalanche problem.

recent observations

Observations from Flagpole Peak and Mt Judah yesterday and Tamarack Peak the day before all revealed a snowpack that consists of melt-freeze crusts with softer snow sandwiched between the crusts. Near the bottom of the snowpack these softer snow layers remain weak and sugary (facets). Many snowpit tests on these layers over the last few days have indicated that if these layers break the resulting fracture could travel along these layers. Observations have also shown that these weaker layers have become moist during the warm weather. Numerous rocks, stumps, logs, and other obstacles / anchors still interrupt these layers in most places. Data collected over the last several days has revealed these weak facets exist on NW-N-NE aspects at elevations ranging from 7600 ft. to 9800 ft. 

Weather and CURRENT CONDITIONS
weather summary

A band of rain showers moved through the northern part of the forecast area yesterday and added about .2 inches of rain to the snowpack. The southwest winds increased overnight and averaged 40 to 50 mph with a gust of 109 mph recorded by one sensor in the past 24 hours. Steady precipitation started north of Tahoe City around midnight last night and fell mostly as rain since snow levels started in the 8000 to 9000 ft. range. So far .6 to 1 inch of total precip has fallen in the northern part of the forecast area. Heavy snow and hurricane force winds will impact the forecast area starting in the north and progressing southward today. Snow levels have fallen to 7500 - 8500 ft. this morning and should drop to around 5500 ft. by midday. The forecast calls for the most intense snowfall and winds this morning with snowfall rates approaching 2-3 inches per hour. 8 to 16 inches of new snow could accumulate today. The storm should weaken some this afternoon before another wave of snow reaches the forecast area tonight. The forecast calls for another 6 to 12 inches of new snow accumulation above 7000 ft. tonight. Temperatures should continue to fall throughout the storm. As the storm exits the region tomorrow, winds and snow showers should decrease, but the cold temperatures and mostly cloudy skies should remain.  

Weather observations from along the Sierra Crest between 8200 ft. and 8800 ft.
0600 temperature: 30 to 38 deg. F.
Max. temperature in the last 24 hours: 36 to 46 deg. F.
Average wind direction during the last 24 hours: Southwest
Average wind speed during the last 24 hours: 40 to 50 mph
Maximum wind gust in the last 24 hours: 109 mph
New snowfall in the last 24 hours: North of Tahoe City: .6-1 inch of rain - Snow accumlation after 4 am 1-2 inches
Total snow depth: 13 to 20 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Thursday Thursday Night Friday
Weather: Rain changing to snow in the morning Snow Cloudy becoming mostly cloudy. 40% chance of snow showers.
Temperatures: 32 to 39 deg. F. 15 to 22 deg. F. 22 to 29 deg. F.
Wind Direction: Southwest Southwest West
Wind Speed: 60 to 65 mph with gusts to 100 mph in the morning decreasing to 45 to 50 mph with gusts to 75 mph in the afternoon 20 to 30 mph with gusts to 45 mph 15 to 25 mph with gusts to 40 mph
Expected snowfall: 8 to 14 in. 6 to 10 in. up to 2 in.
For 8000 ft. to 9000 ft.
Thursday Thursday Night Friday
Weather: Rain changing to snow in the morning Snow Cloudy becoming mostly cloudy. 45% chance of snow showers.
Temperatures: 29 to 36 deg. F. 11 to 18 deg. F. 18 to 25 deg. F.
Wind Direction: Southwest Southwest West
Wind Speed: 70 to 80 mph with gusts to 120 mph 40 to 50 mph with gusts to 75 mph 40 to 45 mph with gusts to 65 mph decreasing to 25 to 30 mph with gusts to 50 mph in the afternoon.
Expected snowfall: 8 to 16 in. 6 to 12 in. 1 to 3 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.