THIS AVALANCHE ADVISORY EXPIRED ON January 24, 2017 @ 6:58 am
Avalanche Advisory published on January 23, 2017 @ 6:58 am
Issued by Andy Anderson - Tahoe National Forest

CONSIDERABLE avalanche danger exists at all elevations due to widespread wind slabs and storm slabs. Widespread human-triggered avalanches are likely today, and some natural avalanches may remain possible. These avalanches could be large, deep, and destructive. Dangerous avalanche conditions exist. Carefully evaluate the snowpack and terrain before committing to any slopes and plan routes to avoid avalanche problems using conservative terrain choices. CONSIDERABLE is the danger rating where the most avalanche fatalities occur.

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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While the best window for natural avalanches may have occurred last night, large, deep, destructive, and widespread human-triggered wind slab avalanches remain likely today. The wind slabs that formed during the storm are deep and likely have not bonded to the snow below them. Wind slabs could exist on wind-loaded W-NW-N-NE-E-SE aspects at any elevation where wind-loading occurred including below treeline terrain. The largest wind slabs could be considered deep slabs. They could have crowns deeper than 4 to 6 ft and would likely be unsurvivable. The deepest and most fragile wind slabs should exist in near and above treeline terrain.

Blowing snow, cornices, wind drifted snow, and other wind created features and textures can help identify where wind slabs may exist. Avoid those areas and the runout zones of potential wind slab avalanches

Avalanche Problem 2: Storm Slab
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Human-triggered storm slab avalanches will remain likely today. Rapid snowfall and fluctuations during the storm created slab layers and weak layers within the storm snow during the last 24 hours. A distinct density change exists at the bottom of the storm snow and could still remain sensitive to human triggering today. Like the wind slabs mentioned above, natural storm slab avalanches may remain possible today, but most of the natural storm slab avalanches should have occurred last night. The largest of these storm slabs could involve most of the new snow and be several feet in depth. Any steep slopes with new snow on them hold the potential for storm slab avalanches

Clues like cracking, collapsing, and whumphing can help identify where storm slabs may exist as can digging into the snowpack with your hands or probing with a pole to look for layers in the storm snow. Use these clues to avoid steep terrain where storm slabs may exist. Storm slabs can exist on tree covered slopes in the openings between trees and may connect across some of these open spaces. 

recent observations

Yesterday observations in Deep Creek (between Squaw and I80), near Watson Lake and in Negro Canyon (Donner Summit), and in Third Creek (Mt. Rose) all showed a dense heavy layer of snow resting on top of a lighter layer of older snow. In Deep Creek and Third Creek, some storm slab avalanche activity was noted on steep sheltered slopes and likely failed at this interface in the upside down snowpack. Skier triggered cracking occurred in sheltered areas in Deep Creek as well. Near the top of Negro Canyon and in Third Creek, ski cuts and kicks triggered shooting cracks in wind loaded terrain and some natural wind slab and ski cut triggered wind slab activity was observed. A snowmobile cut on a N-NE facing test slope near Watson Lake also triggered a small wind slab avalanche. Snowmobiles also triggered a large whumph in the Watson Lake area with noticeable collapsing on a sheltered low angle slope. Wind loaded slopes in Deep Creek held stiff wind slabs and large but fragile cornices. We also received a report of a natural avalanche on Highway 89 during the night that was large enough to bury cars.

Weather and CURRENT CONDITIONS
weather summary

The first wave of the storm yesterday shifted south allowing for a brief break in the weather and less snowfall during the day than expected. 4 to 6 inches of heavy wet snow fell yesterday before about 2 pm when the second wave of the storm hit the region. Snowfall rates increased dramatically at that time and temperatures cooled down. The forecast area received about 12 to 18 inches overnight bring the 24-hour snow totals to 16 to 24 inches. Snow showers and winds started to decrease after midnight and should continue to diminish today. The forecast calls for another 2 to 4 inches of snow with moderate southwest winds. After a brief warming trend yesterday morning, temperatures have been falling steadily. Expect today's highs to be at least 5 degrees colder than yesterday and tonight's lows could drop into the single digits. As the storm departs the area and high pressure begins to approach the region, the forecast calls for the temperatures to remain cold and for the winds to shift to the northeast tomorrow. 

Weather observations from along the Sierra Crest between 8200 ft. and 8800 ft.
0600 temperature: 15 to 23 deg. F.
Max. temperature in the last 24 hours: 23 to 29 deg. F.
Average wind direction during the last 24 hours: South and southwest
Average wind speed during the last 24 hours: 40 mph
Maximum wind gust in the last 24 hours: 157 mph
New snowfall in the last 24 hours: 16 to 24 inches
Total snow depth: ~ 115 to 157 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 with snow showers likely in the morning then a chance of snow showers in the afternoon Cloudy with a chance of snow showers Mostly cloudy with a chance of snow showers in the morning. Snow showers decreasing in the afternoon.
Temperatures: 21 to 26 deg. F. 6 to 16 deg. F. 22 to 27 deg. F.
Wind Direction: Southwest Southwest Variable
Wind Speed: 10 to 15 mph with gusts to 30 mph 10 to 15 mph with gusts to 30 mph Light
Expected snowfall: 2 to 4 in. up to 1 in. 0 in.
For 8000 ft. to 9000 ft.
Monday Monday Night Tuesday
Weather: Cloudy with snow showers likely in the morning then a chance of snow showers in the afternoon Cloudy with a chance of snow showers Mostly cloudy with a chance of snow showers in the morning. Snow showers decreasing in the afternoon.
Temperatures: 17 to 23 deg. F. 6 to 16 deg. F. 18 to 23 deg. F.
Wind Direction: Southwest Southwest Northeast
Wind Speed: 15 to 25 mph with gusts to 55 mph decreasing to 45 mph in the afternoon 15 to 25 mph with gusts to 45 mph decreasing to 35 mph in the afternoon 10 to 15 mph with gusts to 30 mph
Expected snowfall: 2 to 4 in. up to 1 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