THIS AVALANCHE FORECAST EXPIRED ON January 8, 2017 @ 7:00 am
Avalanche Forecast published on January 7, 2017 @ 7:00 am
Issued by Andy Anderson - Tahoe National Forest

UPDATED @ 1:30PM 

Due to snow levels rising sooner than expected, natural and human triggered avalanches are likely this afternoon and tonight. Large destructive deep slabs, wind slabs, and wet slabs that run long distances are likely as more rain and snow impact the region. Very dangerous avalanche conditions exist. Travel in or near avalanche terrain is not recommended. The avalanche danger has increased to HIGH at all elevations due to rain, wind, and snow impacting the region.

4. High

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Above Treeline
Very dangerous avalanche conditions. Travel in avalanche terrain not recommended.

4. High

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Near Treeline
Very dangerous avalanche conditions. Travel in avalanche terrain not recommended.

4. High

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Below Treeline
Very dangerous avalanche conditions. Travel in avalanche terrain not recommended.
    Very dangerous avalanche conditions. Travel in avalanche terrain not recommended.
Avalanche Problem 1: Wind Slab
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New wind slabs will form on wind loaded W-NW-N-NE-E-SE aspects in near and above treeline terrain steeper than 35 degrees today as more snow and wind impact the forecast area. The heavy wet snow that comprises these wind slabs may not bond to the current snow surfaces and human triggered wind slabs will become likely during the day and natural and human triggered wind slabs could become likely overnight. These new wind slabs could involve 2 to 3 ft. of wind loaded snow in some places by this afternoon, and they will continue to grow in size overnight and during the next 36 hours in places above snow line where more snow accumulates. They could also step down and become the deep slabs mentioned below.

Avalanche Problem 2: Deep Slab
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More snow and rain during this storm will add significant load to the snowpack and make it easier for the facets near the 12/15 rain crust to fail. If this layer does fail, large destructive deep slab avalanches will result. These human-triggered deep slab avalanches remain possible today throughout the entire forecast area on W-NW-N-NE-E aspects at all elevations. As more snow and rain continue to load the snowpack over the next 36 hours, natural and human triggered deep slab avalanches will become likely. These slabs could measure anywhere from 4 to 10+ feet deep. Rain on snow could also trigger deep wet slabs on this layer in the next 36 hours.

Deep slabs are unpredictable. Remote triggering with large propagation into any connected terrain remains possible. Large to historic runout zones could occur if a deep slab avalanche was triggered. Ski tracks on a slope will not be an indicator of stability and steep trees will not be a safe zone. Avalanche avoidance and cautious route finding remain the best practice to deal with this deep slab avalanche problem. Deep slabs can occur on less steep slopes and any slopes steeper than 30 degrees are suspect. 

Avalanche Problem 3: Wet Slab
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UPDATE @ 1:30 pm:

Snow levels have climbed to above 8000 ft. in most of the forecast area. Where rain is falling on snow, wet slabs and loose wet avalanches are now likely. As more rain loads the snowpack some wet slabs may step down to the deep slabs mentioned above.

Previous Description published @ 7:00 am:

Heavy wet snow on top of cold light snow means that human triggered storm slabs will be possible today. As more heavy snow accumulates during the storm, human triggered and natural storm slab avalanches will become likely. As the snow changes to rain, rain on snow could also cause widespread avalanche activity. These wet storm slabs could also step down to become deep slabs mentioned above and could occur on slopes steeper than 30 degrees where new snow or rain falls. 

Forecast discussion
PREVIOUS BOTTOM LINE PUBLISHED AT 7:00AM

Human triggered avalanches will become likely again today. Deep slabs, wind slabs, and storm slabs represent today's avalanche problems. Natural and human triggered avalanches will become likely by tonight as the storm continues. Large destructive avalanches could occur. Rain on snow could trigger widespread avalanche activity. CONSIDERABLE avalanche danger exists at all elevations today and the avalanche danger will rise to HIGH tonight or sooner if the snow level rises faster than expected.

recent observations

Yesterday observations in Deep Creek, on Andesite Ridge, and on Red Lake Peak all showed settlement and consolidation in the upper snowpack. Snowpit data from Deep Creek and Andesite ridge revealed that the facets near the 12/15 rain crust remain weak and that fractures can still travel through that layer. In Deep Creek, a whumph occurred on that layer while excavating the pit. Another party in Deep Creek reported some shooting cracks in wind loaded areas. A party on Herlan Peak (east shore) reported large whumphing on N-NW slopes. Ski kicks on wind loaded test slopes on Red Lake Peak and Andesite Ridge as well as a cornice dropped onto a test slope on Andesite Ridge did not trigger signs of instability on wind loaded test slopes. Several other recent avalanches that likely failed on the 12/15 facets on Wed. or Thurs. were also reported from areas including Andesite Ridge, Castle Peak, and Deep Creek. Ski areas also reported still being able to trigger this layer with very large triggers yesterday. 

Weather and CURRENT CONDITIONS
weather summary

A strong storm system will impact the region starting today. Expect warm and wet weather for the duration of this system. Temperatures and snow levels have already started to climb this morning and should reach 8000 ft. by this evening across the forecast area as the warm air associated with this atmospheric river pushes the cold air out to the region. The forecast calls for 4 to 10 inches of snow becoming a mix of rain and snow this afternoon below 8000 ft. and 6 to 12 inches of snow above 8000 ft. today. During the night snow levels will continue to climb reaching around 9000 ft. by the Sunday morning. Precipitation will increase overnight with 1.5 to 3 inches of water expected. Much of this will fall as rain below 8000 ft. with mix of snow and rain below 9000 ft. Areas above 9000 ft. could see another 10 to 18 inches of snow. The forecast calls for temperatures to stay warm through Sunday with snow levels remaining at or above 9000 ft. The heaviest and most intense precipitation will fall during the day tomorrow with another 3 to 6 inches of water forecasted by 4 pm tomorrow. Most of this will fall as rain below 9000 ft. Areas above 9000 ft. could see 1.5 to 3 ft of heavy wet snow. By the end of this storm on Monday morning, the forecast area could see an additional 6 to 12 inches of water. The southwest winds have increased to 30 to 40 mph with gusts to 80 mph this morning and should remain in this range through tomorrow. 

Weather observations from along the Sierra Crest between 8200 ft. and 8800 ft.
0600 temperature: 24 to 29 deg. F.
Max. temperature in the last 24 hours: 27 to 33 deg. F.
Average wind direction during the last 24 hours: Southwest
Average wind speed during the last 24 hours: 30 to 40 mph
Maximum wind gust in the last 24 hours: 81 mph
New snowfall in the last 24 hours: up to 1 inches
Total snow depth: Along the Sierra Crest: 49 to 70 inches | In the Mt. Rose area: 92 inches inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Saturday Saturday Night Sunday
Weather: Snow in the morning changing to rain in the afternoon as snow level rises to ~8000 ft. Rain and snow mix with mostly rain after midnight. Snow levels rising to ~9000 ft. Rain.
Temperatures: 34 to 39 deg. F. 33 to 38 deg. F. 40 to 45 deg. F.
Winds: South shifting to the west in the afternoon Southwest Southwest
Expected snowfall: 4 to 10 in. Snow: up to 3 in. | Rain: 1.5 to 3 in. Rain: 3 to 6 in.
For 8000 ft. to 9000 ft.
Saturday Saturday Night Sunday
Weather: Snow. Snow levels near 8000 ft. Rain and snow mix. Snow levels rising to ~9000 ft. after midnight Rain with some heavy wet snow above snow level. Snow level ~9000 ft.
Temperatures: 32 to 37 deg. F. 30 to 35 deg. F. 37 to 42 deg. F.
Winds: South shifting to west in the afternoon Southwest Southwest
Expected snowfall: 6 to 12 in. Snow above snow line: 10 to 18 in. | Rain below snow line: 1.5 to 3 in. Snow above snow line: 15 to 30 in. | Rain below snow line: 3 to 6 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