THIS AVALANCHE ADVISORY EXPIRED ON March 3, 2018 @ 7:30 am
Avalanche Advisory published on March 2, 2018 @ 7:30 am
Issued by Andy Anderson - Tahoe National Forest

CONSIDERABLE avalanche danger exists at all elevations today due to likely wind slab and storm slab avalanches and possible deep slab avalanches. Another round of intense snowfall and wind impacting the region this afternoon and tonight should cause the avalanche danger to rise to HIGH in near and above treeline terrain. Most avalanche accidents happen at CONSIDERABLE or MODERATE, avoiding avalanche terrain and travelling on slopes less than 30 degrees without any exposure to steeper terrain above is recommended today.

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.

3. Considerable

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Below Treeline
Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.
    Very dangerous avalanche conditions. Travel in avalanche terrain not recommended.
Avalanche Problem 1: Wind Slab
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During this morning's lull in the storm, human-triggered wind slab avalanches will remain likely and natural wind slab avalanches will still be possible. As more snow and wind impact the forecast area this afternoon and tonight, the wind slabs will become deeper, more widespread, and easier to trigger with natural avalanche becoming likely again and human-triggered avalanches becoming very likely. Wind slabs could exist at any elevation today due to the strength of the winds so far during this storm. Wind slab avalanches that do occur could be large and destructive and could run into more sheltered areas like dense stands of trees. 

Use clues like recent avalanche activity, blowing snow, drifted snow, cornices above slopes, and shooting cracks to identify where wind slabs may exist and avoid wind loaded slopes. Wind slabs could exist in unexpected places today. 

Avalanche Problem 2: Storm Slab
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This storm's heavier snow rests on top of older lighter snow creating an upside-down snowpack. Changing conditions during this storm also have created weaknesses within the storm snow. As a result, human-triggered storm slab avalanches will remain likely today on steep sheltered slopes. These avalanches could involve all of the new snow and could occur in areas traditionally thought of as safe like tree-covered slopes.  As another round of intense snowfall impacts the region this afternoon and tonight, storm slab avalanches should become more widespread and easier to trigger with natural storm slab avalanche again becoming likely. 

Recent avalanche activity, human-triggered cracking, and probing into the snow to feel for softer layers below more dense layers can help identify areas where storm slabs may exist. 

Avalanche Problem 3: Deep Slab
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A loose weak layer of snow exists near a crust on NW-N-NE aspects across the forecast area. This layer is now buried several feet below the surface. The loading provided by this storm may be enough to cause this layer to fail making deep slab avalanches a possibility today. These deep slab avalanches would be large and destructive, could propagate across large distances, could release below traditional starting zones, and are difficult to predict.

Snowpit data and probing can help identify where the persistent weak layer may exist. Due to the uncertainty and dire consequences associated with these deep slab avalanches avoiding slopes where they may exist represents the best course of action.  

Due to the amount of new snow that has fallen and the additional snow expected today wind slabs and possibly storm slabs could also exceed 3 feet in slab thickness allow these avalanche problems to behave more like deep slabs (failing deeper, wider, and more destructive than typically anticipated) and not necessarily triggered by the first person on the slope.

advisory discussion

While days like this account for the majority of avalanche accidents, the conditions don't mean that you cannot get out into the mountains. These kinds of conditions require experience and proper planning to manage terrain to avoid the hazards. The lull in the storm may have allowed the avalanche danger to decrease this morning, but today's avalanche problems are real and complicated. The mix of large deep wind slabs, widespread storm slabs and possible deep slabs will make finding safe terrain challenging. All of these avalanche problems could have serious consequences, could fail unexpectedly, could be remotely triggered, and could run into unexpected terrain. For example wind slabs may linger in below treeline areas, storm slabs may exist on tree covered slopes traditionally thought of as safe, and deep slabs could propagate around corners or break below traditional starting zones. Carefully identifying and then avoiding steep areas where instability may exist and lower angle more sheltered areas where you may be exposed to avalanche runout zones should be part of today's travel plan.  As more snow and wind impact the area the avalanche danger will increase again and travel will become even more challenging.  

recent observations

* Widespread wind slab avalanches occurred on Andesite Peak yesterday at all elevations. Some released naturally, some released as a person stepped on low angle terrain above them, and some released as small cornice pieces were dropped onto the wind-loaded slopes. They propagated long distances and crowns measured up to 2 ft. deep by noon yesterday.

* Tender cornices that failed easily in response to ski kicks and more stubborn wind slabs existed on Tamarack Peak. Due to poor visibility, observers could not tell whether or not the cornice collapses triggered wind slab avalanches. Wind loading and wind drifts 3 to 6 ft deep were reported on Relay Peak.

* Widespread storm slab avalanches also occurred on sheltered near and below treeline terrain on Andesite Peak yesterday. Several natural storm slab avalanches had released and triggers ranging from ski cuts on test slopes, to lightly stepping onto test slopes, to touching steep test slopes with a ski pole caused storm slab failures on any steep slopes. Lower angle slopes also cracked and shattered.

* Skier triggered cracking and unstable snowpit test results occurred in below treeline terrain on Andesite Peak and on Tamarack Peak with the interface between the older lighter snow and the new heavier snow acting as the failure layer

* Snowpit tests on Relay Peak targeting the now deeply buried persistent weak layer of loose weak snow near an old crust (near crust facets) yielded unstable results. 

Weather and CURRENT CONDITIONS
weather summary

Yesterday 10 to 15 inches of new snow accumulated during the day and another 10 to 16 inches fell during the night in many areas along the Sierra Crest. So far snow totals from this storm range from 2 to 3 feet above 7000 ft. Gale force SW winds averaging between 70 and 80 mph with recorded gusts over 140 mph have accompanied this snowfall. The forecast calls for a decrease in snowfall this morning before another round of intense snow arrives this afternoon and evening. Heavy snowfall should continue through tonight.  Expect another 4 to 11 inches of snow today above 7000 ft. with 7 to 14 more overnight. Winds should decrease some today but they will still remain strong with average speeds in the 30 to 50 mph range and gusts around 115 mph along the Sierra Crest. The forecast calls decreasing winds and snow tomorrow as the storm exits the area.

Weather observations from along the Sierra Crest between 8200 ft. and 8800 ft.
0600 temperature: 10 to 17 deg. F.
Max. temperature in the last 24 hours: 23 to 28 deg. F.
Average wind direction during the last 24 hours: Southwest
Average wind speed during the last 24 hours: 70 to 80 mph
Maximum wind gust in the last 24 hours: 147 mph
New snowfall in the last 24 hours: 15 to 27 inches
Total snow depth: 47 to 87 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Friday Friday Night Saturday
Weather: Mostly sunny this morning with snow in the afternoon Snow Cloudy with snow showers likely
Temperatures: 22 to 27 deg. F. 13 to 18 deg. F. 20 to 25 deg. F.
Wind Direction: Southwest Southwest Southwest
Wind Speed: 20 to 30 mph with gusts to 60 mph decreasing to 50 mph in the afternoon 15 to 25 mph with gusts to 45 mph Light in the morning increasing to 10 to 15 mph in the afternoon
Expected snowfall: 4 to 10 in. 7 to 13 in. 1 to 3 in.
For 8000 ft. to 9000 ft.
Friday Friday Night Saturday
Weather: Mostly sunny this morning with snow in the afternoon Snow Cloudy with snow showers likely
Temperatures: 17 to 23 deg. F. 9 to 15 deg. F. 15 to 21 deg. F.
Wind Direction: Southwest Southwest Southwest
Wind Speed: 30 to 50 mph with gusts to 115 mph decreasing to 95 mph in the afternoon 25 to 40 mph with gusts to 80 mph decreasing to 70 mph after midnight 15 to 25 mph with gusts to 55 mph decreasing to 35 mph in the afternoon
Expected snowfall: 5 to 11 in. 8 to 14 in. 1 to 4 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