THIS AVALANCHE FORECAST EXPIRED ON March 4, 2018 @ 6:59 am
Avalanche Forecast published on March 3, 2018 @ 6:59 am
Issued by Andy Anderson - Tahoe National Forest

CONSIDERABLE danger exists at all elevations due to likely wind and storm slabs and possible deep slabs. Most avalanche accidents happen at CONSIDERABLE or MODERATE danger. Don't let "powder fever" corrupt good judgment. Make a plan before going into the backcountry that focuses on thorough and careful terrain and snowpack evaluation. Avoiding avalanche terrain and traveling on slopes less than 30 degrees without any exposure to steeper terrain above is recommended today. Avalanches could be large and destructive.

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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Wind slabs that have formed during this storm have grown large and are widespread. Human-triggered wind slab avalanches will remain likely today and natural wind slab avalanches will still be possible. The most sensitive wind slabs should exist on wind-loaded N-NE-E aspects and cross-loaded NW and SE aspects in near and above treeline terrain, but some wind slabs could also linger in below treeline terrain due to the strength of the winds early in this storm. Wind slab avalanches could be larger and more destructive than expected and could run into more sheltered areas like dense stands of trees. 

Recent avalanche activity, blowing snow, drifted snow, cornices above slopes, and shooting cracks can help identify where wind slabs may exist. Use this information to avoid wind-loaded slopes. Wind slabs may exist in unexpected places today. 

Avalanche Problem 2: Storm Slab
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Changing conditions during this storm have created several potential weaknesses within the storm snow including graupel layers and denser layers of snow on top of softer layers of snow (upside-down layering). These storm snow weaknesses mean that human-triggered storm slab avalanches will remain likely today on steep sheltered slopes. These avalanches could involve all or part of the new snow and could occur in areas traditionally thought of as safe like tree-covered slopes.  While most of these should break in the upper layers of the storm snow, some of them could still be large and destructive due to the amount of new storm snow that exists.  

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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Human-triggered deep slab avalanches will be possible today. If they do occur they would likely fail on a loose weak layer of snow buried several feet below the surface near a crust. Observations have found this layer on NW-N-NE aspects across the forecast area. These deep slabs are most likely to react to larger triggers like several people on a slope, large cornice failures, or other avalanches or the right trigger in the right spot on a slope (a trigger point). 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 accumulated during the storm, deep wind slabs and possibly deep storm slabs could behave more like a deep slab avalanche problem (failing deeper, wider, and more destructive than typically anticipated) and not necessarily triggered by the first person on the slope.

Forecast discussion

Most avalanche fatalities happen on days like today. Human-triggered avalanches remain likely and dangerous avalanche conditions exist in the backcountry, but the danger has become more subtle. This doesn't mean that you cannot get out into the mountains. Safe backcountry travel requires training, experience, and proper planning to manage terrain to avoid the hazards. 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, could fail after other tracks already exist on a slope, 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 or fail after multiple people have used the slope. 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. 

recent observations

* Yesterday on Mt. Judah (Donner Summit area), wind-loading had reloaded slopes that had already avalanched and ski kicks on lower angle sections of the reloaded wind slabs up to 30 ft away triggered wind slab avalanches and cornice collapses on the steeper sections of the slopes. Widespread evidence of previous natural avalanches existed. 

*  Observations from the Blue Lakes area (near Carson Pass) found evidence of weaknesses within the storm snow layers including graupel and density changes. 

* The persistent weak layer of loose weak snow near a crust (near crust facets) is now buried several feet below the surface, but observations on Mt. Judah and in the Blue Lakes area indicated that snowmobiles and skiers could still affect that layer. Snowpit tests targetting the persistent weak layer on Mt. Judah did not yield unstable results.  

Weather and CURRENT CONDITIONS
weather summary

Remote sensors and webcams indicate that 16 to 30 inches of new snow has fallen in the last 24 hours with most of that accumulating since yesterday afternoon. This new snow brings storm totals to 3 to 5+ ft of new snow over the last 2.5 days (since Wednesday afternoon). The storm should begin moving out of the area today. The forecast calls for snowfall to decrease and become showery.  Enough instability remains in the atmosphere for these showers to continue through tonight. The forecast calls for 1 to 4 inches of new snow today and another 1 to 4 tonight. The winds have begun to decrease as well and they should continue to diminish over the next 36 hours. Cloud cover should also begin to diminish and expect to see clearing skies over the next 36 hours. 

Weather observations from along the Sierra Crest between 8200 ft. and 8800 ft.
0600 temperature: 12 to 15 deg. F.
Max. temperature in the last 24 hours: 17 to 23 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: 89 mph
New snowfall in the last 24 hours: 16 to 30 inches
Total snow depth: 71 to 96 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Saturday Saturday Night Sunday
Weather: Cloudy with widespread snow showers in the morning and scattered snow showers in the afternoon Cloudy with widespread snow showers in the evening and scattered snow showers after midnight Partly cloudy with a slight chance of snow showers in the morning
Temperatures: 20 to 25 deg. F. 6 to 12 deg. F. 24 to 29 deg. F.
Mid Slope Winds: Southwest Southwest Southwest
Wind Speed: 10 to 15 mph with gusts to 30 mph in the morning 10 to 15 mph in the evening decreasing overnight 10 to 15 mph with gusts to 25 mph in the morning becoming light in the afternoon
Expected snowfall: 1 to 3 1 to 4 0
For 8000 ft. to 9000 ft.
Saturday Saturday Night Sunday
Weather: Cloudy with widespread snow showers in the morning and scattered snow showers in the afternoon Cloudy with widespread snow showers in the evening and scattered snow showers after midnight Partly cloudy. Slight chance of snow showers in the morning.
Temperatures: 15 to 21 deg. F. 3 to 9 deg. F. 20 to 26 deg. F.
Ridge Top Winds: Southwest Southwest West
Wind Speed: 15 to 25 mph with gusts to 50 mph decreasing to 35 mph in the afternoon 10 to 15 mph with gusts to 25 mph 15 to 20 mph with gusts to 35 mph decreasing to 25 mph in the afternoon
Expected snowfall: 1 to 4 1 to 4 0
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

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