THIS AVALANCHE ADVISORY EXPIRED ON January 22, 2016 @ 7:00 am
Avalanche Advisory published on January 21, 2016 @ 7:00 am
Issued by Andy Anderson - Tahoe National Forest

Even though avalanches have become more difficult to trigger, human triggered avalanches remain possible on some NW-N-NE-E-SE aspects at all elevations due to lingering persistent slabs and wind slabs. The persistent slabs could occur on slopes steeper than 32 degrees, while the wind slabs would be more problematic on slopes steeper than 35 degrees. The avalanche danger is MODERATE today, but the consequences of a human triggered avalanche would be serious. Evaluate snow and terrain carefully; identify features of concern.

2. Moderate

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Above Treeline
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.

2. Moderate

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Near Treeline
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.

2. Moderate

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Below Treeline
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
    Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
Avalanche Problem 1: Persistent Slab
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Despite the fact that persistent slabs have become more isolated and difficult to trigger, human triggered persistent slab avalanches that fail on a buried surface hoar layer remain possible on some slopes where buried surface hoar still exists under a 2-3 ft. slab of consolidated snow. Open areas in near and below treeline terrain on N-NE-E aspects represent the most likely places to find buried surface hoar, but it may also still exist on some isolated NW and SE aspects. Larger triggers like large cornice collapses or multiple people on a slope or smaller triggers on the right trigger point such as an area where a shallower snowpack exists or in areas near rocks have better chances of triggering a persistent slab avalanche today. If any persistent slab avalanches do occur, they would be large and have severe consequences. Avalanches that fail on buried surface hoar can propagate long distances, can occur in places traditionally considered safe, and can be triggered from lower angle terrain.

Due to the consequences of these types of avalanches, avoiding steep slopes where reactive surface hoar still exists represents the best way to manage this avalanche problem. Unfortunately, determining where the buried surface hoar still exists and where it is still reactive is difficult. Recent avalanche activity, shooting cracks, collapsing, whumpfing, and snowpit tests can provide some clues. However, a persistent slab triggered by a party on the slope sometimes represents the first and only clue to avalanche instability. 

 

Avalanche Problem 2: Wind Slab
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Wind slabs have also become more difficult to trigger, but like the persistent slabs mentioned above some human triggered wind slabs may still be possible today. Wind loaded slopes on near and above treeeline NW-N-NE-E-SE aspects in couliors, gullies, on hanging snowfields, or in other complex or extreme terrain represent the best places to find lingering wind slabs. Some fragile wind slabs may still linger in other isolated heavily wind loaded areas as well.  

recent observations

Yesterday observations on Rubicon Peak revealed frozen rain crusts ranging from supportable to breakable on the snow surface below 8000 ft. and a layer of dense heavy snow (also sometimes supportable and sometimes breakable) above 8000 ft. Hard wind slabs existed on the near and above treeline slopes exposed to wind loading. While ski cuts on small wind loaded test slopes did not produce results, snowpit tests on the wind slabs did still yield some unstable results. Below these heavier surface layers the most recent snow remained soft and unconsolidated. Snowpit observations also found at least one of the buried surface hoar layers lurking about 70 cm down in the snowpack underneath a firm slab layer. This surface hoar layer consisted of large feathery surface hoar that had not collapsed yet. Snowpit tests on the buried surface hoar layer indicated that triggering this layer has become more difficult. Tests also indicated that if it breaks fractures can still travel along that layer, and failure of this layer could still produce large avalanches in areas where the surface hoar layer exists. 

Weather and CURRENT CONDITIONS
weather summary

A high pressure ridge over the region should keep the weather dry and slightly warmer than normal today. The winds and cloud cover should start to increase during the next 24 hours as another winter storm approaches the region. The forecast calls for sustained south to southwest ridgetop winds at 50 to 60 mph with gusts between 90 and 100 mph tonight and tomorrow. Some light snow could start to fall late tonight or early tomorrow morning as the first wave of the storm reaches the forecast area. Expect snow levels to hover between 6000 and 7000 ft. tonight and tomorrow and up to 3 inches of accumulation for the Sierra above 7000 ft. by the end of the day tomorrow. Snow should continue through Saturday. For more details on this storm check in with the Reno NWS.

Weather observations from along the Sierra Crest between 8200 ft. and 8800 ft.
0600 temperature: 32 to 37 deg. F.
Max. temperature in the last 24 hours: 34 to 39 deg. F.
Average wind direction during the last 24 hours: Southwest to east
Average wind speed during the last 24 hours: 15 mph
Maximum wind gust in the last 24 hours: 40 mph
New snowfall in the last 24 hours: 0 inches
Total snow depth: 62 to 79 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Thursday Thursday Night Friday
Weather: Mostly cloudy Mostly cloudy with a slight chance of snow after midnight Cloudy with a chance of snow in the morning with snow becoming more widespread in the afternoon. Snow levels around 7000 ft.
Temperatures: 40 to 45 deg. F. 27 to 32 deg. F. 32 to 39 deg. F.
Wind Direction: Southeast to south South South
Wind Speed: Light increasing to 10 to 20 mph with gusts to 35 mph in the afternoon 20 to 30 mph with gusts to 40 mph increasing to 30 to 40 mph with gusts to 55 mph after midnight 25 to 45 mph with gusts to 65 mph
Expected snowfall: 0 in. 0 in. up to 2 in.
For 8000 ft. to 9000 ft.
Thursday Thursday Night Friday
Weather: Mostly cloudy Mostly cloudy with a slight chance of snow after midnight Cloudy with a chance of snow in the morning with snow becoming more widespread in the afternoon.
Temperatures: 34 to 40 deg. F. 25 to 30 deg. F. 28 to 34 deg. F.
Wind Direction: Southeast to southwest South South
Wind Speed: 10 to 15 mph increasing to 35 to 40 mph with gusts to 60 mph in the afternoon 45 to 50 mph with gusts to 75 mph increasing to 60 to 65 mph with gusts to 100 mph after midnight 55 to 65 mph with gusts to 95 mph
Expected snowfall: 0 in. 0 in. up 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.