THIS AVALANCHE ADVISORY EXPIRED ON January 23, 2016 @ 6:57 am
Avalanche Advisory published on January 22, 2016 @ 6:57 am
Issued by Andy Anderson - Tahoe National Forest

Despite becoming increasingly difficult to trigger,  human triggered avalanches remain possible on some W-NW-N-NE-E-SE aspects at all elevations on slopes steeper than 32 degrees due to persistent slabs and old wind slabs. The avalanche danger is MODERATE today, but it will quickly increase to CONSIDERABLE due to added weight on the persistent slabs and new wind slabs resulting from more snow and wind impacting the region tonight and tomorrow. Some natural avalanche activity may start to occur during the night.

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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Data and observations keep finding layers of surface hoar under a 2-3 ft. slab of consolidated snow in some open areas on near and below treeline N-NE-E aspects. This layer could still exist on some isolated NW and SE aspects as well. Triggering an avalanche on these buried surface hoar layers has become more difficult, but human triggered persistent slab avalanches that fail on a buried surface hoar layer remain possible on some slopes. As more snow accumulates across the forecast area over the next 36 hours this persistent slab problem should become larger and easier to trigger due to the added weight of the new snow. For today 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. 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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The wind slabs that existed on wind loaded slopes yesterday have become increasingly difficult to trigger. However, some human triggered wind slabs may still be possible today on wind loaded near and above treeeline NW-N-NE-E-SE aspects in couliors, gullies, on hanging snowfields, or in other complex or extreme terrain. New human triggerable wind slabs will also start to form today on wind loaded W-NW-N-NE-E aspects in near and above treeline terrain. These new wind slabs should remain shallow and not extend very far down slope since the forecast only calls for 3 inches of new snow today. These new wind slabs will become larger, more widespread, and easier to trigger during the night and tomorrow as more wind and snow arrive across the forecast area. Some natural avalanche activity involving these new wind slabs could start to occur tonight. 

recent observations

Yesterday in the Tamarack Peak area hard wind slabs still existed above softer snow on wind loaded near and above treeline slopes. Tests like ski cuts, dropping cornice pieces onto the wind slabs, and snowpit tests showed that these wind slabs had become difficult to trigger on most of the wind loaded test slopes. One of these stubborn wind slabs did break on one small wind loaded test slope after repeated ski kicks and jumps. In non wind affects areas snowpit data and general observations did not show any signs of instability. Snowpit data from several snowpits in near and below treeline areas on Tamarack Peak also did not reveal any evidence of a buried surface hoar layer, but one party did report a fractured slope on the north side of the Fireplug Ridge. Details of this fractured slope are unknown.

Weather and CURRENT CONDITIONS
weather summary

South and southwest winds started increasing last night ahead of the winter storm forecasted to reach the region today. The leading edge of the storm made its way into the north end of the forecast area this morning bringing light precipitation. This storm should continue to progress southward today bringing widespread precipitation and ridgetop south winds in the 50 to 70 mph range to the forecast area. Snow (and rain below 7500 ft) should become widespread across the forecast area this afternoon and tonight before starting to decrease again tomorrow afternoon. The forecast calls for up to 3 inches of snow above 7500 ft. today with another 5 to 8 inches above 7000 ft. tonight and 3 to 8 inches tomorrow. In general snow totals should be greatest along the Sierra Crest with up to 20 inches in some places by tomorrow afternoon. At Lake level snow totals should only reach 3 to 7 inches by that time. Snow levels started around 7500 to 7800 ft. and should hover between 6500 and 7500 ft today before falling to 4500 to 5000 ft. during the night. Temperatures should remain cold tomorrow. 

Weather observations from along the Sierra Crest between 8200 ft. and 8800 ft.
0600 temperature: 27 to 34 deg. F.
Max. temperature in the last 24 hours: 33 to 44 deg. F.
Average wind direction during the last 24 hours: South and southwest
Average wind speed during the last 24 hours: 30 to 40 mph
Maximum wind gust in the last 24 hours: 83 mph
New snowfall in the last 24 hours: trace to 0 inches
Total snow depth: 61 to 77 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Friday Friday Night Saturday
Weather: Cloudy with scattered snow showers in the morning with snow becoming more widespread this afternoon Cloudy with rain in the evening. The rain should change to snow overnight. Snow
Temperatures: 32 to 42 deg. F. 20 to 27 deg. F. 25 to 32 deg. F.
Wind Direction: South South Southwest
Wind Speed: 40 to 50 mph with gusts to 75 mph decreasing to 65 mph in the afternoon 35 to 45 mph with gusts to 65 mph 40 to 45 mph with gusts to 65 mph decreasing to 30 to 35 mph with gusts to 50 mph in the afternoon
Expected snowfall: up to 3 in. 4 to 8 in. 3 to 8 in.
For 8000 ft. to 9000 ft.
Friday Friday Night Saturday
Weather: Cloudy with scattered snow showers in the morning with snow becoming more widespread this afternoon Snow Snow
Temperatures: 33 to 40 deg. F. 18 to 25 deg. F. 21 to 28 deg. F.
Wind Direction: South South Southwest
Wind Speed: 55 to 65 mph with gusts to 95 mph 50 to 60 mph with gusts to 95 mph 65 to 70 mph with gusts to 105 mph decreasing to 40 to 45 mph with gusts to 70 mph in the afternoon
Expected snowfall: up to 3 in. 5 to 8 in. 3 to 8 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.