THIS AVALANCHE ADVISORY EXPIRED ON February 20, 2016 @ 6:59 am
Avalanche Advisory published on February 19, 2016 @ 6:59 am
Issued by Andy Anderson - Tahoe National Forest

Pockets of CONSIDERABLE avalanche danger still linger on wind loaded near and above treeline NW-N-NE-E-SE aspects steeper than 35 degrees due wind slabs. MODERATE avalanche danger exists on all other slopes steeper than 35 degrees due to a combination of wind slabs and isolated storm slabs. Large human triggered avalanches remain possible today and may remain likely in some areas. Identify where wind slabs and lingering isolated storm slabs exist and avoid them and their run out zones.

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.

2. Moderate

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Below Treeline
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
    Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.
Avalanche Problem 1: Wind Slab
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Even though the snowfall may have abated yesterday afternoon, the winds did not. They continued to transport snow and wind load leeward slopes all night and will continue to do so today. While the chances for natural wind slab avalanches have diminished, large human triggered wind slabs will remain possible today on wind loaded N-NE-E aspects and cross loaded NW and SE aspects and may remain likely in some places where the most wind loading has occurred or in complex or extreme terrain. Wind loaded near and above treeline terrain will hold the largest wind slabs, but they could also exist in more sheltered locations and in below treeline terrain due to the strength of the winds during this storm. These wind slabs will most likely fail on weaknesses within the storm snow like density changes or graupel layers or the interface between the wind slab and the snow below. Wind slab avalanches that do occur today could be large and could have serious consequences.

Clues like recent wind slab avalanches on similar slopes, shooting cracks on lower angle wind loaded terrain, cornices above a slope, blowing snow, wind drifted snow, ripples in the snow surface, and other wind created textures can help identify where wind slabs may exist. Use observations and clues like these to find where the wind slabs lurk and then avoid those wind slabs and their run out zones.

Avalanche Problem 2: Storm Slab
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Storm slabs should have become more difficult to trigger today since the storm snow weaknesses that they fail on typically gain strength faster than other weak layers. That being said, some storm slab avalanches may remain possible in isolated areas or in complex or extreme terrain where the storm snow weaknesses linger. Couliors, gullies, steep convex rollovers, unsupported slopes, and other steep complicated terrain features on NW-N-NE aspects will hold the best chances for lingering storm slabs. Recent storm slab avalanches on similar slopes, human triggered shooting cracks, test slope failures, and snowpit tests can all provide good information about whether or not storm slabs exist and remain problematic.  

recent observations

Yesterday reports from Jakes Peak told of widespread small natural avalanches that failed during the early morning on a density change near the base of the new snow. These avalanches occurred on 40 to 44 degree slopes in below treeline terrain and remained relatively small and did not propagate very far. Some of the avalanche paths above the Alpine Meadows road also produced natural avalanches between 7 and 7:30 am, but the details of what kind of avalanches and what the failure layers were remain unknown. We also received another vague report of a human triggered avalanche somewhere in Ward Canyon that carried a person a long distance and buried him up to his neck, but we do not have any details on this slide

Observations from the Donner Summit area, on Hidden Peak, and on Jakes Peak also saw some skier triggered shooting cracks that failed on the density change near the base of the storm snow in sheltered below treeline areas or that failed at the base of newly formed wind slabs in wind loaded areas. On Hidden Peak one wind loaded 38 degree test slope failed under the weight of a skier after it had been undercut. Snowpit tests and hand pits on Hidden Peak also yielded unstable results within the storm snow. 

Weather and CURRENT CONDITIONS
weather summary

During the day yesterday another 3 to 4 inches of new snow fell before the snowfall tapered off during the afternoon. This additional snow brought storm totals to 1 to 2 ft. across much of the forecast area. Gale force southwest winds persisted through the day and decreased slightly overnight. Another much weaker system moving into the area this afternoon will cause the winds to increase again and could bring an inch or two of new snow to the area north of Echo Summit this afternoon and evening. This system should depart the area tonight and winds should begin to decrease as a high pressure ridge starts building over the region this weekend. The forecast calls for slightly warmer daytime highs today and another 5 degrees of warming tomorrow.

Weather observations from along the Sierra Crest between 8200 ft. and 8800 ft.
0600 temperature: 20 to 24 deg. F.
Max. temperature in the last 24 hours: 22 to 26 deg. F.
Average wind direction during the last 24 hours: Southwest
Average wind speed during the last 24 hours: 45 to 55 mph
Maximum wind gust in the last 24 hours: 104 mph
New snowfall in the last 24 hours: 3 to 4 inches
Total snow depth: 70 to 100 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 cloudy with a slight chance of snow Mostly cloudy with a chance of snow in the evening and a slight chance of snow after midnight Mostly cloudy
Temperatures: 32 to 38 deg. F. 20 to 28 deg. F. 37 to 42 deg. F.
Wind Direction: Southwest Southwest South
Wind Speed: 25 to 45 mph with gusts to 65 mph 20 to 40 mph with gusts to 60 mph decreasing to 15 to 25 mph with gusts to 40 mph after midnight 10 to 15 mph
Expected snowfall: up to 1 in. up to 1 in. 0 in.
For 8000 ft. to 9000 ft.
Friday Friday Night Saturday
Weather: Mostly cloudy with a slight chance of snow Mostly cloudy with a chance of snow in the evening and a slight chance of snow after midnight Mostly cloudy
Temperatures: 29 to 34 deg. F. 18 to 25 deg. F. 33 to 39 deg. F.
Wind Direction: Southwest Southwest South
Wind Speed: 50 to 60 mph with gusts to 85 mph 55 to 60 mph with gusts to 80 mph decreasing to 35 to 40 mph with gusts to 60 mph after midnight 15 to 20 mph with gusts to 35 mph in the morning
Expected snowfall: up to 1 in. up to 1 in. 0 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.