THIS AVALANCHE ADVISORY EXPIRED ON February 11, 2015 @ 6:58 am
Avalanche Advisory published on February 10, 2015 @ 6:58 am
Issued by Andy Anderson - Tahoe National Forest

Human triggered avalanches remain possible today. Above 8,000 ft, MODERATE avalanche danger exists on NW-N-NE-E-SE aspects on slopes 35 degrees and steeper due to the presence of dense wind slabs and persistent slabs. If any avalanches do occur today, they could break above the person who triggers them and result involve a large amount of heavy dense snow that could easily bury or injure a person. Don't let the new snow lead to careless decision making.

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: Wind Slab
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Wind slabs still exist on wind loaded and cross loaded NW-N-NE-E-SE aspects. The largest of these wind slabs will exist above 8500 ft. in near and above treeline terrain where more new snow has accumulated, but smaller wind slabs may still exist on lower elevation wind loaded slopes. While these slabs should have become more difficult to trigger, human triggered avalanches involving these winds slabs remain possible. Most of these wind slabs would break on layers within the storm snow, some of these slabs could fail on or just below the old snow surfaces and behave like the persistent slabs mentioned below. Above 8500 ft. wind loaded slopes 38 degrees and steeper, couliors, unsupported slopes above cliffs, or slopes in complex or extreme terrain represent the best places to find unstable wind slabs today, but they could still linger on any wind loaded slopes steeper than 35 degrees.

Avalanche Problem 2: Persistent Slab
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The slabs formed by the new snow resting above an old crust with layers of softer snow around it now represent a persistent slab problem. Even though this kind of avalanche has become more difficult to trigger, this kind of avalanche problem warrants extra caution. So far two human-triggered avalanches have failed at the base of the new snow and propagated along the crust or layers around it. Characteristic of persistent slabs these slides broke above the person who triggered them. Typical obvious clues of instability and other tests may not provide reliable data as to whether or not these persistent slabs will break and when they do break they can break well above the trigger and involve a large amount of dense heavy snow. Often these kind of avalanches don't occur until a person hits the right trigger point on a slope even if several other tracks already exist on the slope. NW-N-NE aspects above 8500 ft. where the new snow rests above an old crust with soft weak snow around it represent the most likely places to find these persistent slabs. Areas where a more shallow snowpack exists or trigger points such as shallowly buried rocks or areas near exposed rocks represent the best places to trigger these slabs from.

recent observations

Yesterday a 200 ft. wide avalanche with a 2-3 ft crown occurred on a N-NE facing slope at 9300 ft. on the side of a gully on Relay Peak. This slide only ran downhill for about 100 ft, but it deposited debris 3-5 ft. deep in the gully. It involved all of the new snow that has fallen since Friday and failed on the old crust below the new snow. A snowmobiler riding on the side of the gully appeared to have triggered this slide from below when he or she crossed a more shallow area of the slab above some buried rocks. From this trigger point, it appears that the slide propagated up the hillside. Tests near the crown of this avalanche indicated that the interface between the old crust and the new snow remained weak and that a graupel layer buried about 15 inches from the surface also remained weak. A wind slab avalanche also occurred on the east ridge of Relay Peak on a NE facing slope at about 9500 ft. Other snowpit tests on wind loaded test slopes in the Relay Peak area also showed a weak layer of graupel 1-2 ft deep that fractures could travel along. On the other hand, ski cuts and snowmobile cuts did not yield any results on test slopes in the Relay Peak area.

On Tamarack Peak parties reported no signs of instability. On Andesite Peak up to 8200 ft, observations indicated that the new snow continues to consolidate and bond with itself and the layers below it, and observers reported no signs of instability. Farther south on Red Lake Peak, strong winds continued to transport snow and add snow to the wind slabs throughout the day.

Weather and CURRENT CONDITIONS
weather summary

Snow showers deposited another 2-6 inches of snow across the forecast area yesterday before they ended last evening. The southwest winds also remained moderate to strong for much of the day. During the night the winds decreased and shifted to the northeast ahead of a high pressure ridge building over the region. This high pressure ridge should establish itself over the area today and stop the cooling trend. The forecast calls for sunny skies and warming temperatures with daytime highs climbing back up into the mid to upper 30's today and mid to upper 40's tomorrow above 7000 ft. Winds should remain out of the east and northeast, and they should remain in the 5 to 15 mph range. 

Weather observations from along the Sierra Crest between 8200 ft. and 8800 ft.
0600 temperature: 18 to 25 deg. F.
Max. temperature in the last 24 hours: 27 to 33 deg. F.
Average wind direction during the last 24 hours: Southwest shifting to northeast after midnight
Average wind speed during the last 24 hours: Southwest: 40 mph | Northeast: 15 mph
Maximum wind gust in the last 24 hours: Southwest: 71 mph | Northeast: 30 mph
New snowfall in the last 24 hours: 2 to 6 inches
Total snow depth: 35 to 60 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Tuesday Tuesday Night Wednesday
Weather: Sunny Partly cloudy becoming clear Sunny
Temperatures: 36 to 43 deg. F. 21 to 28 deg. F. 42 to 49 deg. F.
Wind Direction: Variable East Variable
Wind Speed: Light 10 to 15 mph Light
Expected snowfall: 0 in. 0 in. 0 in.
For 8000 ft. to 9000 ft.
Tuesday Tuesday Night Wednesday
Weather: Sunny Partly cloudy becoming clear Sunny
Temperatures: 32 to 39 deg. F. 24 to 31 deg. F. 42 to 49 deg. F.
Wind Direction: East East East
Wind Speed: 0 to 5 mph increasing to 10 to 15 mph in the afternoon 10 to 15 mph with gusts to 25 mph in the evening 10 to 15 mph decreasing in the afternoon
Expected snowfall: 0 in. 0 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.