THIS AVALANCHE ADVISORY EXPIRED ON January 23, 2018 @ 6:54 am
Avalanche Advisory published on January 22, 2018 @ 6:54 am
Issued by Andy Anderson - Tahoe National Forest

MODERATE avalanche danger exists at all elevations due to newly formed wind slabs and unlikely but not impossible persistent slab avalanche problems lingering in isolated areas. In the unlikely event that an isolated human-triggered persistent slab avalanche does occur, it could be large with significant consequences.

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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Small wind slabs sensitive to human-triggering formed near ridgelines on wind-loaded slopes by yesterday afternoon. After a night of continued wind-loading, wind slabs may now exist on wind-loaded N-NE-E aspects and some cross-loaded NW and SE aspects in near and above treeline terrain. Most of these wind slabs should remain small, not involve enough snow to bury a person, and not extend very far from ridgelines. In the most wind-loaded areas or in areas where terrain traps could magnify the consequences of any size avalanche, some of them could have grown large enough to bury a person.

Avalanche Problem 2: Persistent Slab
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While triggering a persistent slab avalanche may be unlikely, snowpit tests continue to yield unstable results indicating that persistent slab avalanches are not impossible for just the right trigger in just the right spot on some isolated near and below treeline NW-N-NE aspects sheltered from previous NE winds. If a person does trigger one of these isolated, difficult to trigger (stubborn) persistent slabs, the resulting avalanche could be large (up to D2 or D3) and destructive. Which isolated slopes may still hold a persistent slab avalanche problem is subject to significant uncertainty and variability with no clear explanation for the variations in stability across similar terrain.

Due to the uncertainty, variability, and potential high consequences of this difficult to trigger (low probability) avalanche problem, avoiding terrain where persistent slabs may exist represents a prudent choice. Collapsing and whumpfing, unstable snowpit test results, and handpits/probing into the snowpack to feel for a loose layer of weak snow under firm slab layers can help identify where persistent slab problems may still exist. Keep in mind that snowpits dug high on the slope may miss evidence of this problem since it is more likely in the mid to lower portions of the slope.

recent observations

* Yesterday, a snowpit test on a N aspect on Tamarack Peak (Mt. Rose backcountry) and several tests on N-NE aspects on Red Lake Peak (Carson Pass area) indicated that while a persistent slab may be difficult to trigger if the weak layer does break the fracture could still travel through it. Other tests from Elephant's Hump (Carson Pass), Upper Blue Lake and Twin Lake areas (South of Carson Pass), and Cold Stream Drainage (Donner Summit area) in the last few days agree with the tests from yesterday. This evidence of stubborn instability exists in near and below treeline terrain on some NW-N-NE aspects sheltered from NE winds. 

* Other tests and observations from additional similar locations on Tamarack Peak (Mt. Rose backcountry) and Castle Peak (Donner Summit) yesterday, as well as observations and data from Andesite Peak (Donner Summit) and National Geographic Bowl (Granite Chief Wilderness) on Saturday, did not reveal any signs of instability associated with the persistent weak layer and snowpit tests indicated a well-consolidated snowpack.

* Ski cuts yesterday afternoon on a small NE facing wind-loaded test slope on Tamarack Peak triggered a small wind slab failure. Winds started to transport snow in that area in the late morning.

Weather and CURRENT CONDITIONS
weather summary

Some light snow showers should continue early this morning as a weak system passes through the area. So far these showers have provided an additional 1 to 2 inches of snow in some places and a dusting of new snow in others. Southwest winds picked up yesterday and continued through the night. As this disturbance exits the region today the winds and cloud cover should decrease, and the snow showers should come to an end without much additional accumulation. The forecast calls for calm and cool weather tonight and tomorrow. 

Weather observations from along the Sierra Crest between 8200 ft. and 8800 ft.
0600 temperature: 21 to 26 deg. F.
Max. temperature in the last 24 hours: 28 to 32 deg. F.
Average wind direction during the last 24 hours: Southwest
Average wind speed during the last 24 hours: 35 mph
Maximum wind gust in the last 24 hours: 78 mph
New snowfall in the last 24 hours: 1 to 2 inches
Total snow depth: 25 to 47 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Monday Monday Night Tuesday
Weather: Mostly cloudy with scattered snow showers in the morning Mostly cloudy Mostly cloudy becoming partly cloudy in the afternoon
Temperatures: 34 to 39 deg. F. 19 to 24 deg. F. 39 to 44 deg. F.
Wind Direction: Southwest Variable Variable
Wind Speed: 15 to 20 mph with gusts to 45 mph decreasing to 35 mph in the afternoon Light with some gusts to 25 mph in the evening Light
Expected snowfall: up to 1 in. 0 in. 0 in.
For 8000 ft. to 9000 ft.
Monday Monday Night Tuesday
Weather: Mostly cloudy with scattered snow showers in the morning Mostly cloudy Mostly cloudy becoming partly cloudy in the afternoon
Temperatures: 30 to 35 deg. F. 18 to 23 deg. F. 35 to 40 deg. F.
Wind Direction: Southwest Southwest Southwest
Wind Speed: 25 to 35 mph with gusts to 60 mph decreasing to 15 to 25 mph with gusts to 45 mph in the afternoon 10 to 15 mph with gusts to 30 mph in the evening decreasing overnight 10 to 15 mph with gusts to 30 mph
Expected snowfall: up to 1 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.

For a recorded version of the Avalanche Advisory call (530) 587-3558 x258