THIS AVALANCHE ADVISORY EXPIRED ON November 27, 2016 @ 6:58 am
Avalanche Advisory published on November 26, 2016 @ 6:58 am
Issued by Andy Anderson - Tahoe National Forest

MODERATE avalanche danger exists on N aspects steeper than 30 degrees in all terrain above 8000 ft. due to possible persistent slabs. MODERATE avalanche danger also exists on NW-N-NE-E-SE aspects in near and above treeline terrain on slopes steeper than 35 degrees due wind slabs. Large persistent slab avalanches are possible on true N aspects in the northern portion of the forecast area.

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 that formed yesterday may remain sensitive to human triggering today. These stiff hard wind slabs may feel firm on the surface and sound hollow when traveled on. In addition to breaking when weighted at the top, these wind slabs can break after one has committed to the slope or when a person reaches the edge of the slab where a thinner slab exists. Most of these wind slabs should remain relatively small and not extend too far downslope from where they form, but even a small avalanche in these low tide conditions would have serious consequences. Some of these wind slabs could have grown large enough to bury a person in the most heavily wind loaded areas. Despite the fact that yesterday's winds already moved most of the snow available for transport, today's S and SW winds could continue to add to these wind slabs. Wind slabs will most likely exist on the near and above treeline NW-N-NE-E-SE aspects near ridgelines. Some wind slabs may exist in unusual places as the S winds take over today. This avalanche problem could exist on slopes 35 degrees and steeper. As more snow and wind impact the forecast area tonight wind slabs will become larger and more widespread.

Avoid areas of recently drifted snow by identifying cornice features and wind pillows. Avoid venturing into steep terrain in areas where human triggered snow surface cracking is observed around the edges of wind slabs.

Avalanche Problem 2: Persistent Slab
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While triggering a persistent slab avalanche may have become slightly more difficult for today, a persistent weak layer (PWL) still exists below the recent snow on N aspects above 8300 ft. on the Sierra Crest and above 9000 ft. in the Mt. Rose area. In places where this PWL has enough snow above it to form a slab, snowpit tests have indicated that if the PWL does break the resulting fracture could propagate along the weak layer and cause a persistent slab avalanche. Tests have yielded unstable results on N aspects along the Sierra Crest north of Tahoe City especially in the Castle Peak area and in the Mount Rose area. Where this weak layer exists, it crosses over sub-ridges, spines, and other features connecting multiple avalanche paths and start zones together. Given the persistent nature and depth of the weak layer, any avalanches that occur could be wide, large, and destructive. This avalanche problem could exist on slopes 30 degrees and steeper. As more snow loads the snowpack tonight, these persistent slabs may become easier to trigger.

Unfortunately, the areas most likely to hold this persistent avalanche problem also represent the areas with the most snow coverage. Informal observations will not prove reliable to determine which slopes are safe to travel on. Often actual avalanches represent the only clue other than snowpit data that persistent slab avalanche problems exist on a slope. These kinds of avalanches can occur after several people have already used a slope so previous tracks do not provide good clues to stability or instability. Dig into the snowpack to see if this layer exists or go the easy route and avoid N aspects that meet the location and elevation criteria for the presence of this avalanche problem. Keep in mind that this is the type of avalanche problem that could be triggered from low on the slope or even remotely from low angle connected terrain.

advisory discussion

The usable snowpack within the forecast area exists along the northern portion of the Sierra Crest. Areas above 7,000' and north of Tahoe City have sufficient amounts of supportable snow for quality over snow travel (2 to 4 feet). Along the Sierra Crest south of Tahoe City, snowpack depths decrease dramatically, even at the highest elevations. In these areas the snowpack is unusable for skiing/boarding/snowmobiling. The Mount Rose area in the NE portion of the forecast area has decent snowcover (~2 feet) in areas that have held snow/ice since mid Oct. This is generally N aspects above 9,300'. Below 9,300' on N aspects or on all other aspects in the Mount Rose area, over snow travel is possible, but impact with rocks is likely with snow depths around 1 foot.

Due to the wide variances in snowcover between the northern and southern portions of the forecast area, this advisory is heavily biased towards the northern areas where the snowpack is usable for over snow travel.

The avalanche center is still working on a limited number of observations so please submit observations through the "submit observations" buttons found throughout this website.

recent observations

Yesterday observations on Castle Peak revealed significant wind transport of the recent snow. Several hard wind slabs existed on the leeward aspects near ridgelines, and large cornices had formed above some of the more exposed N-NE aspects. Ski kicks on wind-loaded test slopes caused some shooting cracks and one wind slab failure that measured about 8 to 12 inches at its deepest and extended downslope for about 10 to 15 ft. Possible debris from what may have been a wind slab avalanche also existed in one of the NE facing gullies on the north side of the Peak. Due to conditions, confirming any details on this possible avalanche proved unrealistic so a great deal of uncertainty exists about this.

Snowpit data showed that the 11/16 facet layer still exists on N aspects above 8300 ft. in the Castle Peak area. Tests targeting this layer yesterday indicated that it may have become more difficult to break in some areas, but if it does break the resulting fracture could easily travel along that layer. Snowpits on more E and NE aspects found that this layer of facets was either non-existent or much less well developed. On the NE aspects above 8000 ft. snowpit observations and tests showed that a thin layer of weaker snow may be forming at the base of the recent snow on top of the first ice crust. 

Weather and CURRENT CONDITIONS
weather summary

Strong south and southwest winds will continue to buffet the region today and tonight as two small low-pressure troughs move through the area late this afternoon and tonight. These systems will keep temperatures in the upper teens to mid 20's above 7000 ft. tonight. Snowfall from these systems could start as early as this afternoon, but most of the precipitation should fall during the night as the second of the two systems pushes through the area. The forecast calls for 4 to 8 inches of new snow above 7000 ft. tonight with snow levels below 5000 ft. Snowfall should begin to taper off tomorrow as the system exits the region, and the winds should start to shift to the northwest and decrease. By the end of the day tomorrow, snowfall totals could reach 3 to 8 inches at Lake level and 6 to 12 inches in the mountains. Temperatures should remain in the 20's for daytime highs tomorrow. For more information check in with the Reno NWS.

Weather observations from along the Sierra Crest between 8200 ft. and 8800 ft.
0600 temperature: 29 to 36 deg. F.
Max. temperature in the last 24 hours: 33 to 41 deg. F.
Average wind direction during the last 24 hours: Southwest to South
Average wind speed during the last 24 hours: 35 to 50 mph
Maximum wind gust in the last 24 hours: 90 mph
New snowfall in the last 24 hours: 0 inches
Total snow depth: 1 to 19 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Saturday Saturday Night Sunday
Weather: Partly cloudy becoming mostly cloudy with a chance of snow in the afternoon. Cloudy with snow likely in the evening. Snow becoming more widespread during the night. Cloudy with snow in the morning. Snow tapering off and becoming isolated snow showers in the afternoon.
Temperatures: 33 to 39 deg. F. 19 to 24 deg. F. 24 to 29 deg. F.
Wind Direction: South Southwest Northwest
Wind Speed: 20 to 35 mph with gusts to 60 mph 20 to 30 mph with gusts to 55 mph 15 to 25 mph with gusts to 50 mph decreasing to 35 mph in the afternoon
Expected snowfall: 0 to 1 in. 4 to 8 in. 2 to 3 in.
For 8000 ft. to 9000 ft.
Saturday Saturday Night Sunday
Weather: Partly cloudy becoming mostly cloudy with a chance of snow in the afternoon. Cloudy with snow likely in the evening. Snow becoming more widespread during the night. Cloudy with snow in the morning. Snow tapering off and becoming isolated snow showers in the afternoon.
Temperatures: 30 to 36 deg. F. 16 to 21 deg. F. 20 to 25 deg. F.
Wind Direction: South Southwest West shifting to the northwest in the afternoon
Wind Speed: 35 to 55 mph with gusts to 90 mph decreasing to 80 mph in the afternoon 30 to 45 mph with gusts to 90 mph 25 to 35 mph with gusts to 85 mph decreasing to 15 to 25 mph with gusts to 50 mph in the afternoon
Expected snowfall: 0 to 1 in. 5 to 8 in. 2 to 4 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