THIS AVALANCHE FORECAST EXPIRED ON November 25, 2016 @ 6:59 am
Avalanche Forecast published on November 23, 2016 @ 6:59 am
Issued by Brandon Schwartz - Tahoe National Forest

This is a two day advisory, valid now through Thanksgiving Day. Avalanche danger is MODERATE on all aspects at elevations above 7,000' on slopes 30 to 35 degrees and steeper due to a mix of wind slabs, persistent slabs, and storm 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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Ridgetop winds increased yesterday afternoon out of the SW and drifting snow was observed near and above treeline. Plenty of recent storm snow was available on the ground for wind transport. New snow last night combined with SW winds will have further wind loaded NW-N-NE-E-SE aspects in near treeline and above treeline areas. New wind slabs are expected in these areas. Ridgetop winds are expected to shift to the W Wednesday afternoon and then back to the SW on Thanksgiving Day. Ridgetop wind speeds are expected to remain moderate to strong in speed and capable of moving snow. This avalanche problem could exist on slopes 35 degrees and steeper.

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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In the northern portion of the forecast area, snowpit data points to a persistent slab avalanche problem. Along the Sierra Crest from about Tahoe City northwards, above 8,300' and in the Mount Rose area above 9,000' to 9,300', evidence of an unstable snowpack exists on N aspects. Snow that was deposited at the very end of October and during the first half of November has become faceted and weak. In areas along the northern Sierra Crest below 9,000' this faceted snow is capped by a problematic rain crust from Nov 16. This weak layer of faceted snow is sandwiched between 1 to 3 feet of recent storm snow on top and basal ice on the bottom. 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 32 degrees and steeper.

Here is the rub. This high consequence avalanche problem exists in the areas that have the best snow coverage in the forecast area. Due to the depth of the weak layer, informal observations are unlikely to highlight instability until an avalanche occurs. Previous tracks are neither a sign of stability or intelligent life. Either get out the shovel and perform snowpit work to determine if this avalanche problem/weak layer exists in the particular area of interest or go the easy route and avoid N aspects that meet the location and elevation criteria for 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.

Avalanche Problem 3: Storm Slab
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While new snow amounts Tuesday night into Wednesday morning were fairly modest at 2 to 6 inches, several reports were received of surface hoar (feathery surface snow) persisting well into the Tuesday afternoon hours in wind sheltered areas on all aspects near and below treeline. Air temperatures in many areas above 7,500' on Tuesday were insufficient to destroy surface hoar and SW winds did not increase to the strong range until around sunset. This leaves a fair bit of uncertainty as to if any areas of surface hoar persisted in an upright position and were buried under the new snow. While any associated avalanches will be small, they would be very easily triggered and could have consequences if a person were caught in an avalanche in a terrain trap such as the sidewall of a large creek bed. This avalanche problem could exist on slopes 30 degrees and steeper.

Forecast 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

Snowpit data collected yesterday on the N side of Castle Peak on a true N aspect at 8,560' pointed towards significant persistent slab instability. Three different types of snowpit tests showed very repeatable evidence of instability on faceted old snow at the base of the 2 feet of recent storm snow from last weekend. Check out the video. Drifting snow was observed in near treeline areas during the afternoon hours. Persisting surface hoar was noted in wind protected areas and as destroyed in wind exposed areas. A few small loose dry avalanches with long run outs were noted on NE aspects on Castle Peak, likely triggered by small cornice collapses. Other reports of persisting surface hoar were received from Tamarack Peak (Mount Rose area) and from Donner Ridge ( Donner Summit area).

 

Weather and CURRENT CONDITIONS
weather summary

The storm system from Tuesday night/Wednesday morning is expected to taper off during the late morning hours Wednesday. Short lived high pressure is expected to build for Thanksgiving Day and last into Friday morning. The next storm system is expected to bring additional snow to the forecast area Friday night into Saturday. Maximum daytime air temperatures above 7,000' are expected to remain in the 20s to near freezing on Wednesday, then warm into the 30s, possibly pushing 40 degrees in some areas on Thursday. Ridgetop winds are expected to remain moderate to strong in speed out of the W to SW for the next few days.

Weather observations from along the Sierra Crest between 8200 ft. and 8800 ft.
0600 temperature: 22 to 28 deg. F.
Max. temperature in the last 24 hours: 28 to 34 deg. F.
Average wind direction during the last 24 hours: SW
Average wind speed during the last 24 hours: 32 mph
Maximum wind gust in the last 24 hours: 95 mph
New snowfall in the last 24 hours: 2 to 6 inches
Total snow depth: Isolated areas to 48 inches, generally 5 to 20 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Wednesday Wednesday Night Thursday
Weather: Cloudy skies, becoming partly cloudy. Snow in the morning. Isolated snow showers in the afternoon. Clear skies. Sunny skies.
Temperatures: 28 to 33 deg. F. 20 to 26 deg. F. 39 to 44 deg. F.
Winds: SW SW S
Expected snowfall: Up to 2 in. 0 in. 0 in.
For 8000 ft. to 9000 ft.
Wednesday Wednesday Night Thursday
Weather: Cloudy skies, becoming partly cloudy. Snow in the morning. Isolated snow showers in the afternoon. Clear skies. Sunny skies.
Temperatures: 24 to 30 deg. F. 20 to 25 deg. F. 34 to 40 deg. F.
Winds: SW to W SW SW
Expected snowfall: Up to 2 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