THIS AVALANCHE ADVISORY EXPIRED ON December 22, 2015 @ 7:00 am
Avalanche Advisory published on December 21, 2015 @ 7:00 am
Issued by Brandon Schwartz - Tahoe National Forest

Increasing avalanche danger will occur throughout the day today and tonight. CONSIDERABLE avalanche danger will exist today both near and above treeline on NW-N-NE-E-SE aspects as well as below treeline on NW-N-NE aspects on slopes 32 degrees and steeper. For all other areas, MODERATE avalanche danger exists. HIGH avalanche danger is expected to form tonight and last into the pre dawn hours tomorrow.

 

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.

3. Considerable

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Below Treeline
Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.
    Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.
Avalanche Problem 1: Wind Slab
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Ridgetop winds continue to increase and transport snow with one natural wind slab avalanche large enough to bury or injure a person reported yesterday on Carpenter Ridge (Independence Lake area) on a N aspect in near treeline terrain at approximately 8,700'. As gale force SW winds combine with high intensity snowfall today, wind slabs will become larger and more widespread. The vast majority of wind slabs will form in near treeline and above treeline terrain on NW-N-NE-E-SE aspects, but winds will be strong enough to form wind slabs in some below treeline locations as well. Uses clues such as blowing snow, cornice formations, wind pillows, and cracking along ridgeline edges to determine where wind slabs exist. Unlike other avalanche problems, wind slabs are constrained by terrain and often have well defined visual boundaries, making them easier to avoid.

Avalanche Problem 2: Persistent Slab
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Rapid new snow loading over the next 24 hours will add significant stress to the existing snowpack and its associated weak layers. In areas along the Sierra Crest where a persistent weak layer of collapsed surface hoar capped by the Dec 13 rain crust exists, large destructive natural and human triggered avalanches could occur. This weak layer is now located about 1.5 to 2.5 feet below the snow surface and is expected be buried by an additional 1 to 3 feet of snow over the next 24 hours. In specific locations where this weak layer exists, snowpit tests have produced repeatable results indicating that propagation is likely to follow failure of this weak layer. This weak layer has been observed over the past week along the Sierra Crest in open areas both near treeline and below treeline on NW-N-NE aspects. This weak layer is known to exist in the Ebbetts Pass, Echo Summit, West Shore Tahoe, Ward Canyon, and Donner Summit areas and may exist in other locations as well.

Conservative, lower angle terrain choices are the best way to mitigate this avalanche problem. Without performing numerous snowpits in the area of question to determine presence or absence of the weak layer, entering steep terrain will be a decision made without using any amount of sound decision making. It doesn't mater if you were in the same terrain yesterday or previously during large storms. This is a locally uncommon weak layer that could lead to some large destructive avalanches below treeline.

There have been some indications of a problematic weak layer of near crust facets forming in the Mount Rose area around the Dec 10 rain crust both above and below treeline on NW-N-NE aspects. Potential exists for this weak layer to become overloaded during this storm cycle resulting in large destructive avalanches.

Avalanche Problem 3: Loose Wet
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Snow levels are currently expected to hold in the 5,500' to 6,000' range. If this comes to fruition, loose wet avalanches will not be a problem. If snow levels rise to 7,000' or higher, widespread loose wet avalanche activity is expected to occur on slopes 35 degrees and steeper as rain is deposited on top of new snow on all aspects. Large roller balls and pinwheels in areas of rain on snow will be indications of this avalanche problem developing.

recent observations

Observations made and received from numerous locations around the forecast area yesterday indicated a wide range of assessed snowpack stability ranging from natural and human triggered wind slab avalanche activity to no signs of wind slab or persistent weak layer instability observed. A natural wind slab avalanche large enough to bury or inure a person was reported from Carpenter Ridge (Independence Lake area) on a N aspect near treeline slope at around 8,700'. A small up to 6 inch deep skier triggered wind slab avalanche was reported from Silver Peak (Pole Creek area) in semi-complex near treeline terrain on a generally E aspect around 8,000'. Avalanche Classes operating on Gold Star Point (Bear Valley area) and on Andesite Ridge (Donner Summit area) assessed generally stable snowpack conditions. TNF/SAC forecasters on Elephant's Hump (Carson Pass area) observed significant amounts of wind transport in near treeline areas with wind slabs up to 8 inches thick in lee areas. Snowpit tests revealed good layer bonding around both the Dec 13 and Dec 10 rain crusts in this area with an absence of collapsed surface hoar underneath the Dec 13 crust.

Weather and CURRENT CONDITIONS
weather summary

A powerful storm system has developed over the past 24 hours and will impact the forecast area today. A significant change in the forecast has occurred with heavy precipitation expected today. New snow amounts of 1 to 3 feet are expected above 7,000' over the next 24 hours. Uncertainly remains regarding snow level, but generally 5,500' to 6,000' is the expected with the potential to rise to 7,000'. Ridgetop winds that were expected to increase to strong to gale force in speed by yesterday afternoon remained moderate in speed until around 9 pm last night. Strong SW winds exist this morning over the upper elevations with gusts of 100 to 120 mph expected today. Remote sensors at all elevations are reporting air temperatures this morning at their maximum for the past 24 hours. Maximum daytime air temperatures are forecast to reach the mid 20s to low 30s today and remain in a similar range tonight.

Weather observations from along the Sierra Crest between 8200 ft. and 8800 ft.
0600 temperature: 27 to 31 deg. F.
Max. temperature in the last 24 hours: 27 to 31 deg. F.
Average wind direction during the last 24 hours: SW
Average wind speed during the last 24 hours: 40 mph
Maximum wind gust in the last 24 hours: 71 mph
New snowfall in the last 24 hours: 2 to 5 inches
Total snow depth: 35 to 42 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Monday Monday Night Tuesday
Weather: Cloudy skies with snow. Cloudy skies with snow. Cloudy skies with snow.
Temperatures: 26 to 33 deg. F. 26 to 33 deg. F. 30 to 37 deg. F.
Wind Direction: SW SW W
Wind Speed: 30 to 35 mph with gusts to 45 mph, increasing to 35 to 45 mph with gusts to 65 mph in the afternoon. 35 to 45 mph with gusts to 65 mph. 30 to 35 mph with gusts to 50 mph, decreasing to 15 to 25 mph with gusts to 35 mph in the afternoon.
Expected snowfall: 5 to 10 in. 8 to 15 in. 3 to 6 in.
For 8000 ft. to 9000 ft.
Monday Monday Night Tuesday
Weather: Cloudy skies with snow. Cloudy skies with snow. Cloudy skies with snow.
Temperatures: 24 to 31 deg. F. 24 to 31 deg. F. 27 to 34 deg. F.
Wind Direction: SW SW W
Wind Speed: 60 to 65 mph with gusts to 100 mph, increasing to 80 to 85 mph with gusts to 120 mph in the afternoon. 80 to 85 mph with gusts to 125 mph decreasing to 65 to 70 mph with gusts to 105 mph after midnight. 60 to 65 mph with gusts to 100 mph, decreasing to 50 to 55 mph with gusts to 85 mph in the afternoon.
Expected snowfall: 5 to 10 in. 10 to 15 in. 3 to 6 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.