THIS AVALANCHE ADVISORY EXPIRED ON December 14, 2015 @ 7:05 am
Avalanche Advisory published on December 13, 2015 @ 7:05 am
Issued by Brandon Schwartz - Tahoe National Forest

CONSIDERABLE avalanche danger will form today above and below treeline on NW-N-NE-E-SE aspects on slopes 32 degrees and steeper. MODERATE avalanche danger for all other areas. Wind slabs and persistent slabs are todays avalanche problems. Significant uncertainty surrounds the potential for buried surface hoar below treeline on NW-N-NE aspects. Where surface hoar is buried, avalanche danger may exceed the regional rating of CONSIDERABLE and avalanche activity may occur on low angle slopes down to 25 degrees.

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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In many areas, significant snow remained on the ground and available for wind transport on windward aspects prior to last night's increase in wind speed. Using this snow on the ground, new wind slabs have already begun to build on NW-N-NE-E-SE aspects. Following the onset of snowfall, these wind slabs will continue to build in size and distribution throughout the day. Gale force winds have the ability to deposit wind slabs far down slope below traditional avalanche start zones as well as creating some areas of wind slab below treeline.

Wind slabs tend to be more easily identifiable than other dry snow avalanche problems as they tend to form in open areas and are confined by terrain features. Cornice formations, pillows of drifted snow below ridgelines, pillows of drifted snow in lee areas behind rocks and trees, or entering areas of suddenly denser new snow are all indications of wind slabs in the immediate area. Avoiding these areas with clues of wind slab formation and/or traveling only in areas less than 30 degrees in slope angle without steeper terrain above are simple effective ways to avoid wind slab avalanches.

Avalanche Problem 2: Persistent Slab
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In some areas the existing snowpack is in good condition to handle new snow loading, in other areas it is not. Persistent weak layers in the form of near crust facets around the Dec 10 rain crust and old rounded facets at the bottom of the snowpack that remain loose grained have been observed in various locations around the forecast area. The presence or absence of this avalanche problem will vary widely from one portion of the forecast area to the next. Suspect areas are NW-N-NE aspects mainly near treeline and below treeline with more isolated potential above treeline.

Failure of facets that are newly formed around the Dec 10 rain crust have the potential to propagate far and wide, wrapping around corners and across drainages. This is because a rain crust is much more uniformly created across the colder NW-N-NE aspects than other typical snowpack layers. Collapsing and whumpfing in low angle terrain are about the only indications of instability that will likely be observed prior to avalanche activity. In most cases it will require shovel, hands, and feet in the snow to deliberately seek out information indicating the presence or absence of problematic faceted weak layers in specific areas at slope scale.

Surface hoar is rarely buried in this part of the world, but when it is it is a major avalanche problem. It frequently forms but is almost always destroyed prior to snowfall by warming air temperatures and strong to gale force winds. Widespread surface hoar was observed in numerous locations yesterday afternoon and observed to escape damage from warming air temperatures. Significant uncertainty exists as to whether or not surface hoar survived last night's winds. If it did, it will remain in large open areas below treeline on NW-N-NE aspects with a good open view of the sky and good shelter from strong to gale force SW winds. If in fact surface hoar survives intact and upright until burial today by rapidly accumulating new snow, significant avalanche problems will ensue. Isolated, localized areas of natural and human triggered avalanche activity on low angle slopes down into the 25 to 30 degree range may occur. Significant cracking, collapsing, and/or whumpfing will occur if this avalanche problem forms today. Quick hand pits are a good tool for searching for buried surface hoar at the old/new snow interface.

recent observations

Observations made and received yesterday from the Donner Summit, Mount Rose, Deep Creek, Echo Summit, Carson Pass, and Bear Valley sections of the forecast area revealed little to no lingering wind slab instability from the last storm. Presence or absence of persistent weak layers on NW-N-NE aspects was highly variable from one location to another. Newly formed facets around the Dec 10 rain crust were observed in the Bear Valley and Carson Pass areas. Snowpit tests indicating potential stability problems under new snow loading conditions on this weak layer as well as on old rounded facets at the bottom of the snowpack. Surface hoar was noted in the majority of areas and was observed to survive destruction in areas protected from sun and increasing SW winds.

Weather and CURRENT CONDITIONS
weather summary

A fast moving storm system is increasing SW winds across the forecast area this morning and will bring high intensity snowfall to the area today. Snowfall will begin in most areas this morning. The majority of the precipitation will occur during a 4 to 6 hour period expected between 10 am and 4 pm with snowfall rates reaching 2 to 3 inches per hour. Maximum daytime air temperatures above 7,000' in the mid 20s to mid 30s are expected to occur this morning followed by cooling air temperatures throughout the day. Gale force SW ridgetop winds are starting to show on remote sensors this morning with maximum wind gusts of 85 to 125 mph expected through this evening. Snowfall will likely linger into the overnight hours. Cold north flow is forecast for tomorrow as the storm system exists the region.

Weather observations from along the Sierra Crest between 8200 ft. and 8800 ft.
0600 temperature: 27 to 30 deg. F.
Max. temperature in the last 24 hours: 30 to 32 deg. F.
Average wind direction during the last 24 hours: SW
Average wind speed during the last 24 hours: 35 mph
Maximum wind gust in the last 24 hours: 74 mph
New snowfall in the last 24 hours: 0 to 1 inches
Total snow depth: 26 to 32 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Sunday Sunday Night Monday
Weather: Cloudy skies with snow. Cloudy skies with snow in the evening. A chance of snow showers after midnight. Mostly cloudy skies with a chance of snow showers.
Temperatures: 28 to 35 deg. F. 10 to 16 deg. F. 17 to 24 deg. F.
Wind Direction: SW W to NW N
Wind Speed: 50 to 60 mph with gusts to 85 mph. 45 to 50 mph with gusts to 75 mph, shifting and decreasing to 15 to 20 mph with gusts to 30 mph after midnight. 15 to 25 mph with gusts to 35 mph.
Expected snowfall: 7 to 11 in. 2 to 4 in. Up to 1 in.
For 8000 ft. to 9000 ft.
Sunday Sunday Night Monday
Weather: Cloudy skies with snow. Cloudy skies with snow in the evening. A chance of snow showers after midnight. Mostly cloudy skies with a chance of snow showers.
Temperatures: 25 to 32 deg. F. 5 to 12 deg. F. 16 to 22 deg. F.
Wind Direction: SW W shifting to NW N
Wind Speed: 70 to 85 mph with gusts to 125 mph. 70 to 75 mph with gusts to 115 mph, shifting and decreasing to 20 to 25 mph with gusts to 40 mph after midnight. 25 to 35 mph with gusts to 55 mph.
Expected snowfall: 8 to 12 in. 2 to 4 in. Up to 1 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.