THIS AVALANCHE FORECAST EXPIRED ON December 8, 2016 @ 6:52 am
Avalanche Forecast published on December 7, 2016 @ 6:52 am
Issued by Brandon Schwartz - Tahoe National Forest

LOW avalanche danger will continue through the day today. Normal caution is advised. Avalanche danger will begin to increase tonight following the onset of snowfall as new wind slabs begin to form in lee areas.

1. Low

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Above Treeline
Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.

1. Low

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Near Treeline
Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.

1. Low

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Below Treeline
Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.
    Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.
Avalanche Problem 1: Wind Slab
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A period of strong SW winds occurred during the day yesterday with minimal amounts of snow on the ground available for wind transport. New wind slab formation is not expected to have occurred on any type of widespread scale, but is not impossible in an isolated location, somewhere within the forecast area. Keep an eye out for signs of recent wind loading such as human triggered cracking, wind pillows, or new cornice formation on NW-N-NE-E-SE aspects in near or above treeline terrain.

Following the onset of snowfall with the approaching storm, new wind slabs are expected to form on a more widespread scale tonight in near and above treeline areas on NW-N-NE-E-SE aspects.

 

Forecast discussion

With a series of storm systems lined up and expected to impact the forecast area, start thinking about how this will potentially affect the facet layer that has been a focus of snowpack monitoring so far this season. There are no guarantees, but day to day snowpit data continues to indicate that this weak layer could become problematic under future loading conditions. Over the next week it is expected that new snow will significantly increase the load on this weak layer. The potential exists for avalanche activity to occur on this layer deep in the snowpack. Again there are no guarantees as to if this will come to fruition. The point is that this is the setup for a persistent deep slab avalanche problem, requiring a very different type of terrain management approach than when dealing with the typical Sierra storm slabs and wind slabs. Have a look at the Avalanche Problem Toolbox and read up on what to expect, how it differs from assessing and managing wind and storm slabs, and how to manage a persistent deep slab problem.

This facet layer exists near the base of the snowpack, usually on top of a ground level ice layer. This weak layer is found on N aspects along the Sierra Crest above about 8,300' and in the Mount Rose area above about about 9,300'.

recent observations
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Observations made yesterday on the N side of Ellis Peak (Blackwood Canyon area) between 7,000' and 8,500' matched well with other recent observations made along the northern portion of the Sierra Crest on N aspects at similar elevation. The problematic facet layer was present near the bottom of the snowpack on top of basal ice (see photo). Snowpit tests indicated that until significant new loading occurred, triggering weak layer failure remains unlikely. However as in other locations, snowpit tests indicated that once weak layer failure is initiated, propagation is likely to occur along this weak layer deep in the snowpack.

Minor amounts of blowing snow were observed on the N side of Ellis Peak in areas that typically receive wind loading, but no new wind slabs of any significant size were observed.

Weather and CURRENT CONDITIONS
weather summary

Cold air has moved into the forecast area with air temperatures in the single digits to teens in nearly all locations this morning. The next in a series of storm systems to impact the forecast area is will arrive later today. Increasing cloud cover is expected ahead of the approaching storm system. Snowfall is expected to begin sometime this afternoon or evening and increase during the overnight hours. Warm air ahead of a wetter, atmospheric river type storm system will move into the region on Thursday. Snow levels are expected to rise on Thursday, switching to rain in the 7,000' to 8,000' range during the afternoon hours. Ridgetop winds are forecast at light to moderate speed out of the S today, increasing to strong in speed out of the SW tonight into Thursday.

CONDITIONS ALONG THE SIERRA CREST BETWEEN 8200 FT. AND 9200 FT. IN THE LAST 24 HOURS
Weather Station Map
Weather Station Table
6am temperature: 9 to 12 deg. F.
Max. temperature: 16 to 30 deg. F.
Average ridgetop wind direction: SW
Average ridgetop wind speed: Prior to 5 pm 35 mph | Since 5 pm 11 mph
Maximum ridgetop wind gust: Prior to 5 pm 67 mph | Since 5 pm 16 mph
New snowfall: 0 to 1 inches
Total snow depth: 10 to 21 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Wednesday Wednesday Night Thursday
Weather: Partly cloudy skies, becoming mostly cloudy. A slight chance of snow in the afternoon. Cloudy skies with snow likely in the evening. Snow after midnight. Cloudy skies with snow through the day. Snow changing to rain in the afternoon.
Temperatures: 27 to 32 deg. F. 23 to 28 deg. F. 37 to 42 deg. F.
Mid Slope Winds: S SW SW
Wind Speed: Light winds 10 to 15 mph with gusts to 25 mph. 10 to 15 mph with gusts to 25 mph, increasing to 25 to 35 mph with gusts to 45 mph in the afternoon.
Expected snowfall: 0 to trace 2 to 4 2 to 6
For 8000 ft. to 9000 ft.
Wednesday Wednesday Night Thursday
Weather: Partly cloudy skies, becoming mostly cloudy. A slight chance of snow in the afternoon. Cloudy skies with snow likely in the evening. Snow after midnight. Cloudy skies with snow.
Temperatures: 25 to 30 deg. F. 22 to 27 deg. F. 34 to 40 deg. F.
Ridge Top Winds: S SW SW
Wind Speed: 10 to 15 mph with gusts to 35 mph. 20 to 30 mph with gusts to 75 mph. 35 to 55 mph with gusts to 90 mph.
Expected snowfall: 0 to trace 2 to 4 3 to 6
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