THIS AVALANCHE ADVISORY EXPIRED ON December 23, 2015 @ 7:01 am
Avalanche Advisory published on December 22, 2015 @ 7:01 am
Issued by Steve Reynaud - Tahoe National Forest

Avalanche danger is considerable on NW-N-NE-E-SE aspects at all elevations on slopes steeper than 32 degrees.  Moderate avalanche danger exists on S-SW-W aspects at all elevations.  Natural avalanches are possible and human triggered avalanches are likely due to winds slabs, storm slabs and persistent slabs.  We have received 2 to 3' of wet, dense, heavy snow over the last 24 hours.  Dangerous avalanche conditions exist.  Careful snowpack evaluation, cautious route finding, and conservative decision making are essential today.

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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2 to 3 feet of new snow has been deposited in the last 24 hours above 7500' along the Sierra Crest.  Strong to gale force SW winds have moved this new snow to NW-N-NE-E-SE aspects and created large wind slabs in excess of 4-5' deep.  With this heavy load of new snow last night expect to see natural avalanche activity related to these wind slabs last night and through the morning hours.  Winds in the 60 to 70mph range with gusts over 100mph will make for wind slabs possible below treeline and much further down slope than normal.  Gale force winds will continue throughout today continuing wind slab development.  Look for obvious clues such as blowing snow, cornice formation, snow surface scouring, and wind pillows to determine where these wind slabs exist.

Avalanche Problem 2: Storm Slab
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2 to 3 feet of new snow has fallen across the Sierra Crest in the last 24 hours.  Temperatures have trended warmer throughout the storm and are expected to peak as the storm exits mid day today.  This warming trend has caused a density change with the heavier (more dense) snow on the top and the lighter (less dense) snow on the bottom.  This density change will allow storm slab avalanches to be a problem for today within the new snow that has fallen over the last 24 hours.  Storm slabs can exist anywhere the recent storm has deposited snow which has not been blown away or scoured by the strong to gale force SW winds.  Signs of avalanche instability for storm slabs could be recent avalanche activity, whumping, shooting cracks and collapsing.

Avalanche Problem 3: Persistent Slab
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Through the forecast area where the persistent weak layer of collapsed surface hoar exists capped by the 12/13 rain crust, large and destructive natural and human triggered avalanches are possible today.  This layer has been observed in the Ebbetts Pass, Echo Peak, West Shore, Ward Canyon, Deep Creek, Donner Summit and potentially in other locations as well.  Snowpack tests targeting this persistent weak layer over the last week have shown consistent results that indicate propagation is likely if failure of the weak layer could occur.

Near crust facets have developed around the 12/10 rain crust in some areas of the forecast region.  Test results indicate that human triggering is unlikely but with a large load of snow, as is currently occurring, it could become possible.

Either of these above situations could cause large destructive avalanches involving in excess of 4-6' of snow.  These persistent slab avalanche problems are not common in our maritime snow climate-the consequences involved would be severe.

recent observations

Recent observations yesterday from Tamarack Peak (Mt. Rose area) and Powderhouse Peak (Luther Pass area) showed 8 to 12'' of new heavy snow fell through the daytime hours. Wind slabs were developing throughout the day with snowpack tests and informal observations pointing to these wind slabs getting larger and weaker.  Gale force winds and blowing snow made for almost zero visibility.  Targeting the 12/10 rain crust on Tamarack Peak showed that weak faceted snow does still exist around this crust and propagation is possible.  While skier triggering may have become unlikely, additional large loads may make it possible to fail this layer.

Weather and CURRENT CONDITIONS
weather summary

Major winter storm that has impacted the Sierra yesterday and last night is slowing winding down this morning.  A second wave of moisture impacted the region early this morning around 3am with winds along the Sierra Crest at 60-80mph and gusts over 100mph.  Snow levels have been fluctuating between 6200-7000' and should top out around 7000' for the remainder of the day.  Wet heavy snow is expected to taper throughout the morning and snow totals along the Sierra Crest are around 2-3 feet.  Winds will be slightly less than yesterday but still in the 60-75mph range with gusts over 100mph along the crest throughout the day.

Weather observations from along the Sierra Crest between 8200 ft. and 8800 ft.
0600 temperature: 32 deg. F.
Max. temperature in the last 24 hours: 32 deg. F.
Average wind direction during the last 24 hours: W
Average wind speed during the last 24 hours: 40 to 65 mph
Maximum wind gust in the last 24 hours: 110 mph
New snowfall in the last 24 hours: 20 to 36 inches
Total snow depth: 48 to 60 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Tuesday Tuesday Night Wednesday
Weather: Cloudy. Snow and rain. Cloudy. Snow and rain likely in the night, then chance of snow after midnight. Mostly cloudy, slight chance of snow in the morning.
Temperatures: 31 to 38 deg. F. 18 to 25 deg. F. 27 to 34 deg. F.
Wind Direction: W W W
Wind Speed: 30 to 40mph with gusts to 55mph. 20 to 30mph with gusts to 55mph. 15 to 25mph with gusts to 35mph.
Expected snowfall: 2 to 4 in. Up to 1 in. 0 in.
For 8000 ft. to 9000 ft.
Tuesday Tuesday Night Wednesday
Weather: Cloudy. Snow. Cloudy. Snow likely in the evening then snow showers likely after midnight. Mostly cloudy. Slight chance of snow in the morning.
Temperatures: 29 to 36 deg. F. 16 to 23 deg. F. 25 to 32 deg. F.
Wind Direction: W W NW
Wind Speed: 65 to 75mph with gusts to 110mph. 60 to 70mph with gusts to 105mph. 40 to 50mph with gusts to 75mph.
Expected snowfall: 2 to 4 in. Up to 2 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.