THIS AVALANCHE ADVISORY EXPIRED ON January 25, 2018 @ 6:59 am
Avalanche Advisory published on January 24, 2018 @ 6:59 am
Issued by Steve Reynaud - Tahoe National Forest

Moderate avalanche danger will exist for most of today due to a wind slab avalanche problem.  Human triggered wind slabs will be possible in near treeline and above treeline terrain.  After the onset of snowfall this afternoon, avalanche danger will quickly rise to considerable due to wind slab, storm slab, and deep persistent slab avalanche problems.  Dangerous avalanche conditions will exist.

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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Strong to gale force S/SW winds overnight will increase through today and build reactive wind slabs on W-NW-N-NE-E-SE aspects in near and above treeline terrain.  In many areas there is substantial available snow for transport on windward slopes to form windslabs today.  Wind slabs will continue to build throughout the afternoon and evening as snowfall begins.  Human triggered avalanches are possible today with natural wind slab avalanches likely this afternoon and evening as new snowfall intensifies.

Look for blowing snow, new cornice formation, and wind pillows as clues to where wind slabs are forming.  Avoid steep wind loaded terrain.

Avalanche Problem 2: Storm Slab
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Storm slabs will become likely on all aspects in near and below treeline terrain after the onset of snowfall this afternoon.  Surface hoar along with shallowly buried surface hoar has been observed in several locations throughout the forecast area.  This surface hoar has been found in open areas in near treeline and below treeline terrain where winds have not destroyed it.  Buried surface hoar is capable of being remotely triggered and can fail on lower angle slopes.  Storm slabs could also have weakness within the new storm snow or at the old snow/new snow interface. 

Look for cracking around skis, shooting cracks, or any signs of cohesion within the new storm snow.  These storm slabs will be in wind protected near and below treeline terrain.  Avoid areas that show signs of unstable snow.

Avalanche Problem 3: Deep Slab
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The weak faceted layer of snow that has affected our forecast area over the last several weeks is now buried under 2 to 3'+ of snow and has become a deep persistent slab avalanche problem.  This problem exists in isolated areas on NW-N-NE aspects in near treeline and below treeline terrain.  Although avalanche activity has been limited, concern exists that this layer could fail with a large load of new snow.  Any avalanche due to this deep slab avalanche problem would have large consequences due to increased slab size involved. 

Deep slab avalanches are unpredictable and hard to collect additional information on.  Tracks on a slope will not be an indicator of stability.  Look for collapsing, cracking, and whumpfing sounds.  Remote triggering is possible, along with failure mid to low slope.  A storm snow related avalanche could have the potential to step down into these weak old snow layers.  Avoid steep avalanche terrain and runout zones where this problem may exist. 

 

recent observations

* Buried surface hoar has been observed on Andesite Peak (Donner Summit area), Silver Peak (Pole Creek area), Lincoln Ridge (Yuba Pass area), and Squaw Ridge (Carson Pass area).  This layer of surface hoar is buried 1 to 2'' below the snow surface.  Many areas also had surface hoar on the snow surface up to yesterday afternoon.

* In isolated areas, the persistent weak layer continues to show unstable results with snowpack tests.  This layer is now buried 2 to 3'+ deep in the snowpack.

* Windward slopes in many areas had substantial available snow for wind transport.

* A mix of rain and snow occurred up to at least 8200 ft. in some areas on Monday, leaving a thin rain crust or slightly heavier snow on the snow surface.

Weather and CURRENT CONDITIONS
weather summary

A cold storm system will bring strong winds with moderate to locally heavy snow to our forecast area today and through tomorrow.  The onset of snow is forecasted for 4pm this afternoon, with the heaviest snow between 5pm to midnight.  8 to 16'' of snow is possible from Tahoe northward above 7000' by Thursday morning.  Gale force S/SW winds are forecasted to continue through today increasing into the afternoon.  SW winds at 6am where in the 50 to 60mph range along the Sierra Crest.  Snowfall rates could peak up to 2 to 3''/hour tonight.

Another round of snow shower activity is possible for Thursday afternoon into the evening.  Heavy convective bands could form from Lake Tahoe northward.  Depending on where these convective bands set up, potential exists for an additional 6 to 12'' of snow in some locations.

We dry out over the weekend with warming temperatures and variably cloudy skies.

Weather observations from along the Sierra Crest between 8200 ft. and 8800 ft.
0600 temperature: 26 to 38 deg. F.
Max. temperature in the last 24 hours: 42 deg. F.
Average wind direction during the last 24 hours: SW
Average wind speed during the last 24 hours: 40 to 50 mph
Maximum wind gust in the last 24 hours: 75 mph
New snowfall in the last 24 hours: 0 inches
Total snow depth: 25 to 46 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Wednesday Wednesday Night Thursday
Weather: Mostly cloudy. Chance of snow in the afternoon. Cloudy. Snow. Mostly cloudy. Scattered snow showers in the morning, then widespread snow showers in the afternoon.
Temperatures: 40 to 45 deg. F. 18 to 23 deg. F. 23 to 28 deg. F.
Wind Direction: S SW SW
Wind Speed: 20 to 30mph with gusts to 65mph increasing to SW 30 to 45mph with gusts to 80mph in the afternoon. 25 to 35mph with gusts to 70mph decreasing to 15 to 25mph with gusts to 60mph after midnight. 15 to 20mph with gusts to 35mph.
Expected snowfall: Likely up to 1 in. | Possible 1 to 3 in. Likely 5 to 11 in. | Possible 11 to 14 in. Likely 2 to 6 in. | Possible 6 to 10 in.
For 8000 ft. to 9000 ft.
Wednesday Wednesday Night Thursday
Weather: Mostly cloudy. Chance of snow in the afternoon. Cloudy. Snow. Mostly cloudy. Scattered snow showers in the morning, then widespread snow showers in the afternoon.
Temperatures: 35 to 41 deg. F. 14 to 19 deg. F. 18 to 23 deg. F.
Wind Direction: S SW SW
Wind Speed: 45 to 65mph. Gusts up to 105mph increasing to 115mph in the afternoon. 40 to 55mph with gusts to 100mph decreasing to 30 to 50mph with gusts to 85mph after midnight. 20 to 35mph. Gusts up to 65mph decreasing to 50mph in the afternoon.
Expected snowfall: Likely up to 1 in. | Possible 1 to 3 in. Likely 6 to 12 in. | Possible 12 to 16 in. Likely 2 to 6 in. | Possible 6 to 10 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