THIS AVALANCHE FORECAST EXPIRED ON December 27, 2020 @ 7:00 am
Avalanche Forecast published on December 26, 2020 @ 7:00 am
Issued by Andy Anderson - Tahoe National Forest - Sierra Avalanche Center

Strong winds and heavy snow will bring back dangerous avalanche conditions. The slabs of new snow will likely overload the old weak layers in sheltered areas. In wind-loaded areas, fragile slabs of wind drifted snow will exist. The complicated avalanche conditions mean that advanced route finding and decision making skills are required for backcountry travel. Traveling on slopes less steep than 30 degrees not connected to steeper slopes represents a safer choice. CONSIDERABLE avalanche danger exists at all elevations.

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: Persistent Slab
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Persistent slabs are back. Prior to this storm, it took several days for the persistent weak layers buried in the snowpack to adjust to the snow above them. Old weak sugary snow (the Dec 11 facets) existed about a foot below the surface on most near and below treeline W-NW-N-NE-E aspects. Observers have even found it in a few isolated areas above treeline. Observers have also reported another thin layer of weak snow closer to the surface. Now 6  to 10 inches of new, dense, heavy snow has accumulated on top of the snowpack in less than 12 hours. This new load will likely push the old weak snow to the breaking point again. Adding a person on top of that new snow would likely provide enough of a trigger to cause a persistent slab avalanche today. These avalanches could break partway down a slope, propagate farther than expected, and people could trigger them remotely from less steep terrain. These avalanches could also have serious consequences as many obstacles still exist in the shallow snowpack.

Persistent slabs represent one of the most difficult avalanche problems to manage. Sometimes cracking and whumpfing can provide clues to instability; sometimes triggering a large avalanche is the only clue to instability you see. Avoiding travel in avalanche terrain where persistent slabs might lurk is the best strategy for dealing with persistent slabs. The best travel advice for this kind of avalanche problem: stick to slopes less steep than 30 degrees that are not connected to steeper slopes for fun snow and better safety margins.

Avalanche Problem 2: Wind Slab
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The strong winds will move the new snow around and pile it up on the leeward slopes. These new slabs of wind drifted snow will form on top of a variety of old snow surfaces including firm wind-scoured and wind-packed snow. The new wind slabs will be fragile today since they have just formed and have not had time to bond to the existing snow surfaces. In some cases, these wind slabs may form on slopes where the persistent weak layers mentioned above exist. Human-triggered wind slab avalanches will be likely on the most heavily wind-loaded slopes. Larger avalanches that could propagate much farther than expected and be remotely triggered could occur where the wind slabs exist above the persistent weak layers. 

Newly developed cornices, wind pillows, and wind created surface textures can all help to identify which slopes may hold wind slabs. Signs of wind slabs instability include recent avalanches, cracking, or signs of denser snow over less dense snow. Avoiding these fragile wind slabs only represents part of the strategy today since the persistent slabs mentioned above are likely to exist in many of the more sheltered terrain. Managing multiple avalanche problems at all elevations will be required today. 

recent observations
Submit Observations

* Strong winds have hammered the snowpack and observers have reported widespread wind-affected snow and wind drifted snow in many places over the last few days.

* Signs of instability and unstable snowpack test results on the Dec 11 persistent weak layer decreased prior to this storm. Despite this overall decrease in instability, this layer still exists and it remains variable with some areas still showing unstable snowpack test results. Data also indicated that additional snow load could reactivate this weak layer.

* Observers have also reported a second potentially problematic layer across the forecast area that was buried on Dec. 17. This thin layer of weak snow continued to show signs of potential instability as of yesterday. 

* Overall data and observations indicate that the snowpack may be too weak to support a new snow load. 

Weather and CURRENT CONDITIONS
weather summary

Clouds and wind moved in yesterday with a few light showers in the morning. By the afternoon the winter storm started to push into the area. It dropped 1 to 2 inches of snow in the late afternoon/early evening and another 4 to 8 inches during the night. The forecast calls for up to 3 more inches before noon. Sensors indicate that the Sierra Crest got more snow than the east side of the lake with the northern part of the forecast area getting slightly more snow than the southern part. Widespread strong SW winds accompanying this storm should continue through today. The storm should move out of the area today leaving colder drier weather for tonight and tomorrow. By Sunday night another storm will begin to impact the region with more snow and wind. Check-in with the Reno NWS for more details. 

CONDITIONS ALONG THE SIERRA CREST BETWEEN 8200 FT. AND 9200 FT. IN THE LAST 24 HOURS
Weather Station Map
Weather Station Table
6am temperature: 22 to 26 deg. F.
Max. temperature: 30 to 35 deg. F.
Average ridgetop wind direction: SW
Average ridgetop wind speed: 45 to 60 mph
Maximum ridgetop wind gust: 112 mph
New snowfall: 5 to 10 inches
Total snow depth: 24 to 36 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Saturday Saturday Night Sunday
Weather: Mostly cloudy then becoming partly cloudy. Snow likely in the morning, then slight chance of snow showers in the afternoon. Snow levels below 7000 feet. Chance of precipitation is 70%. Mostly cloudy. Snow levels below 7000 feet. Chance of precipitation is 0%. Mostly cloudy. Slight chance of snow. Snow levels below 7000 feet. Chance of precipitation is 20%.
Temperatures: 29 to 34 deg. F. 15 to 21 deg. F. 28 to 33 deg. F.
Mid Slope Winds: Southwest 20 to 35 mph with gusts to 50 mph diminishing to southwest 15 to 25 mph with gusts to 40 mph in the afternoon. Light winds. Light winds.
Expected snowfall: 80% probability of 2 to 5 inches. 20% probability up to 2 inches. | SWE = up to 0.30 inch. No accumulation. | SWE = none. No accumulation. | SWE = trace amounts.
For 8000 ft. to 9000 ft.
Saturday Saturday Night Sunday
Weather: Mostly cloudy then becoming partly cloudy. Snow in the morning, then slight chance of snow showers in the afternoon. Snow levels below 7000 feet. Chance of precipitation is 80%. Mostly cloudy. Snow levels below 7000 feet. Chance of precipitation is 0%. Mostly cloudy. Slight chance of snow. Snow levels below 7000 feet. Chance of precipitation is 20%.
Temperatures: 25 to 30 deg. F. 13 to 18 deg. F. 24 to 30 deg. F.
Ridge Top Winds: Southwest 30 to 45 mph with gusts to 85 mph decreasing to 20 to 30 mph with gusts to 65 mph in the afternoon. South around 15 mph with gusts to 35 mph. Southeast around 15 mph with gusts to 35 mph.
Expected snowfall: 80% probability of 2 to 5 inches. 20% probability up to 2 inches. | SWE = 0.15-0.25 inch. No accumulation. | SWE = none. No accumulation. | SWE = trace amounts.
Disclaimer

This avalanche forecast is provided through a partnership between the Tahoe National Forest and the Sierra Avalanche Center. This forecast 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 forecast applies only to backcountry areas outside established ski area boundaries. This forecast describes general avalanche conditions and local variations always occur. This forecast expires 24 hours after the posted time unless otherwise noted. The information in this forecast is provided by the USDA Forest Service who is solely responsible for its content.

For a recorded version of the avalanche forecast call (530) 587-3558 x258