THIS AVALANCHE FORECAST EXPIRED ON December 17, 2020 @ 6:54 am
Avalanche Forecast published on December 16, 2020 @ 6:54 am
Issued by Steve Reynaud - Tahoe National Forest

A weak snowpack structure exists throughout most of the forecast region with a buried persistent weak layer below the recent storm snow.  CONSIDERABLE avalanche danger continues due to this persistent slab avalanche problem and the possibility of human triggered avalanches.  Advanced route finding and backcountry decision making skills are required to avoid this avalanche problem.  Avalanche danger is expected to further rise into tomorrow as a winter storm impacts our area. 

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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    Certain
    Very Likely
    Likely
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    Unlikely
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    Very Large
    Large
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Recent storm snow and wind blown snow exists over the top of a buried persistent weak layer in many areas of the forecast region.  This weak layer is widespread in below and near treeline terrain and isolated in above treeline areas on W-NW-N-NE-E aspects.  The persistent weak layer is buried up to 1 foot deep with whumpfing sounds, cracking, and unstable snowpack test results reported throughout large parts of the forecast area over the last 2 days.  Where this weak layer is present makes up a large percentage of the overall useable terrain with our early season shallow snowpack.  Human triggered avalanches remain possible today with the potential for wide propagation, remote triggering from any connected terrain, and mid slope avalanches.  Additional load from the storm forecasted for tonight and tomorrow is expected to cause this weak layer to fail in some areas.

Managing a persistent slab avalanche problem is challenging.  We know we have a poor snowpack structure and there are many signs of instabilities, but we haven't seen any avalanche activity.  The only way to effectively manage this avalanche problem is to use terrain to avoid areas where this problem is located until it goes away.  Limit terrain choices to under 30 degrees, avoid avalanche run out zones and any connected terrain which may be steeper that could be remotely triggered.  Consequences of any avalanche continue to remain elevated due to shallow snowpack conditions and exposed obstacles.

 

recent observations
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*  The Dec 11 persistent weak layer was formed by the faceting process during our extended dry period from Nov 18 through Dec 11 and is now buried 6 to 12'' below the snow surface.  This buried faceted weak layer is widespread below and near treeline, and isolated above treeline on W-NW-N-NE-E aspects.  Widespread whumpfing, cracking, and unstable snowpack test results have been reported throughout the forecast region over the past 2 days.

*  Light snow fell yesterday in some high elevation areas with a light rain/mist event that was reported up to at least 8500'.  Small roller balls, tree drippings, and generally damp conditions were reported from several locations.

 

Weather and CURRENT CONDITIONS
weather summary

Clouds and wind will be on the increase today ahead of a quick moving storm overnight.  A winter weather advisory is in effect from 10pm tonight until 10am Thursday morning.  3 to 8'' are forecasted for around lake level with up to 12'' above 7000'.  Winds will be shifting from the NE to the SW and increasing to gale force speeds by late tonight.  High pressure is expected to build back into our area for this weekend with mild temperatures and light winds. 

CONDITIONS ALONG THE SIERRA CREST BETWEEN 8200 FT. AND 9200 FT. IN THE LAST 24 HOURS
Weather Station Map
Weather Station Table
6am temperature: 27 to 30 deg. F.
Max. temperature: 32 to 36 deg. F.
Average ridgetop wind direction: NW to NE
Average ridgetop wind speed: 15 to 25 mph
Maximum ridgetop wind gust: 41 mph
New snowfall: Trace snow/rain mix inches
Total snow depth: 22 to 28 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 then becoming mostly cloudy. Snow levels below 7000 feet. Chance of precipitation is 0%. Partly cloudy then becoming mostly cloudy. Slight chance of snow in the evening, then snow after midnight. Snow levels below 7000 feet. Chance of precipitation is 80%. Cloudy. Snow in the morning, then snow likely in the afternoon. Snow levels below 7000 feet. Chance of precipitation is 100%.
Temperatures: 39 to 44. deg. F. 24 to 29. deg. F. 28 to 33. deg. F.
Mid Slope Winds: West around 10 mph increasing to 10 to 20 mph with gusts up to 30 mph by late in the afternoon. Southwest 15 to 25 mph with gusts to 50 mph. Southwest 15 to 25 mph with gusts up to 50 mph decreasing to west 10 to 20 mph in the afternoon.
Expected snowfall: No accumulation. | SWE = none. 80% probability 1 to 3 inches. 20% probability of 3 to 6 inches. | SWE = 0.15-0.40 inch. 80% probability of 3 to 5 inches. 20% probability of 5 to 7 inches. | SWE = 0.25-0.50 inch.
For 8000 ft. to 9000 ft.
Wednesday Wednesday Night Thursday
Weather: Partly cloudy then becoming mostly cloudy. Snow levels below 7000 feet. Chance of precipitation is 0%. Partly cloudy then becoming mostly cloudy. Slight chance of snow in the evening, then snow after midnight. Snow levels below 7000 feet. Chance of precipitation is 80%. Cloudy. Snow in the morning, then snow likely in the afternoon. Snow levels below 7000 feet. Chance of precipitation is 100%.
Temperatures: 36 to 42. deg. F. 20 to 25. deg. F. 23 to 29. deg. F.
Ridge Top Winds: West 10 to 20 mph increasing to 20 to 30 mph with gusts to 50 mph in the afternoon. Southwest 30 to 50 mph. Gusts to 70 mph increasing to 90 mph after midnight. Southwest 30 to 50 mph with gusts to 90 mph becoming west 15 to 25 mph with gusts to 50 mph in the afternoon.
Expected snowfall: No accumulation. | SWE = none. 80% probability of 1 to 3 inches. 20% probability of 3 to 6 inches. | SWE = up to 0.45 inch. 80% probability of 3 to 6 inches. 20% probability of 6 to 8 inches. | SWE = 0.35-0.55 inch.
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