THIS AVALANCHE FORECAST EXPIRED ON December 20, 2020 @ 6:59 am
Avalanche Forecast published on December 19, 2020 @ 6:59 am
Issued by Andy Anderson - Tahoe National Forest - Sierra Avalanche Center

Within the snowpack, lurks old weak sugary snow that can barely support the soft slabs of recent snow. People recreating on steep slopes where this weak snow exists could break this layer possibly triggering large avalanches. This problem is widespread near and below treeline and isolated above treeline. MODERATE avalanche danger exists at all elevations. Traveling on slopes less steep than 30 degrees that are not connected to steeper slopes can provide fun snow and better safety margins.

2. Moderate

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Above Treeline
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.

2. Moderate

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Near Treeline
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.

2. Moderate

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Below Treeline
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
    Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
Avalanche Problem 1: Persistent Slab
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Triggering persistent slab avalanches remains possible today. The layer of weak sugary snow that was buried on December 11th continues to fail as people travel over the snow. Despite no known avalanches on this layer, collapsing, whumpfing, and unstable test results have been widespread near and below treeline on W-NW-N-NE-E aspects. Observers have also found this layer below wind drifted snow in a few isolated areas above treeline. All the right ingredients still exist for persistent slab avalanches. The foot + of recent snow represents the slab. The Dec 11 facets represent the weak layer. A person recreating on the snowpack could provide the trigger. Failures in the facet layer could travel across terrain allowing remote triggering from below, above, to the side, or from any connected terrain. These avalanches could fracture wider than expected and start from unexpected spots on a slope (like midslope instead of the top of the slope). These avalanches would also have serious consequences as many obstacles still exist in the early season snowpack.

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

recent observations
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Observations across the forecast area show that the Dec 11 persistent weak layer remains widespread in near and below treeline terrain on W-NW-N-NE-E aspects. This layer also has been reported in isolated areas above treeline. These facets formed during the extended dry period from Nov 18 through Dec 11. Now recent snowfall has buried them more than a foot below the snow surface in many areas.

Since they were buried on Dec. 11 observers have reported whumpfs and unstable snowpit test failing on this layer every single day in multiple locations. Yesterday was no exception with widespread whumpfs and unstable test results in various areas between Bear Valley and Donner Summit.  

Weather and CURRENT CONDITIONS
weather summary

A clear night with stronger winds along ridgetops and only light winds at the lower elevations allowed cold air to collect in the valleys. This morning sensors reported lower elevation temperatures in the teens and 20's and upper elevation temperatures in the low to mid 30's. This inversion should lift as the day warms up. High-pressure over the area will result in sunny slightly warmer days and cold clear nights through the weekend.

CONDITIONS ALONG THE SIERRA CREST BETWEEN 8200 FT. AND 9200 FT. IN THE LAST 24 HOURS
Weather Station Map
Weather Station Table
6am temperature: 32 to 35 deg. F.
Max. temperature: 36 to 38 deg. F.
Average ridgetop wind direction: ENE shifting to SW during the night
Average ridgetop wind speed: 20 to 30 mph
Maximum ridgetop wind gust: ENE: 79 mph | SW: 38 mph
New snowfall: 0 inches
Total snow depth: 24 to 31 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Saturday Saturday Night Sunday
Weather: Sunny. Snow levels below 7000 feet. Chance of precipitation is 0%. Clear. Snow levels below 7000 feet. Chance of precipitation is 0%. Sunny. Snow levels below 7000 feet increasing to 7000 feet in the afternoon. Chance of precipitation is 0%.
Temperatures: 40 to 45 deg. F. 25 to 31 deg. F. 43 to 48 deg. F.
Mid Slope Winds: Light winds. Light winds. Light winds.
Expected snowfall: No accumulation. | SWE = none. No accumulation. | SWE = none. No accumulation. | SWE = none.
For 8000 ft. to 9000 ft.
Saturday Saturday Night Sunday
Weather: Sunny. Snow levels below 7000 feet. Chance of precipitation is 0%. Clear. Snow levels below 7000 feet. Chance of precipitation is 0%. Sunny. Snow levels below 7000 feet increasing to 7000 feet in the afternoon. Chance of precipitation is 0%.
Temperatures: 37 to 42 deg. F. 24 to 30 deg. F. 39 to 45 deg. F.
Ridge Top Winds: West 15 to 25 mph with gusts to 45 mph. West 15 to 20 mph with gusts to 40 mph. West around 15 mph with gusts to 25 mph in the morning becoming light.
Expected snowfall: No accumulation. | SWE = none. No accumulation. | SWE = none. No accumulation. | SWE = none.
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