THIS AVALANCHE FORECAST EXPIRED ON December 19, 2020 @ 7:00 am
Avalanche Forecast published on December 18, 2020 @ 7:00 am
Issued by Andy Anderson - Tahoe National Forest - Sierra Avalanche Center

Today's biggest concerns exist on steep slopes where old weak sugary snow barely supports the soft slabs of recent snow above it. People could tip the balance and break this layer possibly triggering avalanches. This problem is widespread near and below treeline and isolated above treeline. Shifting winds have also moved slabs of drifted snow into new places at upper elevations. MODERATE avalanche danger exists at all elevations. More accidents happen at MODERATE and CONSIDERABLE than any other danger ratings.

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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A buried layer of weak sugary snow exists on most near and below treeline W-NW-N-NE-E aspects. Observers have even found it in a few isolated areas above treeline. Yesterday brought more reports of collapsing, whumfing, and unstable test results on this persistent weak layer (named the Dec. 11 facet layer). Some data indicating that this layer has started to adjust to the current load was mixed in with these signs of instability, and no one reported new avalanche activity on this layer. While these signs are encouraging, triggering persistent slab avalanches remains possible. All the right ingredients exist: 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. These avalanches could break partway down a slope and propagate farther than expected. 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 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 and whumpfing or snowpit tests 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 remains the best strategy for dealing with persistent slabs. Terrain less steep than 30 degrees that is not connected to steeper slopes offers fun snow and provides a much better safety margin.

Avalanche Problem 2: Wind Slab
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The NE winds have redistributed snow across the ridges scouring previously loaded slopes and forming small slabs of wind drifted snow on new slopes. Wind slabs may exist on almost any aspect since the previous wind slabs may still linger on some slopes. Triggering wind slab avalanches remains a possibility today. Most of these should remain small unless they rest on the persistent weak layer mentioned above. In these cases, larger avalanches that could propagate much farther than expected and be remotely triggered could occur. 

Cornices, wind pillows, and wind created surface textures can all help to identify which slopes may hold wind slabs. Moving into more sheltered terrain or traveling on scoured slopes provides a haven from wind slabs. Unfortunately, scoured slopes are firm and uneven, and the more problematic persistent slabs mentioned above live in the sheltered terrain. The best travel advice is the same as above: 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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*  The Dec 11 persistent weak layer was formed by the faceting process during our extended dry period from Nov 18 through Dec 11. Recent snowfall has buried it more than a foot below the snow surface in many areas. This buried faceted weak layer is widespread below treeline, near treeline, and isolated above treeline on W-NW-N-NE-E aspects. Observers reported more signs of instability including whumpfs on Silver Peak and unstable test results on Silver Peak, in Cabin Creek, and near the Grills.

* No new avalanche activity was reported yesterday.

Weather and CURRENT CONDITIONS
weather summary

Snow showers yesterday morning added another 1 to 4 inches of new snow to the snowpack bringing the storm totals for yesterday's storm to 3 to 6 inches in most places with a few places getting up to 8 inches along the Sierra Crest. As the storm departed the wind shifted to the NE and has been blowing strong enough to move snow along the upper elevation ridgelines since then. High-pressure building over the area should bring sunny skies and a warming trend to the region through the weekend. The winds should also start to decrease and shift back to the W tonight. 

CONDITIONS ALONG THE SIERRA CREST BETWEEN 8200 FT. AND 9200 FT. IN THE LAST 24 HOURS
Weather Station Map
Weather Station Table
6am temperature: 14 to 20 deg. F.
Max. temperature: 23 to 31 deg. F.
Average ridgetop wind direction: SW yesterday morning then NE since then
Average ridgetop wind speed: 20 to 35 mph mph
Maximum ridgetop wind gust: SW: 65 mph | NE: 50 to 70 mph
New snowfall: 1 to 4 inches
Total snow depth: 24 to 33 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Friday Friday Night Saturday
Weather: Sunny. Snow levels below 7000 feet. Chance of precipitation is 0%. Clear then becoming partly cloudy. Snow levels below 7000 feet. Chance of precipitation is 0%. Sunny. Snow levels below 7000 feet. Chance of precipitation is 0%.
Temperatures: 34 to 39 deg. F. 21 to 27 deg. F. 40 to 45 deg. F.
Mid Slope Winds: Light winds. Gusts up to 30 mph in the morning. Light winds. Light winds.
Expected snowfall: No accumulation. | SWE = none. No accumulation. | SWE = none. No accumulation. | SWE = none.
For 8000 ft. to 9000 ft.
Friday Friday Night Saturday
Weather: Sunny. Snow levels below 7000 feet. Chance of precipitation is 0%. Clear then becoming partly cloudy. Snow levels below 7000 feet. Chance of precipitation is 0%. Sunny. Snow levels below 7000 feet. Chance of precipitation is 0%.
Temperatures: 32 to 37 deg. F. 20 to 25 deg. F. 37 to 43 deg. F.
Ridge Top Winds: Northeast 15 to 30 mph with gusts to 55 mph. West 15 to 30 mph. Gusts up to 40 mph increasing to 55 mph after midnight. West 15 to 25 mph. Gusts up to 50 mph decreasing to 40 mph in the afternoon.
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