* Observers reported more unstable test results on the Dec 11 facet layer in areas where the snowpack remains shallow. Unstable test results came in from Elephant's Hump (Carson Pass) and on Signal Peak (lower elevation peak west of Donner Summit).
* On Silver Peak and on Castle Peak in areas with a deeper snowpack few signs of instability were reported. Snowpit data from Silver Peak indicated that the Dec 11 facets continue to slowly gain strength.
* Observers reported variable conditions yesterday with some soft surface snow on sheltered upper elevation slopes around Elephant's Hump, Castle Peak, and Silver Peak. In more exposed areas wind-scoured, wind-packed, and rain crusted surfaces existed. At lower elevations, a sometimes breakable sometimes supportable rain crust existed on most slopes.
Strong SW winds last night and today may form some small slabs of drifted snow on leeward aspects (NW-N-NE-E-SE) near ridgelines. In many areas, not much snow exists for transport so these wind slabs should remain small. In the areas north of I80, more widespread wind slabs may form if new snow accumulates today. Even in these areas, the wind slabs should remain relatively small since only up to 2 inches of new snow is forecast.
Look for blowing snow, new cornice development, and wind pillows as clues to where wind slabs may exist. Even though they may remain small, wind slab avalanches could still have consequences, especially in complex or extreme terrain.
Old weak snow still remains buried in the snowpack. Fortunately, data targeting the persistent weak layer (Dec 11 facets) indicates a strengthening trend, and triggering a persistent slab is growing more difficult in most places. Unfortunately, persistent weak layers are notoriously variable and do not gain strength uniformly. Some areas where the snowpack remains shallow still exhibit signs of instability like whumpfs, shooting cracks, and unstable snowpit tests. Specific areas with reports of a shallow unstable snowpack include the Carson Pass and Ebbetts Pass areas as well as lower elevation areas in other parts of the forecast region. Triggering a persistent slab avalanche may remain possible on a steep slope where the weak layer sits closer to the surface. A person hitting just the right spot (trigger point) on a slope or larger triggers like multiple people on a slope may still be problematic.
Even though the likelihood of triggering one of these avalanches has decreased, the consequences of being caught in one have not. This uncertain, high-consequence, low-probability problem still warrants caution. Terrain less steep than 30 degrees not connected to steeper slopes or terrain without the weak layer can provide fun recreation opportunities with significantly higher safety margins. The snowpack conditions are improving, and bigger, steeper terrain will still be here after the persistent slab problem goes away.
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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.
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