Avalanche forecast

Central Sierra Nevada

Sierra Avalanche Center
View on map

Avalanche danger

Alpine
2 · Moderate
Treeline
2 · Moderate
Below
2 · Moderate
Open full map
Out of date — this forecast expired 6 years ago. You’re viewing past conditions, not the current bulletin.

Highlights

Triggering an avalanche on steep slopes still remains possible today especially in places with a shallower snowpack and old weak snow below the surface. Some fragile slabs of wind drifted snow may also exist in wind-affected terrain. MODERATE avalanche danger exists at all elevations. Conservative terrain choices are recommended when traveling in the backcountry.

Avalanche problems

  • Deep Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    2

    The likelihood of triggering an avalanche has decreased in many areas. However, persistent slab avalanches do remain possible in some places especially in areas 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 of a large cornice failure could also be more problematic. Persistent weak layers do not gain strength uniformly. Most data indicates that the old weak snow buried in the snowpack continues to adjust to the weight of the snow above it, but some data like the whumpfing and test slope failure reported yesterday from Brockway Summit indicate that the Dec 11 facets remain active.

    Persistent slabs behave in unpredictable ways. Signs of instability like whumpfing, cracking, and unstable snowpack test results may or may not exist. Sometimes triggering a large avalanche represents the first and only clue that the slope is unstable. Travel plans built around 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. In many cases, those slopes hold great snow conditions right now.  While this problem will be here for a while longer, it won't last forever. Be patient, the bigger, steeper terrain will still be here after the persistent slab problem goes away. 

    Test Slope Failure on a N facing below treeline slope on Brockway Summit 1/1/2021.

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    1

    Another bout of strong SW winds over the last 24 hours should have moved snow back across the ridges and built slabs of drifted snow on wind-loaded slopes. Some wind transport may continue today. For the most part, these wind slabs should remain small, but some larger ones could exist on the most heavily wind-loaded slopes. Human-triggered wind slabs will be possible today. 

    Newly developed cornices, wind pillows, and wind created surface textures can all help to identify which slopes may hold wind slabs. Identifying where these wind slabs may exist only represents part of the strategy right now since the persistent slabs mentioned above may exist in the more sheltered terrain.

Avalanche Discussion

* Widespread whumpfing and one test slope failure occurred on below treeline N facing terrain near Brockway Summit yesterday. The failure occurred on the Dec. 11 facet layer. The snowpack in this area is more shallow than in other places around the forecast area.  

* Data and observations over the last 2 days from Indian Valley (near Ebbetts Pass), Becker Peak (near Echo Summit), Stoney Ridge (West Shore), Munchkins and Poulson Peak (Hwy 89 between Tahoe City and Truckee), Donner Summit, and near Independence Lake mostly indicate that the snowpack continues to gain strength. The Dec 11 facets remain buried in the snowpack but they are adjusting to the load above them. The same is true of the interface at the bottom of the 12/25 snow.   

* Reports of shallowly buried surface hoar on sheltered slopes have come in from some areas around the forecast region.

* Observers reported soft cold snow on northerly aspects with warmer wetter snow on sunny more southerly aspects. 

Regions covered