Avalanche forecast

Central Sierra Nevada

Sierra Avalanche Center
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Avalanche danger

Thu
Alpine
3 · Considerable
Treeline
3 · Considerable
Below
3 · Considerable
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Highlights

A strong winter storm will enter our area today bringing high intensity snowfall and gale force winds.  Avalanche danger will quickly increase today with  avalanches occurring in a variety of areas by this afternoon.  A period of high avalanche danger may occur in the early evening hours.

Avalanche problems

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    1.5–2.5

    A surface hoar layer and weak snow near a crust are buried 1 to 2 feet below the snow surface and have been observed throughout the forecast region.  These layers have been reactive with avalanche activity occurring during our last 2 storm cycles this past week.  Today's storm will rapidly load these weak layers causing another avalanche cycle to occur today and tomorrow.  Where wind blown snow exists above these weak layers, persistent slab avalanches could be much larger in size.

    Recent avalanches, snow surface cracking, snowpack collapse, or whumpfing sounds will indicate unstable persistent slabs.   These avalanches could be triggered from above, below, or from off to the side of a slope.  Cautious route finding will be required to avoid areas where this avalanche problem exists.

    (1/7/2024)

    This 12-18 inch deep avalanche was remotely triggered from the tracks 20 feet to the side of the avalanche.

    Mt. Rose Wilderness.  Jan 7, 2024

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    1–2.5

    Gale force SW winds today with rapid snowfall will build large wind slabs in near and above treeline areas.  These wind slabs will build quickly with the onset of heavy snow this afternoon.  The vast majority of new wind slabs are expected to form today in above treeline and near treeline areas, but may also form to a lesser extent in open areas below treeline due to the strength of the winds.  Wind slab avalanches are expected to occur today.  Wind slabs would be much larger in size where they form and fail on top of the Jan 2 persistent weak layer.

    Look for areas where blowing snow is depositing and for any areas of snow surface cracking.  Clues such as cornice areas, wind drifts, and loaded vs scoured snow surface conditions can help identify the areas of greatest concern. Exercise increased caution today around and below wind loaded areas. 

    (1/7/2024)

    Wind slab avalanche on Elephants Back, Carson Pass area.  Jan 7, 2024

Avalanche Discussion

More on our persistent slab avalanche problem:

The surface hoar and weak snow near crusts buried on Jan 2 have become the most problematic layers in our snowpack. In sheltered areas, these are ~1-2 ft below the snow surface, before our current storm begins today.  They could be much deeper in places, especially where wind-loaded snow exists above them.

For much of the season, the persistent slab problem focused on faceted snow near the base of the snowpack. Substantial strength gains in the midpack and a deepening of the snowpack along the Sierra Crest have reduced concerns for deep weak layer failure until a major atmospheric river type storm with multiple inches of SWE returns to the forecast area. The eastern side of the forecast area has a weaker and shallower snowpack with more concern for future additional loading.

Regions covered