Bollettino valanghe

Central Sierra Nevada

Sierra Avalanche Center
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Pericolo valanghe

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Alta quota
3 · Marcato
Limite del bosco
3 · Marcato
Sotto il limite
3 · Marcato
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In sintesi

Areas of unstable snow are expected today following the high intensity snowfall, gale force winds, and avalanches that occurred yesterday. Backcountry travelers venturing into avalanche terrain have an increased likelihood of triggering an avalanche today. Consider avoiding slopes adjacent to recent avalanches, below cornice features, other areas where wind drifted snow has been deposited, or any avalanche terrain where snow surface cracking is occurring.

Problemi valanghivi

  • Storm Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    1–2

    Unstable storm slabs were observed yesterday in wind protected areas below treeline as new snow deposited faster than it could bond to itself and to the old snow surface below. Some of yesterday's storm slabs were failing on density changes within the new snow, others were failing on crust at the old/new snow interface. Slabs of unstable storm snow may linger today in wind protected areas on all aspects. Crust, graupel, and/or rime at the old/new snow interface may slow the process of storm slab stabilization.

    If snow surface cracking is occurring or test slopes are failing, pull back from avalanche terrain and onto sub 30 degree slopes without steeper terrain above.

  • Wind Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Likely
    Dimensione
    2–3

    Unstable wind slabs were observed yesterday with wide propagating avalanches occurring on test slopes.  There remains an increased likelihood for backcountry travelers to trigger wind slab avalanches today mainly near treeline and above treeline where wind drifted snow has deposited on NW-N-NE-E-SE aspects. Wind slabs also exist to a lesser extent in below treeline areas due to the strength of the recent SW winds.

    Identify and consider avoiding the avalanche terrain where wind drifted snow has deposited as a thick wind slab. Large to very large avalanches could be triggered today, especially on open slopes below cornices and cliff bands. In addition to recent avalanches, snow surface cracking triggered while making new tracks in the snow is a sign of instability and a signal to pull back from avalanche terrain.

    (1/14/2023)

    The red arrow indicates the far edge of an intentionally triggered wind slab avalanche. The second wind slab avalanche lower on the slope in the foreground was sympathetically triggered by the upper avalanche. Webber Peak, Little Truckee Summit area, CA

    Photo: SAC

Avalanche Discussion

There are numerous rain crusts buried in the snowpack that could have small amounts of faceting around them due to melt layer recrystallization. Some of these crusts are now buried 8 to 17 feet deep in the snowpack making them very difficult to assess with snowpit tests. The vast majority of avalanches at this time are expected to occur at or above the old/new snow interface. An isolated avalanche event up to D4 in size that fails on a crust deep in the snowpack would not be a major surprise during or immediately after this storm cycle. It would be impressive, but not a representation of the vast majority of expected avalanche activity.

Recent Observations

  • Size D1 to D2 wind slab and storm slab avalanches were reported yesterday along with other signs of instability including snow surface cracking.
  • Prior to the recent snowfall, rain crust was widespread on the old snow surface across all aspects below 8,200' to 8,600'.
  • At higher elevations above the areas of rain crust, sun crust exists below the recent storm snow across SE-S-SW aspects up to at least 9,700'.

Regioni coperte