Avalanche forecast

Central Sierra Nevada

Sierra Avalanche Center
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Avalanche danger

Tue
Alpine
3 · Considerable
Treeline
3 · Considerable
Below
3 · Considerable
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Highlights

This is not your average powder day. Human triggered avalanches may be widespread today in open below treeline and near treeline terrain due to surface hoar buried at the base of the most recent new snow. Unstable slabs of wind drifted snow (wind slabs) are also expected above treeline and near treeline. CONSIDERABLE avalanche danger exists for all elevations.

Avalanche problems

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    1–2

    Surface hoar that formed on the snow surface two nights ago and persisted until burial late yesterday afternoon is expected to create highly unstable conditions in open, wind protected below treeline and near treeline terrain. This problematic surface hoar layer is expected on NW-N-NE-E aspects. Surface hoar was likely destroyed yesterday on SE-S-SW-W aspects prior to burial but approach cautiously none the less.

    Even though new snow amounts of 5 to 9 inches will not make for very thick slabs in wind protected areas, don't discount the potential hazard. Avalanches may be triggered from the top, side, or from below on seemingly low angle slopes. Terrain traps could easily create deep burials from shallow slabs so stay out of gullies and creek beds. Very densely treed slopes are the place with the most potential to avoid surface hoar. Any signs of recent avalanches or snow surface cracking are indications of unstable snow in the immediate area.

    Open below treeline terrain like this is ripe for avalanches today due to buried surface hoar.

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    1–2

    Slabs of unstable wind drifted snow have existed for the past two days. Additional snowfall and SW wind last night will have created more drifting and new wind slabs deposited on top of existing wind slabs in above treeline and near treeline terrain. Wind slabs are most likely on NW-N-NE-E-SE aspects but could be encountered anywhere wind drifted snow has deposited. The expected wind slab avalanche size is D2 for most locations today.

    Any slope in above treeline or near treeline terrain that is not scoured down to ice is suspect. Slopes either directly below or adjacent to cornices and wind pillows are highly suspect. Give these areas a wide berth and move around them with an avoidance mindset. Signs of instability such as snow surface cracking or recent avalanches indicate unstable snow is present.

Avalanche Discussion

* Surface hoar ranging from 5 to 20 mm was reported as widespread yesterday in the Donner Summit, Pole Creek, and Echo Summit areas. Smaller surface hoar with less distribution was reported from the Carson Pass area.

* Ongoing blowing snow and unstable wind slabs were observed yesterday near Miess Meadows (Carson Pass) and on Silver Peak (Pole Creek).

* The Dec 11 layer of faceted snow in the lower portion of the snowpack is being monitored for the possibility of a deep slab avalanche problem if the major snowfall event currently expected Tues afternoon-Thurs verifies as forecast.

* With numerous weak layers from the now buried surface hoar, crust-facet sandwiches in the upper snowpack, and the lingering Dec 11 facet layer, the existing snowpack is in poor condition to handle a major new snow load.

Regions covered