Bollettino valanghe

Central Sierra Nevada

Sierra Avalanche Center
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Pericolo valanghe

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Limite del bosco
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Sotto il limite
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In sintesi

On slopes steeper than 30 degrees, CONSIDERABLE avalanche danger exists on NW-N-NE-E-SE aspects at all elevations due to a combination of wind slabs and persistent slabs. Large human triggered avalanches are likely and natural avalanches are possible. Unusual avalanche conditions exist. Avalanches on steeper slopes could be remotely triggered by a person standing in lower angle terrain or may be triggered in places traditionally considered safe. Careful snowpack evaluation, cautious route finding and conservative decision making are essential.

Problemi valanghivi

  • Wind Slabs

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

    Several human triggered wind slab avalanches occurred yesterday in areas where wind slabs sat above a buried surface hoar layer. Last night strong winds and 8 tp 14 inches of new snow added new wind slabs on top of these already fragile old wind slabs. This new wind loading likely doubled the size of some of these old wind slabs. Large human triggered wind slab avalanches will be likely on any wind loaded N-NE-E aspect or cross loaded NW or SE aspect in near and above treeline today. In many areas one or more buried surface hoar layers exist underneath these wind slabs meaning that remote triggering and extensive propagation of wind slabs will continue today. Wind loaded near treeline NW-N-NE-E aspects represent the best places to find wind slabs on top of surface hoar, but this combination of wind slabs and surface hoar exists on some slopes below or above treeline as well. Some wind slab avalanches likely failed naturally during the storm last night, and while natural activity may have diminished, the possibility of natural avalanches will continue this morning.

  • Deep Persistent Slabs

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

    Storms buried surface hoar layers on Jan. 5th, Jan. 9th, and Jan. 13th. In some places all 3 of these buried surface hoar layers still exist in the snowpack, in others only 1 or 2 exist, and in other places none of them exist. So far data has indicated that the Jan. 9th layer remains the most active. A dense layer of heavy snow that fell on the 13th exists above this layer, and now another 8 to 14 inches of new snow that fell last night has also added its weight to the slab above the fragile surface hoar. Yesterday's persistent slabs have grown larger and easier to trigger. Human triggered persistent slab avalanches large enough to bury people are likely on any slope steep enough to slide where buried surface hoar exists today. Avalanches that fail on buried surface hoar can propagate long distances connecting trees or rocks, they can wrap around corners, and they can occur in places traditionally considered safe. People can also trigger failures in the surface hoar layer on lower angle terrain and that failure can propagate into steeper avalanche terrain and cause avalanches. 

    Buried surface hoar does not exist on every slope so this avalanche problem will not exist on all slopes. Unfortunately, determining which slopes the buried surface hoar does exist on is difficult. Recent avalanche activity, shooting cracks, collapsing, whumpfing, and snowpit tests can provide some clues, but sometimes the first clue is an avalanche triggered by the party on the slope. So far observations have shown buried surface hoar layers on open N-NE-E aspects and a few NW and SE aspects.

Avalanche Discussion

Several human triggered avalanches that involved wind slabs or persistent slabs failing on buried surface hoar layers occurred yesterday. On Mt. Tallac the 3rd rider in a party triggered a wind slab avalanche that failed on a buried surface hoar layer (likely the layer buried on Jan. 9). This slide partially buried the rider and occurred on an E facing wind loaded 35 degree slope. Another party remotely triggered a similar avalanche from a low angle ridge on a N facing slope at the top of Negro Canyon. More avalanches were reported on some slopes off of the nordic trails in Tahoe Donner XC. These were likely triggered by groomers working on the low angle nordic trails some distance from those slopes. On Tallac snowpit tests also yielded unstable results on the layer that the avalanche failed on. In Negro Canyon several other signs of instability existed including lots of whumpfing and skier triggered shooting cracks up to 50 ft. long. For more details check out the observations pages.

Evidence of buried surface hoar layers also existed in the Carson Pass area around Frog Lake and on some slopes in Deep Creek. In both of these areas snowpit tests showed unstable results that indicated that fractures could easily travel along the buried Jan 9th surface hoar layer. On slopes in Deep Creek where no evidence of the buried surface hoar existed snowpit tests did not produce unstable results. Across the Lake on Tamarack Peak observations did not find evidence of buried surface hoar, but did find lingering storm snow weaknesses that still resulted in unstable snowpit tests.

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